To discuss your question about vertically and diagonally split vehicles (front
-rear vs. LF&RR - RF&LR),
the best scenario is diagonal. This means that if one circuit of your m/c
fails, you get half of your braking.
In a vertically split system, if your primary (front) circuit fails, you only
get your rear brakes, which as we've
discussed, don't do as much work. There are problems with using diagonal
splits on certain vehicles though.
Vehicles with possible scrub radii will actually pull sharply if you brake a
front and rear diagonal and could
yank you off the road. So large scrub radii vehicles like trucks tend to still
be vertically split.
Historically, I've seen some diagonal vehicles use two prop valves, and others
get very complex valves. Mostly
though, prop valves are a thing of the past. Just about every vehicle made
today has ABS (and by 2012 every
vehicle will, since the government is mandating it). ABS systems can do what
is called DRP, or dynamic
rear proportioning (some window stickers will list this as EBD, electronic
brake distribution). DRP uses
the ABS's wheel information to dynamically adjust the braking to the rear to
always give optimal braking
regardless of the loading or configuration.
Without causing too much more confusion, vehicles which utilize drum and disc
brakes (like the Bricklin),
will actually utilize a combination valve instead of just a prop valve. The
combination valve is a prop for the
rear, and a metering valve for the fronts. Since the inherent design of a disc
brake system makes them activate
much quicker (pads are always seated against the rotor) than drum brakes, a
metering valve is used to "slow"
down the initial pressure so the disc and drum brakes activate together.
Hope this helped.
Seth
#1544
ABS, Traction Control, Stability Control, and Hybrid Brake Systems
Development Engineer
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