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

[Kurt Huhn]

> > Think of it like this, I'll use my truck as an example:
> > The rear end of my truck weighs significantly less than the front.
> > During hard braking, the front will dive, causing even more weight to
> > shift forward.
> 
> Do you mean: "causing even more downward force to be applied to the
> front wheels, resulting in more friction between them and the road
> surface"?
> 
> Of do you mean "I didn't load the truck correctly and more weight will
> shift forward adding even more to the additional downward force on the
> front wheels, resulting in even more friction between them and the road
> surface"?
> 

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.  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.  

Under hard braking, the front dives due to increased force - the vector
of force dictates this.  Now, the action of diving doesn't create force,
but the force results in the front end diving.  I worded that backwards
in my haste to compose the email.

>         :-)
> 
> >   A disk brake setup on the rear would
> > *easily* lock the rears and cause a spectacular spinout
> 
> because of course the dynamic force applied downward on the tires
> remains constant in the direction of motion due to the inertia of the
> vehicle and the fact that its centre of gravity is well above the road.

Not only that, but the vector of force actually changes due to the
dynamic nature of the chassis.  A car isn't a solid object, after all,
and chassis and suspension dynamics come into play here - in a big way.

> 
> >   To an experienced driver, disks are better
> > since they can modulate the brakes to keep all four tires unlocked and
> > stop in impressive times
> 
> Unless you have at least two brake pedals (and two equally co-ordinated
> feet!), or perhaps a very well balanced load in the bed of your truck,
> it's never, ever, going to be a good idea to have manually controlled
> four-wheel disk brakes.

Why then, please explain, do nearly all racecars of all classes have
four-wheel disks, and no electronic braking aids?  Look to NASCAR for an
easy example.  Absolutely NONE of those Winston Cup (and Busch Series)
vehicles have anti-lock of any kind.  Why isn't this a good idea? 
You're assuming that no driver anywhere can use this to their advantage,
and that isn't true.  The average driver with little experience and no
desire to learn, yes.  For professional drivers or those with years of
experience and an open mind, no.

> 
> (In reality most vehicles do have a separate rear-wheel brake control,
> and sometimes it's a pedal and sometimes it's a lever.  I don't know of
> any reason to use it though unless you're either parked or you do wish
> to cause a controlled spin.)

This is a parking brake - it has been mistakenly called an "emergency"
brake in modern vehicles.  It should be used for nothing else except
ensuring your vehicle won't move when it is parked.  This is not (on
modern vehicles anyway) an effective braking mechanism and should never
be used as such.

> 
> > My truck in particular has an odd bit of technology called RWOAL (Rear
> > Wheel Only Anti-Lock).
> 
> Ah, yes, technology to the rescue!  No need for two brake pedals when
> you have rear-wheel disk brakes!
> 

I don't think I clarified that RWOAL brakes are *drum* brakes - not disk
(at least in all current implementations).  They're not even true
anti-lock - which I did clarify.

> > This is a serious piece of heartburn for people that know how to drive,
> > but saves the asses of the unknowing millions from dangerous spinouts.
> 
> I'm sorry, but that's complete and utter BS, and completely and utterly wrong.
> 
> 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.  Since "an object in motion, tends to stay in motion" a large,
heavy vehicle will resist stopping in all manner of conditions.  I've
managed to slide my truck from 5MPH in slippery conditions.  A 5000
pound vehicle does *not* stop easily under any conditions any less than
100% favorable.  Even then, anything that makes the rear lose traction
or lock up (such as a bump or large pebble) will cause the rear to
behave radically different than on a smooth, eve, level surface.

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?

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

> 
> 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.

Simple physics, really.

> 
> > RWOAL is *not* the same as true 4-wheel antilock brakes - which use a
> > batch of  electronic pumps to modulate brake line pressure to all four
> > wheels, keeping them unlocked, but still slowing the car.
> 
> 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 (with a wee bit more weight on and behind the
> rear wheels).
> 

Not in my experience.  A pickup truck will actually brake *better* with
more weight over the rear axle - that's why you see pickup owners
tossing cinderblocks in the bed of their trucks during winter - to try
to increase the weight over the rear axle.  Again - simple physics takes
over - more weight with the coefficient of friction (which remains
realtively constant) equals better stopping power.  If we all drove
around with half a ton of weight in our beds, ther rears would rarely
(if ever) lock under straight line braking.  But we don't, we usually
drive around with empty beds.  If the weight distribution of a truck was
equal front/rear, then RWOAL wouldn't be necessary - but most trucks
have a front/rear weight distribution of 60/40 (or thereabouts).  Add
chassis dynamics, and you get the *radically* different handling
characteristics for pickups.  And locking the front wheels before the
rears wheels, isn't one of those differences.

Kurt