ABS override
12-12-2002, 12:03 AM
#1
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ABS override
I've seen a variety of prepped dual purpose street/track cars fitted with ABS override switches on the dash. And I have found myself getting on the edge of the ABS at the track and wished I could turn it off to get better feel.
I came across an article on the RSAmerica website that talks about the potential to have minimal braking capacity in a rear slide due to the fronts locking up with no weight and the ABS releasing them until the rears catch up. (This recommended a emergency only option, vs the porsche part for the race cars)
On the other side of the coin, everyone in the higher elevations of racing use some sort of ABS/traction control on the track. And in the worst possible case it could potentially help keep the greasy side down.
So what is the general concensus, install an ABS override for the track for better modulation and feel at the threshold, or let the bosch handle the brakes? Has anyone used the Porsche switch? Do you like it?
I came across an article on the RSAmerica website that talks about the potential to have minimal braking capacity in a rear slide due to the fronts locking up with no weight and the ABS releasing them until the rears catch up. (This recommended a emergency only option, vs the porsche part for the race cars)
On the other side of the coin, everyone in the higher elevations of racing use some sort of ABS/traction control on the track. And in the worst possible case it could potentially help keep the greasy side down.
So what is the general concensus, install an ABS override for the track for better modulation and feel at the threshold, or let the bosch handle the brakes? Has anyone used the Porsche switch? Do you like it?
12-12-2002, 01:21 AM
#2
Burning Brakes
The switch is used to turn off the ABS when the car is in an emergency such as spinning or going backwards into a wall. You need fast reflexes to turn it off at the right time. Otherewise you don't turn it off.
12-12-2002, 01:42 AM
#3
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JC,
There are two types of switches, The one with the streamer on it; I just don't know if I would have the ability to identify the potential impact, find the streamer, and get a hold of it with gloves on before hitting the wall. I think I would still be concentrating on driving the car, even backwards.
The switch I am thinking about installing is one that evidentally came on the racing cup cars, which turns off the ABS system entirely. You would turn off the ABS at the start of the race to take it out of the loop.
There are two types of switches, The one with the streamer on it; I just don't know if I would have the ability to identify the potential impact, find the streamer, and get a hold of it with gloves on before hitting the wall. I think I would still be concentrating on driving the car, even backwards.
The switch I am thinking about installing is one that evidentally came on the racing cup cars, which turns off the ABS system entirely. You would turn off the ABS at the start of the race to take it out of the loop.
12-12-2002, 09:04 AM
#4
Burning Brakes
The one on the Cup cars is what I was referring to. It is a punch button on the dash. It is only used in an emergency when the car is spinnng or off in dirt or backwards. You don't use it on Cup cars to disable ABS before a race. You want ABS.
12-12-2002, 09:26 AM
#5
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The main reason we put the ABS switches in our cars is to reset the ABS after it gets confused from going off track or over very large gators. When the suspension is hammered from rough condition the ABS will turn itself off and the only way to get it back on is to cycle the power to it by either turning the ignition off and then on again (which of course slows you down on the track 
) or simply cycle the ABS switch. It is also used occasionally for the reasons mentioned above but to be honest I have never used mine unless it was to reset the ABS or to get a more accurate feeling when adjusting brake bias.
) or simply cycle the ABS switch. It is also used occasionally for the reasons mentioned above but to be honest I have never used mine unless it was to reset the ABS or to get a more accurate feeling when adjusting brake bias.
12-12-2002, 09:51 AM
#6
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If using the switch only to reset the ABS ECU after hitting the gator strips, the most appropriate way to do so would be with a momentary-off pushbutton switch - hit and forget it, rather than two motions. If using as described to disable ABS for the length of the race, then yes, you'd want the switch with 2 distinct positions. If used in a Cup car as described - hit it in an Oh $ht moment, then specifically you'd want a "mushroom" style switch, mounted prominently on the dash, works like an emergency off switch on heavy machinery, easy to slap at to turn off the ABS without having to hunt for a switch and figure out its position. Just some notes on switch type.
