Leaf spring shackles and bushing flex . . .

[mikedc]

Watching an old video of a 2nd-gen Camaro set up for PT:

https://www.youtube.com/watch?v=y0IvH74NqeM

Look how much the lower spring shackle is moving laterally. That seems like a lot. The rear tires look pretty sticky for a street car, but still.

Now I'm wondering how much movement there is with milder street radials. Has anybody else looked into this?



Yeah, I know you can take all the rubber out of the shackles with poly/Delrin. But I like keeping rubber bushings in street cars. Racecars have used wider-top shackles for decades and that seems like a better fix.

[dontlifttoshift]

There is no way I would try to fix an axle lateral location problem by changing bushing material on either end of the leaf springs. The compromises are large, as you already noted, and the gains are minimal.

[mikedc]

I'm thinking of the fact that leaf axles can benefit from a Panhard/Watts bar. Lateral flex is definitely a thing with leafs overall.

I wonder if wider-top shackles would be worth adding on a street car. It wouldn't help as much as a Panhard/Watts but it's a much less severe modification. You would only need to add a longer mounting tube to the subframe rail. (On the old Mopar NASCARs, they left the inner-facing side of the shackle unchanged, and just added all the width to the outer side.)

And of course the benefit would be bigger if the shackles were longer than stock.

[dusterbd13]

We found clamping and spring sliders to be highly effective in lateral location with the amc build. Always thought id have to do a panhard, but never did.

[hquackenboss]

I am not known to active members on this forum, but I have considerable direct experience. I was part of the group led by Herb Adams that raced in the Trans-Am series, and although i wasn't a Pontiac employee, I led suspension/brakes in 1972 when our Firebird finished 2nd in the Trans-Am series. We used a leaf spring pack that had a relatively low rate, about 180 lbs/inch, with most of the stiffness between the axle and the front spring eye. We used a spherical bearing in the front spring eye, and production rubber bushings in the rear spring eye. We used a modified front spring eye bracket with the spring eye hole relocated the maximum we could without modifying the floor, which was about 3/4 of an inch. This modification is known as the Herb Adams mod, but I was the one that invented it. We used a longer rear shackle. If you imagined a line in the side view through the two spring eyes, the line was angled so that it was pointing higher toward the front of the car than stock. We did not use any lateral location device, although competitors used either Panhard rods or Watts linkages to control lateral location. We finished 2nd in the Trans-Am series despite not running all the races, and it being the first year we ran the car. We also did not use lowering blocks at the axle, and used production hardware to attach the axle to the springs. Lowering blocks increase the problem of axle tramp under acceleration, and hop under braking. The combination of a spherical bearing in the front, eliminating the deflection of rubber (or even urethane) bushings, the rigid attachment at the axle provided plenty of lateral control. Angling the spring by lowering the car at the front spring eye, offset by raising the car with the rear spring eye improves the ability to get back to the power coming out a corner because of increased anti-squat. Using a tubular bushing of urethane or Delrin or steel in the front eye caused bind when the car leans during cornering Triming the car at a rake provides both slight aero downforce and drag reduction. The best angle based on coast down testing at Pontiac is about a 2 degree forward rake when the car is accelerating under power is about optimum. The car was stable in cornering, and easy to drive. I went to driver's school in the car, and drove it in testing. This car pulled 1.24G on the skid pad at the GM tech center, which is less than what you can do with the best of today's ultra-performance street tires, so the loads on the suspension that cause deflection of the spring are less than the best pro-touring autocrossers experience in today's world. Herb Adams sold a lot of equivalent parts under the VSE brand to 2nd Gen Camaros and Firebirds for street use. Only an experienced tester would notice any increased harshness or noise with the spherical bearing in daily driving. Herb's son, Matt, provides the same parts today.

[mikedc]

I am not known to active members on this forum, but I have considerable direct experience. I was part of the group led by Herb Adams that raced in the Trans-Am series, and although i wasn't a Pontiac employee, I led suspension/brakes in 1972 when our Firebird finished 2nd in the Trans-Am series. We used a leaf spring pack that had a relatively low rate, about 180 lbs/inch, with most of the stiffness between the axle and the front spring eye. We used a spherical bearing in the front spring eye, and production rubber bushings in the rear spring eye. We used a modified front spring eye bracket with the spring eye hole relocated the maximum we could without modifying the floor, which was about 3/4 of an inch. This modification is known as the Herb Adams mod, but I was the one that invented it. We used a longer rear shackle. If you imagined a line in the side view through the two spring eyes, the line was angled so that it was pointing higher toward the front of the car than stock. We did not use any lateral location device, although competitors used either Panhard rods or Watts linkages to control lateral location. We finished 2nd in the Trans-Am series despite not running all the races, and it being the first year we ran the car. We also did not use lowering blocks at the axle, and used production hardware to attach the axle to the springs. Lowering blocks increase the problem of axle tramp under acceleration, and hop under braking. The combination of a spherical bearing in the front, eliminating the deflection of rubber (or even urethane) bushings, the rigid attachment at the axle provided plenty of lateral control. Angling the spring by lowering the car at the front spring eye, offset by raising the car with the rear spring eye improves the ability to get back to the power coming out a corner because of increased anti-squat. Using a tubular bushing of urethane or Delrin or steel in the front eye caused bind when the car leans during cornering Triming the car at a rake provides both slight aero downforce and drag reduction. The best angle based on coast down testing at Pontiac is about a 2 degree forward rake when the car is accelerating under power is about optimum. The car was stable in cornering, and easy to drive. I went to driver's school in the car, and drove it in testing. This car pulled 1.24G on the skid pad at the GM tech center, which is less than what you can do with the best of today's ultra-performance street tires, so the loads on the suspension that cause deflection of the spring are less than the best pro-touring autocrossers experience in today's world. Herb Adams sold a lot of equivalent parts under the VSE brand to 2nd Gen Camaros and Firebirds for street use. Only an experienced tester would notice any increased harshness or noise with the spherical bearing in daily driving. Herb's son, Matt, provides the same parts today.

Wow, that is some experience!

I've seen that leaf spring angle thing on some other race cars. Raise the front spring eye + lower the rear eye/shackle, to tilt the whole thing. I figured it would help put the leaf spring geometry back in the ballpark of stock. Old racecars typically had the body raked down a couple degrees in front, which throws off the geometry at both ends a little bit.


It's interesting that your car was competitive without a panhard/watts even though competitors ran them. I suppose the spherical front spring eyes helped a lot with that.

And the tires back then. Less lateral G-forces on the car in general. Nowadays the tires stick like glue. Everything in the frame & suspension gets pushed & pulled so hard.