Rear wheel hop during braking..

[zlexiss]

Autocrossing today on the tarmac at Libby Air Field. Wonderful sticky concrete, like driving on a sheet of sandpaper.

At the end of straights, I've been hitting the brakes hard. The fronts bear down and don't even complain. The back end starts shuddering like crazy. Enough to pop me out of second gear, leaving me revving nothing but air coming out of the apex.

Anyone deal with this well, how did you fix it? Im thinking the control of new shocks (some revalves from Sam Strano maybe, Bilstein HD's in the min) will help matters some, but what else might help out here?

As far as tire pressure, I'm running about 39 psi front, 34 psi rear - shoe polish has been showing the rears are perfect as far as cornering goes, right to the edge of the shoulder blocks.

-Jeff

[Arctc Wolf]

lower control arms, shocks/springs, torque arm, and sfc's will all help u out. I'd suggest lca's first, since they're the cheapest and most effective.

[Dominic]

<blockquote>quote:</font><hr>Originally posted by Arctc Wolf:
lower control arms, shocks/springs, torque arm, and sfc's will all help u out. I'd suggest lca's first, since they're the cheapest and most effective.<hr></blockquote>


This is not going to solve his one problem.

First of all, how old are the pads, are they stock? Stock pads will shudder when they get heated and glazed, I know this from experience... this shuddering is bad yeah.

Shocks are always great, but if it is brake related they're not going to help. Depending on what class you run in, and how comfortable you are changing classes, choose your LCAs carefully. My rod ended LCAs are awesome but kick me up into SM right out of ESP. :( Oh well, screw it, I love them too much lol.

It doesn't do this on the street does it?

[zlexiss]

I figured that this wouldn't be as clearcut as hop on acceleration :)

Pads and rotors are still stock, 38k miles. Checked them last month when rotating tires, and lots of life left on all four corners. Rotors are clean, no warping or surface problems.

I'm not sure if it does it on the street - I usually don't brake that hard. I'll give it a try on the way home today, when they're nice and cool. It's hit a couple times on asphalt autox courses, usually when I brake hard after the finish gate. On the concrete I was getting it all over the course, even during the first brake use of a run.

If the pads help this, any recs on which work well for us? Hawks, EBC? I need something streetable, autox is the worst I do to them. Rotors stay pretty hot between runs when it's over 100F out there though.

As far as classing, I'd like to stay in STX, not sure what LCA's I'm allowed to use there. When I mentioned shocks, I was hoping not for elimination of the problem, but at least control and mitigation of the effects.

When I was out on the course yesterday, I saw a Formula do the same hop once, and he got a little sideways while hopping too. Lost a small chunk of rubber off a tire.

[zlexiss]

Well, I did some hard braking on the way home. With the rotors cool (2 miles of 60 mph driving before the stop), the front tires scrubbed and complained a bit in the braking, but the rear was fine. Same with 2 other stops in similar conditions. I haven't changed the tire pressures since racing.

Looks like hot brakes are a factor here. Dom, aside from the rears shuddering when they get hot, could it be caused by the fronts fading more than the backs as they heat up? If the backs continue to grab well as the fronts fade, they might be locking up some, and the ABS may just suck at handling it.

Either way, sounds like I'm due for some new pads. Any suggestions for a set that streets well (cold perf) and can also handle hard auto-x in 110F heat? I want to stay with solid rotors..

-Jeff

[HAZ-Matt]

The rear could be locking earlier than the front, adn the shudder may just be the ABA pulsing... although I couldn't say since I wasn't in the car at the time.

[Dominic]

I would try running some Hawk pads all the way around. You can go HPS or HP Plus depending on how aggressive you want. Generally people like running HPS in the back, HP Plus in the front, or stock in the rear with one of those two up front.

You might try sanding the corners of the pads and the faces a little in case they are glazed. I don't know why they would hop, but the ABS can sometimes do weird things when used in racing situations :(

I'll think about it further, I'm not sure what to tell you at this point. Throwing money at it wont fix it [img]smile.gif[/img]

Try posting on the ls1.com autoX board, they might be of more help in this severe situation.

[Jason McCallister]

Could be the brake bias...too much in the rear. Unfortunatly, it cannot be changed with factory brake system.

Also pull the ABS fuse or pull the E-brake up one notch to disable the ABS.

[zlexiss]

I'm sure the low trac light kicked in, but don't know if the abs caused it or was flipping out when the wheels left the ground.

And when I said hop, I meant it - the back was jumping up and down, wheels off the pavement.

Dom: thanks for the ls1 forum hint, I'll have to register over there and ask. I need an aluminum driveshaft anyways :)

Maybe Sam Strano knows something about this.. I'll see if I can ring him up.

-Jeff