Last weekend at Big Willow, i noticed that my car would twitch under threshold braking, particularly braking for T1, T3, and T9. I thought it might have been the brake compounds being used (spec VR/spec VX), but i swapped my pads out for stock ones last night and i'm still getting the same twitchy problem. The car brakes fine in a straight line and then the abs kicks in and the car twitches to the left. Anyone else have this problem before?

Things i think it could be:
-preload ratio front:rear
-In december, i scored my driver side front rotor when my pad life wore out and i was using the backing plates temporarily to get home. Thus, rotor thickness isn't the same left to right since the backing plate was eating the driver front (although i don't think there is much difference). I also never took the rotor to get resurfaced.
-Compression/rebound setting

 

Twitch meaning what?

 

Twitch as in the car will suddenly veer one direction and upset the balance of my car. This would sometimes result in the rear end kicking out, causing me to fight oversteer on corner entry.

 

You mean like your rearend swings out or is unstable or you mean the front end is pulling in one direction? You may need to increase low speed compression in your front shocks to reduce unloading the rears or your tires may be the problem, wear or pressure. Veering or pulling at the front wheels is a different problem than trouble keeping the back end behind you with different fixes so that's my best guess.

 

I'm sorry for not being clear. In reality, i'm not 100% sure what my car is doing, but what it feels like to me is that the front end is pulling in one direction violently when the ABS kicks in, which is what kicks the rear end out. I forgot to mention that my rear tires are fresh while my fronts are near balding with 5 events on them now. I figured that since both front tires are equally as bald, it shouldn't really be a problem? I'll try decreasing my front low speed compression, thanks

Maxrev also drove my car and noticed the same problem. Since he is better at describing the dynamics of a vehicle, hopefully he can chime in.

 

Interesting. My car used to pull to the right under mild to hard braking, but as soon as ABS kicked in the car would slow in a straight line. Never figured out what it was. Changed shocks, tires/rims and even swapped my Wilwood calipers from left to right and nothing helped.

To eliminate tires/rims being the culprit, swap some stock rims/tires on if you still have 'em. Or swap left to right and right to left to see if your car behaves the same in a different direction. Sometimes a bent rim or one tire being damaged on the inside (or heat cycled way more than the other) can cause this.

Are your pads wearing evenly? Perhaps there's less pressure in the brake system on one side vs. the other? Blockage / contamination in the lines?

 

I find old tires will cause this. You'll need to try a fresh set of rubber and see if the problem persists. You can try a touch of toe in on the front also.

Other things it could be: uneven corner weight, excessive front damper rebound, uneven castor.

Get your alignment and corner weights checked to make sure they are correct, put on some new tires and the next time you are at the track try adjusting the front shocks increasing low speed compression force and reducing low speed rebound a little at a time logging the results.

Unless you maintain a log of your settings, the conditions and your feedback about how the car is handling your settings will always be random and you'll start over from scratch every time. After a while you'll be able to go back through your notes and pick the right setup for the track and conditions and make only fine adjustments from there.

Twitchy usually refers to rear-end instability (the back end "twitches" and doesn't want to follow the front). Darty usually referes to front-end instabilty (the front end wants to "dart" in one direction or another). I think what you are describing is "darty" only because I assume you have the same problem I do.

HTH

 

Or, it coud be something as simple as a sticking caliper. Before going through the trouble of recalibrating your entire suspension, make sure that the pistons on both front brakes move smoothly. Also remove and regrease the caliper pins to make sure the whole caliper can slide freely from side to side.

If you go through that and still have the problem, then I'd look at tire pressures, alignment, and then suspension settings.

 

Thanks guys, you guys all brought up some interesting points.

Nobody: unfortunately i only have one set of rims. But, i will beable to try to swap the fronts and see if that changes anything. The pads appear to be wearing pretty evenly and in regards to the fluid, i haven't checked yet.

Cthree: I actually got my alignment done about two months ago but never got the car corner weighted. It seems though that the corners weights couldn't be off that much to throw the balance off too much though? Next time i'm out, i'll try playing with the front shock settings to see if that changes anything. It just stinks cause i finally got my suspension dialed in to the way i liked it too .

Ludedude: Good suggestion. I know this may be a dumb question, but how do i check to see if both pistons are moving freely in the front? When the caliper is off, i can't move the piston with my hand.

I'm going to go ahead and change out my front rotors since i already got my new ones from GFL. I'll let you guys know how that goes, thanks again.

 

I use a big set of channel locks.