As to whether or not to do it, there are a number of considerations, and one most important item has not been discussed. How is the ABS calibrated??? Who tuned it, and for what? Doesn't seem like a relatively street car, with ABS tuned for street suspension and tires, would work too well on track with high-rate springs and race tires. I'd figure to ditch it and use an adjustable prop valve in that case; you should do back to back tests with hot tires in race config and compare stopping distances. Unless you like driving with the ABS on, as many people do. OTOH, if you've got an ABS tuned for race tires and suspension, as I'm hearing comes standard with the Cup cars, then you'd want to leave it on unless absolutely necessary. You need to assess your individual situation based on car, driving style, etc., and come to your own conclusions. Testing would be wise, to back that up.
As to whether or not to do it, there are a number of considerations, and one most important item has not been discussed. How is the ABS calibrated??? Who tuned it, and for what? Doesn't seem like a relatively street car, with ABS tuned for street suspension and tires, would work too well on track with high-rate springs and race tires. I'd figure to ditch it and use an adjustable prop valve in that case; you should do back to back tests with hot tires in race config and compare stopping distances. Unless you like driving with the ABS on, as many people do. OTOH, if you've got an ABS tuned for race tires and suspension, as I'm hearing comes standard with the Cup cars, then you'd want to leave it on unless absolutely necessary. You need to assess your individual situation based on car, driving style, etc., and come to your own conclusions. Testing would be wise, to back that up.
12-12-2002, 10:42 AM
#7
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I always mount an ABS reset switch for the reasons mentioned above -- sometimes the system gets confused and shuts down due to unusual fluctuations in wheel speeds. The reset switch lets you turn it back on without having to cycle the ignition key.
Porsche's race cars which used ABS always had different software. These were less sensitive to hitting gators etc. However these systems still faulted occaisionally.
The right way to set up a disaster situation cut out switch is to use a pressure switch in one of the brake lines. Porsche did this in the 993 RSR and probably other cars as well. If you pushed the brake pedal really, really hard, like you will when you see the wall coming, the system will shut off due to the action of the pressure switch. I think the switch was set to open at 100 bar or so. You REALLY had to push to set it off.
I am a big proponent of ABS in race cars. The one huge advantage that never can be denied is that ABS automatically proportions braking power correctly to each front wheel. Without it, you are limited by the unladen inside front wheel locking up as you turn in. If you are using all the capability of the car when you brake and turn in, this is an insurmountable advantage. You can always outbrake the guy who doesn't have ABS.
The thing to remember is that the activation of the ABS system is by no means the limit of deceleration. It is telling you that one wheel is nearing lockup. You likely still have three other wheels not yet giving their all to slow the car. So when the pedal begins to vibrate, you can certainly push harder and decelerate more.
Chris Cervelli
Technodyne Inc.
Porsche's race cars which used ABS always had different software. These were less sensitive to hitting gators etc. However these systems still faulted occaisionally.
The right way to set up a disaster situation cut out switch is to use a pressure switch in one of the brake lines. Porsche did this in the 993 RSR and probably other cars as well. If you pushed the brake pedal really, really hard, like you will when you see the wall coming, the system will shut off due to the action of the pressure switch. I think the switch was set to open at 100 bar or so. You REALLY had to push to set it off.
I am a big proponent of ABS in race cars. The one huge advantage that never can be denied is that ABS automatically proportions braking power correctly to each front wheel. Without it, you are limited by the unladen inside front wheel locking up as you turn in. If you are using all the capability of the car when you brake and turn in, this is an insurmountable advantage. You can always outbrake the guy who doesn't have ABS.
The thing to remember is that the activation of the ABS system is by no means the limit of deceleration. It is telling you that one wheel is nearing lockup. You likely still have three other wheels not yet giving their all to slow the car. So when the pedal begins to vibrate, you can certainly push harder and decelerate more.
Chris Cervelli
Technodyne Inc.
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12-12-2002, 11:32 AM
#8
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I wouldn't use a pressure switch set at 100bar, in case anyone's thinking about doing this - that's way too low, could easily be reached in the process of cycling the wheels in ABS control. Better to go 160-180bar or even higher. Which is probably about where they were at. Depending on Cp of your brakes and tires and all, you might be cycling your wheels at 100+bar - especially if the pads are fading!!! LOL Of course, 80 bar is more typical, at least for a street car.
