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914Driver
914Driver SuperDork
4/29/10 6:39 a.m.

I have a car with coils-struts up front and leaf springs out back. I can't find any performance aftermarket leaf springs and am thinking about another type of set up.

What are the pros, cons and fabrication skills required to install a 4 link or a trailing arm type suspension? Is this the best option?

Thanx, Dan

John Brown
John Brown SuperDork
4/29/10 7:32 a.m.

Dan, A "spring shop" can tailor your springs for you. If your primary springs are good they can add different tension springs to the pack to tailor to your needs.

I deal with a place called State Spring locally if you don't have anyone.

914Driver
914Driver SuperDork
4/29/10 9:11 a.m.

There's one here in Albany called Watkins Spring. I just don't know what to tell them. The car will be light with huge horsepower, I want it to handle but not hop around because it's too stiff.

I figured since I have a blank page, lets look at options.

Dan

ReverendDexter
ReverendDexter Dork
4/29/10 9:27 a.m.

Well, what spring rate would you be looking at with coil springs to make it handle but not hop around because it's too stiff? That's what I'd be telling the spring guys. That, and what height you want the car to rest at.

Only thing I don't like about leaf springs is I'm not really sure how you control axle wrap.

alfadriver
alfadriver Dork
4/29/10 9:27 a.m.
914Driver wrote: I have a car with coils-struts up front and leaf springs out back. I can't find any performance aftermarket leaf springs and am thinking about another type of set up. What are the pros, cons and fabrication skills required to install a 4 link or a trailing arm type suspension? Is this the best option? Thanx, Dan

With what you have, you can do a 3 link + panhard rod set up easier than the 4 link with new coil spings. It's what Shelby did with the Mustang. Not perfect, since the leaf springs are not totally solid vs. a lower arm, but I think it will work quite well.

And if you put the upper link in the right spot, it will counteract quite a bit of the torque wedge you'll get from the engine. (the engine torque will lift one of the wheels- that's torque wedge)

There were some posts on Corner Carvers about this, http://www.corner-carvers.com/forums/showthread.php?t=2474

And this link hidden in that discussion can be useful- http://www.racetec.cc/shope/tim.17.htm

I first read about this design idea in Racecar Engineering- but I don't know if the original article is still there. It was a series about assymetric race cars, and beam axles- really interesting. It was quite a while ago, though.

Eric

ReverendDexter
ReverendDexter Dork
4/29/10 9:29 a.m.
alfadriver wrote: With what you have, you can do a 3 link + panhard rod set up easier than the 4 link with new coil spings. It's what Shelby did with the Mustang.

Wha-huh? I wasn't aware Shelby ever changed the rear geometry. I must research this...

914Driver
914Driver SuperDork
4/29/10 9:47 a.m.

There are traction bars for leaf springs to prevent axle wrap. They bolt to the underside of the spring and push on the frame forward of the spring.

If I keep the leafs {leaves?} can or should I do a panhard bar anyway?

What height do I want the car? If there's a mouse in the middle of my lane, I want to shave his back.

Dan

John Brown
John Brown SuperDork
4/29/10 10:03 a.m.

Look under a Datsun SPL311 Roadster and mimic the locator used in that suspension. Simple easy effective.

For ride height I would look at tire and wheel selection on car then fit to that requirement, then tune to that fitment. All out race is a bit different than what I would do though. In all out race you go geometry and tuning first ride height as needed.

alfadriver
alfadriver Dork
4/29/10 10:06 a.m.
914Driver wrote: If I keep the leafs {leaves?} can or should I do a panhard bar anyway? Dan

I would. While there would be a degree of bind, it's still better than relying on the leaves for lateral movement. If that's the prime worry- you can do a Watts, but I've seen a few leaf set ups with a panhard rod. The shakles still allow enough movement.

And you know that the upper 3rd arm is NOT the traction arms- this is an upper locater, which will end up on the passenger side of the axle, near the center of the diff.

If the traction arms do a good enough job on wind up, and the upper third link + panhard rod- it should handle pretty well. Just make sure the panhard is below the center line of the axle, so that it's moment will counter the side traction moment from the wheels- above the CL, and it will help try to lift the inside tire (draw a FBD).

Eric

iceracer
iceracer Dork
4/29/10 11:00 a.m.

Leaf springs are the easiest to change rates. Add or remove a leaf. Even go half leaf in front of the axle. Can be rolled flatter. Spacers help lower the car.

ReverendDexter
ReverendDexter Dork
4/29/10 11:07 a.m.
iceracer wrote: Spacers help lower the car.

...at the cost of decreased leverage against axle wrap. It's like the 4x4 guys that do block lifts. Cheap and easy, but asking to break stuff.

914Driver
914Driver SuperDork
4/29/10 12:08 p.m.

Maybe I'm confused about Panhard Bars. I pictured something runing from the top on the diff to the nearest frame in a horizontal manner with heim joints at each end.

Alfa, you suggest the panhard be below the axle CL, how do you do that? The entire bar or just the axle connecting point?

alfadriver
alfadriver Dork
4/29/10 12:44 p.m.

I'm bad at pictures. Yes, I'm suggesting that the entire bar is below the axle CL- you have to make a guess at some point what the geometry would be, but you can use 3-4 holes on the body part to locate it. The one issue with a rod with leaf springs is that the entire set up has to be within the two springs- which does allow more lateral movement since the arm is shorter.

Where can I post some pics?

Maybe this will work- But the bar is horizontal when the rear axle is loaded.

Eric

John Brown
John Brown SuperDork
4/29/10 12:45 p.m.

