Originally Posted by MatthewMiller

For autocrossing, I think the camber loss due to the insane KPIA (or SAI) is at least half the issue. For most road courses, it's probably less of an issue because the steering angles (and therefore the camber loss) are usually lower. The other oddity on the later front ends is the gobs of anti-dive geometry they have. You see this visibly as the upper control arm being angled severely upward toward the front where it mounts to the frame (the front mounting bolt is much higher than the rear). This makes for lots of push on braking: where most cars turn in better with some trail braking, the later C4 just plows until you get off the brakes. This issue could be corrected in other ways besides a wholesale swap to the older front subframe, if one were so inclined. Basically, you'd need to find a way to drop the front mounting bolt hole for the upper A-arm about an inch. Then you'd want to increase your front spring somewhat to compensate for the higher dive and increase turn-in on the brakes. I contemplated doing this, but for my purposes it would have probably been better to do the whole swap.

The previous owner of my car had amassed all the parts to do the swap. He never got around to it, and sold it to another C4 autocrosser in his region. That guy did do the swap, and some national champion caliber drivers have pronounced that car as the best handling car they've ever driven (and they've driven hundreds of cars competitively). It seems to me that an early front end a later rear end are the perfect combo. If I hadn't sold the car, I'd probably be working on that swap right now...

Originally Posted by MatthewMiller

I'll tell you what I know, but keep in mind I'm not an expert. No problem. I appreciate your help with eliminating a few of the holes in my understanding of suspension/steering dynamics.


The binding up really depends on how the anti-dive is accomplished. If the car has a lower A-arm angled downward toward the front (which is equally effective at creating anti-dive), then it will be less compliant over bumps. That's because in order for the lower ball joint to move up in compression (as it would going over a bump) it also would have to move forward, but the bump you're hitting to cause that is trying to drive the ball joint backward (unless you're driving in reverse!). So that's a bad way of doing it. I think a C4's lower arm is pretty level from front to rear, though. The upper arm being angled upward toward the front actually should make it more compliant over bumps, because as the ball joint moves upward in compression it also moves back a bit. Luckily, that's how a C4 does anti-dive.

It does act like the front spring rate is increased though, because it understeers on the brakes. I've heard anti-drive referred to as a "geometric spring," which is what it feels like to me. But I've not been able to completely work out how this would happen. The geometry should add a constant vertical force into the frame that only varies with braking force, and so one would expect the spring and sway bar rates to still determine roll resistance (which is what determines handling balance). However, as the car dives or rolls, the CG would get lower which increases the anti-dive even if the force and vector angle stay the same. I don't know if that accounts for all this effect or not, but in some way it seems to add to the effective spring rate. Note that if you took out the anti-dive and compensated with a stiffer front spring, you'd have the same effective spring rate as with anti-dive. However, you would have presumably compensated by also increasing the rear spring or sway bar rate or by decreasing your front swaybar rate in order to keep the balance. What you end up with, then, is a more consistent handling balance vs the car with lots of anti-dive, which may be close to neutral off the brakes but understeers a lot with the brakes on.


That's actually a point I hadn't considered. As it sits, the upper ball joint does move backward in compression, which the direction it moves when the car leans to that side. It's probably not insignificant: if the arm were canted upward at 45-deg the ball joint would move backward as much as it moved upward. It's not close to 45-deg, but it may be close to 20-deg (the early C4 appears to have around 12-deg). If so, it could gain roughly a degree of caster for every inch of compression travel. That's generally considered beneficial, at least with as much KPIA as is built into the later cars. That said, I'd rather build in a higher amount of static caster (even using custom or SPC upper arms if necessary) and get rid of at least some anti-dive in order to get more consistent handling, than to rely on dynamic caster gain from anti-dive geometry.

Originally Posted by 69427

Originally Posted by 69427

Originally Posted by MatthewMiller

I did that by modifying the UCA shafts to move the arms backward 1/2". I ended up with 8.3-deg caster, and with my offset LCA bushigns I could still get -3 camber or more. IIRC, I think that 1/2" gave me about 2-deg extra caster, so that's a rough rule of thumb. I do think it helped my front end grip in tighter autocross turns.


Please be sure to let us know what you find!

Originally Posted by DMITTZ

Hey MatthewMiller, so you found adding more caster (8.3- caster) to the front added more grip?

I've got the SPC adjustable upper control arms on my car as well as the full banski stuff in the rear, I played around with alignment settings a bunch to find what worked best.

I tried 10 then 8 then 6, then 6.5 degrees of caster along several camber and toe settings at each of the caster levels and ended up finding that at 10 and 8 degrees of caster I got a sort of 'skipping' in the front tires on hard turns. The problem was resolved by dialing it back to 6.5 degrees of caster. With my suspension being so adjustable I can do pretty well any settings I want but found in the front 6.5 degrees caster, -2.7degrees camber and 1/16 toe out was perfect for me. My uses are street and auto-X. I'm tempted to add more toe out as I love the turn-in and don't find any problems with wandering but don't want to wear out my tires to fast.

My C4 has the 'late suspension' BTW.

I wonder if moving the whole UCA backwards vs just using adjustable arms gives different grip results in regards to adding caster...any thought on this?

Originally Posted by MatthewMiller

On my car I did find more front grip in tight turns, where the steering angles are high. I believe this is because the caster is making up for some of the camber that's lost due to the crazy-high steering axis inclination (kingpin inclination) on the 88-96 front ends. It also adds a bit of inside-tire "jacking" which should help keep more load on the inside tire during a turn (the less weight transfer, the better the overall grip). I never had the skipping effect you're talking about, but that could be highly depending on tires, inflation pressures, shock settings, spring and sway bar rates, etc. I've epxerienced it in other cars, so I know what you're talking about. I was running a little over -3 camber on my setup, FWIW.

I don't think there should be any difference in geometry between setting the UCA back on its shaft or mount vs making an arm with an rearward-offset ball joint (which is effectively what you did with the SPC arms). Either way, the pivot axis should be at the same angles and the arc through which the ball joint travels should be identical. I was actually contemplating having custom UCAs made to accomplish the same thing, to try even more caster.

It would be worth trying an offset UCA of some sort, even if you decide less-than-max caster is desirable. You can set your UCA offset as far back as possible, and then use shim differential at the pivot shaft. When we splay the shafts outward toward the front (which what the stock caster adjustment of differential shim stacks accomplish), it introduces bump steer that can't be fully corrected. So in your case - with the SPC arms - I'd set the ball joint backward as far as possible (watch for interference with the strut tower!) and reduce the caster at the pivot shaft instead.

Originally Posted by ChrisLSx

Do you have a build thread or any pictures of how you did your conversion? I’ve just about narrowed down to running an early C4 front and probably something akin to a Greenwood style rear, so now it’s planning how to actually do the conversion without make a mess of everything.