I've been trying to get a catch to write a brake system primer for the
magazine
and just can't seem to find the time.
The short answer is that a Dennis and Stephen are correct, prop valves
decrease
the amount of pressure to the rear. And yes, the short answer is that the
rears don't
do as much of the braking.
The longer answer is that the front and rear actually do similar amounts of
work
at low decelerations ( low braking forces) because most vehicles are around a
50/50 weight distribution (I'll touch on things like pick ups in a second).
It's the load
transfer that occurs at high deceleration which causes the fronts to be
capable of doing
more braking (more weight over the the front axle). Without a proportioning
valve, you'd
end up locking up your rear brakes before your front brakes. Production prop
valves typically
have a "low decel" and "high decel" gain. As long as the brake pressure from
the master
cylinder is less than a certain amount (typically around 400-650psi), you'll
get the same
pressure to the front and the rear.As soon as you apply more pressure than
this threshold,
the prop changes the slope, and you get a ratio of the pressure to the rear
(typically
between 0.6-0.8)
When you talk about trucks, they tend to have different problems. Due to the
wide range
of weights that can be seen on a pick up truck, they tend to employ what is
called an LSPV,
or Load Sensing Prop Valve. These have an arm which moves depending on the
load, and will
increase the kneepoint I described above with increasing rear load. This keeps
the rears from
locking early when the pickup is empty, yet allows more braking from the rear
when the pickup
bed is full.
to be continued
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