Okay boys, it's like this:

At 1200 miles my wife and I attended a high-speed (several 3d-gear sections) autocross on a hot summer weekend. This was the second outing for the car. The course length was on the order of 1:20 and, with a small turnout, we had about 3 minutes between runs. On the 7th run, I began experiencing what feels like ABS pulsing, accompanied by a very hard pedal and very poor stopping power. On day two, the brake problem became evident after a couple of runs. Thinking I had exceeded the capability of the stock pads, upon returning home, I ordered and installed a set of Porterfield RS-4 fronts. I didn't engage in a disciplined/aggressive bedding-in process, but drove them daily for a week or so, occasionally (5 or 6 times) working them very hard, and them letting them cool.

At the next event the problem again became apparent almost immediately. This past weekend, at a more normal, lower-speed event, the problem continued. On my later runs, I braked much sooner and used light brake application, which did not bring on the problem, but I was probably at 30-40% braking effort.

All runs were made with the TCS system turned off and the brake-light circuit disconnected at the clutch pedal, to allow left-foot braking.

I reconnected the brake-light circuit and made a couple of time-onlies without left-foot braking (there was a challenge!), but with Lotusboy in the passenger seat. The pulsing was less prevalent, but still there, and John was sure the effect was occurring at the rear. On my final run, under very hard braking, the ABS system seemed to be in full effect in the front, and functioning normally (noise, pulsing, aggressive stopping).

More strangeness: on that final run -- with no brake application -- the steering wheel shuddered for a couple of seconds in a sweeper, then returned to normal.

Brake force distribution problem... ?

Has anyone had any experiences like this?

 

I've never had this problem during autocross or track days, including very hot long track days. I have Porterfield 4RS also in my Brembo brakes.

Of course I never 'disconnected the brake light circuit at the clutch pedal'. In fact I've never heard of anyone doing that.

I wonder what Dave Z has to say about this problem.

 

Could a brake rotor be warped? might seem like ABS shudder and might be felt through the steering wheel if it was front rotor?

I'm not an expert on this. just thinking out loud.

 

Ya, I was thinking warped rotor. Have you had the rest of the system checked out?

 

Dave Z thinks it's 'classic pad material deposition' on the rotors, so I'll be cleaning them up and rebedding the brakes. He could be right: with the fronts not working very well, and me then pushing hard enough to actuate the ABS in the rear, I'd have really crappy stopping perfomance and ABS-pulsing, which is exactly what I've been experiencing. I'll report back -- and thanks for the input.

 

I think you went into "Ice Mode", was there a dip when you braked hard? I had it happen one time, on a course that had a dip in a hard braking zone. Corvettes have a similar problem, John Ames said to try a 2 phase brake vs a push it thru the floor brake. Initial soft contact, then brake hard.

 

No dips, flat, smooth pavement, but I'll add your input to my 'database' for future reference. Thanks.

 

Mark,


Check page 84 of the "brake control system" of the manual, it talks about when the ABS comes on when you wouldn't expect it to. There are other mentionings in this area about the computer turning on just the rear brakes to stabilize the car in severe cornering situations. Also mentioned is how to disarm the computer to "test" the basic function of the mechanical parts of the brakes.
The whole section on our type (enthusiast model) is the TCS/ABS which is pages 44-89 of the brake control system part of the shop manual.

John

 

Awesome. Thanks, John.

 

Lateapex: Have you cleaned and rebedded the brakes yet? and what did you notice?

 

Might get that done tonight -- but will be done by Saturday latest (have an event Sunday).

 

Had the dreaded "Ice Mode" happen last weekend again, we had a very loose surface to deaL with, just before a turn. If I hit the pedal hard, then it would go into the mode, no dash light. If I used a double tap braking it would not go into the mode. Also had a short braking zone after the finish and it would mode there on me.

 

Latest: I sanded the front rotors and pads, and bedded-in the brakes, then attended the Alfa Club lapping day this past weekend. The problem seems to have been cured. On to the next issue.

Overall, I think the brakes should be better. The course we run @ Bremerton Raceway is 1:10, w/three heavy stops per lap: 110-20; 80-35; and 85-15. I did 15 laps per session, and went flat-out for about ten laps. By the last couple of laps in each session I was experiencing some soft-pedal and loss of stopping power.

I understand the soft pedal is overheated fluid. Would slotted rotors alleviate some of this? And would braided lines help the feel?

Can anyone else who has run similar conditions chime in with their experience?

I am actually considering aftermarket brakes, now that I have car that generates a lot of speed at these events. But do I need to go that route?

In terms of sheer stopping power, I can add the following anecdote. I was using Porterfield pads and was on the stock tires. One of the cars chasing me was a supercharged 2nd-generation Miata, on 032Rs. I pulled him on the straights, and was at least as fast through the high-speed turns, but he could close-up like a madman in the braking zones. Perhaps it was as much the tire disparity as a brake issue...

But it was all a blast, nonetheless.

 

I sure hope you sanded them with garnet paper. Ordinary sandpaper has aluminum oxide which reacts with the iron rotors when hot and can cause problems. If you used brake cleaner and or compressed air after using sandpaper, you MIGHT have avoided the problems.Slotted rotors will not help at all with soft pedal. If you're boiling the fluid you either need to reduce brake temperatures or raise the boiling point of your fluid. The latter is much cheaper. I suggest bleeding with Motul 600. It has a boiling point more than 50 degrees higher than ATE Super Blue Racing or TYP 200.

For the rear brakes, you can reduce rotor and pad temperatures significantly by cutting away the rotor backing plate with a pair of tin snips.

The stainless lines will help with pedal feel, but they are not a substitute for changing to a higher temperature fluid. The lines are used to solve a DIFFERENT problem that also causes a soft pedal. That problem is the inherent flex of the factory rubber lines. At least the factory line flex is consistant, though. Boiling fluid can cause symptoms ranging from a slightly soft pedal to total loss of braking. Not yet. You still have a few tools at your disposal that are MUCH cheaper. Start with fluid and brake lines. Then use a real track pad, such as the Porterfield R4. The R4S is not a track pad at all. In fact, it's not even an outstanding street pad and it's a real shame they chose to use the good name of their excellent club race pad (the R4) to try and market a mediocre street pad. The R4S pad has caused lots of confusion due to the similar name and has ruined many a race weekend. Do NOT use the R4S on the track.

 

If you are still able to kick in the ABS under hard braking once the tires are warmed up...your brakes aren't the issue...the tires are.