Ramo7769

I'm still struggling with my setup. I have VBP springs 1100# front, 800# rear. C5 Z06 sway bars. (Strano front bar 1 5/16" ordered, patiently waiting) DRM Bilsteins. Z06 rear 18x10.5s square with Dunlop Direzza ZIIs. I cross corner balanced and aligned myself.

Thanks to Matt_27 for the alignment advice at NCM where he kicked my ***. I took out the caster and maxed out the camber up front. I also run a bit of toe out (2 flats per side from zero) in the front. That helped the front end bite and got rid of some of the low speed push. I just ran this alignment for the first time at a Toledo Express event over the weekend. Rear alignment is max camber and 0 toe.

The issue I'm having is that the car still pushes in the tightest corners in Toledo, which is a big site, and it is twitchy and oversteery in transitions and power on. I had a better driver co-drive with me and he agreed that the car doesn't give enough confidence to drive hard... yet still pushes at low speed.

I'll also say that the car still pitches quite a lot. I don't believe the rear spring is considerably stiffer than a Z06 spring. And more front spring still wouldn't hurt imo. I unload the rear a lot in braking and the car is unstable. Need to really focus on braking straight or you're very easily tail out. And then

I'm open to advice but my next steps will otherwise be: Tuning rear toe-in this weekend and also changing to the stock rear sway bar, changing one variable at a time. Aside from that, accepting some level of low speed push being inherent to the C5 and adding some front end stiffness to get transitions and mid-high speed handling better. I have the Strano bar coming and I'm also waiting on a VBP ~1200# spring.

Obviously part of my issue is the Dunlop. They have over 100 autocross runs, a track session, and street driving on them. They obviously aren't competitive, but I want to make the car handle better before I throw Bridgestones on and start wasting them with a bad setup.

I would like to improve the handling of the car and have RE-71Rs mounted in time for the Toledo Pro Solo 6/28. Aside from spending a few thousand dollars on shocks, I'll try anything and am open to advice. It would be appreciated.

ltborg

Rear toe in will help all of your issues except the mid corner push. Remember that with soft bushings, under braking the car will toe out in the rear, contributing to the instability. Your setup will change quite a bit going from one tire brand to another so if you want to switch, I would recommend switching, then tuning for those tires.

Ramo7769

Good point on the bushings. I probably should have tried some toe-in a long time ago. And on the tire issue, yeah... if I set the car up as fast as I could for the Dunlop, it'd probably be too pushy for the RE-71R.

I'll try some toe adjustments on the track where I am going to melt the remains of my Dunlops. And then order the Bridgestones sooner rather than later before I start adding stiffness to the front end.

Thanks for the input. Do you have any further suggestion for low speed push? knowing my front alignment settings are roughly -3.0deg camber, -0.5deg toe and what ever that results in for caster? Other than a high amount of yaw inertia which we can't do much about, I would think the only other factor that could help it would be a looser differential.

troyguitar

My C5Z pushes like a FWD car going into slow corners, I don't know what to do about it either. It's totally neutral everywhere else but that push makes it a chore to autox. I am hoping that moving to a square tire setup will cure most of it since I'm running 265/305 staggered crap currently. I guess I could keep throwing camber at the front too, I'm only at 2.5 degrees not maxed.

Xian

Yup, I'd switch tires and then start tuning. The Rival-S I tried on the rear this weekend was quite a bit more progressive than the Z2SS. The 71R's up front had the "pointy-ness" of the Z2SS but with slightly better braking. They also tended not to be as temperature sensitive as the Z2SS. That brings me to a decent point; any chance that you're overheating one or both ends of the car during the run and alternately causing push and serious looseness?

LateBreak

I have a very similar setup in my car and found that the Z06 rear bar was too much for the Dunlops, so I unhooked it first as a Band-Aid at the track but have since switched to the C5 Base rear bar and really like the feel.

*This is with C5 Z06 Springs front and rear, C5Z front bar, stock bushings, DRM Bilsteins with Poly bushings, and solid front spring adjusters.

LateBreak

I forgot that this was a convertible until reading the original post for some background. Do you think that the chassis may be flexing and causing some of your issues? That would be my fear if you find that the adjustments aren't helping.

Ramo7769

Actually, I'm not the OP and I drive a FRC. Thanks for the input though. Hopefully I'm not thread jacking, but it seems like pretty open STU setup discussion so I refrained from starting a new thread.

In response to the other poster regarding tires over heating. I struggle with overheating tires, but my dynamic issues show from run #1 with the tires at ambient temp.

Where I'm at is accepting some of the inherent low speed understeer of the C5 and getting the car optimized and easier to handle at mid-high speeds. I'm going to go after rear toe, front spring, front swaybar, tires, one variable at a time. Thanks again.

jesup16

I'd follow Lane's advice. Rear toe-in will help. I've always ran some rear toe-in, even on my STU prepped STi. I think that will help keep the rear in check. I'd start there and run the 'stone RE71r's

I just switched to the 'stones and they are awesome! I never owned the ZII's, but I did drive them a couple times (I was never a fan) and I can tell you there will be a difference, obviously in favor of the 'stones...I'm not going to tell all the differences..

I think everyone's given good advice. Toe-in and tires to start. The only other thing I can think of is braking...which is likely just a fraction of your described struggles. I'm thinking this could be mix of your technique and brake setup:

What pad's are you running?
Do you have aftermarket calipers, rotors, etc?
Are you trail braking?
How do you "attack" the brakes?

