trackjunky

You bring up an interesting point in that once I thoroughly bled the brakes I couldn't duplicate the problem. Maybe the issue rears it's head when the fluid gets hot or starts to contain some air or water.

J richard

this is a pretty old thread but applies to the same issue, Chris Cervelli made a few suggestions on a pressure limit switch for "O Sh*T" override... still might be a reasonable option....

https://rennlist.com/forums/racing-a...t=abs+override

looks like this still doesn't have a good solution other than the full race ABS...

onefastviking

Mglobe - Air in the clutch system wouldn't cause it.

Trackjunky - Air in the brake system could cause a similiar problem although I think it would still more feel like a pedal to floor vs a hard/good pedal that just doesn't stop the car. You will know the difference for sure when you experience it.

JRichard - I think the switch whether pressure or manual would come in too late and just be a damage control switch. If a pressure switch it would be critical to test the pressures to get the right switch.

924RACR

Not exactly. Sorry. Gotta take issue with that.

It's an ABS system for a car that was not capable of pulling those kind of G's. Not that it couldn't be made to work just as well for that setup - but it wasn't. It's tuned for a completely different setup. We try to engineer in as much robustness as possible, but everything has its limits. Then throw in questionable tire size changes (YES those matter - more than ANYTHING else!), you're on your own.

You either need to get a proper Motorsport ABS setup, or consider learning to live without it.

Oh, yeah, one more point: it's rather a big mistake to suggest that the ABS systems in the more recent cars, like the 996's, 997's etc is essentially the same as what's in a 951 or similar vintage cars. They have indeed changed quite a bit.

jakermc

With regards to tire size, I know Jay runs a 315 R6 and the original poster runs a 305 R100, which are both about the same size. John - what are you running these days? I wonder if my narrower 285 R100 rear helps to hide the problem a little longer?

I did actually feel it at TWS entering turn 4 before making a shock adjustment, then it was not an issue. My initial settings were fairly screwed up so not surprised I confused the system. I think a street course would do crazy things to the system, the Houston impact makes a lot more sense to me now! ECR could get interesting ....

onefastviking

Maybe I didn't word it correctly, but I think we are saying the same things.
I agree that it's an ABS system that wasn't designed for what we are doing when racing it. I understand the tire size does make a difference however the problem has happened even when that isn't an issue. I also agree the ABS is much better than the earlier systems, although the improved systems are also is not designed for what we are doing, it also was designed for street conditions, with street tires. - I know there are mechanical as well as software issues here that come into play.

I remember something about you working for Bosch so I am sure you understand the systems much better than I. My experience is more making what I need to work for the track, and I've been able to work around the old ABS system. I have had experience with the real Motorsports system and the system many are using as a motorsports system so I am also pretty familiar with them as well.

chris walrod

I am wondering if this issue is more related to the sampling rate of the ABS system or related to the spring rate of the tire, or both?

I've had similar issues with the 993 on-track where it seemed like the 'black ice' condition. In playing around with front damper settings I've found it to be less of an issue. This leads me to believe this may have something to do with the frequency of the tire and its ability to 'sync' with the friction so to speak. In other words, the pulse frequency of the ABS system is similar to the frequency of the tire where the ABS releases pressure when the tire is making most friction, then applying back the pressure when the tire is making the least friction.

va122

I use the same tire and I haven't found a problem, but mine is a GT3

Darren

Keep looking, it's there.

924RACR

Not exactly... the "sampling rate" to me would refer to the rate at which the sensor inputs are read into the algorithm... which are pretty much the same on most or all of these systems. The rate you're perceiving is actually the pressure increase/decrease rates, which do vary... and those are just a few of the things in the calibration that will end up at odds with the mods to some of these cars.

Of course, it's more complex than that... this is just the tip of the 'berg...

Correct - I do work for Bosch, in ABS/TCS/ESC systems... Cheers, I think we're on the same page now.

BGB Motorsports

If you want to discuss this further at length, feel free to email me. We ran the OEM ABS at Daytona twice without issue. None of the drivers felt the rock hard pedal with our 997s prior to switching to the Grand-Am permitted alternate ABS, but really only ran it with the OEM ABS for two weekends. After that we went to the aftermarket Teves system that was allowed. I have to say that when we ran the 996 in 2004-2006, we never had any issues. It wasn't until Darren Law or maybe Brian Cunningham crashed a 997 heavily at LRP that we all were given an alternate system. The recurring complaint was that the pedal would go rock hard and not stop the car. A similar thing happened to Spencer Pumpelly at Laguna the race before.

So, your problems would be the first time i've heard complaints from someone running a 996. With those big calipers you have, my first reaction would be that the mechanical grip of the tire isn't sufficient enough to handle the initial shock from the braking input. But you have slicks, so I have to rule that out. I've heard that some people are having issues as a result of tire circumference. I haven't read this entire post but i think that you guys have already addressed that too. Other than bumps in the track surface, excessive spring rate or not enough rate or incorrect shock settings (none of which seem to be the cause of your problem), I don't know what else it could be. I only have answers you'd have about alternative solutions. Alternate ABS systems can be plumbed and installed but it's at least a $5K gesture at a minimum. the Bosch system needs a $10K number in front of it so that's certainly the more expensive option. We're sticking right now with the OEM ABS from Porsche on the 2009 Cayman cars but I'm afraid eventually we'll hit a wall with the potential of that system. We're entertaining Bosch or BMW as a replacement.

I wish that i had some further input for you. All i can tell you right now is that the newer Mark II ABS systems that we see in the DFI Caymans don't seem to have the same issues as the 05-08 997 cars did. While the car is still not all that happy trying to slow down and turn while using ABS, it hasn't given us any reason to believe it's a safety concern.

Feel free to send me an email if you have further thoughts/comments.

John

va122

It was a problem with the 680 rears, but not with the 645s for me

mglobe

Viking the problem Bill and I experienced was pedal to the floor, not good pedal and no brakes. Not the same issue that the OP brought up.

trackjunky

John, I knew you'd chime in here. I remember you and I discussing this via e-mail once where I foolishly asked about overriding the system and going non-ABS. You had data on that and really suggested that that was a very bad option as well.

NJ-GT

Hoosier R100, GrandAm GS slicks are low grip slicks, grip level similar to a V710 or A6.

I would like to read the feedback from drivers using the Michelin Blue Cups, Dunlop soft or GoodYear 250 radial on a street 996/997.

With my two previous GT3 the ABS has been a problem. 996 on GY, pirelli ,yoko, michelin slicks, 997 on michelin blue cups.