One further note on ABS function, specifically detail regarding the difference between race setup and street. Not only are there substantial differences due to setup, the tire is key. A street tire will typically give up at between 8-10% wheel slip, and go towards lock, so the ABS will try to cycle the wheel at between 6-8%. Typically. Race tires, as you might imagine, will typically acheive peak (and a higher peak, of course) decel at a deeper slip. If you're running race tires on a street ABS cal, you'll never reach optimal decel. This would be the case where the very experienced driver without ABS could blow by the less experienced guy using ABS. I'd suspect I could probably out-brake and pass under braking in a 951 w/ ABS disabled vs. someone in an identical car w/ ABS on, due to experience threshold braking and based on the expectation that the old ABS is a street tune, not a race tune as in the Cup cars. Not really even bragging here!
On a personal note, as words of advice... don't lend your non-ABS car to a guy who's used to racing w/ ABS, unless you've taken care of putting junk tires on before he hits the track... 'cause they'll be junk when he gets off track! LOL - happened to me once...
One further note on ABS function, specifically detail regarding the difference between race setup and street. Not only are there substantial differences due to setup, the tire is key. A street tire will typically give up at between 8-10% wheel slip, and go towards lock, so the ABS will try to cycle the wheel at between 6-8%. Typically. Race tires, as you might imagine, will typically acheive peak (and a higher peak, of course) decel at a deeper slip. If you're running race tires on a street ABS cal, you'll never reach optimal decel. This would be the case where the very experienced driver without ABS could blow by the less experienced guy using ABS. I'd suspect I could probably out-brake and pass under braking in a 951 w/ ABS disabled vs. someone in an identical car w/ ABS on, due to experience threshold braking and based on the expectation that the old ABS is a street tune, not a race tune as in the Cup cars. Not really even bragging here!
On a personal note, as words of advice... don't lend your non-ABS car to a guy who's used to racing w/ ABS, unless you've taken care of putting junk tires on before he hits the track... 'cause they'll be junk when he gets off track! LOL - happened to me once...
12-12-2002, 05:04 PM
#9
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Hello,
I could use an ABS reset light.
In one of the practice session for the '01 Summit
Point Club Race, I braked too late for T1 and went
over the gators. I dropped two wheels into the dirt
and continued. The ABS light came on, so I knew the
ABS was off. No proble, just drive smooth until the
session ends.
Down the chute, on the brakes hard to slow for the
hard left that leads into the carousel. Next thing
I know is that I am sliding backwards through the
dirt! Trying to trail brake with the ABS off did
not work well for me!
I like ABS on my car (89 944 turbo) a lot.
[quote]Originally posted by 924RACR:
<strong>differences due to setup, the tire is key. A street tire will typically give up at between 8-10% wheel slip, and go towards lock, so the ABS will try to cycle the wheel at between 6-8%. Typically. Race tires, as you might imagine, will typically acheive peak (and a higher peak, of course) decel at a deeper slip. If you're running race tires on a street ABS cal, you'll never reach optimal decel. This would be the case where the </strong><hr></blockquote>
Why not? The ABS should not activate until a wheel
has locked. With sticky tires, the ABS will activate
later than on street tires. When the ABS activates
you may be able to beat it, but isn't the whole
point of threshold braking to stay just below
the lockup point?
Barry
I could use an ABS reset light.
In one of the practice session for the '01 Summit
Point Club Race, I braked too late for T1 and went
over the gators. I dropped two wheels into the dirt
and continued. The ABS light came on, so I knew the
ABS was off. No proble, just drive smooth until the
session ends.
Down the chute, on the brakes hard to slow for the
hard left that leads into the carousel. Next thing
I know is that I am sliding backwards through the
dirt! Trying to trail brake with the ABS off did
not work well for me!
I like ABS on my car (89 944 turbo) a lot.
[quote]Originally posted by 924RACR:
<strong>differences due to setup, the tire is key. A street tire will typically give up at between 8-10% wheel slip, and go towards lock, so the ABS will try to cycle the wheel at between 6-8%. Typically. Race tires, as you might imagine, will typically acheive peak (and a higher peak, of course) decel at a deeper slip. If you're running race tires on a street ABS cal, you'll never reach optimal decel. This would be the case where the </strong><hr></blockquote>
Why not? The ABS should not activate until a wheel
has locked. With sticky tires, the ABS will activate
later than on street tires. When the ABS activates
you may be able to beat it, but isn't the whole
point of threshold braking to stay just below
the lockup point?
