Suspension Mythbusting: Solid Axle vs. IRS

Everyone fixates on track handling, but on the street, what the car has makes an enormous difference in ride comfort due to the sprung-to-unsprung weight ratios. The lighter the car, the more pronounced it becomes and on Lotus Seven type cars (or even Miatas), it's honestly the difference between a comfortable car and one you don't want to drive on an even slightly bumpy road.

Well, i see i am not needed here 

I do think it's kind of funny that most of what people do to their cars to make them "handle better" is basically just turning their independent suspensions into more and more like a solid axle. For instance installing super stiff springs or sway bars to try to keep camber from changing through roll.

I have plans for building an fsae car for SCCA autocross in the future and I'm definitely going to be using solid axles (with probably a de Dion in the rear) for sheer simplicity. There are actually tons of benefits to it compared to independent stuff. For instance, it takes far fewer suspension pickup points on the chassis. You also get consistent predictable camber behavior. It also makes a great platform to mount sprung arrow elements to, which is totally legal in that class.

kb58 said:

Everyone fixates on track handling, but on the street, what the car has makes an enormous in difference in ride comfort due to the difference in sprung-to-unsprung weight ratios. The lighter the car, the more pronounced it becomes and on Lotus Seven type cars (or even Miatas), it's honestly the difference between a comfortable car and one you don't want to drive on an even slightly bumpy road.

I've put a solid axle in the rear of my V8 MG, which has Miata front suspension and very similar weight and balance to the Miata. I'm having a hard time getting the ride quality in the rear where I want it, and I think it's in large part due to that massive axle bouncing around.

In reply to Keith Tanner :

Based on the fun I had loading the Explorer axle for the Ranchero out of the van, I'm pretty sure it's having a substantial impact on the sprung:unsprung ratio... It's not apples to apples comparing it to the A1 VW knuckle on the shelf, but I can't help noticing when I pass them both...

A bit off topic, but loading into the van with the help of an exuberant but cavalier assistant reminded me how much I prefer doing that sort of thing myself. Even if the total time it takes is longer, the number of digits retained per unit time is better.

Solid axles can definitely be a problem in the unsprung weight department, but other than that, they're generally simpler to get setup in a way that works well.  Even a solid front axle isn't a bad thing if it's done well. 

I prefer IRS for the street. On a smooth track, I am not sure it really matters too much. On the street the suspension has way too much to deal with, potholes, camber changes, surface changes, dirt, ice, snow, rain, and that is before you try to turn

I’ll tell you this, my ford 8.8 lifts the inside tire on off camber corners on the street. It sucks in the rain or snow, since you can’t whomp on the gas to pull out in traffic. 

freetors said:

I do think it's kind of funny that most of what people do to their cars to make them "handle better" is basically just turning their independent suspensions into more and more like a solid axle. For instance installing super stiff springs or sway bars to try to keep camber from changing through roll.

In a way, that is actually making them the opposite of a solid axle, because bump on one end will result in droop in the other and vice versa, since the springs are by necessity not located over the contact patches.  It is sort of like having a built in Z bar.

If it is not apparent, i consider this to be a virtue and not a vice.  Especially when judicious amounts of anti squat are present and the goal is to accelerate as hard as practical over rough terrain.

In reply to Trackmouse :

Isn't that more of a tuning problem than a stick axle problem? 

In reply to BrokenYugo :

Sounds like the rear roll center is too high or the front suspension isn't stiff enough in roll.  Maybe becaus ITs roll center is too low...

Trackmouse said:

I’ll tell you this, my ford 8.8 lifts the inside tire on off camber corners on the street. It sucks in the rain or snow, since you can’t whomp on the gas to pull out in traffic. 

That's not a solid axle specific problem.  It's a "too much weight transfer off the inside rear in turns" problem.  Could be a mix of too much caster, too much rear sway bar, not enough front bar or a few other issues. 

Knurled. said:

freetors said:

I do think it's kind of funny that most of what people do to their cars to make them "handle better" is basically just turning their independent suspensions into more and more like a solid axle. For instance installing super stiff springs or sway bars to try to keep camber from changing through roll.

In a way, that is actually making them the opposite of a solid axle, because bump on one end will result in droop in the other and vice versa, since the springs are by necessity not located over the contact patches.  It is sort of like having a built in Z bar.

 

If it is not apparent, i consider this to be a virtue and not a vice.  Especially when judicious amounts of anti squat are present and the goal is to accelerate as hard as practical over rough terrain.

It's kind of funny you mention Z-bars. I'm actually planning on incorporating longitudinal z-bars (probably in the form of lightweight center pivot leaf springs) and having a monoshock/spring at the front and rear. The theory is that it should articulate like a mini rock crawler and keep load variations on the tires to an absolute minimum while driving on uneven pavement.

My Datsun roadster project and I thank you for the article. 

Michael

I think the main advantage for street cars besides unsprung weight (and load floor height which is why even truck based SUVs went IRS) is that if you encounter a bump while loaded up hard in a corner, you are less likely to spin out of control. 

I once went a road rally with a car with a stiffened torsion beam that worked well in autocross. On actual hilly curvy (bumpy) roads, i felt it was dangerous to keep pace with IRS cars that were closer to stock. That's anecdotal, but it's the difference between a flat track and a real road. 

When I asked about the benefits of independent suspension, I got a completely different answer!

https://www.youtube.com/watch?v=fWOmhngbHQc

(Linked, and not embedded because while it is just a commercial from Nissan it does push the NSFW boundaries)

Also:

So if the torque arms are better for weight transfer, and the IRS is better for unsprung weight and not having your shoes tied together... Does the answer become "De-Dion"?

Hungary Bill said:

Also:

So if the torque arms are better for weight transfer, and the IRS is better for unsprung weight and not having your shoes tied together... Does the answer become "De-Dion"?

De Dion doesn't give you any anti squat because the drivetrain torque loadings are divorced from the suspension.  All a de Deon is, is a really heavy IRS.

I like what BMW did for the E36's ti chassis cars.  It's the old trailing arm suspension but they mounted the front on some bushings with a ton of vertical compliance.  So you get some anti squat as you accelerate and the nose of the diff pushes the subframe up, altering the geometry for a higher instant center and therefore more anti squat.  Not a lot, but some, which is better than none.  And then when braking, the brake torque on the trailing arms pushes the subframe down, minimizing the brake hop that you can get with an excessively high/short instant center.

You fix the brake hop problem on solid axle cars by floating the calipers on bearings that have their own linkage to the chassis, so brake torque doesn't go through the rearend the same way...

Vigo said:

I think the main advantage for street cars besides unsprung weight (and load floor height which is why even truck based SUVs went IRS) is that if you encounter a bump while loaded up hard in a corner, you are less likely to spin out of control. 

I once went a road rally with a car with a stiffened torsion beam that worked well in autocross. On actual hilly curvy roads, i felt it was dangerous to keep pace with IRS cars that were closer to stock. That's anecdotal, but it's the difference between a flat track and a real road. 

A well set up solid axle that's not way too heavy for the car it's under isn't too bad about that.  Keeping enough sidewall on the tires helps a lot too, as it'll let the tires soak up the small bumps.  Having shocks that are well matched to the springs and chassis makes a big difference on this one.  Not using too much rear sway bar helps a lot too.  

Before building my Locost (and still now, 12 years later), I waffled back and forth between a stick axle and IRS.

My stick-axle Locost has been on the road, travelled 500km on the Trans Canada to autocross events and back, commuted as a daily for a time, and I've driven it everywhere, rain, shine, snow, even dirt roads (-that- sucked).

