philooo

So this weekend I went autocross with the local PCA club and when I made my start of the line, I really pushed it pretty hard. I mean 3 or 4k rpm then good clutch slip to pass the power as fast as possible, but still keeping it smooth. In other words I did not drop the clutch like a bomb.

For info the ground was made out of concrete slate.

So when I made my start I head a pretty strong knocking in the rear of the car, as if the wheels where kind of dribbling/rebounding from the ground. Like 3 or 4 knocks. Did not feel like an engine noise more, as if someone was giving 3 strong hammer tap to rear end.

Can you guys comment on what cause this phenomenon and how harmfull can it be, if any.

I remember hearing the same sounds when trying to apply full throttle on a dodge viper in second gear once. The car would simply stop moving and wheels turning around.

Phil

ivangene

I also had that befoe on the 996...it scary and I never figured out what it was...

my guess was poor ground contact and shocks that bounced but it didnt feel bouncy...just made a sound like you described. - I changed my launch habbits

other possible things, motor mounts, trans mounts ?

philooo

I am glad I am not crazy and that you experienced it too. I had great lap time so I wonder if those 'hard start' helped my time or if I should avoid them for a matter of speed as well.

The thing is that the dribble/bounce sound is pretty fast while the car indeed feel totally stable.

ivangene

I thought "axle" at first (or CV) but checked the heck out of those and could see any issues. I never changed motor mounts and that is one of the other things I thought it could be (or trans mount)

im my case it was a slight turn in the launch too but by the time I got on track it was gone...I have to say very "distracting" too

balefire

this sounds like "wheel hop" or axle hop.
here is a pretty good explanation.
https://rennlist.com/forums/928-foru...ml#post7158396
concrete makes it worse because it is a grippier surface, making it harder to break free of traction.
it is bad for the car.
i have wheel hop too at 3-4k.
it may require a higher rpm clutch dump or improved suspension components to resolve.

fpb111

You need Traction bars and a box of spare clutch parts.

philooo

to illustrate. that is how it sounds from the inside.

http://www.youtube.com/watch?v=giUP4O-Gjr8

Seems that the problem comes from the wheels moving forward under acceleration, change the toe and then grip get reduced and wheels spins, then toe get back to good value, grip get restored, wheel moves forward... and so on...

after a little reading it seems that quite a few things can break after that kind of stunt, so better keep it to a minimum.

I am curious to hear comment from people who worked their wheel hop out of the car

Most common answer are more rigid bushings. considering I am about to do my RoW-M030 suspension upgrade I wonder if I should try that route, but which bushing to change ?all of them ? or the one located on the control arms are the most important ?

ivangene

did you buy the ROW already?

philooo

yes I have it here, just waiting for the right time to put it on.

ivangene

awe - cool!

Pac996

Biking you'll find interesting facts like concrete has the same grip as wet asphalt. Might plan accordingly for the launching the car.

DreamCarrera

Your RoW-M030 kit should have new bushings included in the kit.

I would assume the new(RoW) bushings would be more rigid than the stock components but I am not sure of this.

philooo

With the RoW kit comes bushing for the sway bars and the 'shock stopper', but I assume that the bushing that need to be replaced are the one located within the control arms, so that they don't move under acceleration.

When I talked to suncoast there was +-$500 of bushing that I could have bought, but my car having only 30k miles I think it was not necessary. AND I have no clue how easy it is to replace those as I have not seen any DIY on this and I have no clue how to remove bushing in the first place

peavynation

Mine does this too (tried to launch in the rain to see if the ABS/PSM would do anything). I don't think it's suspension-induced wheel hop more than it's the ABS and/or PSM trying to keep traction by applying the brakes. The wheels break loose then get traction again when the brakes are applies. Wheel hop is the same thing, but the mechanism that causes the stick-slip is the suspension travel allowing the wheel to loose traction then come slamming down and gaining it again.
The PSM (or another light) would flash on when this "hopping" was happening, so I know the Pcar knew about it.

Or, maybe I'm just crazy and it has nothing to do with the traction control???? But mine does do it too. I've got a C4, not sure if it makes a diff for the discussion.

balefire

reviving an old thread.
any ideas what bushings could be replaced that might help reduce wheel hop?
thanks!