Ben951S

I understand (basically) how braking works, ABS vs. Threshold braking, how ABS allows one to steer and brake, and how to properly threshold brake.

Question - I have ABS (89). Under dry, 'ideal' conditions where no steering input is required, should I threshold brake, or just hold the brakes down and let ABS work? I understood the tradeoff of steering vs not if it was needed, but in the above scenario, it's not (steering). Would threshold braking be advised here?

Tedro951

Threshold is better in the dry.

Cole

Here is what you need to understand!

Think about this in terms of traction. You have "100 units" of available traction to do any action in the car. You can use all 100 to turn with, use all 100 to brake with or divide the units into some combo of turning and braking. (60/40 etc). As soon as you try to use anything over 100 (101) the car is sliding. This is the "threshold" the space between 100-101.

Threshold braking is using ALL 100 units to brake with. Which leaves 0 units left to steer the car. ABS was never designed to stop a car faster. It was designed to give you some of those units of traction back so that you can steer the car. When a wheel sensor determines that YOU have crossed the threshold it RELEASES brake pressure until it is down to say 80 units or so of traction allowing you 20 or so units to steer with. The problem here is that YOU still have your foot on the brake and quickly cross the threshold again, ABS releases it, you cross it again, ABS release it, on and on. This is the sensation you feel in the pedal.

The problem here is that the ABS system is constantly releasing your brake pressure to give you steering control. This means that the amount of braking you are actually doing is less than if you are using threshold braking effectively and holding the system at the 99-100 mark.

Does not matter if the road is wet or dry. They system is doing the same thing. What matters is your ability to hold it at the threshold wet or dry. It takes more skill to hold it at the threshold without going over on a slicker surface (wet, snow, ice etc)

Van

Cole's right on. You can brake harder threshold braking than engaging the ABS.

Cole

.....I will add that modern ABS systems are much better than old ones. You should practice "Threshold Braking" and use the ABS system to tell you when you have "crossed the line". Ultimately using the system as a backup to your own skill. While the systems have gotten better, they do still fail from time to time and you should have the skills to not need the system. Too many out there using it as a crutch to not learn or practice appropriate skills.

kombatrok

ABS... Whats that?

Cole

A system to allow bad drivers to think they have super powers

Yomi

I don't think I buy the argument about ABS being designed to leave a bunch of traction unused so you can steer. [Edit: I think our conclusions are identical, we're just arriving there differently] If you can threshold brake well, that's better. But most average (non-track, usually non-enthusiast) drivers will either not press hard enough (say 60%), or stomp on the pedal and initiate a slide. So ABS will brake until it crosses the threshold, back off some, apply pressure again, and so on. It's dynamically finding the threshold every second or so. This lets you steer since it will keep adjusting. The downside is that it's rather stupid about threshold braking, so an experienced driver can do a better job. The main upside is that my mom can mash the pedal and get far, far better braking than she would without ABS, regardless of conditions. Another upside is that a good ABS system can get me 100 - epsilon percent of traction with a pretty small epsilon in almost all conditions and let me worry about other things like watching my line, steering, etc. -- a lot of good stuff for the penalty of the difference between the ABS's epsilon and my threshold braking skill's epsilon. On the road there's no question which I want. On the track or autocross ... debatable but harder to argue against ABS these days.

Oh, another good reason I know when autocrossing. One bad braking mistake can cost you $200-1000 in tires if you don't have ABS. A lot of people I know figure it's better to sacrifice a little bit of time for that.

[Edit2: apologies for the rather rambling paragraph]

JET951

Thats not quite true, you can ruin tyres with and without ABS, we have seen many stuffed tyres from ABS cars even the new porsche's.

Cole

Buy it or not, those are the facts. Keeping the wheels below the threshold keeps the car under control. Being able to steer around an object is a very nice tool to have.

You are right about panicking, uneducated drivers just mashing the pedal. Those are the ones they panic, smash the pedal and then try to steer without releasing the brake first. Would be nice to have a system that allowed then to do that! :-)

I've taught performance driving since the early 90s and we have worked very closely with several of the manufacturers and engineers.


We also taught on a real skid pad. The average car could be stopped 3-4 CAR LENGTHS sooner from 20mph on our skid pad with threshold breaking over just pounding the ABS. They would only stop maybe 1/2-1 car length sooner (with the ABS) over just pounding the brakes and sliding to a stop in a non ABS car. The real advantage in ABS is the ability to steer.



Have you taken apart an ABS system? Up until the recent systems that include stability controls there is no internal method to "apply pressure again" only release it.

DanR 1201

[QUOTE=Yomi;8691982]...So ABS will brake until it crosses the threshold, back off some, apply pressure again, and so on. It's dynamically finding the threshold every second or so...

I thought ABS doesn't brake UNTIL the threshold has been crossed and a wheel or more has locked. Doesn't it then come into application and momentarily release the brake? Is that why Jet would be seeing ruined tyres because insensitive drivers haven't recognized when ABS is working and need to back off a little.

What's an epsilon?

TurboTommy

????????
I find that a little hard to believe.

Cole

Keep in mind this is on a skip pad, not normal dry asphalt.

Ben951S

It also weighs a lot less and has a s~ ton of more rubber... bad comparison for this thread. Looking forward to reading all the replies tomorrow!

Black51

Is that like this "traction control" I've been hearing about too?