Barry
12-12-2002, 08:52 PM
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Actually the pressure switch goes on the input brake line from the master cylinder, so whatever the ABS hydro unit is trying to do doesn't affect it at all. I am pretty sure that 100 bar is well out of the range of normal braking. I did a test one time with a pressure gauge. Big Red calipers flex very noticably at something like 800 psi (say 60 bar). I felt that the pedal pressure required to achieve that was way beyond what I used on the track.
With small brakes such as what you are using the pressures may be much higher. I have no experience with anything but late model 4 piston calipers.
The business about the ABS control range software gets very, very complicated. I think that in reality, the systems are programmed to intervene quite a bit before optimal slip. If the front tires were braked all the way to optimal slip, there would be no traction left for steering. We all know you can steer these cars quite well even with ABS activated.
I agree that the later ABS systems work much better than the systems on 944s. I agree that perhaps you would be able to outbrake the ABS car on occasion, if the road was perfectly flat and the brake zone was perfectly straight. Try outbraking the ABS car going into Turn 17 at Sebring for a graphic demonstration.
ABS remains and will remain a solid advantage that the competitive racer would be crazy not to take advantage of, if allowed.
Chris Cervelli
Technodyne Inc.
With small brakes such as what you are using the pressures may be much higher. I have no experience with anything but late model 4 piston calipers.
The business about the ABS control range software gets very, very complicated. I think that in reality, the systems are programmed to intervene quite a bit before optimal slip. If the front tires were braked all the way to optimal slip, there would be no traction left for steering. We all know you can steer these cars quite well even with ABS activated.
I agree that the later ABS systems work much better than the systems on 944s. I agree that perhaps you would be able to outbrake the ABS car on occasion, if the road was perfectly flat and the brake zone was perfectly straight. Try outbraking the ABS car going into Turn 17 at Sebring for a graphic demonstration.
ABS remains and will remain a solid advantage that the competitive racer would be crazy not to take advantage of, if allowed.
Chris Cervelli
Technodyne Inc.
12-12-2002, 11:45 PM
#11
Three Wheelin'
I did some experimenting with the ABS on and off at a recent high speed auto-x. I had complained that the 02/03 ABS was way to sensitive and just plain not designed for track use. My testing proved this. I could stop in 50% of the distance and with infinitely better control without the ABS. This was well and good until I locked a wheel and was not even close anymore. I need to find a balance in between. My 99 has infinitely better ABS for high performance driving. I have been told to tap the brakes hard to set the front and them mash them, but this obviously costs time. My 03 still had the stock pads. I am switching to Pagid Black RS14's and blue fluid and hope this will help the premature lockup and resulting ABS. I would prefer to run with ABS, as long as it works for what I need.
12-13-2002, 01:54 PM
#12
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Actually, Barry, your understanding of how ABS works is not correct, sorry. Chris has gotten pretty much the right answer regarding slip targets - it's that whole friction circle thing, and we must (when tuning) reserve some friction for lateral control/stability.
In case you aren't already aware (some here are), I develop/tune/test ABS, TCS, and ESP for a living... for that great company, Bosch.
Yes, you are correct in that threshold braking is intended to be braking below lockup - or more specifically at the peak of the mu-slip curve of the tire, where max decel can be found. ABS will attempt to function similarly.
Chris, regarding your pressure notes. Those pressures are low. We (naturally) have fully instrumented vehicles with pressure transducers at the master and each wheel. Pressures can get up to and above 260bar. YMMV based on your complete brake package, though, of course. I've seen plenty of street cars that will fade the pads and require up to 120-140 bar to lock the front wheels.
You're quite right regarding the later ABS vs earlier ABS - the software and hardware both continue to improve. This continues to be the case.
Ed - for reference, what tires were you using at that autox?
In case you aren't already aware (some here are), I develop/tune/test ABS, TCS, and ESP for a living... for that great company, Bosch.
