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

[Kurt Huhn]

> >  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).
> 

Okay - lets try to meet halfway here.  The coefficient of friction never
changes it is a constant - physical fact.  However, other factors do
change - weight, force, vector of said force, etc, all combine with the
coefficient of friction to change the *actual* frctional force applied
to the pavement by the tire.  You are correct though, the harder two
surfaces get pressed together, the greater the frictional force (*not*
coefficiant of friction) is.

Now, where were we going with this...I forgot.

> > Under hard braking, the front dives due to increased force - the vector
> > of force dictates this.
> 
> Exactly, and thus the increased C.o.F.

Increased frictional force - the CoF never changes.  The CoF is a value
given to any given material in order to calculate the frictional force
of CoF, surface_area, force, vector of force, etc - in an equation I
can't recall, but probably readily accessable via Google.


> > 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.
> 

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.


> 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.
> 

Oh god.  Okay.  I'm a bad driver.  But I can assure that I have more
damn fun on the road than you.  I also know how to manuver my vehicle in
and out of situations as they arise.  I constantly adjust my speed to
match conditions, and I occasionally lock my rear wheels as a method of
avoiding those situations.  I went to school to learn this, and I've put
it to test on the racetrack and off-road.

To say that locking the rears is an indication of driving too fast or
mechanical failure is very shortsighted.  There are loads of situations
where locking the rear is preferable, and these happen to me about twice
a month.


> >  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)
> 

Not exactly.  Every damn time I locked the rears on my truck, it was on
purpose and I knew it was going to happen - every damn time.  When I use
the 5mph example, that was during the single snow storm we had here in
RI and I wanted to test the limits of my vehicle, as I do every year at
the first snowfall.  I go to an empty parking lot and spin that fucker
like a dreidel on crack.  It helps me learn, again, how my vehicle
behaves in slippery conditions.


> > > 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.
> 

A locked tire will *never* provide more stopping power than a properly
modulated tire on the very edge of locking.  This is because the static
coefficient of friction of any given material is greater than the
kinetic coefficient of friction.  I've probably bastardized those terms,
but the basic gist is correct.  An object sliding will take far longer
to stop, than one that is not - the example being a rotating mass of
rubber...

I felt like finding reference material to back myself up, this website
does a good job:
http://hyperphysics.phy-astr.gsu.edu/hbase/frict2.html

Google returns several others...


> 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!).
> 

By locking your tires, you actually took far longer to stop than if you
had properly modulated them at the brink of actually locking, when the
forces at work would have conspired to create greater force between your
tires and the road, stopping you sooner.  See the above website...

> > > 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?
> 

Here it appears that I misread your statement.  However, for the great
millions of unlearned pickup drivers, 4-wheel antilock would be a vast
improvement of RWOAL.  Please re-read my description of RWOAL as I
explained it, because I think you may have misunderstood.

RWOAL is not *true* antilock.  It pulses the brakes 3 times in one
second with excess pressure in the master cylinder, then lets go
completely if the rear is still locked - letting the rear tires
free-wheel with ZERO braking pressure.  In this situation it is also
possible for the fronts to also let go, due to reduced brake-line
pressure.  For people that don't know better, this is itself a hazard -
and 4-wheel antilock would be far better.

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

Adding to that we could say that:
4-wheel antilock is best for unlearned drivers
RWOAL is better than nothing for unlearned drivers

I think we've gotten so far removed form that original post, that now
were simply arguing for the sake of argument.

Kurt