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~ Dear SOL'ers
~
~ I've been thinking about designing a progressive brake system for my
~ car, and I would like some other opinions. It's my street car, but with
~ wide tyres and near race spec brakes, it stops well. The intent is to
~ produce a near even brake split at light pedal application, and a large
~ front bias at heavy pedal application.
Ah -- I see. I was all set to say "I've got a progressive brake system;
if you push the pedal easy, the car stops slowly, and if you stomp on it
hard, the car stops quickly." But you mean a progressive BIAS...
~ I'd connect the master cylinder pushrod to an adjustable lever arm actuating
~ a conventional pressure reducing valve. System variables would be: Initial
~ and final brake bias settings, and Rate of bias increase.
~ My question is, is there a proportioning valve that is actuated by a lever
~ arm, and would it be able to handle the cyclic bias changing I'm proposing?
Standard prop valves do have a lever, but it's designed to be manipulated
by the driver to compensate for the tires going away or for weight shifting
as fuel load lightens; I don't know anyone who frobs the proportioning
while simultaneously trying to brake at ten-tenths. I personally would
like to be half a lap away from you for the first few races you run with
this setup, assuming that the Stewards Of the Meet don't just call in a
tac-nuke strike from orbit when they find out what you're doing.
The Proper British method to do this would be to run an auxiliary
hydraulic line that effectively sensed the pedal position, and
used this to move a slave cylinder that adjusted the handle on
the bias valve. I suppose some outfits might have tried to use
a solenoid, but they don't leak. A moral degenerate might
simply use a throttle cable connected to the brake line and the
bias valve, with a REALLY BIG return spring and some hard stops
to keep the travel within limits, but that's for the kind of people
who stop the car to put up the top when it starts raining. Not good
form, old bean, simply not good form. And of course the Carpal
Tunnel Brigade would hook up a potentiometer to an ADC, send
digital values (at a higher resolution than the pot reads) down the
wire to a modified Winchester read-write head controller that maps
pedal position to the default tracks on the disk drive's BIOS map in
such a way that the back-and-forth motion of the r-w head moves
the bias lever to the desired setting. The rest, assuming components
you can pick up at Fry's for probably less than $10 (plus the
requisite brain damage from breathing lead solder fumes), is a simple
matter of programming. Besides, BIOS and bias lend themselves to
some truly wicked puns in the source code.
~ Am I completely off my rocker? Should I just retrofit ABS? Has anyone
~ else thought about this concept?
Let's ignore whether or not it's necessary, let alone beneficial, for
a bit. Life may not be simpler that way, but it's more enjoyable.
The best way to get fine control would be to fit a dual-master system,
such as many Tilton competition brake setups, with a pressure limiter
on each, set so that the rear system reaches maximum pressure (that is,
stops pushing the pads) at a lower pedal setup than the front. If you
are of a particularly meticulous mind, you could hook it up as follows:
#1 master to FRONT brakes ONLY
#2 master to proportioning valve that varies pressure to FRONT and REAR
The advantage there is that you'd still end up with progressively more
pressure to the front, but you could use the prop valve to vary the
amount of front-rear split on the #2 master to get the right feel at
the rear. Since this is street-driven, you probably wouldn't need
to worry about dynamically changing to compensate for tire wear and
fuel load, but the adjustability would mean you could dial for the
feel you really like. Or of course it might just give you something
to fiddle with while you drive, or an excuse to reach over and goose
your passenger's thigh if you like. :-)
To do this, you'll need a four-piston front brake caliper. You can
find these in junkyards, pulling them off a Volvo 140 or newer. Yes,
there are two lines going to each front caliper on 140-series Bricks.
This also has an articulated brake pad, BTW, so that each of the
four pistons has its own pressure characteristics. I'll let you know
what it's like to work on after I bleed the system on my 144E...
Of course, if you want to change the rate with pedal travel (which may
or may not be the same as pedal PRESSURE), you could rig up a non-linear
bell crank on one or the other master cylinders. In theory, you could
even set it up so that the #1 had a curve with a negative exponent
and #2 had a positive exponent causing #1 to start slow and then gain
pressure faster, while #2 started with high response and then fell off
after the midpoint. Imagining how that would feel to drive makes me
sympathize with Sir Isaac Newton when he contemplated the three-body
problem...
The four-piston caliper has the advantage that it might actually
improve your braking. :-) Calculating the pressure/volume required
and choosing the appropriate size master cylinder to get the pedal
feel and braking you're after is left as an exercise for the user.
--Scott "Hey, who's this little guy standing on my shoulders" Fisher
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