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?

tmtraylor

I thought the wheel-hop issues are more with the drag racing crowd.
I doubt the "more aggressive" options are necessary. I use the stock motor and diff mounts on my C5Z. I think the motor mounts leak their oil when bad so you can "see" that. The rear mount for the diff just being a hunk of rubber maybe not so easy to confirm, but you don't hear too much of that part being bad.

The time and effort on the toe links will serve you well whether or not that is the source of your issue. Three days at Sebring beat up my car enough to put the rear toe out at more than 1/8". The next event at Road Atlanta and I thought the car wanted me dead... we adjusted to 1/8" toe in and the car went back to normal. No "wheel hop" but it sure wasn't fun to drive with the rear toe'd out.

Make sure to check /replace the inner tie rod on the rear as well as the outer. Good luck !

Tim

davidfarmer

Only time I’ve ever had wheel hop in a c5 was due to a damage shock absorber

Quickshift_C5

I've only had wheel hop in C5's from aggressive starts from a dead stop, never while moving. This is including high horsepower rolling starts, autocross, and road racing on both stock and lightly modified suspension.

emptnest

Bill Dearborn

That isn't the same as the wheel hop experienced when the rear tires spin. As the tire heats up the grip levels change and you reach a point where the tire starts stuttering in a series of slips/stops which sets up a harmonic causing the rear wheel to literally jump up and down while the car is barely moving. It literally feels and sounds like the rear end is going to rip out of the car.

What kind of condition is your diff in? Maybe you have some issues with the clutches in the posi? If the clutches are locking up so they don't permit sufficient slip you could get more noise and clunking in a tighter turn than during a wide turn. The LSD fluid you used should prevent that but if there is an issue in the diff it may need to be serviced.

Bill

emptnest

I agree that the differential might warrant a closer look.

Your comments reminded me of these additional 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 rear 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.

63Corvette

Shocks! I recommend stock 04 Z06 shocks or Bilstein Sport shocks.

jlaw1313@comcast.net

2004 C5 coupe with Z06 sways, springs and shocks. Intermediate driver. NT-01 on OEM Z06 wheels. I run with no traction control but leave active handling on. Never experienced wheel hop but did have some chatter mostly turning left from a standstill under heavy throttle after about 12 HPDE track days. Changed out Redline Severe Gear with no friction modifier for ACDelco #88862624 (no friction modifier required). Problem solved. NT-01 I use 28 lb cold pressure and end up 36 lb hot all around. As Mr Farmer said check dampeners.

crimlwC6

I would suspect shocks first then LSD. I was told my LSD was shot but I've kept racing for another 2 years on it. With the exception of trying to put heat in the tires leaving grid in first gear I don't get wheel hop (I did upgrade to penske coilovers 2 years ago). The only time I had people telling me they could see wheel hop or what I observed when sharing the car with another driver was solved by the penskes. I think you have a bad shock or two.

emptnest

This was at 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.

Will soon be doing Z06 shocks and sways, with metal end links. Should I go with Z06 rear spring too? (I dont envision going to coil overs)

emptnest

I'll try switching to the ACDelco LSD fluid. Thanks for that tip. Sometimes aftermarket fluids (my Amsoil) turn out to not be the upgrade that was intended.....

sccaGT1racer

Did you have your traction control turned off? I had a friend describing the same thing to me this weekend and it ended up being the active handling coming on in tight turns at summit point.

duk007

Was this at the HPDE on Saturday? I was there in a silver c5z. I was there all day so I must have seen you at some time ( there were several silver c5’s) mine has a number plate “072”. Im an instructor there, reach out if you see me there , I’d be more than happy to go out for a lap or two and help diagnose the problem. I’ve had rear end noise in my own car in the past and I know what, bearings, traction control, and dif noises sound/feel like. Happy to help.

jlaw1313@comcast.net

Will soon be doing Z06 shocks and sways, with metal end links. Should I go with Z06 rear spring too? (I dont envision going to coil overs)[/QUOTE]

If you decide to upgrade with Z06 springs I would replace both front and rear. This would keep weight tranfer balanced. Alot of suspension engineering to consider. Way over my head. I let GM do the math. The Z06 springs, sways w/ metal endlinks, and shocks feel planted and well balanced to me for my skill level. I am very satisfied with the NT-01 tires; 275/40/17 front 305/35/18 rear on Z06 wheels with this set up. I would consider myself advanced intermidiate. I am considering a shock upgrade and moving to a square setup for tire sizes.

jaredtxrx

Some cars are super picky with fluid. Corvettes tend to like the ACDelco fluid for the differential the best but the transmission takes any old synthetic.

Quickshift_C5

63Corvette

If you decide to upgrade with Z06 springs I would replace both front and rear. This would keep weight tranfer balanced. Alot of suspension engineering to consider. Way over my head. I let GM do the math. The Z06 springs, sways w/ metal endlinks, and shocks feel planted and well balanced to me for my skill level. I am very satisfied with the NT-01 tires; 275/40/17 front 305/35/18 rear on Z06 wheels with this set up. I would consider myself advanced intermidiate. I am considering a shock upgrade and moving to a square setup for tire sizes.[/QUOTE]

Suspension design should be considered as a fully engineered PACKAGE. do NOT remove/replace one item at a time.
NOTE: All Z06 shocks are NOT the same. Use ONLY 2004 Z06 shocks.

emptnest

I have unhooked the steering wheel position sensor (SWPS) to deactivate active handling. That seems to have also deactivated traction control; pushing the traction control button no longer causes any change in dash display, and the amber traction symbol (near the speedometer) is constantly lit.

To hijack my own thread, regarding effects of unhooking the SWPS, the threads on that subject are not clear; some say that TC remains active while others say that it is deactivated along with active handling. Prior to this, I have been unconcerned about that, as I am willing to drive without the nannies (ABS remains active).

But this experience makes me wonder: is Traction control still supposed to be available with the SWPS disconnected, and if so, might my "wheel hop" vibration be a faulty TC system trying to kick in and do its job?

emptnest

63 Corvette: I note that you specifically recommend the 2004 shocks.

I see a lot of forum members (but most of them are not necessarily track guys, and most certainly don't have your experience - which I respect) recommend C6 Z06 shocks, C6 Z51 sways, and C5 Z06 springs. Do you recommend staying with 2004 Z06 for all components? And if so, why?