[geeks] Drum versus disk brakes vs. rear-wheel anti-lock

[wa2egp at att.net]

> First of all you're confusing what's going on here.  In reality this
> isn't a simple physics experiment.  These are not two perfect surfaces
> of identical materials.  There's also no such thing as a "perfectly
> modulated tire on the very edge of locking".  Even a computer controlled
> ABS will cause some skidding (unless maybe it has a separate accurate
> ground speed indicator and a very accurate way to calculate the speed of
> the tire surface at its somewhat dynamic circumference).  In order for
> the ABS computer to know a skid has started the axel rotation must come
> to a complete stop for some tiny interval of time.

One thing that may be forgotten is that in a locked wheel skid the rubber heats up
and the coefficent of friction changes which is less likely to happen when the
tire surface does not move in relation to the road.
 
> No human driver can ever hope to manage to reliably and repeatable
> obtain perfect manual modulation of the brakes and always obtain a
> shorter stopping distance than a locked wheel skid under perfect road
> and tire conditions.  The reason people are taught to pump the brakes
> when they don't have ABS is because that's what works best in _adverse_
> conditions where the coefficient of kinetic friction is far lower than
> the coefficient of static friction.  Of course if you really know what
> you're doing then pumping the brakes is just what you do at the very
> beginning to measure the ideal pedal force, and then you apply that as
> steadily as you can and up to the maximum limit where you can sense the
> skid starting again.  Personally I'll take computer-controlled ABS any
> day I can get it -- I don't have tiny video cameras on my tires to see
> when they stop turning and I can't hear them skid very well on wet
> surfaces either, and I can't measure the brake fluid pressure accurately
> enough to maintain it at an ideal level when I'm in an emergency
> situation.
Does ABS have that same capability to the degree you're talking about?

> While what you say is true for many materials, especially if there are
> other factors such as lubrication of some sort, it's not nearly so much
> of a difference for for a proper tire on clean, dry, firm, pavement.
See my comment above.

> Perhaps you're thinking of all those driver training films that show ABS
> pulsed brakes as stopping a vehicle in less distance than a locked wheel
> skid.  What you're missing is the fact that in those demonstrations
> there is always some material on the road surface which will result in a
> lower kinetic coefficient of friction.
Have there been any including stopping using brakes in the normal fashion?


> Pay particularly close attention to the relatively tiny loss of breaking
> force in the first graph on the left and the caption which reads:
> 
> 	"On a dry road it may not matter so much if your tires
> 	lock and slide."
But there is a difference, the question is if it is enough to make a difference.

> A truly learned driver will accept true ABS, RWOAL, and any other
> technology that will make their vehicle safer (but they'll still learn
> how to properly use a manual mechanical "parking" brake as an emergency
> brake).  A truly learned driver will know how to make the best use of
> vehicles with such features, and will also learn how to do without them.
> 
> I hope you (or any other reader) don't have to take this personally, but
> I really cannot stand to listen to any idiot who thinks he or she knows
> how to operate brakes in an accident situation better than an ABS
> computer!  It's simply physically and physiologically, and often
> psychologically, impossible!
A truly learned driver would avoid situations which require heavy braking.
You also cannot make that statement about ABS are always better than a
human driver.  That is just as idiotic especially with cost-cutting, device
failure, substitution of inferior parts, biased testing and the worse failure,
loose nut behind the wheel.  I don't have ABS.  I don't need it and I don't
really want it.