MatthewMiller

1996 C4 here, used for street driving and autocross. It's lowered with VBP extreme springs, Koni Sport shocks on the rear, "beam plates" on the C-beam, Exotic Motorsports toe links, Banski camber rods, and DRM trailing arm plates. The car came to me with all of this installed, and did not axle hop. I verified with the previous owner that he never experienced hop, either. When I got the car it came with stock trailing arms installed.

Some time last year it began to have axle hop on aggressive launches, with the stock trailing arms. If I feather the clutch a little more, it doesn't happen. And it never happens once under way, even if I break the tires loose. Occasionally, when first beginning a drive (just regular street driving, not hard-core competition runs), I'll hear a little "pop" from somewhere in back. It's only ever a single pop, and it only happens once, at the very beginning of the drive. I have no idea if that sound is related to the wheel hop.

I had Banski rod-end trailing arms sitting in a box, so I installed those a couple weeks ago; thinking maybe the rubber bushings were getting worn and this would stop the hop. No such luck: at an autocross after the swap, it hopped just as bad as ever. No change. I usually race on BFG Rival S tires, but the hop has also happened with less grippy tires on it.

Today put the car on jack stands and started really looking for a cause. I can't seem to find any obvious problem. Here's what I've checked out so far:

• The rear shocks are fine. Their mounting bolts were all tight, bushings looked fine.
• The trailing arms are all still in good shape, and they are solidly bolted to their mounts. Ditto for their brackets.
• Ditto for the camber rods.
• The batwing looks good, with no visible cracks, and the rubber bushings holding it in look great. I tried to pry on the steel tabs that it attaches to, to see if any spot welds were broken. They seem to be fine.
• I made sure the bolts on both ends of the C-beam are tight. They weren't loose, but I put somewhere between 1/8 and 1/4 turn on the rear bolts just to make sure.
• I see no cracks or other problems with either upright.

I haven't put it back on the ground yet, because I doubt I've fixed anything. I'm wondering if anyone else has ever experienced this? Is there any common cause of wheel hop in C4s? What else should I be inspecting? All input is appreciated.

383vett

MatthewMiller

I'm pretty sure it's usually in a straight line, and always from a standing start. FWIW, the fluid that's in the diff now is the same fluid that was in there when it wasn't hopping.

On another note, I have some different rear shocks that have way more compression damping than the Konis (custom-valved Bilstiens). If I don't find any obvious cause, I will also try putting those one just to make sure the Konis aren't the issue.

gerardvg

Hi
I have Banski trailing arms and strut rods, I have never experienced axle hop in my c4 over the last 20 years.
I would check the sway bar bushes and joints, they can pop out and affect spring tension a little on each wheel.

Check the trailing arms, measure each at center of bolt holes, if they are not clamped tight and have slipped or one side is uneven that may be it ?

Another thing to check is are the tires out of round? jack the rear up both wheels of the ground and spin the wheels.
Tread separation or high spots could be an issue, however the vibration would be real bad.

Should be a simple issue, check Diff batwing to body mount bolts and bushes. Don't forget to check for wheel bearing play or universal joints.

Good luck

MatthewMiller

I forgot to include it, but I checked those and they are all good. Also, it's done this with three different sway bars installed, so probably not a sway bar issue.

All good there.

It's done with four different sets of tires. The Rivals are actually really round and well balanced, too.

Those all seem to be good.

I think the bearings are fine for now. I've replaced them before, and these would have been almost new when the hopping started. But I will double-check.

The U-joints...I need to check those, especially since the halfshafts also serve as the upper lateral links. I'm not exactly sure how to check them, though. Do I have to remove the wheel bearings to get the shafts free, and then just check for play? I'll read up on it in the FSM tomorrow morning.

84 4+3

The way I always did u-joints was have someone go from forward to reverse while holding the brakes and look to see if the u-joints move without anything else loading up. This would be on the driveshaft but I imagine it would also work on the axles. If they are really bad they will click or pop.

WVZR-1

This experience hints that maybe it's time to actually service the differential. Pinions, washers, pin, clutch pack etc. Have you ever checked the 'break-away'?

https://www.corvetteforum.com/forums...post1591070166

MatthewMiller

I have not checked this. I will do that. Thanks. If the breakaway torque is still within reason (above 50lbs according to rklessdriver in that thread you linked), does that mean that all is probably well inside the diff? Is there something else I can/should check with it all still installed in the car? Or do I have to take it all out and open up to inspect it?

WVZR-1

I'm assuming this is a D44. How many miles? You should be able to feel the pinions/washers with a twist of wheel/tire forward and back. There's also pinion/ring gear you'll feel but generally pinions feel differently. You could lock the pinion to check.

