Tedro951

This doesn't get too deep into it, but is a quick read.

http://www.drivingfast.net/car-control/braking.htm

Yomi

Good advice.

Granted. I don't see the argument much in autocross any more, but in 2003-2005 there were a lot of cars without and some with, and some models where it was an option. I saw more tires flatspotted from non-ABS systems, and don't remember seeing any from ABS ones. But you're correct that ABS isn't a cure-all for this.

I agree wholeheartedly with the end result. My impression is not that they brake at, say, 80% of maximum traction so they can leave 20% always available for a theoretical turn which is what I was disagreeing with. They're trying to dynamically come as close as they can to the traction limit and are constantly adjusting. For example, the article Tedro links to indicates "By rapidly applying and releasing the brakes in pulses when wheel lock is detected." This is what I indicated. You say "there is no internal method to "apply pressure again" only release it." which doesn't quite match that statement. However, I think if we make an assumption that the driver has the pedal pressed down such that the force locks up a wheel, the ABS can release pressure until the wheel is moving again, then can stop releasing pressure, and repeat. The effect is identical even though the system only has a "release" mechanism.

I read my statement as the same as yours -- perhaps I worded it poorly to give the impression of a more active system (it reads to me basically the same as the article Tedro linked to). The ABS determines from wheel movement that the threshold has been crossed, it backs off braking until traction has been regained, then lets pressure back, and so on. It's cycling over and under the limit. The tire does slide some, but only for a very short time -- not multiple seconds in a single spot like some drivers might do with a non-ABS system if they're not paying attention.

epsilon = some small number. It's a math thing.

A driver doing good threshold braking in the dry can perhaps hold the tires such that the most loaded one is not locked and the sum of each tire is very close to the best we can get. ABS will go back and forth between some (or all) wheel lock and not locked -- that's a lot better than just holding them all locked, and better than poor threshold braking of 80%. ABS can be a little smarter in that it can treat each wheel independently -- important if the weight and brake balance leaves one set of tires with lots more braking ability unused when the other set locks up. On the other hand, ABS doesn't have a clue about what the situation is: wet, dry, ice, uphill, downhill, rocks, sand, potholes coming up, steering input about to be applied, etc. They're making a system that does a good job in all of these conditions, which means it's not perfect in our hypothetical dry straight example. A driver can apply his knowledge of all of these to do a better job, assuming he has the knowledge and skills. You can read plenty of stories about the dreaded "ice mode" Brembo systems will go into sometimes, whereas a driver knows that obviously in 100 degree weather, a slight bump doesn't mean we've hit a patch of ice and isn't going to decide to pull braking back to 25% for a while.

Cole

Unfortunately only the newer systems are like this. Many of the older systems would either relieve pressure to the whole system based off of one wheel locking up, or treated the rear wheel pair as one channel, so if you lifted a rear under hard "turning and braking" it would release the pressure to the other rear wheel too. (they mostly did this in SUV/truck platforms. Ford for example mounts one sensor on the ring gear of the rear axle)

Just something to be aware of in older cars. The newer stuff can do some really cool things with stability programs integrated into the ABS.

Ben951S

How is the 944/951 abs? Is it discrete wheel control? It felt like that on more than one occasion, but it was also under proper threshold braking, and probably only one wheel was locking to begin with.

Cole

I honestly don't know. My car is an 86, so no ABS. (never looked at the ABS system on my 928S4)

I would imagine it's at least 4 sensors. No idea how it responds.

jerome951

In my '89 I believe there is a sensor at each wheel detecting lockup but the rears are treated as a single circuit (since only 1 brake line to the rear exits the ABS pump). The fronts are treated independently.

One thing we need to be careful of is making sweeping statements that may not be true in all circumstances. True, a car that is properly setup and balanced for the track with a well-functioning brake system on a level track may stop shorter with threshold braking than engaging ABS, but --- what about the car with off cornerweights ,or too much bias to the rear, or on a cambered, bumpy, or unlevel track. It is conceivable that the total braking force from the 4 tires under these conditions could be higher when ABS is engaged (since 1 tire's braking grip can be somewhat sacrificed to continue to increase grip of the other 3) as opposed to managing grip of the tire with the least grip (i.e. the tire that is about to lock up under threshold braking). You can't always assume that in threshold braking you have all 4 tires at the limit of grip.

Van

The 951 system is a "3 channel system" - that means it can operate the front wheels independently, the the rear wheels operate as a pair.

To have traction control, you need a 4 channel system, so it can control each rear wheel independently.