I've come to a couple conclusions:

1) If you want comfort, you really wouldn't pick a Lotus 7 or replica thereof.  Comfort is not their forte.

2) If, on the street, the stick axle is a limited factor, you are likely traveling faster than you legally should be going.  If it got you in trouble, it's going to be a big "off."

3) I'm not totally convinced an IRS is going to net a "lighter overall" weight of vehicle, and maximum lightness is what I want.

I don't believe that swapping TO an IRS is going to be worthwhile for -me-. If I was building a project and I had to source an axle, I'd probably go IRS, but the stick works well enough, and if it came with your donor vehicle, or it's already under your project vehicle, it will work fine.

SkinnyG said:

If you want comfort, you really wouldn't pick a Lotus 7 or replica thereof.  Comfort is not their forte.

Ha!  No kidding.  I really, really thought I was tough enough to daily drive a Cobra replica -- but spending the day with one nuked that idea.  Comfort is not their forte.  My commute involves a 40 mile mix of highway driving at high speeds and bumper-to-bumper traffic, and a Cobra is uncomfortable at both.

SkinnyG said:

Before building my Locost (and still now, 12 years later), I waffled back and forth between a stick axle and IRS.

My stick-axle Locost has been on the road, travelled 500km on the Trans Canada to autocross events and back, commuted as a daily for a time, and I've driven it everywhere, rain, shine, snow, even dirt roads (-that- sucked).

I've come to a couple conclusions:

1) If you want comfort, you really wouldn't pick a Lotus 7 or replica thereof.  Comfort is not their forte.

2) If, on the street, the stick axle is a limited factor, you are likely traveling faster than you legally should be going.  If it got you in trouble, it's going to be a big "off."

3) I'm not totally convinced an IRS is going to net a "lighter overall" weight of vehicle, and maximum lightness is what I want.

I don't believe that swapping TO an IRS is going to be worthwhile for -me-. If I was building a project and I had to source an axle, I'd probably go IRS, but the stick works well enough, and if it came with your donor vehicle, or it's already under your project vehicle, it will work fine.

"Comfort" is a very subjective term and not something everyone's going to agree on. Your experience with it only being an issue at high speed is probably safety-related, but I want to stick to what's comfortable. I have real-world back-to-back experience with both as well and can say that a heavy rear axle and light car will always make for uncomfortable driving, regardless of speed.

For comparison, my brother has a Seven clone, front LS3 engine and straight rear axle. I have a mid-engine "Seven", with IRS. Both weigh the same, have the same power, and are nearly the same size. We each drove both cars down the same street at about 30 mph. At one point, my brother said I needed to slow down for a bump - a 1"-high step in pavement height. I saw it and didn't slow, and my brother was dumbfounded how he could hardly feel the bump. Later when I drove his car, I understood why he spoke up, as hitting the same bump at the same speed was like driving over a curb - it hurt.

Two things were at work, obviously the straight axle versus IRS, but also sprung-to-unsprung weight. His car is front engine while mine is mid, so my car has a lot more weight over the tires that most affect ride comfort, so the weight ratio is a lot higher. Also, because he's using an LS3, he needs a heavy duty rear axle assembly, and this one weighs 150 lbs just for the axle alone, without brakes, wheels, or tires. As a result, the weight riding directly on the ground is a fairly large fraction of the sprung mass riding on it. The result is that a far greater fraction of a given bump is fed straight through into the chassis - and occupants.

It's interesting that this thread came up now, as just last week we were discussing him converting to IRS. I asked why, because the reason matters. If it's for a faster lap time, I doubt the conversion would help, and might even hurt. On the street though is a completely different story and it absolutely will improve the ride.

So at the end of the day, what's "better" and more "comfortable" are however you define them to be.

kb58 said:

For comparison, my brother has a Seven clone, front LS3 engine and

I had the same car as Kurt's brother (with a smaller motor). It would happily run with ZO6 Vette's at Sears Point or Laguna Seca, but struggle to keep up with 1st gen Miatas on a poorly maintained twisty road. The difference: purely that rear axle. 

Excessive unsprung weight is still excessive unsprung weight.

That is why I went to moderately extreme lengths to try to get a 9" as light as the Mazda 7" that I am taking out.  I still need to find a scale but I am pretty sure the Mazda rear is 160lb with brakes.  I still have a couple places to shed weight but I already hit all of the cheap ones.

And why I want to build a Ford 7.5" for the RX-3.  Them suckers is light!

Yeah, like Kreb said, on-track, my brother's car (a Stalker) runs as fast or faster than just about anything - on a smooth track.

So, if you can get a really light axle with a strong enough differential to weigh the same as the unsprung weight of an IRS setup, then a straight axle will have the same ride quality on the street as an IRS setup, and will probably also be better on-track. The hard part is making it that light. As mentioned, how much of a difference it makes relates directly with how much weight is riding on it. If you do the swap on a heavy car like the Dodge Hellcat (4400 lbs!!!), I doubt the driver would really notice the difference. That said, it's interesting that the Hellcat is IRS, so maybe a strong enough (read: really heavy) straight rear axle would still have a noticeable impact on ride quality. It would be really interesting to hear from the Dodge engineers about why they chose IRS. My guess it's marketing, that their competition has it, not necessarily that it's "better."

It would be really interesting to hear from the Dodge engineers about why they chose IRS. My guess it's marketing, that their competition has it, not necessarily that it's "better."

It's because they made the Challenger by taking an entry-lux family car (that sold substantially on comfort) and making it less practical while not spending money on new underpinnings. 

I've read some on this subject.  There are (as my computer teacher from Korea at Blinn used to say) ad-a-vantages and dis-ad-a-vantages to both.  What are you building?  What are your goals?

All suspension design comes down to keeping the tire at the optimal "sticky" angle to the road.  Smith has a graph of a race tire in it, and the peak grip was at -1 degree camber.  It was kinda flat from 0 to -1.5, if I recall (look it up yourself or better yet, do your own measurements on your tires).

IRS:

Advantages: 

• Hitting a bump with the inside will will not upset the camber of the outside wheel.  When cornering, the outside wheel is more important in traction than the inside wheel because the weight has shifted to it.  Keeping that outside wheel at -1 degree WITH RESPECT TO THE ROAD, NOT THE CAR is the most important thing.  Hit a bump like a track corner rumble strip on the inside with a IRS and you don't upset the important outside tire to road angle.
• Unsprung weight.  The pumpkin is bolted to the chassis, thus sprung.  Less unsprung weight is considered better.
• You can design the inside camber curve to go positive in droop, keeping it at -1 degree with respect to the road.

Disadvantages:

• More complex to design and build
• Building a camber curve that optimizes for cornering will result in less than optimal camber for a drag launch.  That is, when the car squats down (suspension in bump), the built in gain in camber will give you, say, going from -1 to -3 degrees squatted camber, and now you are outside the optimal traction area of the tire to road angle curve.

Live axle:

Advantages:

• Simple to design.  Want -1 degree camber?  Bend the tubes until that's what you got.  Done.  You'll (kinda, see below) always have -1 degree tire to road camber.
• Better drag launch.  The tire to road angle does not change with squat. If you have enough HP to have squat down the straight part of the track, you'll have better traction with a live axle.

Disadvantages:

• Unsprung weight.  The pumpkin is now unsprung. That's like 80+ lbs that didn't have to be there.
• Taking that corner on the track, you clip the inside, because Apex, and the inside tire is on the rumble strip, bouncing up and down.  That movement is transferred to the important outside tire, which now is changing camber all over the place, limiting traction.

Briefly, DeDion.  For those unfamiliar with it, a DeDion axle has the pumpkin bolted to the chassis, axles going to the hubs, and the 2 hubs are mounted to a tube connecting them.  I think Caterhams were using DeDion axles for a long time.

