Stock sports pads at auto-x

ChrisSlicks

Anyone else tried to use the stock pads at autocross? They seem to start off okay but turn to crap by the end of the run. Braking into the stop box the ABS was going into a kind of ice mode where the pedal was still firm but the ABS was pulsing and the car was barely slowing down. I slid through the stop box one time because the car just didn't want to stop, a rather disconcerting feeling!

Just from putting my hands over the brakes there was a big temperature difference front to rear. Do you think that the ABS system would get confused in this scenario and start activating prematurely?

I've ordered some Hawk HP+ pads so we'll see how those do compared to stock.

imag

Man, I'd expect that from an OE session, but I'm surprised autox even got the stock pads to fade. How fast was the course?

ChrisSlicks

The course was long at over 2 minutes, and probably about 1.5 miles. There were 2 heavy braking sections where you had to brake from about 60 to 20, everything else was just moderate braking.

Seems weird to me as I never had this problem in my G35 and it had the wimpy sized stock brakes which seemed to love the heat. Now this car is lighter and has bigger brakes, doesn't make sense.

imag

Weird. Well I'll be curious how the Hawk's do. I was going to follow the Carbotech advice myself, but that would be for OT, not autox... and I have to get my car first

ChrisSlicks

Carbotech has an auto-cross compound as well but I don't have any experience with it.

For track it's hard to beat carbotech, lots of brake tuning capacity with the various compounds. I think DD went with XP10's front and XP8's rear on his setup.

The HACK

If your pedal was firm, and the ABS is pulsing, that means it's not the pads, since it's still got enough grip against the rotor to "lock-up" the rotors to cause the ABS sensor to release the pads. Sounds more like your tires are over-heated, and relatively little pressure from the pad was causing the tires to break traction.

ChrisSlicks

I think your right about the pads but I don't think the tires were overheated as it was doing it on a cold day on the 2nd run. It was worse on a hot day with the sticky pads but it I don't think it's the tires as there was plenty of lateral grip and it never felt like the tires were breaking traction when braking (no chatter, skip or chirp).

The HACK

Huh. That's interesting...Was the course relatively clean? The only other explanation I have, is if you hit a patch of dirt/sand, causing the tires to lose grip, or if someone spilled coolant in front of you. The symptom you described would indicate the brake system is functioning as it should.

ChrisSlicks

The course was very clean. We walked the course in the morning and it was probably cleaner than I've ever seen it thanks to heavy rains earlier in the week. There's not much sand around so the course usually stays very clean unless someone goes off course into the grass and drags some dirt back on.

But I don't think surface is the issue as the problem is repeatable on a clean piece of asphalt on the street. You can make several stops 60-10 were it will be perfect and then as things heat up it gets progressively worse. It could just be my imagination but appeared as though everything was fine until heat soak reached the calipers and brake fluid. I.e. you could make repeated stops and get the rotors hot and everything is fine, then just cruise and let the heat soak in and then things start to go bad.

I took it to the dealer this morning and they couldn't find anything yet. But this isn't surprising as you have to drive it pretty hard on the street to reproduce the conditions. Here's a link to another thread on the issue after I realized that pads were not an issue.

Braking problem, malfunctioning ABS

The HACK

Okay, I think I know where the problem is. And you may be right initially, that the PADS are the problem.

The pulsing sensation you feel in the pedal wasn't from the ABS system. It's from the pads skipping across the surface of the rotor due to over-heating between the pads and the rotors. The pads "glazed," meaning heat near the melting temperature of the pad material allowed the brake dust to fill all the little surface imperfections and made it mirror like, and the result is instead of grabbing onto the rotor surface and providing friction to slow the car down, it just glides over the rotor and sort of just bounces, like trying to slide two pieces of mirror together.

The result is you'll feel an odd, almost ABS like vibration when the pads knock back into the caliper from the surface imperfections of the rotor, pedal is firm but car's not slowing down. Once the pads cool off and the brake dust that fill the nooks and crannies can no longer stick to the little nooks and crannies the brake works again.

Heat soaking into the fluid would result in a soft pedal, so I'm not sure if it's the cause here.

At least that's what I think happened. A more aggressive, track oriented pad will likely cure the issue.

ChrisSlicks

I also tried the Hawk HP+ which is a much more aggressive pad designed for auto-x and light track duty. These did the same thing, perhaps even worse. There was no sign of glazing on the rotor and I pulled the pads when I got back home and they looked fine, although any pad glaze would have worn away by then.

I never had anything like this problem with my G35 and it was a heavier car with smaller brakes (single piston caliper). It stopped like a champ and the pads seemed to love the heat.

The HACK

Odd. I'm at wits end. I thought I had gone through all the possible brake problems on my 350Z (I even manage to burn through an entire set of rear pads in a single day). Maybe there's something else going on here I can't explain.

ChrisSlicks

Thanks, I do appreciate the ideas.

No one else with a 370 has reported this problem yet so I'm thinking it has to be something unusual.

imag

Yeah, I'm starting to wonder if it's an ABS sensor or logic problem, somehow triggered by high heat...

ChrisSlicks

I did pull and inspect the wheel speed sensors while I was in there and they looked perfectly fine. There is no separate ABS sensor that I'm aware of.