Re: brakes and lonnnng downhills (longish)

[Chris Newbold <cnewbold@transarc.com>]

DeWitt Harrison wrote:
> 
> Not exactly. I was suggesting that everything is fine, but, due to
> the adjustable bias design,  if you work the brakes hard in a long
> downhill situation with a couple of lard butts - I mean adults -
> in the back seat, the rear brakes are going to get hot. On the
> other hand, if the rear brakes still became warmer than the
> fronts without rear seat passengers, then the bias valve's control
> arm may have come loose from its moorings or otherwise
> become stuck in maximum pressure to the rear position.

I'm not sure I buy this... Here's why:

  The amount of heat generated by the rotor/caliper/pad at any corner
  of the car is proportional to the hydraulic force applied to the caliper.

If the bias valve were to fail "wide open", you'd expect roughly equal
hydraulic force to be applied to all calipers and, consequently, roughly
equal amounts of braking force to be generated. (I'm ignoring the
implications of design differences between front and rear calipers, 
differences in front and rear pad compound, etc.)

Since the bias valve cannot fail in a manner which results in lesser
hyrdaulic pressure to the front calipers than to the rears, the "worst
case" is the aforementioned "wide open" failure, with equal hydraulic
pressure all-round.

  The percentage of the vehicle weight "over" a given wheel affects _only_
  the amount of braking force which may be applied by the rotor/caliper/pad
  before the wheel locks up. 

  That is, it gives you the _ability_ to generate more heat; in order
  to _actually_ do so, however, you must also increase the hydraulic
  force applied.

If Phil's loaded 200 + passengers had a bias valve with this "wide open"
failure, a perfect 50/50 weight distribution, and if Phil didn't lock up
any wheels on the way down, we'd expect each rotor/caliper/pad to generate
an equal amount of heat:

	equal hydraulic pressure = equal braking forece = equal heat

Of course, Phil's loaded 200 didn't have a perfect 50/50 weight distribution.
However, this should not affect the amout of heat generated!

With equal braking force at all corners, the only effect of imperfect weight
disitribution will be to increase the amount of braking force which may be
applied to the "heavy axle" before lockup results and to reduce the amount
of braking force which may be applied to the "light axle" before lockup
results.

So, right up until lockup occurs, the situation looks exactly the same as
with a 50/50 weight distribution: we're applying equal hydraulic pressure
to each caliper resulting in equal amounts of braking force and therefore
heat.

In order for Phil's rear brakes to have generated more heat than the fronts,
they would have had to be applying more braking force than the fronts.
With a rear-heavy load, it would be theoretically possible to apply greater
braking force to the rear before lockup. However, since the front brakes
receive no less hydraulic pressure than the rear, the _front_ brakes would have
locked up!

I'm going to bet that Phil had (close to) equal hydraulic pressure available
at all four corners (because of the weight and the automatic bias valve)
and so generated roughly equal amounts of heat at all four corners. The
rear wheels were hotter at the end due to a relative lack of airflow
compared to the front and smaller/lighter/not ventilated rotors (resulting
in more heat transfer to the wheel than in front).

Okay. Tear me to shreds!

-Chris
1993 90CS 70k miles