Yes, you are correct in that threshold braking is intended to be braking below lockup - or more specifically at the peak of the mu-slip curve of the tire, where max decel can be found. ABS will attempt to function similarly.
Chris, regarding your pressure notes. Those pressures are low. We (naturally) have fully instrumented vehicles with pressure transducers at the master and each wheel. Pressures can get up to and above 260bar. YMMV based on your complete brake package, though, of course. I've seen plenty of street cars that will fade the pads and require up to 120-140 bar to lock the front wheels.
You're quite right regarding the later ABS vs earlier ABS - the software and hardware both continue to improve. This continues to be the case.
Ed - for reference, what tires were you using at that autox?
12-13-2002, 09:25 PM
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If you work for Bosch maybe you can answer a question that has plagued me for years:
Why are all Porsche ABS braking systems so front biased?
Even the Porsche race cars don't seem to let you get anywhere near rear threshold braking. I realize that there is a tradeoff between rear braking power and stability, but it seems that with a modern Porsche rear suspension you could get away with much more rear brake power and shorten the stopping distance.
On my 944 Turbo racecars I have always fiddled with caliper sizing to get more rear brake and been happy with the results: more brake power for shorter stopping distances and still enough stability to go in to the corner nice and hot.
What am I missing?
Chris Cervelli
Technodyne Inc.
P.S. What does a brake engineer have to say about stainless over teflon brake lines?
Why are all Porsche ABS braking systems so front biased?
Even the Porsche race cars don't seem to let you get anywhere near rear threshold braking. I realize that there is a tradeoff between rear braking power and stability, but it seems that with a modern Porsche rear suspension you could get away with much more rear brake power and shorten the stopping distance.
On my 944 Turbo racecars I have always fiddled with caliper sizing to get more rear brake and been happy with the results: more brake power for shorter stopping distances and still enough stability to go in to the corner nice and hot.
What am I missing?
Chris Cervelli
Technodyne Inc.
P.S. What does a brake engineer have to say about stainless over teflon brake lines?
12-13-2002, 09:46 PM
#14
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Thread Starter
924RACR is getting to my concerns, I am running stock 93RSA brakes with Pagid Orange, and V700's 235/275's and H/R turbo springs/Bilstein setup. I had been feeling the ABS in some corners when I knew I was nowhere near the limit and the warning light has come on a few times, after finding I was lifting and inside wheel I am assuming the ABS was doing it's think trying to keep the inside from locking up and getting scattered.
But now the question that Ed leads to: at the track (practice or DE) don't you think that you would hone your abilities much better if you didn't have the ABS setting an lower threshold for lockup? I can understand in a race situation the best advantage should be taken, but most everyone here seems to think that the street ABS is a compromise. You tend to push to the threshold of the ABS initiation rather than the true braking threshold (924RACR's argument).
So I get the "oh-s**t" switch for reset and offtrack shunt, and/or get a remapped ecu and try to keep all four on the ground?
And it sounds like a dumb question, but with the tendancy for Pcars to swap ends, what happens to the poor slob who spins on the street? You would think that porsche would have considered the problem/liability for that one! <img src="graemlins/c.gif" border="0" alt="[ouch]" />
But now the question that Ed leads to: at the track (practice or DE) don't you think that you would hone your abilities much better if you didn't have the ABS setting an lower threshold for lockup? I can understand in a race situation the best advantage should be taken, but most everyone here seems to think that the street ABS is a compromise. You tend to push to the threshold of the ABS initiation rather than the true braking threshold (924RACR's argument).
So I get the "oh-s**t" switch for reset and offtrack shunt, and/or get a remapped ecu and try to keep all four on the ground?
And it sounds like a dumb question, but with the tendancy for Pcars to swap ends, what happens to the poor slob who spins on the street? You would think that porsche would have considered the problem/liability for that one! <img src="graemlins/c.gif" border="0" alt="[ouch]" />
12-13-2002, 09:47 PM
#15
Three Wheelin'
Michelin Pilot Cups. They are not well suited for Auto-X, but my one experience with the car at Lime Rock had it ABSing in every brake zone even with the Cups dialed in to the right pressues and sticking incredibly well. The system appeared to be sensing that microsecond lockup when the brakes are first applied and then left the system in ABS which increases stopping distances.