Advantages:

• Unsprung weight is less than live axle, more like IRS.
• In Squat (bump), behaves like a live axle with no camber change.

Disadvantages:

• Has the same problems as a live axle in regards to the inside tire upsetting the outside.

There's a reason drag race cars use live axles and road race cars use IRS.

Smith also said that it was his observation that race tracks weren't all that smooth.  Advantage IRS.

I honestly believe a well engineered de Dion setup could be as light or lighter than an equivalent IRS. Most all the de Dion pictures I can find depict de Dion as having a large, heavy tube spanning the width of the axle. However this can easily be replaced by a very light and stiff steel tubular framework that's three dimensional. I once saw a picture of a very elegant setup (I believe it was a Ferrari Lemans prototype car?) but I can't find it now.

Here is a decent system IMO. BTW, this is currently the fastest autocross car in the country. This lets you adjust camber and toe just as easily as an independent system too.

Dr. Hess said:

Smith also said that it was his observation that race tracks weren't all that smooth.  Advantage IRS.

 

Agreed - or, if you look at it another way, having a car that can deal with rough surfaces means you can go places on the racetrack that other people can't.

Vigo said:

It would be really interesting to hear from the Dodge engineers about why they chose IRS. My guess it's marketing, that their competition has it, not necessarily that it's "better."

It's because they made the Challenger by taking an entry-lux family car (that sold substantially on comfort) and making it less practical while not spending money on new underpinnings. 

No, because the Challenger had IRS from day one. It's based on the LX platform  (I think that's the code) which also had IRS from day one. And the LX was based on a Daimler chassis that had IRS from day one. It wasn't an add on like the New Edge Cobra. 

One place where solid axles rule the day (and night).

freetors said:

Here is a decent system IMO. BTW, this is currently the fastest autocross car in the country. This lets you adjust camber and toe just as easily as an independent system too.

I really, really like that. That is a very tempting way to go on Locost #2.

If this were Grassroots dragsports, we'd be talking about whether a Ford 8.8 was a good solution to get rid of the IRS in an M3.

As said, a DeDion implemented with a space frame can be far lighter than a huge tube stressed entirely in bending.

Could also be done as a stressed skin torque box (essentially a monocoque structure), but it is far more difficult to package,

since the diff will be enclosed within it, and you must have openings for things like the driveshaft, axles, etc.

(ISTR that Jim Hall tried something like that on one of the later Chapparals)

Think of a Classical DeDion as two vertical links with hubs that carry the wheels, rigidly connected by a stiff beam.

Even if there is built in pre-set camber or toe, the rigid beam is responsible for all wheel location and control, just like a live axle.

But lets think a moment about what is really required, and what we might do differently to achieve the same goal:

1) Camber control; just like a live axle, each tire should be equal, if one side changes the other side does too by same amount.

2) Toe control; just like a live axle, each tire should be equal, if one side changes the other side does too by same amount.

3) Caster control ... caster control ... caster control ... hmmm, maybe not.

What if our rigid 'beam' was stiff in the horizontal plane to control toe, and stiff in the vertical plane to control camber,

but was torsionally flexible?

For 'caster' control, could use a 4 link setup, a link connected from the chassis to the top and bottom of each 'vertical link'.

That would handle suspension fore/aft location, as well as tire forces reacting from braking and acceleration.

When the wheels are at different heights, ie one wheel is compressed, the other wheel is extended, the two 'castor' angles will differ.

But the 'beam' is torsionally flexible enough to twist and accommodate the few degrees needed, remaining stiff for toe and camber control.

What does that change buy us?

Such a flexible beam could be formed as a 'U' channel, or a 'T' or 'L' section, or even a light spaceframe having triangulation

in only 2 of the three dimensions, designed for limited torsional compliance.

All three of those structures could be very light in weight, yet open enough to enable packaging with access to the diff, driveshaft, and axles.

"But what about lateral location of the DeDion assembly, how can such a configuration handle side loading from cornering?"

One method is a Watts linkage, but employing a chassis mounted bell crank rather than a suspension mounted one. (reduces unsprung weight!)

The side links from the bell crank, rather than attaching to the chassis, would attach to the 'vertical links' instead.

For additional packaging flexibility, the bellcrank could be laid flat on top of or underneath the diff, rather than standing up.

Such a setup could combine the light weight and low unsprung weight of an IRS, with the effective tire location and minimal camber

of a live axle, in a relatively simple and straightforward configuration.

If cleverly implemented, it would offer easy pre-set adjustment of camber and toe (as I believe the autocross car setup shows).

I call this concept the 'Floppy D'.

In reply to clshore :

basically you are talking about a "twist beam" as used by many FWD cars, but as a RWD DeDion axle.

freetors said:

Here is a decent system IMO. BTW, this is currently the fastest autocross car in the country. This lets you adjust camber and toe just as easily as an independent system too.

Thanks for posting this. I am generally a proponent of the DeDion rear and had thought about similar such adjustable configurations before, including connecting it to the chassis via Satchell link, but had never seen one as such in practice.  Very cool.  I am somewhat surprised that they were able to get away with putting the bending loads from the toe and camber links into such a small diameter tube, but it's obviously working on that car.

ncjay said:

One place where solid axles rule the day (and night).

All successful front engined rear drive stage rally cars have been solid axle.

I run a for 9" solid axle in my 2500 lb modified over some on the roughest asphalt you can imagine (hillclimb).  Its a 3 link setup with very soft springs and no sway bars front or rear for all of the funny attitudes and bumps that I encounter.  Its actually a surprisingly capable setup, even with all the bumps, off camber corners and wicked crowned roads.  Every time I think about an IRS, I imagine it exploding with all of the shock loads.  Its hard to beat solid axle for strength per $.

Appleseed said:

Vigo said:

It would be really interesting to hear from the Dodge engineers about why they chose IRS. My guess it's marketing, that their competition has it, not necessarily that it's "better."

It's because they made the Challenger by taking an entry-lux family car (that sold substantially on comfort) and making it less practical while not spending money on new underpinnings. 

No, because the Challenger had IRS from day one. It's based on the LX platform  (I think that's the code) which also had IRS from day one. And the LX was based on a Daimler chassis that had IRS from day one. It wasn't an add on like the New Edge Cobra. 

Owning a 2012 SRT8 Challenger I will say that the IRS is wonderful at bombing down rough VT country roads.  The IRS soaks up the bumps with ease, in a way that even a well tuned solid axle would have much trouble with.

Dr. Hess said:

I've read some on this subject.  There are (as my computer teacher from Korea at Blinn used to say) ad-a-vantages and dis-ad-a-vantages to both.  What are you building?  What are your goals?

All suspension design comes down to keeping the tire at the optimal "sticky" angle to the road.  Smith has a graph of a race tire in it, and the peak grip was at -1 degree camber.  It was kinda flat from 0 to -1.5, if I recall (look it up yourself or better yet, do your own measurements on your tires).

IRS:

Advantages: 

• Hitting a bump with the inside will will not upset the camber of the outside wheel.  When cornering, the outside wheel is more important in traction than the inside wheel because the weight has shifted to it.  Keeping that outside wheel at -1 degree WITH RESPECT TO THE ROAD, NOT THE CAR is the most important thing.  Hit a bump like a track corner rumble strip on the inside with a IRS and you don't upset the important outside tire to road angle.
• Unsprung weight.  The pumpkin is bolted to the chassis, thus sprung.  Less unsprung weight is considered better.
• You can design the inside camber curve to go positive in droop, keeping it at -1 degree with respect to the road.

