GrantB

I know there have been a lot of threads on this subject, but I've been fighting with mid-late corner oversteer for a while, and I want to make absolutely sure I'm not hitting the bumpstops.

First, does anyone know what stock C5Z ride heights are? I know GM recommends lowering a stock car 15mm for track use, so that seems like good settings to start with.

My car is a '99 FRC with '04 C5Z suspension, pfadt bushings and pfadt anti-roll bars (currently set full stiff in front and full soft in the rear, due to oversteer problems). Raceweight (with a half tank and me in the car) is 3227 lbs, 52.2% front bias.

The OE tires have diameters of 25.4" and and 26.1". The tires I am running (275/40/17 and 315/30/18 Kumho XSs) have diameters of 25.7" and 25.5". So relative to stock sizes, the car will ride 0.15" higher in front and 0.3" lower in the rear.

The ride heights I was planning on running were 4.8" front and 5.0" rear, as measured right in front of and behind the lift points on the frame rails. I didn't want to give it too much rake, because I thought he difference in tire sizes might cause issues with geometry and the roll axis. Obviously these heights will change a bit after I corner weight the car, but those are my targets. Do those seem reasonable? Does anyone know what the "lower bound" of rear ride height is? Does anyone ever trim the factory bump stops a bit?

I'm also swapping out the (slightly binding) VP&B bushings with pfadt ones, to make sure they aren't contributing to oversteer. The alignment I'm going to run is -2.0/-1.2 camber, 6 degrees caster, and -1/8"/+1/8" toe.

Thanks for any help.

gkmccready

I hope you have these toe settings backwards... you want about 1/16"-1/8" toe IN in the rear. I've heard of autocrossers running up to 1/4" in out back even... I chased an unhappy car with various swaybar combos before putting on poly bushings and setting 1/8" toe-in in the rear. Night and day difference.

Also, FWIW, my C6 Z51 came 5" to the jacking puck holes front and rear. Almost everybody told me to lower the car at least 3/4" and that I'd have to cut some of the rear spring adjuster to get it there.

GrantB

Right, I guess I got my signs backwards. I might only go with 1/16" toe out up front. Good to hear toe made such a difference, are these cars particularly sensitive to it? I never felt such a marked difference in other cars, so I generally just ended up running zero toe on cars that saw street use.

I already had poly bushings in the car, but the left rear always made some noise (I think maybe some of the poly pieces were over-sized), and the upper fronts started squeeking pretty bad too, after ~8k miles. I'm swapping out VB&P bushings for pfadt's, that have grease grooves on the inside. I've already done the fronts, and it made a huge difference in noise (silent now). I'm hoping the reduction in binding will make a difference.

My next track event is this weekend, so I'll post up the DAQ results, compared to my previous setup.

Hmm, thanks. There is no way I could get 4.25" of ride height up front, unless I removed the adjustment bolts completely. 4.8" is as low as it will go with the shorter West Coast Corvette bushings. Much of the weight I've taken out has been in the front, so on the stock bushings the lowest I could get it was around 5.2".

Gordy M

The factory rake settings are set using a parallel chassis and the tires making the rake angle. That is 1/4 to 5/16". By setting you rake angle with the new tires installed to .2" at the jack points you should be 5/16 to 3/8" of rake at the "z" points. This should dramatically improve your high speed cornering. After you have everything set you should then re-set your cross weights.

Last C5

My current ride heights are 5.0" rear and 4.7" front with weight in the driver's seat. Caster is 6.70 deg and Camber is -2.3 deg front -1.3 deg rear with -0.04 deg front tow and -0.10 deg rear tow. I'm running 275/40 and 305/35 Toyo 888s. Car handles fine.

davidfarmer

is this Throttle induced oversteer, or is it happening mid corner before you get on the throttle????

GrantB

Thanks for all the help, everyone.

Last C5, if I bolted my rears onto your car it would drop the rear down to 4.6". I'm uncertain how changes in rake affect mechanical grip. There is the shift in CG, change in camber curves and roll axis. I know what affect moving the CG has, but what about the other stuff? If you decrease tire diameter but keep the ride height the same, how does that affect the handling of the car?

Its a bit of a long story, as I've been fiddling with the car for a while. As it sits now (with the pfadt bars set to maximum understeer and very little rake), it tends to push a bit before throttle application. Applying even a little bit of throttle (past simple maintenance power) causes the rear to jump out.

