Rear brake lockup?

#1

Sunday’s race was cut a bit short for me when the rear brakes locked up completely in turn five. I was on them pretty hard, ABS was engaged, and when I got back on the gas there was very little acceleration. The engine seemed to be functioning properly, but the car got slower and slower. I finally made it back to the pit at full throttle in first gear at about 3 MPH. It sat for 10 or 15 minutes, and when I got back in to check on it again the problem was completely gone, normal operation had resumed.

Anyone have any ideas what this might be? I’m fairly certain that both rear rotors were smoking, so I don’t think it was isolated to just one side or the other, which leads me to believe it’s hydraulic. Anyone ever experienced anything like this? We had a similar problem with our 318 project car a while back, come to think of it, but it only manifested itself once.

#2

GRMScott wrote:

[quote]Sunday’s race was cut a bit short for me when the rear brakes locked up completely in turn five. I was on them pretty hard, ABS was engaged, and when I got back on the gas there was very little acceleration. The engine seemed to be functioning properly, but the car got slower and slower. I finally made it back to the pit at full throttle in first gear at about 3 MPH. It sat for 10 or 15 minutes, and when I got back in to check on it again the problem was completely gone, normal operation had resumed.

Anyone have any ideas what this might be? I’m fairly certain that both rear rotors were smoking, so I don’t think it was isolated to just one side or the other, which leads me to believe it’s hydraulic. Anyone ever experienced anything like this? We had a similar problem with our 318 project car a while back, come to think of it, but it only manifested itself once.[/quote]

check your ABS sensors in each wheel hub. mine are constantly getting dirty and negatively impacting the behavior of the system.

#3

If both rears locked up I think I’d tend to suspect that the cause lies in the ABS pump/valves.

#4

Scott,
I spoke with Tim on the phone today… Try removing the fuse to the ABS system to see if the issue continues.

Cleaning the wheel speed sensors is also not a bad idea however I believe that a failed wheel speed sensor will not result in lockup.

Walter

#5

Scott: have you positively checked that the rear calipers aren’t seized / corroded? I’ve had a few of them "freeze" in the "clamped" position with no advance notice.

#6

Scott,

You’re supposed to release the e-brake after using it through turn 5! :wink:

#7

If they’re seized they’re being sneaky about it, because they worked just fine after 15 or so minutes of cooling. No excess drag whatsoever now, perfectly normal operation. I kind of like having ABS, but if pulling the fuse is the answer so be it. Unfortunately, if it’s anything like Tim and Margie’s problem, it won’t be easily recreatable (pretty sure that’s not a real word).

Believe me, the first thing I did was check the parking brake!

#8

This might not be the answer, but I had a similar problem with a different car before, and it was due to the brake shoes getting stuck inside the drums for lack of lubrication and excessive dirt.
I don’t know if you car still have the old fashion drums and shoes in the rear in addition to the discs, but my donor car did, and the first thing I did was getting rid of all of that crap.

#9

It does still have rear drums, but the rotors are brand new and it was the disc pads that were generating the friction, they left some scoring as they overheated.

#10

I just saw your write up on the GRM home page. I was hoping to see you guys at the NASA event last weekend at Sebring. Sadly, there were only two E30s there and neither were Spec E30 cars. One was a GTS car with which they had problems all weekend and the other was a nice little light weight 318is that was running in HDPE. Based on the speed that guy was running there is no question that a 318is could be competitive at the right (lower than 6 cyl) weight.

Anyway, regarding your brake problem, I had a similar problem on a street E21 years ago and it turned out to be a bad master cylinder. This would be the first place to look. I replaced mine and the problem went away.

Good Luck,

#11

Scott:

I had no rear pad wear after several race weekends…and the other drivers were out-braking me.

I replaced the master cylinder and the rears are working fine now.

I don’t know if a bad MC can cause locking but it’s something to consider.

'Good to see you at Summit.

Carter