Example: I decided to auto-x on track pads I left on from the weekend before (cuz I was too lazy to change em). They were too aggressive when hot (scrubbed too much speed) and not good when they were cold. I switch back to my auto-x pads the next weekend and was back to normal. I've run different pad compounds on the front and rear of cars, too.

edwardo99

I've found that 2.5 degrees in the front is plenty for 265 street tires (Dunlop Z2* to date, 275 Stones next) This is what I can get on my FRC in BS and you have 1" more wheel support than I do. My car pushes only slightly in slow corners, neutral at mid-speeds, starts to get loose only at high speed, but is still perfectly controllable at track speeds as long as the front toe is at zero. This is normal behavior for the Vette, as far as I know.

@Ramo7769: the number of experienced autocrossers I've heard of in the last 5 years that run a C5 or C6 Vette with zero rear toe is exactly 0. Your symptoms are what most people would expect with zero rear toe-in, as others have pointed out.

Now, when you say you have -.5 deg toe in the front, is that total and do you mean out or in? that's about 1/4" per side if you mean total.

One last thing: I can (stupidly) make my car push badly just by having too much front shock rebound as compared to compression. I had to learn this the hard way at the Bowling Green tour. 3 clicks different (out of 22) made a huge difference on about the smoothest surface imaginable. My point is this: I think that the tires are unlikely to be the source of a severe handling balance problem and therefore unlikely to be the cure.

ltborg

All cars will have a balance shift with speed. On a non-aero car, it will be tighter low speed than high speed. This is a fundamental aspect of vehicle dynamics and not limited to just the Corvette. It is also why I prefer a looser car; you can rotate it at low and high speeds, you just need to be on your game at high speeds.

I did this too testing shocks. Compression feels great, but the car will have no grip in sweepers. Too much rebound can lock the car on the bump stops and potentially create a low grip situation also. If you have adjustable shocks, try turning them down equally front and rear to give yourself some more mechanical grip back.

Ramo7769

The next thing I try will definitely be rear toe in. I have toe out up front to help with the low speed push since the C5 doesn't have much ackermann.

Damping could help but I think fundamental weight transfer changes in the way of springs and swaybars is where my issue is. I was apprehensive to stiffen the front end with the low speed issue, but that seems to be something to drive around.

edwardo99

Good luck with figuring it out. I have the big Strano bar, very high low-speed compression in revalved shocks, stock springs of course, but no problem with understeer.

Keep in mind that the damping and driver inputs control how fast the weight transfer happens more than springs & bars. I think it's pretty easy to shock overload the front tires, streets especially. The stiffer the springs, the stiffer the bar, the more compression damping... all that makes it great for transitioning but even easier to induce understeer. When I drove Lane's car last year I had to really slow down my inputs to match the tuning of the car. Apparently some folks tended to spin that car when they co-drove (I came close!) but I thought the car was well balanced... it was just so freakin' responsive it took some getting used to.

Ramo7769

This is a good point that it is a fundamental aspect of vehicle dynamics and it's an interesting topic. In my vehicle dynamics class, I was able to calculate a threshold speed in which the car would become "out of control" with oversteer. One thought I had regarding this that explains part of this issue is that the lower the speed of cornering, the higher the steering angle the limit is found. Because of that, the lateral force from the front tires has a larger X (aft) component relative to the vehicle that is working to accelerate (negative) the vehicle longitudinally rather than accelerating the vehicle laterally.

And like you said, all cars exhibit this. But, the Corvette has more factors, like a tight 2-way clutch diff, low ackermann, and a relatively large amount of yaw inertia.

Ramo7769

Good input, thanks. Don't get me wrong, having well calibrated damping I'm sure would help some of my issues. But I'm having issues with steady state too.

I'm not well versed in shock tuning. I feel that I could learn it with some time and adjustables, but you mentioned having revalves on your car. What type do you have and what sort of pricing have you found for revalving?

troyguitar

Appreciate the feedback. I only have single adjustables so at this point the only tuning available is shock rebound or simply entering corners more slowly - it's entirely possible that I'm just trying to go faster than the tires (265 RE-11's) are ever going to handle. This weekend I'm attending a school out here and hopefully someone who knows more than me will be able to get in the car and tell me whether it's a driver issue or a setup issue.

Ramo7769

Some toe out in the front helps too. In tight corners the fronts fight each other a bit due to lack of ackermann. 3 flats of toe out per side that I adjust at the event seems to help me.

Too much caster was also hurting me, it seems. I had -2.0deg camber and 7deg caster and had more issues. When I changed to maxed camber and unknown (but less) caster, low speed performance improved. I benchmarked against John Laughlin who is nationally competitive in FS and found that I closed the gap by 0.8s on average, on a 50s course.

troyguitar

I think that's a lot more toe than I'm running. I have roughly 1/16" total measured 21" apart. If my brain is working properly this afternoon that's 0.17 degrees total.

I really have no clue where my caster is at this point, I didn't even attempt to measure it. It was at 6 degrees before I added a degree of camber - tried to move the front and rear eccentrics by about the same amount but realistically the caster could be anywhere.

Ramo7769

Just checking in to say holy crap, why have I not tried toe in the rear before last weekend. I don't know the toe angle I used. I just string it to zero, mark the tie rod and then turned 2 flats per side toe-in. It made an unbelievable difference and really settled the car down, especially on entry.

Apocolipse

Welcome to CF

These guys know their stuff.