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

[Greg A. Woods]

[ On Tuesday, March 12, 2002 at 12:38:25 (-0500), Kurt Huhn wrote: ]
> Subject: [geeks] Drum versus disk brakes (Was RE: car question RESOLVED)
>
> 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"?

	:-)

>   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.

Also, if you're lucky the wheel-base will be wide enough to prevent a
roll-over when the vehicle is sideways and even more tire surface is
available to create even more friction with the road surface (because of
course the tires will crumple sideways more easily than in-line).

>   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.

(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.)

> 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!

> 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).

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?

> 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).

-- 
								Greg A. Woods

+1 416 218-0098;  <gwoods at acm.org>;  <g.a.woods at ieee.org>;  <woods at robohack.ca>
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