A friend of mine drove it and said its as if the rears are overheated, which I think is a good description of the feeling. In other RWD cars I've driven, smoothing increasing throttle causes understeer as weight transfers to the rear tires, eventually followed by oversteer (if the car has enough power, the tires aren't really sticky, etc). My car just feels like it skips the understeer and goes right to oversteer, every time.

Grip in a straight line is fine, with the car doing ~11.8-12.0 @ 121 to 124. When the car was on NT-01s, longitudinal gees tended to peak around the same as my '08 ACR Viper (on PS Cups), around 0.85 gee, yet the ACR absolutely rapes it out of corners (~5 mph in a 60 MPH corner). Of course the FRC doesn't have a huge wing out back, so I'm not sure what to make of that comparison. I also use it as more of a street car than anything else, so I don't want to put springs on it as stiff as the ACR's (I'm happy with the OE C5Z suspension).

fatbillybob

What is your rear toe?

GrantB

Only about 1/32" total. I never felt toe made that much of a difference in other cars I'd driven, so left it mild. Whatever the case, I'll be using 1/8" this weekend, and should see any benefits in the data.

fatbillybob

The rear toe is important to handling and must be changed on a car that has been lowered. Factory is 0 rear toe actually -0.02 +-0.20 degrees IIRC. Toe can change as the suspension travels through its range. The number I quote is stock with stock springs. You may have more travel and alter the toe more and effect handling. Sometimes adding the best cam, best head, best piston etc does not work as a package and you are slower than stock. Honestly I have tracked with guys who have all the good stuff yet slower than stock. GM did a pretty darn good job when setting up the vettes. We need to be very careful when we re-engineer with our small wallets compared to GM's big wallet. Also, rake changes over/understeer. If you lower the rear alot like 1" you will instantly feel the rear grip more. I don't advocate an inch but use that as a measure that everyone can definately feel instantly in the first turn. Also, before you change things jack the car up and put it on stands and check that everything is where it is supposed to be and installed right and nothing is broken. You can try every coilover made for man and never solve your problem if it turns out to be something else in the car causing it.

davidfarmer

I agree that more rear toe will help some. with only 1/32", as soon as you apply throttle, you induce the rear tires to actually toe OUT, which will cause the rear tires to fight each other. I'd put a solid 1/8", assuming it's a track only car.

If you have scales, you might want to make sure you aren't ridiculously off, and that the bars are neutral. Since smoothly transferring weight to the rear of a Corvette (during transition to throttle) tends to make the car more stable (by adding weight to the "light" end), you obviously have something disrupting this.

GrantB

I just put the pfadt bushings in, and noticed a huge increase in how freely the lower control arms move. I think some binding may have been going on in the rear.

Are you saying the rear tires of a C5 (and C6?) toe out under bump? Looking at the tie rod and LCA with everything apart its hard for me to judge, but I just assumed it toed in under normal bump travel. If not, is there a part commonly used by racers to correct this? I admit I've never adjusted suspension geometry in any of my cars, but always thought a bit of toe-in under bump was beneficial, and how all sports cars were tuned from the factory.

I don't suppose anyone has put the C5 into SusProg3D (or similar)?

I do have scales and adjustable bar end-links. Corner-weighing the car made a noticeable difference, especially under braking. The car is actually more of a DD, and is not my primary track car. However, I'd say about 2/3rds of its tire wear comes from a monthly track event at a local test track, so I'm not concerned with toe-induced wear so much.

gkmccready

Yes, but it's mostly due to the bushing material, I think, and not the geometry. The harder the bushing material the less toe-in you need to run. Rubber bushings = lots of toe-in. Poly bushings = some toe-in. Spherical bearings = ??.

GrantB

The combination of pfadt bushings, a new alignment (with more toe-in) and me leaving the rear sway bar disconnected after corner weighting helped enormously. I carried 1.5 more MPH out of the course's increasing-radius corner, and beat my previous best lap time in the car by 0.7 seconds. I hope to pick up more time tomorrow when I reconnected the rear bar.

I think it may be the geometry. While the car was getting aligned, I pulled down and pushed up on the rear sway bar. Pulling down reduced toe (in, I think, though admittedly the machine wasn't clear that we were in fact dialing toe-in into the car), while pushing up increased it.

GrantB

Reconnecting the rear sway bar netted 0.2 seconds, with the car running a 55.22 vs. a 55.4. General handling, especially transitions, improved. The ability to enter decreasing-radius (i.e., trail-braking) corners suffered, as did powering out of 2nd-gear corners.

Maybe I just need to add more rear tire and take more weight out of the front of the car.