emptnest

I am an advanced driver in HPDE, with this being my first season driving a new (to me) 1999 FRC. I previously drove a stock Z07 C4. The FRC has stock rotors and calipers, SS caliper pistons, Raybestos ST 43 pads, SS brake lines, DRM brake ducts, and ECS spindle ducts. The suspension is stock Z51, with 66,000 miles, riding on 315 x 17 NT01’s all around. The rubber bushings look good, but the stock endlinks are kinda chewed up (torn boots) and slightly twisted.

I just finished my first track day in the new FRC. While I expected the stock suspension to be soft and the car to possibly be difficult to control, overall I was pleased. The car stayed planted and behaved well; but I was only driving at 7/10ths most of the day, it being a new car, first day of the season, etc.

Here’s the issue: for the first time ever, I experienced wheel hop as I would aggressively accelerate out of a turn. I found the hop to be more noticeable with tighter turns; I could reduce it by taking a wider line through the corner. Of course, it was also reduced with lower RPM’s.

A quick forum search informs me that this is fairly common on the C5, and that it is usually cured with suspension upgrades including springs, shocks, and sways. More aggressive upgrades include the PFADT transmission / differential mount, a Transmission brace, and possibly even motor mounts.

But before I go down that road, I want to rule a few simple things out. My preseason work included a few things that might be related:

1- New wheel bearing hubs were installed all around. On the rear, that of course meant that the drive axles were removed from the hubs with associated movement in/out of the differential (the axles were not dis-assembled or removed).

2- Differential fluid was drained and replaced with Amsoil severe gear 75W, including about 3 ounces of limited slip additive.

3- And most suspect: a new wheel alignment was done, and I think the shop gave me rear toe OUT, when I wanted toe IN. On the front, I wanted toe out, and it came back at zero. I challenged the shop to explain the results, and all I got was gibberish. They couldn’t even explain whether the rear was toe IN or OUT. Obviously, I need a new shop and alignment.

The car handled well, drove and braked straight, with no vibrations, other than the hop.

I have proper alignment specs for track duty – this thread isn’t about that. The question is did the rear toe OUT (or any of the other preseason work) possibly cause the wheel hop? Or should I just accelerate my schedule and start beefing up the suspension components right away?

emptnest

UstaB-GS549

Something is not right. Bad shock?

In 12 years of track time I have never experienced wheel hop coming out of a corner. Stock Z06 suspension, engine and all sizes of scrubs.

Are you slow enough to get wheel spin? From a standing start at Autobahn CC in the pit lane on the south course I could get wheel hop with a hard launch. Something about track surface. The solution was to use less of the right pedal.

Toe out in rear will make the car unstable under heavy braking.

emptnest

Two more observations:

The vibration was absent or less intense with lower tire pressure. In each run, as the tires heated up, the vibration increased.

Also, the rear tires gained 5-6 more psi than the front. In my first run, fronts gained 5 psi, and rears gained 10. I bled them back to my 36 front and 34 target pressures. In subsequent runs, the fronts finished on target at 36, while the rears finished around 39 (despite having been bled down to 34 hot immediately after the previous session). Being new to the C5 on track, I'm not sure if that is unusual.

emptnest

Tires never spun. Tach was smooth each time. Not even a tire chirp.

UstaB-GS549

Any play top to bottom with rear wheel off the ground? The stock GM end links have a no-lube plastic liner which can degrade quicker due to close proximity to heat of rotor when doing multiple laps.

Was this corner specific? and which track. Someone else might know something about track condition, etc.

emptnest

Pittsburgh - Turns 1, 8, and 13. Most noticeable at T13, which I enter and exit at high RPM (enter going uphill, levels out at or shortly after apex). T 1 and T 8 are entered with a lighter throttle than T13. Interestingly, all 3 of these turns are left handers, on a clockwise track.

In a different thread, someone suggested the posi traction clutches might be bad. Looks like maybe the left side.

End links definitely need replaced. Just waiting to do it with sways and shocks. No play at 12 and 6 with wheels off the ground.

jjc508520

Traction control off?

emptnest

Bill Curlee

Here are the simple and most cost effective things that you can do to fix the problems that you are having. Very similar to the issues I was having on my 35,000 mile 02 ZO6.

Change the chocks to C6 ZO6 Shocks. Made a significant improvement in high speed maneuvers.

Install a PFADT style differential mount.

Get rid of the worn out engine mounts and install new OEM mounts or up grade to poly mounts.

If you don't have metal sway bar end links, install them.

punz

Many years ago, I had a noise/ vibration as you described. Not all the time, certain turns and accelerating, after changing the drivers side half shaft. Drove me nuts, thinking of what I might have not done right. After a few times jacking up and pulling the tire and inspecting and not finding anything, I was ready to start pulling the shaft back out, thinking it may be defective. It ended up being the emergency brake cable hitting the inner part of the tire/rim. The is car lowered, tubbed and 335's. Figured it out on the road, laying on the ground, looking and feeling around. I guess the cable relocated a little and hit at certain loads. When jacked up it had lots of clearance. Couldn't believe it, time consuming easy fix.

Bill Curlee

To check the clutches and clutch beleview springs,, Jack ONE rear wheel off the ground, E Brake OFF, trans in N... Rotate the wheel that is off the ground. It should have resistance like the E Brake is on in either direction. Measure the Break A Way torque in the CW direction.

Do the same procedure to the other wheel.

They should BOTH be/feel the same. If one is significantly different than the other, there's an issue. Either bad clutches OR a broken beleview spring.

Bill

emptnest

Bill Curlee

As long as they are equal, all is good