Seth,
I'm putting Cadillac disk brakes in the rear and keeping the Kelsey brakes
in front. What kind of proportioning valve would you recommend?
George
-----Original Message-----
From: owner-bricklin@autox.team.net [mailto:owner-bricklin@autox.team.net]
On Behalf Of Seth
Sent: Wednesday, April 07, 2004 7:09 PM
To: bricklin@autox.team.net
Subject: Re: AMC parts; proportioning valve, etc.
You need to be careful changing prop/combo valves. Since the brick has
disc/drum, you need a combo valve (prop and meter). The metering slows down
the inital apply rate of the disc brakes since inherently disc brakes apply
faster than drums do to the engagement distance drums require. Getting the
metering right is important, but not as critical as getting the
proportioning right. The prop valve has two distinct parameters when sizing
them, the slope and knee point. The knee point is the point at which the
ratio goes from 1:1 to the slope ratio. The brake configuration (caliper
size, rotor effective diameter, lining material, etc) and vehicle weight
distribution will determine your Z critical (the vehicle deceleration at
which your vehicle goes from front biased to rear biased). Typically, a
brake system is designed to be inherently rear biased (for parking brake
effectiveness and failed systems and such) so a prop valve is used to move
the bias to as close to neutral as poss!
ible for all loadings while remaining slightly front biased. So, trying to
cut to the chase, changing your prop to an aftermarket can cause you to be
rear biased which caused worse brake performance and can cause the vehicle
to oversteer since the rear wheels will lock before the front ones. An
aftermarket prop can also cause you to be front biased which is safer
because it doesn't induce oversteer, but it still decreases overall
potential braking. That's why the prop for the Brick is unique. As a side
note, most new vehicles don't have a prop valve because they have ABS. ABS
can electronically control the rear pressure and does a better job, giving a
neutral bias for all loading conditions.
Seth
ABS/TC/Stability Control Vehicle Development Engineer
#1544
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