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RE: brakes and lonnnng downhills (longish)

The analysis below of Chris', which I feel cannot be easily culled to limit
BW, is a good one, but skips one important step.  Given that the hydraulic
pressures are equal, the piston force equals the piston area times the
hydraulic pressure.  The pad force, for infinitely stiff pads, equals the
piston force.  So, assuming no lockup, if the rear pistons are larger than
the fronts, there would be more work done at the rear than the front, if the
pads at each end are the same distance from the axle centers.  (Work = force
x distance).  So, we need piston sizes and radii to the pad centers to
continue this.  Of course, the real question is which end locks up first.

                .... Kirby   (Kirby A. Smith)
                              2 x 1988 90q
                          New Hampshire USA

> ----------
> From: 	Chris Newbold[SMTP:cnewbold@transarc.com]
> Sent: 	Friday, October 09, 1998 4:34 PM
> To: 	DeWitt Harrison
> Cc: 	Smith, Kirby A; quattro@coimbra.ans.net
> Subject: 	Re: brakes and lonnnng downhills (longish)
> 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