Disadvantages:

• More complex to design and build
• Building a camber curve that optimizes for cornering will result in less than optimal camber for a drag launch.  That is, when the car squats down (suspension in bump), the built in gain in camber will give you, say, going from -1 to -3 degrees squatted camber, and now you are outside the optimal traction area of the tire to road angle curve.

 

Live axle:

Advantages:

• Simple to design.  Want -1 degree camber?  Bend the tubes until that's what you got.  Done.  You'll (kinda, see below) always have -1 degree tire to road camber.
• Better drag launch.  The tire to road angle does not change with squat. If you have enough HP to have squat down the straight part of the track, you'll have better traction with a live axle.

Disadvantages:

• Unsprung weight.  The pumpkin is now unsprung. That's like 80+ lbs that didn't have to be there.
• Taking that corner on the track, you clip the inside, because Apex, and the inside tire is on the rumble strip, bouncing up and down.  That movement is transferred to the important outside tire, which now is changing camber all over the place, limiting traction.

You have good points but you are missing the main advantage that live axles have over IRS, and the very subject of the article that this thread is based on:  The ability for a live axle to exploit drivetrain torque. 

I have admittedly little smooth-surface experience, but I do have a lot of experience on irregular surface.  In my experience there, live axle is king, specifically because it is irregular surface. 

With an IRS, the drivetrain has no effect on suspension loadings.  You accelerate, you brake, none of that really affects the contact patches, other than gross weight-transfer means.  Rotation on turn in has to be compromised against acceleration on corner exit.

With a solid axle, you can play with the instant center so that drivetrain torque forces the tires into the ground.  This is "anti squat".  You get an effect where the harder you are accelerating, the more grip you have, because the very act of trying to put power down will create grip.  This is something that is impossible to get with an IRS in effect significant enough to be worthwhile.  In addition,  the effect is greater the rougher the surface is, because of the articulation of a solid axle.   The end result is a vehicle that can rotate in when under braking, and will accelerate straighter the more power is being fed in.  Or, colloquially, a win-win scenario.

You also have made a large incorrect assumption, that one wheel bumps only affect one wheel with an IRS.  A one wheel bump affects the whole chassis, with that wheel as the epicenter, no matter what kind of suspension a car has.  A solid axle at least pays the dividend of redistributing some of the load to the other rear wheel via leverage against the springs/shocks, rather than unloading it via chassis lift.  From a NVH aspect, the solid axle is bad because a one wheel bump affects both sides of the spring/shock department.  From a grip aspect, it is good because it gives more grip to the non bump side. 

Knurled. said:

ncjay said:

One place where solid axles rule the day (and night).

 

All successful front engined rear drive stage rally cars have been solid axle.

Hate to break it to you, but no. The Fiat 131 was the last successful Front Engined Rear Drive Rally car, it had an IRS suspension even if the road car did not. The Fiat 124 spider was successful in it's class (It was overshadowed by the all mighty Stratos) was also equipped with an independent rear suspension. Again, both these cars used a "stick" rear suspension on the regular road cars that normal people bought, but once gone over by Abarth, they received an IRS. After them came the mid-engined and AWD cars that still dominate today

mad_machine said:

Knurled. said:

ncjay said:

One place where solid axles rule the day (and night).

 

All successful front engined rear drive stage rally cars have been solid axle.

Hate to break it to you, but no. The Fiat 131 was the last successful Front Engined Rear Drive Rally car, it had an IRS suspension even if the road car did not. The Fiat 124 spider was successful in it's class (It was overshadowed by the all mighty Stratos) was also equipped with an independent rear suspension. Again, both these cars used a "stick" rear suspension on the regular road cars that normal people bought, but once gone over by Abarth, they received an IRS. After them came the mid-engined and AWD cars that still dominate today

The last truly successful RWD rally car was the MkII Escort.  Successful including privateers and not just factory backed throw-money-at-it cars.

The last truly good Group B RWD car was the Toyota Celica, which was more or less a copy of the Escort chassis-wise, with a lot of power.

With an IRS design, you can build in anti-squat pretty easy.  I put some in on my Locost.

Also, that whole driveline torque loading up the suspension -- more pressure on the contact patch thing, you are forgetting about the no free lunch.  That more pressure on, say, the left contact patch comes at the price of less pressure on the right contact patch.  Eventually, you reach a limit of that left contact patch's grip earlier than you would if both contact patches had equal weight on them.  It may help some in exiting, say in this example, a right turn, but you would have less grip in exiting a left turn.  Or vice versa, depending on limits of adhesion and forces involved.

Driven5 said:

freetors said:

Here is a decent system IMO. BTW, this is currently the fastest autocross car in the country. This lets you adjust camber and toe just as easily as an independent system too.

Thanks for posting this. I am generally a proponent of the DeDion rear and had thought about similar such adjustable configurations before, including connecting it to the chassis via Satchell link, but had never seen one as such in practice.  Very cool.  I am somewhat surprised that they were able to get away with putting the bending loads from the toe and camber links into such a small diameter tube, but it's obviously working on that car.

I had a think about this and they may be have designed in some compliance/flexibility so that they get some camber gain in cornering. It's definitely not the stiffest way they could've built it. They could have run a link laterally across the top. But then camber adjustment method has to change.

That car produces over 1000 pounds of downforce (possibly much more, only the owners know the truth) so the cornering loads would be massive.

In reply to freetors :

With a solid axle or DeDion, you don't want camber gain in cornering.  It would only push you off the top grip of the curve (-1 degree, say).

The whole camber gain thing, my philosophy:  As the Meat Loaf song goes, "Where the rubber meets the road."  That's what's important.  We see in Teh Mag all the time tech articles on setting up a car with more front static negative camber.  Some cars, especially those with strut suspension, do well in a turn with a front static camber of -3 or -4, but look at what's really happening.  If the strut suspension is giving zero or very little camber change WITH RESPECT TO THE CHASSIS in a turn as the suspension loads up, then WITH RESPECT TO THE GROUND you are changing the tire camber.  Just picking numbers here, say you start out at -1 camber, lean over 4 degrees of chassis roll and you are adding 4 degrees to the tire camber, you are now at +3 camber WITH RESPECT TO THE GROUND and way off the peak grip of -1.  Start out at -4, add 4 and you are now at 0, and still in the high grip zone.  This is also why some cars do better with really stiff springs.  When you do that, you are removing the body roll which is limiting the amount of camber change seen at the road surface.  Instead of rolling 4 degrees, you are rolling 1 degree and going from -1 to 0 that way, masking a poor suspension design, or a suspension design that is operating outside it's design limits or optimal goal zone.

Hence the quote,

Any suspension will work, if you don't let it.

Same thing with upping the diameter of the Anti-Roll-Bars.  You are effectively increasing the spring rate on that side by transferring the load to the other side, reducing body roll, reducing the camber change on the outside wheel (with respect to the ground).

mad_machine said:

In reply to clshore :

basically you are talking about a "twist beam" as used by many FWD cars, but as a RWD DeDion axle.

 Somewhat similar, but with significantly different design goals and implementation:

The goal of the twist beam is minimal production cost, along with packaging that achieves minimal intrusion into the passenger/cargo space.

Wheel camber and toe are managed by a beam having minimal cross section, wheel 'caster' and fore/aft location is managed by a single

trailing link on each side, rigidly attached to the the hubs. Lateral wheel location is provided by bending in the joint between the trailing link

on each side where they join the beam.

This imparts significant bending loads that require both the beam and links to be somewhat heavy, along with the joints between them.

The goal of the Floppy-D is to achieve the camber behavior of a Live axle (and DeDion), combined with unsprung weight comparable to IRS.

