ABS causing a wooden brake pedal and crash
09-10-2010, 01:58 PM
#1
Nordschleife Master
Thread Starter
ABS causing a wooden brake pedal and crash
Posting this on behalf of a friend who races a 964C2 on our (UK) Porsche club championship. He would welcome any ideas of what could be wrong.
Here's the video he mentions. This part of the track is well known for crashes.
http://www.youtube.com/watch?v=6GwNWnBL274
Bit of background first, coming down to Castle Combes Quarry corner at 140mph on the first proper lap in the Club Race, I hit the brakes and got a solid brake pedal, pressing harder didn't slow the car much so I whacked the back of the car in front, lost control and then bounced off the barriers. There is a lovely video clip on you tube, look for Porsche Club Crash Quarry.
Anyway, in the 2nd race I was getting the same thing but if I came off the brakes and reapplied them the woodeness went away. I didn't have this problem in qualifying.
So after discussing my accident down the pub last night, it was suggested that this was a known problem with ABS cars and the long bumpy straight leading down to the corner. This is upsetting the ABS somehow but by tapping the brake before the corner, the ABS sorts itself out and the brakes are fine.
Has anyone else heard of this phenomenon ? And if so how can the ABS cause a solid feeling pedal ? I’ve had a quick google for it but didn’t find any info.
A couple of the other club racers have also had 'strange' things happen with the brakes at the same corner.
The car is a little battered by the way but I went on to get 2nd in the 2nd race and was the fastest 911 so I guess that means the wheels are still pointing in the correct direction!
Anyway, in the 2nd race I was getting the same thing but if I came off the brakes and reapplied them the woodeness went away. I didn't have this problem in qualifying.
So after discussing my accident down the pub last night, it was suggested that this was a known problem with ABS cars and the long bumpy straight leading down to the corner. This is upsetting the ABS somehow but by tapping the brake before the corner, the ABS sorts itself out and the brakes are fine.
Has anyone else heard of this phenomenon ? And if so how can the ABS cause a solid feeling pedal ? I’ve had a quick google for it but didn’t find any info.
A couple of the other club racers have also had 'strange' things happen with the brakes at the same corner.
The car is a little battered by the way but I went on to get 2nd in the 2nd race and was the fastest 911 so I guess that means the wheels are still pointing in the correct direction!
http://www.youtube.com/watch?v=6GwNWnBL274
09-10-2010, 02:37 PM
#3
Racer
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I am no specialist on this!!!!!!!!!!!!!!!!!!!!!!!!!!
hi,
When I was going through hell with the bloody pdsa lights and trying everything to get them out I did change the brake booster pump switch for another which was faulty and it performed a wooden pedal.
I crept home basically with hardly any brakes scary!!!!!!!!!!!!!!!! but my c4 had another switch and the two relays and a full system bleed to get it fixed.
I feel for the guy this has happened to, as cruising home at 30mph with the problem was scary anything above that was a serious underwear soiling moment!!!!!!!!!!
I think if the relays are loose or the wiring underneath is earthing out that could explain a intermittent fault.I hope he sorts it as I would need very large ***** to get back in and trust the car.
Gaz
okay thick me,,c2 I have a couple of mates with c2's who's abs relay have gone **** up!!!!!!!!!
When I was going through hell with the bloody pdsa lights and trying everything to get them out I did change the brake booster pump switch for another which was faulty and it performed a wooden pedal.
I crept home basically with hardly any brakes scary!!!!!!!!!!!!!!!! but my c4 had another switch and the two relays and a full system bleed to get it fixed.
I feel for the guy this has happened to, as cruising home at 30mph with the problem was scary anything above that was a serious underwear soiling moment!!!!!!!!!!
I think if the relays are loose or the wiring underneath is earthing out that could explain a intermittent fault.I hope he sorts it as I would need very large ***** to get back in and trust the car.
Gaz
okay thick me,,c2 I have a couple of mates with c2's who's abs relay have gone **** up!!!!!!!!!
Last edited by screamingflat6; 09-10-2010 at 02:41 PM. Reason: speed reading!!!!!!!!!!!
09-10-2010, 02:58 PM
#4
Nordschleife Master
Thread Starter
Hey Gaz,
Thanks for the input and don't worry about Richard a little incident like this won't put him off. He's been racing for years and will be back in the car for the next round at Silverstone. At least he had the 993 to slow him down and prevent more damage being done!
Thanks for the input and don't worry about Richard a little incident like this won't put him off. He's been racing for years and will be back in the car for the next round at Silverstone. At least he had the 993 to slow him down and prevent more damage being done!
