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

[Greg A. Woods]

[ On Tuesday, March 12, 2002 at 21:58:53 (-0800), Kurt Huhn wrote: ]
> Subject: Re: [geeks] Drum versus disk brakes vs. rear-wheel anti-lock
>
> Okay - lets try to meet halfway here.  The coefficient of friction never
> changes it is a constant - physical fact.

Yes, OK, I agree there -- I was confused by your other confusing use of
the terms.

> Okay - I implied pickup trucks.  I submit that there are pickup truck
> drivers that *can* use 4-wheel disk without electronic aid and do it
> well - regardless of weight distribution.  You have to learn how - and
> that's the point I was making originally.  You can learn how - but most
> people don't want to.

I'm afraid the physics just are not with you there though.

Some kind of technology must come to the rescue in order to deal with
the differing amount of pressure required on the rear brake pads
depending on the weight distribution.

(Both my truck and my motorcycle have a "dive sensor" to help me manage.)

> Oh god.  Okay.  I'm a bad driver.  But I can assure that I have more
> damn fun on the road than you.

remind me to try to avoid using the same road as you at the same time!

I don't go on the roads to have fun -- I want to get safely from point A
to point B in an optimum amount of time.  I don't always agree with
posted speed limits, but I don't pretend I can second guess the experts
either (unless I've done more study of a section of road than they
apparently have).  I don't always correctly estimate the conditions and
the state of my vehicle, but I'm not flying a helicopter either.

> A locked tire will *never* provide more stopping power than a properly
> modulated tire on the very edge of locking.

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.

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.

>  This is because the static
> coefficient of friction of any given material is greater than the
> kinetic coefficient of friction.

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.

Richard Feynman himself said that "[under ideal circumstances] it is
very hard to show any difference."

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.

Indeed reference I've researched on the WWW and in my physics text say
that the coefficient of kinetic friction is only "typically" less than
the coefficient of static friction.  I'd tend to believe Feynman even if
I didn't do any other research.  Note also that the coefficient of
kinetic friction may vary depending on the speed the surfaces pass
across each other.

Try this page for instance:

	http://hyperphysics.phy-astr.gsu.edu/hbase/mechanics/frictire.html

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."

Note too that the region of optimal pressure on the break pedal is very
narrow in that graph -- probably inhumanly narrow for any emergency
situation.  If you know the conditions are good you'll do just as well
to lock the tires for as long as doing so won't affect your steering
control.  If you are lucky your window will be down enough for you to
hear the difference between a total lockup and the condition where the
wheels still turn slightly even though you're mostly skidding.  That'll
give you minimal steering control with very decent braking force and it
won't wear a flat spot on your tires.

Note in contrast that the region of pedal pressure for better braking
force under wet or otherwise adverse conditions is incredibly wide in
comparison and easy for even a human to attain even during an emergency
with only a few tiny pulses of the pedal and with only the human eye and
inner ear to measure the effective braking force.

> Recall, my original argument was this:
> 4-wheel disk with no antilock is better for learned drivers than RWOAL

You're still wrong.  4-wheel disks in a pickup are still going to need
modulation and pressure control to adapt to changing weight distribution.
An engineer cannot ethically sign off on 4-wheel disks on a pickup truck.

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!

-- 
								Greg A. Woods

+1 416 218-0098;  <gwoods at acm.org>;  <g.a.woods at ieee.org>;  <woods at robohack.ca>
Planix, Inc. <woods at planix.com>; VE3TCP; Secrets of the Weird <woods at weird.com>