tuna55
tuna55 PowerDork
1/20/14 5:21 a.m.

This is jumping ahead a bit on the 72 GMC build thread. I want to mess with the rear suspension. Primarily for looks and updating the handling. I want to make the truck not push like a truck.

To re-cap, GMCs from that era typically have leaf springs while Chevrolets typically have coils. I don't want to switch.

I am willing to raise or lower the spring mounting points and tilt them inboard/outboard.

My understanding, to make the thing handle better, is to promote less roll understeer. To do this, I take the front spring hanger and raise it, but not higher than the rear shackle (and spring mounting eye). My understanding also says that I should strive for parallel spring mounting, when viewed above. Springs typically toe in as mounted by the factory, and that further increases roll understeer. I will also pursue de-arching the springs when my mount(s) is(are) relocated to set the ride height as desired.

Is my understanding correct? Has anyone documented the results of such tweaking? Does anyone know autocross/road race leaf spring setups just for comparison?

Folks typically do flip kits and long shackles for ride height in the rear, and in my opinion both of those moves promote terrible handling.

bentwrench
bentwrench New Reader
1/20/14 6:37 a.m.

1 ton front sway bar

tuna55
tuna55 PowerDork
1/20/14 7:00 a.m.

Sway bars are a given, at least in front. I don't really fully understand what to do with the leaf spring rear to make everything as good as it can be since I'll be in there already.

44Dwarf
44Dwarf SuperDork
1/20/14 10:18 a.m.

This sounds like a good question for John Block over on the W.A.R. board. Weekend auto racer.

iceracer
iceracer UberDork
1/20/14 11:03 a.m.

Memory fails me but I do remember that raising or lowering the front mount will change the roll couple. Under steer- soften front, stiffen rear. The biggest problem with leaf springs is roll steer. Best way to reduce this is to use the old "Chrysler" style spring with the axle mount towards the front of the spring. A flat spring will promote a very loose rear end. Some of the stuff I remember from the old modifieds.

nicksta43
nicksta43 UltraDork
1/20/14 11:18 a.m.

Herb Adams had some tricks he used on his T/A race cars back in the day. My memory is failing me on what exactly he did. If it was mine I'd change it out for a set of center drive truck arms from hot rods to hell.

Ransom
Ransom UberDork
1/20/14 11:21 a.m.

I started to ponder what I could work out about changes to roll steer and roll center height, but quickly realized that there's a lot to leaf springs, and I'd be closer to useful by shutting up and providing a possibly interesting link... Though not comprehensive over all considerations.

http://www.pro-touring.com/threads/7385-Calculating-Rear-Roll-Center-for-Leaf-Spring-Cars

emodspitfire
emodspitfire Reader
1/20/14 11:25 a.m.

"How to make your car handle" by Fred Puhn has a good chapter on tweaking/improving leaf spring cars.

tuna55
tuna55 PowerDork
1/20/14 12:35 p.m.

I have read that and the Herb Adams book. I get that higher means less roll understeer. I don't see the results of this, any issues with it, how much is too much, or is anyone's places the springs parallel as viewed from above.

jimbbski
jimbbski HalfDork
1/20/14 4:11 p.m.

From my experience and that was on first a Capri Mk I and later on a Ford Ranger PU. On the Capri I didn't raise the front mount but I did de-arch the springs and then longer rear shackles to get the leaves level when viewed from the side. To fine turn the ride height I used lowering blocks at the leaf to axel mount. Finally I went with a third upper link to limit axel rotation/pinion angle. You need to keep the front half of the leaf from wrapping up under acceleration or braking.

You can also go with traction bars on each leaf but if you have room and the fabrication skills the third upper link is best. Early Shelby GT350 had two such bars that actually came through the rear floor to mounts inside the car where the back seat would be. They were covered over with sheet metal. This is what I did on the Capri.

The Ranger got traction bars and they worked but added to un-sprung weight. On a PU truck that was not much but the Capri was a dedicated Auto-X car and un-sprung weight mattered there.

As for sway bars, you will need them but they are tuning devices and not a crutch for a poor handling car.

RealMiniDriver
RealMiniDriver UltraDork
1/20/14 7:56 p.m.

This is a good read, too. Found in Ransom's link.

tuna55
tuna55 PowerDork
1/21/14 7:54 a.m.

Alright, I am just going to raise the mounting point by a couple of inches. Swaybars should help with the rest.

whenry
whenry HalfDork
1/21/14 9:07 a.m.

a panhard rod will also keep the axle located along with a sway bar.

aussiesmg
aussiesmg MegaDork
1/21/14 9:47 a.m.
whenry wrote: a panhard rod will also keep the axle located along with a sway bar.

I ran two upper control arms and a panhard on my RX3 race car, reset the spring rates, used Spax adjustable shocks and with the three extra locaters it was very good at controlling axle tramp, wind up and simply put the power down much better.

I also clamped the front of the leaves to stop them sliding, this stops the axle shortening under load.

This was in a class where the suspension locating points were not able to be changed

tuna55
tuna55 PowerDork
3/19/14 9:51 a.m.

Folks, one more on this subject. Now that I have fancy new drop springs (one eye appears to be reversed to achieve the drop) with fancy new rubber bushings, is it worth throwing the poly bushings I bought in?

jimbbski
jimbbski HalfDork
3/19/14 1:13 p.m.

For leaf spring I would only if it were a race car. If to be driven on the street then keep the rubber bushings. They don't deflect that much unless they are huge! Most leaf bushings I've seen have been small and/or pretty firm to begin with. In the past most leaf spring cars were built with body on frame so the bushings didn't have to absorb all road noise so they were not that soft. Whatever got past them the frame to body bushings stopped.

Appleseed
Appleseed UltimaDork
3/20/14 1:22 p.m.

I'd go for it. There is little deflection in a leaf set up, unless the goal is huge articulation. I have them in the back of my 4Runner, and I don't notice any more road noise.

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