09-10-2010, 04:23 PM
#5
I have had that same thing happen at an autocross on really bumpy pavement.
The theory was that the bumps "confused" the ABS into thinking that the grip level was much lower than it really was, possibly because one or more wheel was in very light contact the moment the ABS released that wheel to let it turn. The suggested course of action was to do exactly as described above, come off the brake and then go back on. That is tough thing to make yourself do when you really want the car to be slowing down.
The theory was that the bumps "confused" the ABS into thinking that the grip level was much lower than it really was, possibly because one or more wheel was in very light contact the moment the ABS released that wheel to let it turn. The suggested course of action was to do exactly as described above, come off the brake and then go back on. That is tough thing to make yourself do when you really want the car to be slowing down.
09-10-2010, 05:18 PM
#6
Rennlist Member
Hydraulic or vacuum assist car? My '92 C4 (hydraulic assist) would do it consistently into the uphill T3 at Willow Springs. Confused the hell out of me, until I figured the brake fluid was marginally low. The part of the reservoir dedicated to supplying the hydraulic pump was going dry on hard uphill braking, and oddly neither of the other uphill braking corners in my track rotation (T5 and T9 at Thunderhill, T2 at Sears) caused a problem.
09-10-2010, 06:18 PM
#7
Racer
Happens to me all the time on a couple of bumpy bends near home. The peddle goes hard, brakes lose stopping power and ABS is very slow (seems like an eternity if u panic) in reacting and returning stopping power. You can scare yourself silly thinking you have no brakes when it happens the first few times!
It seems to me that when the front goes light on the bumps the ABS cuts in instantly and does more damage than good. Best thing to do is avoid being hard on the brakes when the car becomes unsettled... The trick is to hit the brakes hard when the weight transfer on the front is in a downward direction. I think that was all that was happening when it was working better for you in the 2nd race (you were releasing the brakes when bobbing upwards and then getting on them hard again when the front was loaded again).
Otherwise trust the ABS, keep your foot hard on the brake and wait until the stopping power returns (but that takes far too long in race condition)... can't really see any other solutions unless you can make the ABS "smarter".
It seems to me that when the front goes light on the bumps the ABS cuts in instantly and does more damage than good. Best thing to do is avoid being hard on the brakes when the car becomes unsettled... The trick is to hit the brakes hard when the weight transfer on the front is in a downward direction. I think that was all that was happening when it was working better for you in the 2nd race (you were releasing the brakes when bobbing upwards and then getting on them hard again when the front was loaded again).
Otherwise trust the ABS, keep your foot hard on the brake and wait until the stopping power returns (but that takes far too long in race condition)... can't really see any other solutions unless you can make the ABS "smarter".
Last edited by Tuscany964; 09-10-2010 at 06:40 PM.
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09-10-2010, 07:53 PM
#9
Three Wheelin'
Brake booster switch definately. Happened to me big time and i drove the car back from a wales run.
Happened again recently but in putting the spare wheel in the switch wasn't locked in place and exactly the same symptoms. Pushed it in this time and all was well when the pressure built up.
Happened again recently but in putting the spare wheel in the switch wasn't locked in place and exactly the same symptoms. Pushed it in this time and all was well when the pressure built up.
09-13-2010, 06:23 AM
#10
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Hi Guys,
Well it was me driving the car... thanks for the input. Now I understand the issue the brakes don't worry me but it would be good to get rid of the problem.
As it's a C2 is doesn't have all that brake booster thingy stuff in it.
I think it was the long bumpy straight heading into the corner that upset the ABS. I checked all the ABS sensors yesterday the front ones were very gunged up with metal filings and dirt so I've cleaned those off. The outer casing to the wiring to the front sensors has melted a bit due to the proximity of the disc but the wires seem intact.
I'm going to check that the relay is a nice tight fit and that the ABS ECU connection is good and clean.
One option might be to use an RS ABS ECU as it has a slightly different program specifically for race use.
Cheers
Richard
Well it was me driving the car... thanks for the input. Now I understand the issue the brakes don't worry me but it would be good to get rid of the problem.
As it's a C2 is doesn't have all that brake booster thingy stuff in it.
I think it was the long bumpy straight heading into the corner that upset the ABS. I checked all the ABS sensors yesterday the front ones were very gunged up with metal filings and dirt so I've cleaned those off. The outer casing to the wiring to the front sensors has melted a bit due to the proximity of the disc but the wires seem intact.
I'm going to check that the relay is a nice tight fit and that the ABS ECU connection is good and clean.
One option might be to use an RS ABS ECU as it has a slightly different program specifically for race use.
Cheers
Richard
