This weekend I attended the MAP Proving Grounds lapping day and I encountered some very strange braking behavior that almost put me into the weeds on a couple different occasions.

Here is my setup and the scenario....

EVO9 with ST47 front pads, ST43 rear pads, 245 Hoosier Slicks.

Long high speed straight with speeds reaching over 120mph. I go to stab the brakes and I get almost instant ABS feed back preventing me from shaving speed. I had to back of the pedal and initiate braking with a very light foot to get the car to slow to below 100mph. After speeds dropped to 100mph I could input the proper pressure to decel as one would expect with the combo I'm running.

Brakes worked great from 100-0 and like absolute crap from 130-100. I was so upset that I actually pulled the ABS fuse and that worked wonders. I was able to brake hard without issue from 130-100 but I did smoke a few tires once below 60mph.

Additionally once ABS enabled braking did function it felt as if the car wanted to "dance" left to right. At first I attributed this to the uneven loading of the stock bilstien struts or possible out of whack alignment but after pulling the ABS fuse this issue also seemed to resolve itself.

On the way home it tried a 135mph stoppie with my Hawk HPS pads and street tires and experienced the same behavior. The ABS wants to engage instantly and once that passes the rear wants to step out requiring corrective steering inputs.

Please keep in mind that the brakes are always bled using a power bleeder and i've encountered the problem after two full power bleeds. Help.

 

abs light on? Sounds like a wheel speed sensor.

 

Are you slamming on the brakes??
It really sounds like you’re slamming on the brake and lock up the tires. You should never do that. My stopping distance decrease when I apply brakes slowly and then increase braking pressure quickly.

 

^ NO ABS light until I pulled the fuse.

Also not slapping on the brakes. My normal stab when transitioning but definitely not a ninja kick though I have done that in the past and never had to fight ABS especially at 130mph.

 

I'd guess some kind of sensor/sensor wire malfunction. Or the ABS rings that the sensors use to get the signal. On my old VW, they rusted/got dirty and car was not getting the proper signal back.

 

Could this be a result of front end dive under hard braking with sticky tires? Could the rears be engaging the ABS as a result of lightened load?

Can we log ABS sensors individually via ACD computer?

 

It is possibly the wheel speed sensor. Common reason race cars will not use ABS. I do like ABS on dry pavement but we installed a control switch on the dash. It works but we need more testing to see if it will deleat ABS during its operation.

 

I experienced a very similar issue last fall at Mid Ohio. It seemed no matter how lightly I applied the brake at the end of the long straight the entire car wanted to "dance" to the right and just about spun the car around twice at high speed. Very scary feeling when it happens at 125+ mph.

For some reason Mid-Ohio was the only track it happened to me at and has not happened since then. I'm hoping the issue does not return next visit there.

I never did diagnose the issue. If it happens again I might pull the ABS fuse as well to see if that helps.

Maybe for me this was just an aero loose condition due to running the small APR wing at the time?? Hopefully with my new wing setup there will be less chance of that happening.

 

I would check out the surfaces of all 4 of the abs sensors. Any metallic debris can distort the signal between the sensor and the tooth wheel. At high speeds the sensor frequency is high so if signal quality is poor it will likely think it's locking up. As wheel speed comes down the frequency drops to a point where quality is enough to brake hard without thinking it's abs.

That's the first thing I would check anyway

 

pedal box with no booster is a best and the most predictable set up> you will be scared for a first time, but after you got used to it, you will never look back.
ABS is just a plague for racers at factory level abs systems.

 

I don't even know what a pedal box is! I've never had issues like this before. The car has always stopped straight and rarely engaged ABS unless I was panic stopping with street tires.

I will check those sensors to determine their condition.

 

you spend a lots of money on the car , so the pedal box as weird is sounds will be an upgrade and safety for heavy track use.

this is just an example. i dont have this neither just to get an idea.
I have a custom made one by RKTmpotors. It is dead on and never fails.

behind the shock tower, and under the cage on the fire wall

 

Hey rob would you have a pic of the actual pedal cluster in the cabin? I'm loving that set up. And what makes it scary for the first time?

 

feels like when you turn of the car and run out the brake booster then the pads warming up later and it is perfect. Need some time to get used to it.

you cant see much from inside , that is a beauty of RKT motors work

looks oem for an average guy , the real deal is in the engine bay. but you can see the BIAS bar top on the brake pedal assembly.

(this picture is before RKT started rebuilding the car)

 

That looks pretty fan dancy but I've never had a problem before .... I'm curious why this would start now. My main concern is the immediate abs feeling when going for the brakes at 130mph.