Care is taken to avoid stressing major components in bending, instead resolving the forces by triangulation, and employing multiple links

with spherical joints to directly take the suspension loads into the chassis, as in IRS design, to save unsprung weight.

Cost and packaging are definitely secondary issues for Floppy-D.

Here's a question: Have any production cars employed 'twist beam' on a driven rear axle?

I know that there have been AWD versions of some FWD econoboxes that employed 'twist beam' rear suspension. 

I put a 3 link modified 8.8 in the Rice Rod because that's what another team was selling for $50 at the $2016 Challenge flea market.

My $2018 Challenge car will probably have IRS because that's what the donor car comes with. 

Neither are running now or in the forseeable future, so I don't have much to say on that 

Dr. Hess said:

Also, that whole driveline torque loading up the suspension -- more pressure on the contact patch thing, you are forgetting about the no free lunch.  That more pressure on, say, the left contact patch comes at the price of less pressure on the right contact patch. 

It's the opposite in my experience.  The anti squat equalizes the forces on the tires.  If one tire is "light", accelerating will push it into the ground.

The effect over rough roads is that the car stabilizes when you accelerate.  Sounds like what people describe 911s as being like, actually   Accelerate out of problems and don't dare lift.

clshore said:Here's a question: Have any production cars employed 'twist beam' on a driven rear axle?

I know that there have been AWD versions of some FWD econoboxes that employed 'twist beam' rear suspension. 

Smart cars have a transverse engine nestled inside a twist beam.

Blew my mind the first time I saw one.

Perversely, when VW converted some of their cars to AWD in the 1980s, they repurposed some Super Beetle parts to make a trailing arm setup.  From a handling standpoint, trailing arms have even more toe-loss issues than twist beams do, and VW invented "kinematics" in part to alleviate that problem with twist beams.  (Those funky ramped bushings that they used to use?  Those will yaw the beam under suspension load to negate some of the load induced toe out)  Nose heavy AWD cars need all the help they can get with rotation though so this is fine.  Makes them handle a bit spooky when there's 600lb of stuff in the back though.

In reply to Knurled. :

I'm not sure if what you are experiencing can be attributed to anti-squat or weight transfer.  And, with an IRS, you would likely have less problems over the rough roads to start with.

Has the same problems as a live axle in regards to the inside tire upsetting the outside.

I drew that up once. If you nail a bump big enough that camber of the opposite wheel is an issue, that end of the car is probably going to have daylight under both tires anyway.

A 2-3" bump at 72" is about 2 degrees, isn't it?  a 2 degree sudden (really sudden) camber change can't be a good thing for a tire/road interface.  If your starting camber was 0, adding 1.5 or 2 degrees positive is going to put you in the less grip part of your curve.  If you're starting at -1, you might get away with it.  Anyway, it may be more of the upset/sudden change/vibration in the outside tire than just the raw angle change and I fully admit not being an expert, just a student.

Lapping Hallett in the Esprit (RIP), I used to clip those bumps with the inside wheels and not really even notice them.  Not the turtles, but the bumps on the turns.  Can you do that with a solid axle?

In reply to maschinenbau :

I think the Suzuki SX4 uses a twist beam in the rear. 

I'm really curious how the Novak F500/F-Mod cars work in the rear suspension area. I totally understand how the elastomer pucks are both spring and damper but the four link in addition to the swingarm deal for lateral location seems like it would be well overconstrained. I get that the car has only maybe 2" of travel at most but I still think any bind is bad.

EDIT: On second thought maybe the key to this is the birdcage bearing deals on the axle. The axle mounts would then be able to twist relative to the other side unlike a conventional solid axle. Too many variable to run through my brain!

Dr. Hess said:

In reply to Knurled. :

I'm not sure if what you are experiencing can be attributed to anti-squat or weight transfer.  And, with an IRS, you would likely have less problems over the rough roads to start with.

We're not talking about accelerations hard enough for weight transfer to matter much.

Of course, it is tricky to separate because the anti squat allows you to generate more grip with allows you to accelerate harder which creates more weight transfer to create more grip with.  But you need that initial push.

I will allow that IRSs tend to be a lot heavier than a solid axle, which increases the amount of weight over the drive wheels...

Back in my old drag racing days, we tweaked the rear suspension to create lift, as in the body pushing the rear axle down when the body went up as opposed to body squatting and taking weight off the rear axle. Mopar drag cars had leaf springs in the rear, and simply clamping each leaf to prevent spring wrap changed the dynamic from squat to lift.. Coil spring rear suspension required a bit more engineering with bars. Of course, that was nearly 40 years ago, and I'm sure the ideology and engineering has changed dramatically...

I ran two race cars with the same power unit on the same tracks.  To maintain adhesion on the medium bumpy track surfaces we had, I needed to use an LSD on the live axle car, while the IRS car made do without that and did the same sort of cornering speeds.

Short answer - smooth track, not much difference.  Bumpy track, advantage IRS.

Both my modern sports cars have IRS; none of my 4 vintage sports cars have IRS.

I always found solid axle cars to be a lot easier to initiate sliding in the rear, but as a consequence stability in corners, especially when bumps were involved, was reduced.

That said, all my cars now have IRS, and I don't intend to go back to a live axle, unless I ever build a drag car. Even then, the new Dodge Demon runs an IRS, so it can't be that bad in the drag racing world...

I learned some things reading this thread. Explains why swapping from a Ford 7.5 to an 8.8 made the rear of my car ride worse. Everything's the exact same aside from 70lbs of extra unsprung weight. 

Dr. Hess said:

A 2-3" bump at 72" is about 2 degrees, isn't it?  

A 2-3" bump is a massive upset to the chassis no matter what the suspension type is.  At that point, at speed, damper tuning is more important to roadholding than mere camber figures. 

Smooth inputs need to happen at both ends of the suspension.

For reference, a 1" high bump at speed will make you feel like stopping to make sure all the tires still have air in them...

Knurled. said:

Excessive unsprung weight is still excessive unsprung weight.

 

That is why I went to moderately extreme lengths to try to get a 9" as light as the Mazda 7" that I am taking out.  I still need to find a scale but I am pretty sure the Mazda rear is 160lb with brakes.  I still have a couple places to shed weight but I already hit all of the cheap ones.

 

And why I want to build a Ford 7.5" for the RX-3.  Them suckers is light!

Just to complete this thought.

Old and busted on the right:  140lb as you see it.

New hotness on the left: 181lb as you see it.

Mind, neither rear has brakes, and the calipers for the 9" (VW/Lucas aluminum units) are waaaay lighter than the Mazda GSL-SE unobtainium, and the "GSL-SE" rear has a light Kia open diff instead of a limited slip, and the 9" has an un-lightened 5.43 that may get changed out for a lightened gearset in the mid-4s range. 

But the point is, the 9" parts should be more or less immortal, while the lighter Mazda bits were "I hope it lasts long enough to get me home so I can scrounge up some more stuff next weekend on car-part.com".

I think I have the only road race car (sedan) with an old Halibrand quick change that has a Jacobs Ladder. It has tremendous unsprung weight. The empty magnesium center section was heavier than I expected when we had it apart. Then add the shafts, spur gears, R&P, bearings(lbs and $s) and 1930s Ford truck axle tubes.I use curbs and rumble strips have gone over/outside the strips with no adverse issues.

The axle shafts started life with a 5 on 4.25" bolt circle.
 The splined end is the same as Volvo but the bearings
are much smaller. These were from some production car from the 60s or 70s but I don't know what.

