Pro-Squat & Braking (Early Suspension)

[neophile_17]

Hello guys,

First a little history on what started this post. This past weekend I joined my uncle's LeMons team at his first race with a '76 BMW 2002. I had a suspicion that the handling might be better than the Fiero but what really got me was the brakes. I had convinced myself that the only way I could improve on our brakes was ABS. Seeing what that car can do I'm forced to admit (again) that the Fiero suspension is holding us back.

The symptom- under heavy braking the back end gets very light so we modulate the pedal to the drivers current fear threshold. I took Will's idea for Anti-Dive and implemented something similar a couple years ago. I also massaged the UCAs to get some additional camber and caster. I don't think I could improve the front geometry much more without building a new front sub-frame, control arms, or both (may be my best option). However, I didn't worry about anti-squat and actually made it a little worse by shaving the rear cradle "bushings" (aluminum) and leaving the front alone. I justified this because we don't make mountains of torque and turn a tallish tire. What I didn't consider was how pro-squat would affect braking. I've got myself pretty well convinced that the pro-squat geometry is jacking the rear under braking but I'm not a chassis engineer. Does it make sense that the rear is lifting under braking due to pro-squat geometry?

Thanks!

[The Dark Side of Will]

Are you running the stock proportioning valve?

[neophile_17]

The stock proportioning valve was removed and an adjustable valve was installed in line with the rear brakes.

[The Dark Side of Will]

What's your brake setup at each end? Rotor diameter and piston size. I assume you're running the same pad compound.

[neophile_17]

Same pad shape and same compound all around. "Metric" calipers with Porterfield ST-43 pads. Front Rotors are 11.6" and rear are 12" which is just about proportional to tire diameter so similar effective torque. Maybe the slight rake makes the pro-squat worse? Hadn't thought about that before.

[The Dark Side of Will]

Pro-Squat causes forward tractive loads to compress the suspension and rearward tractive loads to extend it. So pro-squat in the rear can make brake dive worse. If you've made the rear cradle bushings thinner, you've made pro-squat worse. One of the mods done to the original Fiero Indy Pace Car was to shim the rear cradle bushings to reduce pro-squat. If you shim yours thicker than stock, you may find the problem gets better. This does NOT affect ride height, but will have a small effect on roll center height. You can also make up eccentric forward cradle bushings and raise the front end of the cradle to counteract the roll-center impact of lowering the rear of the cradle.

[neophile_17]

I originally cut the rear bushings down to improve the dynamic camber and I'd prefer not to go backwards. I had though about the offset bushings but I'm not sure I would gain enough. I'm considering chopping the from ears on the frame and then welding them back on straight as seen below.

Bare_Subframe_Anti_Squat.jpg (124.78 KiB) Viewed 4124 times

[The Dark Side of Will]

I think you'd be the first. Be careful... a little can go a long way.

Watch out for top-side clearances around the valve covers and intake manifold.

[The Dark Side of Will]

The potential problem being that too much anti-squat can give you wheel hop on hard braking.

[neophile_17]

Appreciate all the tips! Considering this is a racecar with no ribbing on the deck-lid and easily extended hood pins I'm going to go for it. I did some more digging since no one had immediate and justified objections to my plan. My search took me to Blooz's calculations and I think I'm pretty safe taking the kink out of the forward arms on the early cradle. With the shaved rear cradle "bushings" I estimate my anti-squat to be less than that of a stock 88 cradle. That doesn't seem to be a disaster and my current setup is definitely costing us time.

Thanks for taking the time to think this through with me! I'll check back in with the track results if I can get this executed along with our other maintenance and upgrades (mostly safety) before August.

Sam

[neophile_17]

Sorry for the late update! In the midst of a struggling engine program that was ultimately due to a an oil cooler with a pin hole leak this modification faded into the background. That is a shame because it was a big win! The car feels many times better and we can trail brake with confidence which is something we had given up on as not worth the risk. I don't know how much kink can be taken out for a street car with a regular deck lid. I never reviewed the data but I think I think all drivers picked up over a second around NHMS.

