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

[Greg A. Woods]

[ On Tuesday, March 12, 2002 at 14:39:11 (-0500), Kurt Huhn wrote: ]
> Subject: Re: [geeks] Drum versus disk brakes vs. rear-wheel anti-lock
>
> This on an unloaded truck.  No payload - only the weight of the frame
> and bed (and axle, yadda yadda).  Of course, nitpicking aside, the front
> brakes in any vehicle will ultimately have more braking power than the
> rears due to angle of acceleration and vectors of force.  I think that's
> what you were trying to say anyway.

Well sort of -- I am nitpicking, and I'm trying to use real physics
instead of a simulation thereof too.....  :-)

>  The coeficient of friction never
> changes, but because of the increased downward force on the front tires,
> they won't skid as easily, resulting in better braking and the ability
> to add more force to the brakeing mechanism.  

Perhaps you'd like to think that last statement over again too.

One of the reasons there's more friction between the front tires and the
road when braking is of course because there's more tire surface applied
to the road due to the way the front tire crumples more than the rear
(due to the amount and angle of the force applied to it of course).

However the other reason is because the harder you press two (dry)
surfaces together, the more friction exists between them.  I.e. the
coefficient of friction definitely increases proportional to the force
which presses the surfaces together (and this is usually true whether
the surfaces are already in motion or not, unless/until perhaps the
friction generates enough heat to melt one or both of the surfaces and
thus lubricate them, which is, as far as I know, normally not true of
burning rubber on dry pavement).

> Under hard braking, the front dives due to increased force - the vector
> of force dictates this.

Exactly, and thus the increased C.o.F.

> Why then, please explain, do nearly all racecars of all classes have
> four-wheel disks, and no electronic braking aids?

Because they're not built like pickup trucks and designed to carry a
load in the rear bed.  Race cars are designed to have very well balanced
weight applied to each tire.  Pickup trucks only have such well balanced
loads in what turns out to be rather rare circumstances (_very_ rare in
the case of my truck, at least vs. miles driven).

> You're assuming that no driver anywhere can use this to their advantage,
> and that isn't true.

No, I'm talking about pickup trucks, just like you were talking about
pickup trucks.

> This is a parking brake - it has been mistakenly called an "emergency"
> brake in modern vehicles.

That's no "mistake", as you will know if you've learned from my example.

> > If your rear wheels are locking then you were moving too fast for
> > the conditions (i.e. you're not a good driver).
> 
> Not true.

Well first of all I want to be sure you understand that driving too fast
for the conditions is the very definition of a bad driver, or at least
one type of bad driver!  Yes, conditions can change quickly and
unexpectedly, which is why good drivers are prepared to modify their
driving behaviour at all times.  Accidents will still happen of course,
since it's impossible for any driver to control all circumstances.

Now what I meant to imply there was that in situations where you are
supposed to be in control, and where the potential for dangerous
conditions to develop is quite predictable, then locking your _rear_
wheels is either an indication you were driving too fast for the
conditions, or of a mechanical failure.

>  I've managed to slide my truck from 5MPH in slippery conditions.

Then quite literally you _were_ driving too fast for the conditions and
the vehicle.  (unless perhaps you took the sliding into account and used
it to your advantage, but the way you phrased it I doubt that was the
case)

> There's also the danger of having to compensate for other people's
> stupidity.  Someone cuts you off and have to slam on the brakes - the
> tires lock - were you the one being irresponsible?

Accidents and failures do happen, and sometimes there's not even any
clear way to lay blame, not even if the conditions were monitored
perfectly and all actions of all drivers were recorded perfectly for
analysis.

> Your experience with unequal weight distribution is clearly limited, so
> I'll excuse the ignorance - but you must remove the arrogance from your
> statements.

I've probably driven more different types of vehicles in more different
types of conditions than most people on this list.  About the only thing
I haven't driven that's designed at least in part to travel normally on
standard roads is a modern tank or personnel carrier (though I've
operated quite a variety of heavy equipment designed primarily for
off-road operation).  (I'm not rated for air brakes, but I've driven
such vehicles a small amount anyway.)  Most of my miles have been in
pickup trucks too, I think (though most of my hours have probably been
on farm tractors, even though I haven't operated a tractor for quite a
few years now).

> > Certainly your front wheels will lock before your rear wheels unless you
> > have an unbalanced and too-heavy load.  If your front wheels are also
> > locking then you're not a good driver either, right?
> 
> Not at all.  This is totally untrue.  Due to the vectors of force, the
> coefficient of friction, the pavement qualities, the chassis and
> suspension dynamics, this is all variable from vehicle to vehicle.  On a
> with an unequal weight distribution, with more weight over the front
> tires, the rears will lock before the front.  That's why technology such
> as RWOAL exists - to try to combate that dangerous situation for folks
> that don't understand this.

You really seem to have missed my point.  A good driver will unlock the
front wheels unless he or she is able to maintain control and is certain
that the locked tires will provide more stopping power under the
circumstances.

For example when I lost my brakes on the hill I was forced to lock my
rear wheels with the emergency brake and only pulse them to re-steer
them into my lane (it was on dry pavement and even though I had applied
the emergency brake while still using the engine as a brake, I still
under-estimated the amount of braking required -- I'd never had to
practice that maneuver on such a steep long hill with heavy fast-moving
rush-hour traffic ahead of and behind me before!).

> > On a pickup truck a good driver does not need 4-wheel anti-lock brakes
> > -- rear-wheel only anti-lock brakes are more than sufficient, esp. given
> > that most pickup trucks are driven almost all of the time without a
> > properly balanced load.
> 
> Not in my experience.  A pickup truck will actually brake *better* with
> more weight over the rear axle

You're contradicting yourself.  What part of my sentence above did you
not read?

-- 
								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>