[Author Prev][Author Next][Thread Prev][Thread Next][Author Index][Thread Index]

Re: Why did tires 'chirp' in panic stop?

Static friction is -always- higher.  Slipping tires don't grip as 
well, contrary to what most hot-rodders think.  Hence ABS.  The 
switch is for gravel or extremely dense snow, if I recall past 

In order for ABS to activate, it must sense a difference in speed. 
Once that happens, it will turn on.  The squeal you hear is the tire 
slipping; it stops because ABS turns off the "juice" to that wheel.

The problem with ABS is it isn't very "gentle"; it has two settings, 
on and off.  There's no inbetween.  This causes the ABS system to 
just bounce; every time it turns the brakes on, the system locks the 
wheels immediately because you're instantly going from no braking 
action to full clamping on the rotor.  The wheel slips; ABS turns off 
the brake.  Wheel starts spining.  ABS slams the brake back on. 
Wheel stops.

In extremely slippery conditions, you'll get much shorter stopping 
distances by feeling out your limits and only applying the brakes to 
that limit.  I've proven that a car can stop in considerably less 
distance if you break up to a threshold determined by 
experimentation(ie, right when you pull out of the driveway, for 
example.)  Slamming on the brakes increased my braking distance by 
almost 2x.

Then, of course, there's the highly unlikely(at speed) probability 
that you would lock all 4 wheels at exactly the same time.  If this 
happens, ABS will simply "think" the car has screeched to a halt 
thanks to Super Wonder Brakes.

ABS was originally designed for planes, and adapted to cars later. 
Planes don't have to take curves when they land, cars have to take 
corners all the time, and people tend to swerve when they make a 
panic move.  ABS is due for a serious re-thinking.  There is, 
however, no alternative until we develop better(non frictional) 
breaking systems, or until we integrate sensors that determine 
pressure exerted by the caliper, etc.

There will probably be great advances when we are all driving hybrid 
or electric vehicles; the electric motor and advanced AWD systems 
will present very interesting possibilities for non-frictional 
braking systems that will allow a computer to control braking to a 
wheel with a greater degree of control.


> Doug;
>     I had a fellow step out in front of my '91 200q as I was cruising down a
> Toronto street at 30 mph. I stood on the brakes and the ABS activated (on
> dry pavement). The tires did not even squeak. The unfortunate pedestrian
> froze in front of my car when he saw me coming and stood there with eyes as
> large as saucers, no doubt wondering why the tires weren't squealing. I
> stopped about 12" in front of him. Lucky chap.
>     I would have stopped sooner if the ABS hadn't activated. ABS lets you
> maintain steering control, but usually lenghtens your stopping distance. In
> my case, traffic density and parked cars precluded me from steering
> anywhere. It was not a good feeling cruising up on this guy with the ABS
> hammering away. I think I would have felt better with a little rubber smoke
> in the air! Guess that's why the antilock switch is there.
>     I wouldn't worry about a few chirps, but the tires shouldn't squeal with
> the ABS activated. The chirps may indicate you are getting maximum braking
> effort from your system.

Brett Dikeman
"Diplomacy is the art of telling someone to go to hell and making 
them happy to be on their way." - Mark Twain

"Oh no.  Not again." - The bowl of Petunias

"... it is important to realize that any lock can be picked with a 
big enough hammer." -- Sun System & Network Admin manual

Statler: "Hey, this one sounds cute."
Waldorf: "You old fool, that's not the personals, it's the obituaries!"