If it ain't never been serviced? I actually believe that likely a very large % of the D44 failures can be attributed to 'lack of maintenance'. Just because it passes a minimal 'breakaway' doesn't mean there's no differential wear/issues. Fluid changes ain't maintenance.

Your particular use would hint 'regular' maintenance and maybe it's way past time.

MatthewMiller

It's a D44, right. It's been serviced in the past and has around 10k miles on it. It's had no high-speed track events on it since the last service, and that's usually the hardest on the parts in terms of heat. It never gets drag raced or power shifted. Autocrossing is usually a put-in-2nd-and-leave-it kind of event. All that said, this is a 396 with decent power and a light flywheel, so the diff is putting up with more force than it was designed to. I'll check for slop in the gears as you suggested. Thanks.

WVZR-1

What was considered 'serviced'? Bearings, seals etc OR? Is this an OE ratio and what year? Early D44 are shimmed with shims and later C4 D44 do actually have a crush-collar.

Many who do bearings and seals look no further. Generally speaking when servicing a D44 the pinion washers should just be replaced.

Total known miles?

MatthewMiller

Not exactly sure how far the service went - it was the previous owner's at the time. I will have to find out. It's the OE 1996 D44 with the OE 3.45 gears. Total miles on it are same as the car: about 55k, most of which had stock or mildly elevated levels of power. Probably only the last 15k had the current power levels (probably roughly 425rwhp).

Tom400CFI

This is weird. I've NEVER seen a C4 have wheel hop. C6? Oh HELL yeah. But not a C4.

I don't think it's the diff -even if worn...there is not compliance in a diff, worn or not, and I don't think it's suspension (since your all Banski and the stock suspension isn't even prone to hop) or shocks (I know you'd be a stickler about shocks, and even worn shock C4's don't wheel hop).

What parts of the drive train/suspension have compliance that would allow the out-of-control behavior of a spring?
*Trailing arms? No
*Diff housing? No, not if your beam bolts are tight and the beam isn't cracked
*batwing bushings? No, they're spread too far apart and have enough "leverage" to prevent that kind of movement.
*Shocks? You have good shocks.
*Motor trans mounts? They're rubber, pretty narrow mounted and have to contend with up to over 1000 lb-ft of tq. What if they start bouncing?

Does your shifter move left and right when it's hopping? I would look at motor mounts...and I'd probably mount a Gopro or some facsimile thereof, under the car and do so burn outs to see what is "bouncing" to exacerbate, rather then dampen the wheel hop frequency.

Tom400CFI

I made this vid about 5 years ago for Shakedown067. We were wondering how much diff reaction tq deflection is normal in a C4. How much should the C-Beam allow.

Anyway, there is a ton of wasted time in this vid, but look at ~7:25 and ~8:50.




I'd think if you had wheel hop going on, a few vids w/a camera strategically placed would help you ID the root cause pretty quick.

C409

..... I would try the shocks ... most likely not enough compression ... are both sides hopping ? ... What size are your tires and how much air pressure ? .....

C409

..... Could be a cracked spring ... some of those VBP set-ups use a cantilevered mounting apparatus that could be hiding the fracture .....

MatthewMiller

Oh, this is an interesting thought! My diagnostic thinking is like your checklist: there just aren't very many points that control this kind of oscillation in a C4. It's a pretty simple (thankfully!) design. But the motor mounts are definitely one of those points, and I had not thought about them.

I honestly can't say about the shifter - not sure I've ever been paying attention to it when it happened. C-beam appears to be in good shape, but I should check the motor mounts. Since they're covered by heat shielding, I guess the best way to check them is to unbolt them from the frame and actually look at them. I may try the GoPro route, too. I was pondering that last night.

I can't tell for sure, but it feels like both sides are hopping. The main autocross tires are 315/30/18 Rival S tires, at a variety of pressures ranging from 29-35psi (depends on a lot of things). But it's also happened with Hoosier A7s (315/35/17), Toyo 888s (315/35/17), and crappy 14-year-old Avon street tires in stock sizes and ~30-35psi.

I will double-check the spring. The rear spring is mounted in the normal, stock fashion though.

Thanks all for the ideas. This is exactly why I started this thread!

C409

..... I have never experienced nor have I witnessed a C4 with wheel hop ... C5's and C6's are notorious but that's more due to the a-arm style IRS vs the 4-link set-up in our cars ... I cannot imagine how an engine mount would cause a loss of wheel control on an IRS car ... too many insulators ... I would focus on the rear suspension , particularly the shocks and spring .....

383vett

Before I switched to a straight axle, I made well over a thousand 1/4 mile runs with different shocks, springs and tires and never experienced axle hop in my 84. My C5Z and C6Z definitely did. C7Z not so much. I'm not sure why you are experiencing hop. Result of axle hop in my C6Z...

rocco16

Check all the U-joints very carefully.