Most of what I've gleaned from midget and sprint car sources isn't applicable or isn't quite right.
Some say the Jacobs Ladder is on the right for turning left. My car turns equally left and right in
the dry. Once long ago it wouldn't turn in the wet.

I wish I had the free track time for testing that I had 20 years ago when I did a lot of instructing.

I'm sure I can change the rear roll center with adjustments on the Jacobs Ladder. Exactly what does what I don't know. From what I've learned the way it's set up now the roll center is somewhere next door. The "straps"/ 2 tubes with heim joints on each end are close to parallel instead of a "V" pointing to the roll center.

I can drive it the way I have it now and have fun. I'm even surprised at some of the cars I've passed in corners with it.

In reply to Knurled. :

How the heck did you get a 9" that light!?

rslifkin said:

In reply to Knurled. :

How the heck did you get a 9" that light!?

Aluminum pumpkin.  The butt dyno equivalent of the scale says that the Yukon aluminum pumpkin with an aluminum Daytona-type pinion housing, WITH a 5.43 (heavy!) and unknown brand NASCAR takeout Detroit locker (light!) weighs as much as a bare iron 9" pumpkin and pinion housing.

It hurt to spend the money.  That hurt only lasts as long as it takes to pay for it.  The hurt of having a really heavy rearend lasts for years.

Knurled. said: The hurt of having a really heavy rearend lasts for years.

Giggle giggle

So what are some donor cars with really light rear axles? I've toyed around with the idea of putting on a four link system on the MGB but I also wanted to get rid of rear unsprung weight. That factory rear axle is seriously heavy for a car with less than 100hp.

In reply to OJR :

Keep in Mind not only is the QC Much heavier it also requires 50Hp more to turn the gears w/more Drag.

GTXVette said:

Keep in Mind not only is the QC Much heavier it also requires 50Hp more to turn the gears w/more Drag.

So a 49 hp engine can't even get the axle to turn? 

The spur gears are extra drag. It has a lot more ($s) bearings to add drag than a normal R & P but I'm sure not 50hp.

I can turn it by the driveshaft with one hand!

I didn't design, plan or build this. I'm just stubborn enough to keep it as it was. I have a copy of a grid sheet from 1982 that could have been it's last race before I got it in kit form in 1998.

Long ago the " something competition" or "Competition Something" shop (in CT) that built Paul Newmans first 510 built a different make sedan with this setup. Then Al bought that car and wrecked it in his first race. Then he built this car using many of the unusual for an SCCA sedan parts from the wreck.

The photo shows the Jacobs Ladder and the holes for adjustments.

Looks like the theory to that is basically the same as a scrunched up panhard bar with a lot more rod ends!

freetors said:

Looks like the theory to that is basically the same as a scrunched up panhard bar with a lot more rod ends!

 Yes it's like a panhard bar but much longer. You add the lengths of all the bits. This gives much less side movement than the arc of a panhard bar. The Jacobs ladder has 5 points with 3 holes each. The roll center can be changed from adjustments here. I'd like to know what does what before I change it more than I have.

The rod ends....AARRRGGG!

I have left hand taps I've used on customer and other parts/projects.

All of the rod ends on this car  are right hand thread.

freetors said:

Looks like the theory to that is basically the same as a scrunched up panhard bar with a lot more rod ends!

Or a Watts link that has been folded in on itself.

freetors said:

So what are some donor cars with really light rear axles? I've toyed around with the idea of putting on a four link system on the MGB but I also wanted to get rid of rear unsprung weight. That factory rear axle is seriously heavy for a car with less than 100hp.

The Ford 7.5 is reasonably strong and light enough.  The reason I like it for the RX-3 is the RX-3 has a track width of only 50".  If you get an early Ranger axle, if you cut the (left?) side shorter and use two (right?) side axles, you end up with something in the 52" width range, with junkyardable/Dorman-able parts availability.

Alternatively, I also like the idea of grabbing a Trailblazer rearend and having axles made with 9" bearing ends.  Upside:  Magnesium centersection of dubious strength (but good enough for 100hp, eh?)  Downside:  You gotta get custom axles made no matter what, which means money.  Dutchman's pretty cheap at $400-ish for a pair depending on options.  But there's an upside to that:  You can have them made to YOUR specs.  So whatever bolt pattern or brake width dimensions you want.

For what it's worth, the Ivan Stewart era Toyota stadium trucks used magnesium case Dana 60 rearends.  4-link mounts on the case were bolted to it rather than welded.  Neat stuff.

OJR said:

freetors said:

Looks like the theory to that is basically the same as a scrunched up panhard bar with a lot more rod ends!

 Yes it's like a panhard bar but much longer. You add the lengths of all the bits. This gives much less side movement than the arc of a panhard bar. The Jacobs ladder has 5 points with 3 holes each. The roll center can be changed from adjustments here. I'd like to know what does what before I change it more than I have.

 

It looks to me like what you are doing by adjusting the locating points on the center piece is akin to adjusting instant center on a 4 link, except laterally.

The trig nerd part of me is foaming at the mouth regarding the possibilities!

This car had a solid rear axle.  It was so fast at Daytona and LeMans in the 24 hour races, they changed rules and the tracks (chicanes were added right after this car appeared on the scene).  Seriously, look it up.  The FIA and IMSA hated this car.  It's a good thing the car had transmission problems in many of the races or they would've hated it even more. 

No doubt IRS is better technology, but sometimes simplicity is beautiful too (and fast). 

Reading through all the comments I'm surprised that rear steer wasn't discussed. I'm no expert but rear steer is something that I've been observing and working with in my chassis adjustments. Mostly so far it has been counter steer or neutral. With the relocated chassis side upper arm I will actually get some rear steer.

In reply to asphalt_gundam :

No matter what has been said so far, I really like and appreciate your project!  It's a great example of doing something you enjoy.

In reply to asphalt_gundam :

Mazda made a running change in the solid axle RX-7s, moving the lower links' front pivot down significantly (20mm?).  This was to create roll understeer.  Chassis roll would cause the axle to steer to the inside, creating a nice safe negative feedback loop.

In practice, the years with this geometry ('84-85) feel less crisp in their handling compared to the early cars.  They also have more wheelhop because of the instant center change, and the geometric bind in the rear suspension got worse, so the '84-85 years were a lot more prone to ripping the upper link mounts off of the body.  It's a four link with parallel lower links and fairly short, angled upper links, and a Watts link, and the roll center described by the upper links is not the same as the one defined by the Watts, so everything binds up with axle articulation...  It worked well enough with the original pivot mounting points and the 18mm rear sway bar to keep it from articulating much.

SkinnyG said:

Before building my Locost (and still now, 12 years later), I waffled back and forth between a stick axle and IRS.

My stick-axle Locost has been on the road, travelled 500km on the Trans Canada to autocross events and back, commuted as a daily for a time, and I've driven it everywhere, rain, shine, snow, even dirt roads (-that- sucked).

I've come to a couple conclusions:

1) If you want comfort, you really wouldn't pick a Lotus 7 or replica thereof.  Comfort is not their forte.

2) If, on the street, the stick axle is a limited factor, you are likely traveling faster than you legally should be going.  If it got you in trouble, it's going to be a big "off."

3) I'm not totally convinced an IRS is going to net a "lighter overall" weight of vehicle, and maximum lightness is what I want.

I don't believe that swapping TO an IRS is going to be worthwhile for -me-. If I was building a project and I had to source an axle, I'd probably go IRS, but the stick works well enough, and if it came with your donor vehicle, or it's already under your project vehicle, it will work fine.

An IRS can be lighter than a "stick" axle. But the right parts need to be selected.  The chief weight loss is the housing. Linkages and springs can be the same.  Plus you can  that the heavy disk and calipers and mount them inboard. Reduce unsprung weight.  

