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Knurled.
Knurled. MegaDork
1/12/18 6:19 p.m.

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!

kb58
kb58 SuperDork
1/12/18 6:21 p.m.

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."

Vigo
Vigo UltimaDork
1/12/18 6:55 p.m.

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. 

Dr. Hess
Dr. Hess MegaDork
1/12/18 7:03 p.m.

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.

Dr. Hess
Dr. Hess MegaDork
1/12/18 7:17 p.m.

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

 

freetors
freetors New Reader
1/12/18 8:12 p.m.

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.

Keith Tanner
Keith Tanner MegaDork
1/12/18 8:17 p.m.
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.

Appleseed
Appleseed MegaDork
1/12/18 8:35 p.m.
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. 

ncjay
ncjay SuperDork
1/12/18 8:46 p.m.

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

SkinnyG
SkinnyG SuperDork
1/12/18 9:08 p.m.
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.

Streetwiseguy
Streetwiseguy UltimaDork
1/12/18 9:19 p.m.

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.

clshore
clshore New Reader
1/12/18 10:06 p.m.

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'.

mad_machine
mad_machine MegaDork
1/12/18 10:29 p.m.

In reply to clshore :

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

Driven5
Driven5 SuperDork
1/13/18 12:39 a.m.
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.

Knurled.
Knurled. MegaDork
1/13/18 4:58 a.m.
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.

Challenger392
Challenger392 New Reader
1/13/18 7:02 a.m.

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 $.

Challenger392
Challenger392 New Reader
1/13/18 7:14 a.m.
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.

Knurled.
Knurled. MegaDork
1/13/18 7:32 a.m.
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. 

mad_machine
mad_machine MegaDork
1/13/18 7:32 a.m.
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

Knurled.
Knurled. MegaDork
1/13/18 7:45 a.m.
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.

Dr. Hess
Dr. Hess MegaDork
1/13/18 7:57 a.m.

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

Dr. Hess
Dr. Hess MegaDork
1/13/18 8:18 a.m.

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.

freetors
freetors New Reader
1/13/18 8:39 a.m.
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.

Dr. Hess
Dr. Hess MegaDork
1/13/18 8:55 a.m.

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.

bentwrench
bentwrench Dork
1/13/18 10:13 a.m.

Hence the quote,

 

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

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