Re: brakes and lonnnng downhills (longish)

[DeWitt Harrison <de@aztek-eng.com>]

At 04:34 PM 10/9/98 -0400, Chris Newbold wrote:
>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:
>
>[ snip argument that rear brakes cannot generate more heat than the fronts ]
>
>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!

I'm not sure we really disagree, here. I think you have covered more
ground than I. And, as you point out, the temperature rise of the wheels
must take into account the rates of heat dissipation from the brakes and
wheels as well as heat generation.

With respect to your heat generation discussion, I think you should
include relative brake pad areas as well as pressures. If you do, it
will be even more difficult for either of us to rationalize the rear wheels
feeling warmer than the fronts after prolonged braking.

DeWitt Harrison
Boulder, CO
88 5kcstq