Every once in a while, like one time a track day, I get this bad hopping feeling as I slow down from a long straight into a tight turn. I'm not quite sure what it is, but it's a pretty violent THUNK THUNK THUNK THUNK as the rear end seems to be skipping off the ground (not that I actually have seen that). There's a chance it's happening when I do my 4->3 downshift a bit late, but I know the clutch pedal is completely disengaged, though I am heel-toeing the shift. But it's on a very hard braking zone and happening before I turn in.

Any thoughts as to what this might be?

The car's got Bilsteins/ProKit/Spohn TA along with the usual kinds of things.

Dave

 

This is the dreaded rear axle hop so prevalent in these cars. Since you already have an aftermarket TA it could be caused by any number of things; rear shocks to stiff in rebound, overly aggressive rear pads, too much rear brake bias, etc. I would inspect the TA to make sure it isn't bent, you wouldn't imagine how the one hanging on my "Wall Of Shame" is bent!

 

I'll check out the TA, thanks. The brake bias comment strikes a bell though. I've been having problems with the brakes getting soft on some of these hard turns, which I've blamed on the fronts, and haven't really considered a rear brake issue. If the fronts go soft, can the bias be stronger toward the rear, causing this axle hop? Does that make any sense? Or when the "fronts go soft" is it actually the whole system that's fading, so the rears aren't proportionally any stronger now than when the brakes are fine?

Dave

 

I guss you'd have to be more specific when you say the fronts go soft. Do you mean the pedal is getting soft and going to the floor? If so that's usually a symptom of fluid boiling. If it (the fluid boiling) gets bad enough, yes the braking would all occur at the rear, similar to what would happen if a front brake line blew. I would try a less aggressive rear pad and bleed the system with a good DOT3 fluid, and maybe get some air to the front calipers.

 

Oh I'm definitely boiling the fluid, I just hadn't put together this model of the fronts going away, leaving the rears strong, which then caused the axle hop. All sounds right and my "late shift" which corresponds to later/harder braking reinforces the scenario.

My ducts are off at the moment, so when they go back on I'm hopeful this problem will go away. Thanks for the help.

Dave

 

The LS1 cars are notorious for this under hard braking. Something about their ABS system makes it alot worse for them vs an LT1 car. They try to bandaid it with less agressive pads, shock tuning, aftermarket torque arms, etc. but I dont think they've ever been able to eliminate it completely.

Anyhow personally I would suspect the your TQ arm as Spohn isn't exactly known for their roadracing parts. I know its a PITA but if all else fails I would try swapping it out for a stocker to see if it helps any.

 

Dave, I had bad wheel hop under hard braking with the CMC car - even with my adjustable brake proportioning valve, there was too much bias to the rear. My sponsor CME in Vacaville solved the problem by running a less aggressive pad in back(GM Durastops) and actually grinding off some of the surface of the rear pads (trick learned from Bob Bondurant).