This may sound silly, but it may be a good idea to see how far off from good your OE baseline is. You may just need a speck of tuning and some good shocks.

ReverendDexter
ReverendDexter Dork
4/29/10 1:09 p.m.

Wait... I thought with a panhard bar that the rollcenter was determined by either the height of the center of the bar or the height of the bar where it crossed the vehicle center line? Hence why the MM TA/PhB setup for a fox-chassis mustang has such a low roll center (and thus requires significantly stiffer springs to keep roll under control).

Or am I smoking my very select brand of crack again?

klipless
klipless Reader
4/29/10 1:19 p.m.
ReverendDexter wrote: Wait... I thought with a panhard bar that the rollcenter was determined by either the height of the center of the bar or the height of the bar where it crossed the vehicle center line? Hence why the MM TA/PhB setup for a fox-chassis mustang has such a low roll center (and thus requires significantly stiffer springs to keep roll under control). Or am I smoking my very select brand of crack again?

I believe that it is the height of the bar where it crosses the vehicle center line. But maybe I am smoking what you are.

alfadriver
alfadriver Dork
4/29/10 2:54 p.m.
ReverendDexter wrote: Wait... I thought with a panhard bar that the rollcenter was determined by either the height of the center of the bar or the height of the bar where it crossed the vehicle center line? Hence why the MM TA/PhB setup for a fox-chassis mustang has such a low roll center (and thus requires significantly stiffer springs to keep roll under control). Or am I smoking my very select brand of crack again?

You are right- I'm combatting a different problem.

Look at dan's pictures above- now draw force lines- one on each wheel, and then a counter one on the panhard rod.

If you calculate the moment of those three forces around center of the axle- then you can get an idea how the basic geometry will try to lift or lower the inside axle. If the panhard is above the AXLE CL, then all of the forces add up to a large moment, which picks up the inside rear wheel, if the panhard is below the axle CL, then the bar force will counter the moment caused by the wheels to pick up the inside wheel.

As you correcly point out, the lower roll center means that you need more roll stiffness from either springs or sway bar.

But what I'm pointing out is unique to a beam axle- whether driven or not.

Eric

Don49
Don49 New Reader
4/29/10 3:00 p.m.

In regards to a panhard bar causing bing with a leaf spring rear. When I built the rear suspension for my race car, I only had room for a 36" bar. With 4'' of travel the deflection was approximately 1/32". Unless you have a very large amount of travel it shouldn't be a factor

Knurled
Knurled Reader
4/29/10 4:52 p.m.
klipless wrote:
ReverendDexter wrote: Wait... I thought with a panhard bar that the rollcenter was determined by either the height of the center of the bar or the height of the bar where it crossed the vehicle center line? Hence why the MM TA/PhB setup for a fox-chassis mustang has such a low roll center (and thus requires significantly stiffer springs to keep roll under control). Or am I smoking my very select brand of crack again?
I believe that it is the height of the bar where it crosses the vehicle center line. But maybe I am smoking what you are.

Technically, you need to find the point where the upper and lower links would meet, and from there draw a line to the point on the Panhard where it crosses vehicle centerline, then find where THAT line crosses the axle centerline.

I think.

It's all in Fred Puhn's book.

emodspitfire
emodspitfire New Reader
4/29/10 5:51 p.m.

+1 on referencing Fred Puhn's book. It will get you into the ballpark on basic geometry.

Remember to consider bump steer when (Not if) you re-arc the leaf springs.

Rog

turboswede
turboswede SuperDork
4/29/10 6:50 p.m.

Why not a Watt's link?

Or a Mumford link?

Just curious as to the pluses and minuses to them both....

Jensenman
Jensenman SuperDork
4/29/10 7:18 p.m.

Watt's and Mumford links require a lot of fabrication and take up a lot more room. Were it me, I'd go for a 3 link with a Panhard and use coilover shocks. Oh, wait- that's what I did with the Jensenator.

Since the car will be way low, you should be able to use the original front leaf mount points for the two outer links, then you have to fabricate the axle brackets for the rear of the lower links, the rear of the top link, then the Panhard and front of the upper link brackets on the car. Make the Panhard as long as possible and have it level at your chosen ride height, the lower the better as this does decide the roll center.

The two lower links should be parallel to the ground at ride height, that will dictate the axle bracket design for those. The upper link ideally should be paralell to the lowers but reality usually points the front of it downward unless you are willing to put the bracket inside the car. That's not unheard of; the Tri Point 3 link for a 1st gen RX7 is done that way. It will probably be shorter than the bottom links, as long as it's around 70% of the length of the lower links it should not cause any weird geometry problems.

44Dwarf
44Dwarf HalfDork
4/30/10 6:27 a.m.

Roll center for a panhard is at mid point between the attachment points.

If you panhard a leaf it can bind BUT depends on the leanght of the bar longer is better less side shift thus less bind in the bushings. You can replace two of the 4 bushings with "mono balls" or "pivot bushings" like from Afco and the like to keep the bind out.

I've got comouter programs that would work with this dan if you need help.

914Driver
914Driver SuperDork
4/30/10 6:55 a.m.

Thanks Tony. It appears the top of the pumpkin will be between the frame rails, so if I run a bar from the inside of the frame rail to the top center of the pumpkin it will be horizontal. Heim joints will allow some up & down movement.

I may take Mr. Brown's advice and do nothing until I see what I have. Yes it has stock 40 year old leafs out back, but the car is considerable lighter than stock.

Dan

44Dwarf
44Dwarf HalfDork
4/30/10 1:29 p.m.

I would geuss you'd want it below or at axle centerline to corner but this will also tend to give more body roll as theres more stuff above the center of rotation...

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