I see a lot of pictures that look like this:

When people talk about a solid axle being lighter than an IRS setup.  But that's misleading.  The comparable setup for a solid axle is really more like this:

I'd certainly grant that a solid axle CAN be lighter than an IRS, but as in all things it comes down to how it's designed.

Plus the difference in unsprung weight (IRS will usually be lighter there) and not transferring motion to the opposite wheel goes a long way.  Sure a solid axle can be made to work if there are limitations on budget, design, rules, etc.  But it's not what I would pick if I were making a clean sheet optimal design.

In reply to BA5 :

Once I start to see solid axles in Formula  1  Indy car etc. I'll agree with the premise. 
     Other wise if the rules limit it to  solid axles I'll be looking at IRS. 

In reply to BA5 :

Yes that top photo is misleading.  That's a 99-04 Cobra IRS which had serious design limitations.  It was designed to fit a solid axle chassis with no modification to the chassis.  This caused the control arms and linkages to be less than optimal.  

Next to it is an 8.8 rear end without the control arms.  It's installation does not match your 2nd drawing.  The 2nd drawing looks like a 3rd or 4th gen F body installation.

Sadly for the sake of this thread, the Mustang 8.8 is probably lighter than the Cobra IRS even with control arms, springs and shocks which are not shown.  

Tons of 99-04 Cobras were solid axle swapped for street / drag use.  

This thread really highlights the compromises that are inherent in pretty much any design decision.  One of the compromises we haven't' touched on is the evil twin of anti-squat on acceleration which is wheel hop on deceleration.  An extreme example of that can be seen on NASCAR Cup cars. To be fair the truck arm suspension in those is there due to history and the rules not because it's a particularly great suspension but any suspension design that relies on engine torque to push the axle down on acceleration is going to lift the axle on deceleration.

I'd be interested in a very in depth article on solid axles. Even over on the pro touring side of things there's lots of argument about which suspension design is the way to go. 3 link (centered or offset) vs 4 link (parallel or triangulated) vs torque arm vs truck arm and watts link vs panhard bar. All have pros and cons, all can weigh different and vary in complexity. 

I think the only real setback that can't be gotten around in the IRS vs Solid is transfer of bump influence from one wheel to the other. I have a ford 9" rear now and it is much heavier than the stock GM 7.5 was. However...going to an aluminum center, light weight ring and pinion, light weight locker, aluminum daytona pinion support, drilled axles, light weight brakes, and a fabricated housing I've researched to be 45-50lbs of weight savings (still around an estimated 120-130lbs with out arms for an rear axle that can support 700HP+). Half of which is rotating weight. Floater style housings can be built with radius drive axles to have camber and toe. Then comes in the various pros/cons of each individual suspension design. A long arm 3 link and torque arm seem to be the top contenders for a track/auto x type car with the torque arm being easier to package in most cars. The other designs have proven that they can work well too. This is where rear steer comes in as something that can be tuned on a solid axle easily vs an IRS. 

I agree that IRS is the best performance option in a track setting but there are plenty of cars out there that don't have that as an option without massive work in cutting/welding/fabrication. I find it hard to believe that an IRS could be a viable option in the cost/work vs performance gain when starting with a factory solid axle car.

Back to the beginning of my post I would be very curious to see some back to back, real world (track, autox, and street) testing of IRS vs 3 link/panhard vs 3 link watts, vs 4 link pan and watts, vs torque arm in fully prepared scenarios on the same (or as close as possible) car(s). 

APEowner said:

This thread really highlights the compromises that are inherent in pretty much any design decision.  One of the compromises we haven't' touched on is the evil twin of anti-squat on acceleration which is wheel hop on deceleration.  An extreme example of that can be seen on NASCAR Cup cars. To be fair the truck arm suspension in those is there due to history and the rules not because it's a particularly great suspension but any suspension design that relies on engine torque to push the axle down on acceleration is going to lift the axle on deceleration.

One of the cars that got into the wall back at Heartland with Gridlife was a truck arm suspension...he'd told me it was twitchy on the brakes and it went around on him in a decel corner. 

I have my triangulated 4 link set up to load the rear on accel because otherwise the car goes nowhere. On the brakes it does get a little light but in places that help to get some rotation during trail braking. I'd suspect that a torque arm is going to be a whole other feel than either of these.

asphalt_gundam said:

APEowner said:

This thread really highlights the compromises that are inherent in pretty much any design decision.  One of the compromises we haven't' touched on is the evil twin of anti-squat on acceleration which is wheel hop on deceleration.  An extreme example of that can be seen on NASCAR Cup cars. To be fair the truck arm suspension in those is there due to history and the rules not because it's a particularly great suspension but any suspension design that relies on engine torque to push the axle down on acceleration is going to lift the axle on deceleration.

One of the cars that got into the wall back at Heartland with Gridlife was a truck arm suspension...he'd told me it was twitchy on the brakes and it went around on him in a decel corner. 

I have my triangulated 4 link set up to load the rear on accel because otherwise the car goes nowhere. On the brakes it does get a little light but in places that help to get some rotation during trail braking. I'd suspect that a torque arm is going to be a whole other feel than either of these.

As you've discovered it's generally faster overall to dial in the anti-squat on accel and drive around the deccel compromise.  It does help the car rotate on entry and by playing with the brake bias you can adjust how much.  You just have to be sure that your downshifts don't completely unload the rear axle.  That's what usually gets people in trouble.

In reply to APEowner :

For track racing it's generally cheaper to put brake someone than to put accelerate them. 
It's not as ego satisfying  or as simple to just blow by them under power  but it's a heck of a lot harder to comeback past you when you take their line away and require them to slow and wait for you. 
   I've also found guys with more power than others seldom are as good at  figuring out tactics. Then even with the same brakes as you they will not always be in a position to use that extra power.  

APEowner said:

asphalt_gundam said:

APEowner said:

This thread really highlights the compromises that are inherent in pretty much any design decision.  One of the compromises we haven't' touched on is the evil twin of anti-squat on acceleration which is wheel hop on deceleration.  An extreme example of that can be seen on NASCAR Cup cars. To be fair the truck arm suspension in those is there due to history and the rules not because it's a particularly great suspension but any suspension design that relies on engine torque to push the axle down on acceleration is going to lift the axle on deceleration.

One of the cars that got into the wall back at Heartland with Gridlife was a truck arm suspension...he'd told me it was twitchy on the brakes and it went around on him in a decel corner. 

I have my triangulated 4 link set up to load the rear on accel because otherwise the car goes nowhere. On the brakes it does get a little light but in places that help to get some rotation during trail braking. I'd suspect that a torque arm is going to be a whole other feel than either of these.

As you've discovered it's generally faster overall to dial in the anti-squat on accel and drive around the deccel compromise.  It does help the car rotate on entry and by playing with the brake bias you can adjust how much.  You just have to be sure that your downshifts don't completely unload the rear axle.  That's what usually gets people in trouble.

For the issue you are covering, design in anti-wedge into the geometry.  The third member will be offset from center, and pointed a little down- but dealing with engine torque is very possible.

asphalt_gundam said:

I'd be interested in a very in depth article on solid axles. 

Back to the beginning of my post I would be very curious to see some back to back, real world (track, autox, and street) testing of IRS vs 3 link/panhard vs 3 link watts, vs 4 link pan and watts, vs torque arm in fully prepared scenarios on the same (or as close as possible) car(s). 

Thanks to Ford and the aftermarket you can actually do this very test in a Fox Mustang from 79-2004.  It'd be a labor of love and cost quite a bit of money though.  If I had a sponsor, I could pull this off.  It'd take quite a bit of time.  At the end of the day though, I think all the solutions for a Fox Mustang would be closer than you think.