Sam

[The Dark Side of Will]

Sorry for the late update! In the midst of a struggling engine program that was ultimately due to a an oil cooler with a pin hole leak this modification faded into the background. That is a shame because it was a big win! The car feels many times better and we can trail brake with confidence which is something we had given up on as not worth the risk. I don't know how much kink can be taken out for a street car with a regular deck lid. I never reviewed the data but I think I think all drivers picked up over a second around NHMS.

Sam

This is really cool to hear. Thanks for sharing!

The actual Indy Pace Car had shimmed rear cradle mounts to reduce pro-squat geometry. I figure that when the guys who build race cars zero in on that modification, there's probably something to it. It's great that you've had success with it!

[The Dark Side of Will]

Also, how far did you go with it? Are the control arm forward pivots higher than the rear pivots?

Does your LeMons team have a website?

[neophile_17]

Also, how far did you go with it? Are the control arm forward pivots higher than the rear pivots?

Does your LeMons team have a website?

The forward control arm pivot is higher than the rear pivot. From a side profile, drawing a line between the pivots they point slightly below the cars center of gravity. This creates a force line that is very similar to the 88 suspension.

We had a website but it never got used and has been gone for probably 10 years now. There was/is a facebook group that was mostly maintained by an old girlfriend but has been neglected for 5 years or more.

[ericjon262]

Thanks for posting the follow up, it's always good to see that something worked as desired.

[The Dark Side of Will]

Also, how far did you go with it? Are the control arm forward pivots higher than the rear pivots?

The forward control arm pivot is higher than the rear pivot. From a side profile, drawing a line between the pivots they point slightly below the cars center of gravity. This creates a force line that is very similar to the 88 suspension.

Thanks!

I don't think I have actual measurements of the pivot heights with the stock configuration... I guess I need to snag those measurements. I'm pretty sure I can't go as far as you did while having a NorthStar and a right-side decklid hinge box. I was just hoping I could get the inner pivots level, but I'm also interested in driving it extensively before and after to get a good feel for how it changes.

Great point about trail braking! The pro-squat extends the rear suspension more than it otherwise would, which elevates the rear and changes the roll center and camber... makes sense that would get tamer with better anti-squat.

[neophile_17]

Thanks!

I don't think I have actual measurements of the pivot heights with the stock configuration... I guess I need to snag those measurements. I'm pretty sure I can't go as far as you did while having a NorthStar and a right-side decklid hinge box. I was just hoping I could get the inner pivots level, but I'm also interested in driving it extensively before and after to get a good feel for how it changes.

Great point about trail braking! The pro-squat extends the rear suspension more than it otherwise would, which elevates the rear and changes the roll center and camber... makes sense that would get tamer with better anti-squat.

I would love to hear an expert opinion on this! It worked out better than I had hoped and I'm curious to see how well it transfers to a street driven car.

[neophile_17]

The actual Indy Pace Car had shimmed rear cradle mounts to reduce pro-squat geometry. I figure that when the guys who build race cars zero in on that modification, there's probably something to it. It's great that you've had success with it!

I've been thinking about duplicating this on my street car. Any idea how thick these shims were? I made up a couple that are 20mm but I'd be curious to know how this compares to those Indy parts.

[ericjon262]

The actual Indy Pace Car had shimmed rear cradle mounts to reduce pro-squat geometry. I figure that when the guys who build race cars zero in on that modification, there's probably something to it. It's great that you've had success with it!

I've been thinking about duplicating this on my street car. Any idea how thick these shims were? I made up a couple that are 20mm but I'd be curious to know how this compares to those Indy parts.

IIRC, he's working on doing this on The Mule, but he's also working on a ton of other things for it, and I'm posting this to rag on him about not working on it...

(Says me on my high horse with 2,000,000 unfinished projects/ideas)

[neophile_17]

IIRC, he's working on doing this on The Mule, but he's also working on a ton of other things for it, and I'm posting this to rag on him about not working on it...

(Says me on my high horse with 2,000,000 unfinished projects/ideas)

Yeah I know a little about project compounding.

I managed to find 20mm longer Cradle bolts in my bolt bucket and it prompted me to split that up by size which is progress in it's own right. I did a rough calculation and 20mm will only help the angle by a little over 1 degree. Probably not a huge difference but combined with offset bushings up front it could be significant. For now I'm going to move forward with the rest of my engine swap and leave the offset front bushings for a later date.