Re: [DML] Brake proportioning valve

[William T Wilson <fluffy@xxxx>]

On Thu, 1 Aug 2002, content22207 wrote:

> I'm afraid Josh is correct on a car w/ rear drums. Without the
> proportioning valve, braking is rather squirrely (trust me -- I've

Brake proportioning valves, or some sort of engineering intended to
control the balance of braking force, are necessary on all cars intended
to have high performance braking. The proportioning valve might not have
been necessary on the DeLorean because of the rear weight bias, but I
guarantee they did not just ignore it.

There are many forces at work with braking, weight transfer foremost among
them. All cars transfer weight to the front wheels under hard braking,
which is visible by the nose of the car dipping down. Just how much
weight is transferred depends on the weight distribution of the car and
the amount of braking force (and has nothing to do with the suspension).

Because of this, the front wheels require more braking force than the rear
wheels. If there is (hypothetically) 2000 pounds of weight on the front
axle and 1000 on the rear, the front brakes need twice as much braking
force as the rears. On the DMC, under hard braking, the weight probably
ends up about 50/50 front to rear. Keep in mind that with modern high
performance tires and brake pads, with no other upgrades, the DeLorean
should be able to decelerate with nearly 1G of braking force. In terms of
weight transfer, this is equivalent to parking on a 45 degree incline! 
It will transfer a lot of weight to the front wheels.

On top of this, it is extremely bad to have the rear wheels lock before
the front wheels. If the rear wheels lock first the car will most likely
spin. If the car should happen to have a rear weight bias, it would be
guaranteed.

> tried it). On 4 wheel disc however does not seem to be necessary (I've
> done that too, but it *IS* a 5,000 pound Lincoln). Guess it's because
> hydraulic fluid is self adjusting.

The Lincoln probably has almost as much of a front weight bias as the
DeLorean has a rear weight bias. In the case of the Lincoln, much more
work is done by the front brakes. At least, in the original design :}

Have you ever had to make a maximum effort panic stop? If you have
4-wheel ABS, then bypassing the proportioning valve just makes more work
for the ABS system and more pedal pressure for the same amount of braking
(under maximum braking conditions - under normal ones, it wouldn't affect
pedal pressure). If you don't have 4 wheel ABS, and you ever have to
really brake hard, you might get an unpleasant surprise.