The factory quadrabind would be the worst no doubt.  A Griggs or Maximum Motorsports torque arm, panhhard bar, modified 4 or 3 link would be a big leap forward.  There are even Watts links available for the Fox chassis (or were).  A Cobra sourced IRS with MM mods might be a bit better but it'd definitely ride better.  None are optimal though, because Fox Mustang chassis. 

Shown is a suspension bushing set for a Nissan 350Z, a rear IRS car engineered with soooo many bushings. YIKES! PROTHANE wants to remind, that with more stock rubber bushings under the race car, the LESS CHANCE of everything keeping in alignment during racing. Alleviate the problem by replacing them with performance urethane ones, which are firmer in the right areas, for better traction. 

No weight savings here! Hopefully a more comfortable ride=more driver confidence in this case. We will see!

In reply to ncjay :

Very highly developed for the application and those tall sidewalls (and relatively low tire pressures) help.  Someone tried IRS and couldn't afford the developement time.  Also, there was no info to fall back on.  They were completely on their own.

In reply to frenchyd :

So msch depends on packaging now.  The front suspension on Formula 1 cars has a high front roll center which is usually undesirable, but what they gain in other areas must offset it.

frenchyd said:

An IRS can be lighter than a "stick" axle. But the right parts need to be selected.  The chief weight loss is the housing. Linkages and springs can be the same.  Plus you can  that the heavy disk and calipers and mount them inboard. Reduce unsprung weight.  

On that last point, you introduce some possibly insurmountable cooling problems by tucking the brakes up under the car. I have friends that raced XKEs and have gone to heroic lengths trying to get the rear bakes cool enough to function as intended - ducts, heat sinks bolted to the calipers. etc., etc.

Apparently the vintage racing club they were running with didn't allow a simple relocation of the calipers to the outside ends of the axles out by the wheels.

In reply to frenchyd :

The inboard brake heat can be dealt with.  Let me start by saying that's a lot of heat to put right next to the differential. So you have to deal with it or every race you'll be replacing seals and fluids. 
     The trick is to duct cool air in and.  AND hot air out.  With the disks fixed to the diff you can duct air right to the rotors/calipers.  However the ducts out need to be much bigger in diameter than  cool air ducts.  In order to Pick up heat that's radiated  as well.  Duct it out the truck lid.  That's the trick Air flowing through the area.   I use 4-6 inch truck exhaust tubing.  It makes a very neat look. 
Luckily, the rear brakes don't work as hard as the fronts do.  So you don't need to pick up that high pressure air at the fron.  You can pick it off NACA ducts in the rear widows.  Or the high pressure area under the car. ( or both)   

Nobody wants that crap!

Interesting article and comments.

My 65 Corvette roadster is probably unique and I'm pretty sure no one has another street car like it. All the running gear started out Chrysler.  A 1957 Chrysler 392 Hemi with dual Carter AFB's.  Originally a Chrysler 4 spd which I've replaced with a TEMEC 5 spd.  Then a Chrysler 8.75" solid axle, limited slip and 3.70 gears with after market disc brakes.  I've changed the rear suspension to 4 link and coil over shocks.  The weight is 48% on the rear, which is not to bad with that big Hemi up front.  This summer I'll be considering a rear sway bar after a little testing.

asphalt_gundam said:

I'd be interested in a very in depth article on solid axles. Even over on the pro touring side of things there's lots of argument about which suspension design is the way to go. 3 link (centered or offset) vs 4 link (parallel or triangulated) vs torque arm vs truck arm and watts link vs panhard bar. All have pros and cons, all can weigh different and vary in complexity. 

I think the only real setback that can't be gotten around in the IRS vs Solid is transfer of bump influence from one wheel to the other. I have a ford 9" rear now and it is much heavier than the stock GM 7.5 was. However...going to an aluminum center, light weight ring and pinion, light weight locker, aluminum daytona pinion support, drilled axles, light weight brakes, and a fabricated housing I've researched to be 45-50lbs of weight savings (still around an estimated 120-130lbs with out arms for an rear axle that can support 700HP+). Half of which is rotating weight. Floater style housings can be built with radius drive axles to have camber and toe. Then comes in the various pros/cons of each individual suspension design. A long arm 3 link and torque arm seem to be the top contenders for a track/auto x type car with the torque arm being easier to package in most cars. The other designs have proven that they can work well too. This is where rear steer comes in as something that can be tuned on a solid axle easily vs an IRS. 

I agree that IRS is the best performance option in a track setting but there are plenty of cars out there that don't have that as an option without massive work in cutting/welding/fabrication. I find it hard to believe that an IRS could be a viable option in the cost/work vs performance gain when starting with a factory solid axle car.

Back to the beginning of my post I would be very curious to see some back to back, real world (track, autox, and street) testing of IRS vs 3 link/panhard vs 3 link watts, vs 4 link pan and watts, vs torque arm in fully prepared scenarios on the same (or as close as possible) car(s). 

The only place you'd ever see that is maybe a factory with unlimited funds.  
      Pure performance oriented cars are all designed with IRS and the chassis is built accordingly. 
    The closest you will ever see is when the D type Jaguar near the end of its life was converted from a Straight axle to a IRS.   A one off with a tiny budget.  
 That engineer ( sorry his name escapes me )  drew up a design in 1957 that is the basis of every Jaguar from then until 2000.  
     And is still used with minor modifications today. 
    It's noteworthy that prior to the introduction of the Jaguar XKE. Racing sports cars were usually straight axle.   Following Jaguar in 1961 Corvette in 1963  most Sports cars went to IRS. 
 

Kolsen911 said:

Interesting article and comments.

My 65 Corvette roadster is probably unique and I'm pretty sure no one has another street car like it. All the running gear started out Chrysler.  A 1957 Chrysler 392 Hemi with dual Carter AFB's.  Originally a Chrysler 4 spd which I've replaced with a TEMEC 5 spd.  Then a Chrysler 8.75" solid axle, limited slip and 3.70 gears with after market disc brakes.  I've changed the rear suspension to 4 link and coil over shocks.  The weight is 48% on the rear, which is not to bad with that big Hemi up front.  This summer I'll be considering a rear sway bar after a little testing.

     Drag racing has different needs and trade offs than road racing.   That heavy Hemi  likely isn't that much heavier than a Chevy 454 of the same type ( iron block , Iron Heads ). 

Ok I may be getting old but yesterday I looked at an Exocet Miata kit car and I liked the idea of just taking something with a tried and tested setup that works. But then again I am very content with the way my stock Mazda RX-8 handles when driven fast. It just works, was cheep even new, has wide tires, revs like crazy and is just FUN to drive. 

Remember, I am old (but not slow) and my first car that I bought (with my money - $25) was a original Fiat 500 with no starter and very low compression so I could visit my girl friend in Hamburg that was three hours away from college in Germany. 

Bite me!  

my 84 alfa spider has a soild rear axle,, the pick up points at T bar from bodyshell to the top of the diff...  and 2 lower trailing arms from the bodyshell  to under the axle tube... i am planning to use a panhard link to the rear axle,, and design a tourqe arm rear suspension..i must design both of those.. the panhard will be easy.. got most of it made now.. but the tourqe arm part will be hard.. i plane to mount it to the tranny W/C t5.. has 3 mounting holes in the lower rear casing.. my rear roll center is at the top of the diff.. with the panhard system,, it should be at axle centreline.... the alfa spider, handles well enough.. but can be made better..i have moved my front roll centre from being underground to about 50 mm above the ground