Current setup = OEM rotors/calipers, ST43 pads front & carbotech xp10 rear, OEM shims, SS lines & Motul RBF 600/AMSOil brake fluid. Running 255/40/17 Hankook RS3's on 17x9.5" wheels.

The problem I'm having seems to only come into play when I'm braking uphill from about 60 or more. The higher the speed the more pronounced the issue. Braking down hill or on a level surface isn't an issue at all.

This started about half way thru the last DRY track day I had @ NJMP Lightning course 2 weeks ago. What happened was when I hit the brakes on the straight to enter turn into T1 (which is an uphill turn coming from about 120mph to 80mph), the whole car shifted about 5-8' to the right for no good reason. I have this on video and can post if it will help diagnose. Luckly for me its a right hand turn and I was far left, so I was able to just ride it out. This happened totally out of the blue with no warning. I assumed there was something on track at that spot, since I didn't have that issue anywhere else, and just adjusted my line accordingly. Oddly, no matter where on track I started my braking for that turn (far left, middle or even inside right) the car would still step out to the right on me. T1 isn't the hardest brake zone either, I brake much harder for T7 which is about 115-60, but its a downhill left turn instead of an uphill right like T1.

When I got back into the pits I was talking with some of the people who were out there at the same time with me and none of them felt anything on track at that spot. So it was suggested that my rear pads are probably grabbing too hard and the fronts not enough. Sounds logicial to me and since my front are almost to the backing plate I figured I would throw these fancy new pads I have for testing in and see what happened.

Fast forward to last week. I have these new pads bedded in with a nice transfer layer of material on fresh rotors and I decided take them out to my usual spot for a quick test, I'm not even thinking about the issue I had the previous week @ Lightning. Get the car up to speed and do a couple quick stops on level ground with no issues. The pads feel great! I assume all is well and turn around to head back home. As im heading up a hill towards my garage I see deer to my right, so I start to brake and instantly the car kicks sideways like there is oil or ice on the road. I almost lost control of the car @ 55mph, lol.

So I get home and try to figure out what the problem is. I take it to my shop and they tripple check my alignment but everything is where it should be. Toe is correct, camber is right where we want it and nothing is loose. They check ride height and thats good too. Shock settings are the same and from a visual standpoint everything looks fine. I did a track day in the WET this past saturday and had no issues but I wasn't driving the car near as hard as I do in the dry so I wouldn't expect there to be.

So that leaves me where I am right now... scratching my head looking for answers. I'm thinking, and this is a total guess, that I might have an issue with one of the calipers on the left side of the car since I'm being pulled to the right. i still have the OEM calipers that I've never rebuilt or done anything to other than inspect.

One really odd thing I should mention is that i'm seeing some pretty severe pad taper in the REAR. By severe I mean the top of the rear pad is at about 25% material left and the bottom of the rear pad is about 75% material left. This is uniform with both the left and right side of the car. There isn't any front-rear taper of the rear pads. I also only have minimal top to bottom taper on the front pads and even less front-rear taper on the fronts.

Another thing worth mentioning is that my rear rotors are about 1 track day away from being replaced. The min thickness is something like 20mm and I'm at about 21.5mm. Both sides are the same thickness so everything is wearing evenly from side to side.

I have a two day event coming up this weekend and I'd really like to get some ideas as to what the problem is here. Thanks for reading.

 

Had the EXACT same issue 2 events ago buddy. Turns out the right front wheel was sometimes locking up and actually braking MORE then the left wheel causing the whole car to jerk to the right. Twice it put me 4 in the grass after a 100+ straight. Upon inspection the front right was also darker and had more brake dust then the left. No ABS light so I assumed the failbo's like you did. Took everything apart and it all works still. After some talking with a racing engineer I fried ABS sensor on that wheel. No light because it's still "working" but it's way out of wack. It happens as you know, esp with the heat in there. I replaced mine and haven't had the issue since. Also wrap it while you got it off.

 

Busy night here. Bookmarking and will read in depth tomorrow AM with my morning coffee.

