ABS question, mechanically oriented.

Congradulations. Now you know what abs feels like. I agree with your assessment that it sounds like some damage is occuring, but I assure you, none is. Perhaps someone with more mechanical knowledge than I can explain it better, but in short what is happening is, you activate the brake and pump fluid to the pads causing them to grip the disc to stop. The sensors detect approaching lock-up and briefly release the hydraulics. What you are feeling and hearing is this release. The first abs brakes where worse than today's, giving you much kick-back through the brake pedal, and rachet noise from the brakes. But, nothing is being damaged.

What you experienced is what *everyone* does when they *use* the abs. It is far better to experience this in a non-emergency condition, because your first inclination is to lift from the pedal.

Whenever I rent a car (usually on fishing trips) I immediately find out whether it has abs, either by asking the agent, or check the panel at start-up. If I am still unsure, I will put the right side in some dirt and jump on the brakes. It's important to know this because the operation of non-abs is exactly opposite of what you do *with* abs.

And most important - remember to steer! Dave

Nothing. You're feeling the brakes being rapidly cycled on and off. Perfectly normal.

-- C.R. Krieger (Been there; sideways)

If you drove like that all the time, you'd wear out your brakes :-)

Seriously, though, although the ABS is only designed for emergencies to prevent skidding, the quattro system uses the same components and is very reliable even in continuous winter use.

It's not aproaching lock-up, it's lock-up! That's what you hear on a dry road and you'll even leave marks on the road which the cops use to determine your initial speed.

Ronald

Well, one out of two isn't bad, Ronald. I'll go along with your "lock-up" but most abs experts use the term "approaching lock-up". It locks-up for the shortest of time (milliseconds?). The tire tracks you see in the road are not caused by the abs at lock-up. Otherwise you would see tire marks once on every revolution the same as you would see if the tire was on grass or dirt. Dave

Ok, this is sort of funny now.

I have known (through inattentive driving and such) what ABS feels like for years now. It's also caused me to have no illusions of my personally being able to out perform it - especially while steering. I most likely can not without the proper training, practice, and road conditions.

After yesterday's event, I am just now wondering (new thread) what the weak point in the mechanical system might be, regarding wear.

In other words, if I want to practice max stopping/steering maneuvers within slick or snowy parking lots, am I wearing anything out aside from my brake pads and rotors?

The answer seems to be "no", but it was worth asking.

I'm one of the converted, believe me.

Grover

You are causing wear, of course, but you are not hurting anything. The wear would be on the pads and the rotor. The school cars (Ford Taurus) take a big beating each day they are used, but brakes last a long time, even under school conditions.

Dave

You did what most people should do. Slam on them and see what happens. Do that in the snow too.

It (should!) be designed to take the load.

Arguably there is potentially more damage to the surface of the pads (the on/off is arguably more violent and frequent than a typical human driver would apply), but - IMHO - I would very, very much doubt that it has a significant difference on the service interval for the pads.

Theoretically, depending upon the materials used, it could shorten the life of the calliper seals. Mind you, they should have failed any rational test/inspection w-a-y before then.

I politely disagree.

The actual travel of the calipers when the ABS opens and re-applies brake pressure is IMO a few hundreds of a Millimeter. The pressure on the pad's fluctuates, but for that not a lot of travel is needed anyway. This travel is IMO covered in the flexibility of the seals itself, there's not much real relative movement there. If you look at it the travel of the calipers is mostly caused by the wear of the pads, making them thinner and the calipers moving out.

I think the ABS control valve and the tubes running to the wheels will vibrate but I haven't heard or read about mechanical damages to these systems.

Regards

Wolfgang

I'm thinking crack propagation. That said, I would reverently hope that the callipers had been overhauled w-a-y before then..

The pipes should be OK, as long as they're not copper (work hardening)

H1K

In article , R@L writes

Although when the wheels actually lock-up, they obviously have to slow down to zero rotational speed, I believe this slow down happens so quickly that not even the fastest ABS system is actually capable of detecting the rapid speed drop and releasing the calipers in time to stop it. Basically by the time the calipers are released, the wheel has already stopped rotating. Obviously the faster the system reacts, the less time the wheels will spend "stopped", but no system can stop it completely.

In article , Grover writes

I don't believe *any* human being is capable of out-performing a decent ABS system.

The one obvious wear issue is the tyres themselves (tires for our US cousins :) I doubt a full bore ABS assisted stop from high speed does much good to your tyres at all, but then this is relative. Compared to a long skid on totally locked wheels, the tyre wear is obviously less and distributed around the circumference of the tyre rather than in a single spot, but there's obviously more wear involved than if ABS were never invoked in the first place.

Then I believe that you should re-read the comments, take any additional training that (may) be required, and prove yourself wrong.

Tyre wear should be fairly neutral, I'd have thought. Locking the wheels would induce more wear (the phrase "flat-spot" ring a bell? ;o)

The number of flat-spots that an ABS system would induce is - IMHO - a very minor amount of wear in the overall wear-pattern of a tyre, assuming "normal" usage and material. I think that we agree on that one! :o)

H1K

Yeah, what he says. I know lots of drivers that can beat abs. It is nothing more than training. At a school teaching teens defensive driving in Vermont, after a lane change exercise using abs, we turned off the abs and did it the old way with threshold braking. My first student did it perfectly at 45 mph. I upped the speed to 55 and again he did it as well as anyone I've ever observed using threshold braking. This kid was 18 and he was good. We normally didn't go above 55 without abs, but he was so good I took him through at 60. Again, perfect.

We questioned him later and found out that he lived on a farm and drove the school bus. His farm was on a hill and at the bottom was a stopsign: you had to turn left or right or go into a river. He taught himself threshold braking on that hill in the worst of weather. Dave

Actually, I think you're incorrect. I know that the engineers decide

*how much* rotational difference is tolerated before the ABS is engaged. BMW used "25%", last time I'd heard anything in particular. Now what *that* means I'm not quite sure. We didn't ask for enough of an explanation to completely understand it. Any *more* anal retentive engineer/trivia freaks out there?

-- C.R. Krieger (Been there; done that)

In my opinion triggering the ABS once in a while can't do much harm but rather help "moving up" rear calipers which tend to hang and cause excessive wear/increased fuel consumption.

Rune

..and hitting the odd wall head-on at 60mph helps to give the front bumper practice in the event of a "real" accident..? ;o)

(Quite apart from that, if a caliper "sticks" then it's more likely to be in the retracted position, IMHO)

H1K

Respectfully disagreeing, IME, there's always enough hydraulic pressure to push the pistons *out* and very little in terms of piston seal flex to bring them *back*. That's why the simplest visual indicator of a stuck piston is excessively worn pads on that wheel.

-- C.R. Krieger (Been there; broke that)