 

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So the pull is caused by the ABS sensor not really working on that wheel...which results in the effect that the ABS works on the other 3 brakes, leaving the one wheel to brake more, slightly lock up and pull the car to the right?.
interesting..
I note that it happens more with the load off the wheel ( uphill ), allowing the one brake to lock more and skid the tire...

yep I can see that.....you sure no codes were thrown for the ABS sensor? That isnt supported thru the main ECU is it? You have to pull that separately At least I thought so..

Well the test is to replace that one sensor and reset the ABS system..if it goes away then I would say you found the real issue.

Thanks for sharing...

 

AIB, has anyone been able to follow you and witness this also? Just wondering if anyone can confirm your locking up a wheel, if that is indeed whats going on.

 

That sounds like the bridge of the calipers have been distorted by too much heat in the rears and allowed the caliper to severely spread. However you might check the pistons to see if they are sticking first. also check to see if the rotor is tapered towards the outside of the rotor. ...if the rotor is square and true, then swap in new pads then measure to see if the top of the piston is contacting the pad first..if it is then the calipers are distorted and will need replacing.

It sounds like you may have more going on though than just the brakes and taper or distortion......see the post above where one guy traced it to an ABS sensor that failed and caused the severe pull with weight off the front end of the car.

 

it sounds like it's completely driver error :P

seriously though. there's a good chance that's it's abs sensor, but you said the abs light is not on. my RS doesn't have ABS and tends to lock the rear first, even with street pads in the rear. when i drove Chu's car at Lightning, the ABS light came on mid sessions and the rear got very loose in the hard braking area. his ABS light went off after we parked the car but would come back on again during mid session after some hard braking. good thing im used to driving without abs. pull the ABS fuse and do some hard braking to see if it's doing the same thing when you brake on level surface.

 

And back.

I agree with checking all ABS sensors. I've had cars lose ABS and damn near lose drive-ability due to a sensor not reading correctly - no ABS light either. I solved that issue by pulling the ABS fuse and test driving in a parking lot. Sure enough, car stopped something awful and sideways. Then it was a matter of solving which line it was. Ordering 1 front and 1 rear, swapping 1 at a time to find location (mine was a front left).

As far as rear pad wear. Have you taken the rear pads off and inspected the calipers and dust boots? Sounds like the piston has an impedance either within its stroke or on the surface of part of it. Might just need a good cleaning and re-assembly.

 

XP10 rear seems pretty aggressive for rear considering you're running ST43 front. From my experience, I don't feel that the ST43 have as much torque as the XP10.

As for rear pad tapering, sounds like a bad case of calipers spreading. If you could take temperature across the rear rotor to see if there's a huge temperature difference between inner and outer diameter of the rotors. I have an infrared temp gun that we can use to do some testing at the track this weekend.

 

thanks everyone for the input. the ABS issue seems like a logical place to start. I found the procedure in my service manual, it seems pretty straight forward. I'll hopefully get to inspect and test everything tonight.

 

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We concur that ABS is the most likely culprit. I would check the sensors...you may very well have fried something.

 

Sorry for not updating sooner, I pulled my back out the other day and haven't been able to spend as much time as I would have liked on diagnosing this issue.

Last night I was feeling a little better and was able to pull the front sensors off the car and clean them up along with the ABS wheel/sprocket. The sensors were covered in rubber, small pieces of metal and grease but didn't look damaged in anyway. Hopefully that makes a difference. I couldn't get the back ones to come out, so I just blew compressed air in there and cleaned them up the best I could.

I have two track days this weekend on two different tracks so I should be able to repeat the issue if its still there. BEing that I'm not as 'agile' as I was last week prior to my back injury, if the issue is still there I plan to just pull the ABS fuse and drive around it until I can recover enough to fully tests the system.

I'll update this thread with my findings. Thanks everyone for the input.

 

I can totally confirm I had the exact problem which was a faulty ABS sensor. I replaced ALL four sensors, still had the problem, when I just pulled the fuse on the ABS and disabled it completely - worked like a charm after that. Obviously you won't have ABS but even on pouring rain track days I didn't have an issue.

Hope that helps.