The temperature approach will also be subject to variation given the
differential heating and cooling aspects of vented rotors compared to solids,
as well as ducting or airstream variables, pad compounds, etc. The
proportioning valve is a very handy tool if you are starting with a balance
situation that is rear biased, since proportioning valves (unlike the bias
adjusters on master cylinders) are usually pressure-limiting devices, simply
reducing the rear pressure and effort. Like the knobs for cockpit adjustable
cylinders, the nice part about a proportioning valve within the drivers reach
is the ability to make changes on the fly.
You may find that you have to fiddle with the relative master cylinder sizes
(or wheel cylinders if you have rear drums) to get to a rear-heavy staring
point for pressure-limiting type systems. Many production-based cars have
pressure-limiting or pressure-delaying devices installed that can be removed
to help get to the desired staring point for a racecar.
Of course, if you are a high roller like Joe Alexander you would have a bias
bar on the cylinders and then fine tune it all with the proportioning valve in
the cockpit. Ultimately though, as Kas mentioned recently, you want to be
able stand on the brakes real hard, but then get off of 'em a bit sooner and
let that sticky rubber slow you down as you turn in. You work hard to make
the brakes work well, and then you work hard to learn how to not slow down so
much.
Dave
David Talbott, President
Architectural Reproductions, Inc.
Portland, Oregon
Toll Free (888) 440-8007
www.archrepro.com
----- Original Message -----
From: Mike
To: fot@autox.team.net
Sent: Wednesday, April 04, 2007 10:28 AM
Subject: [Fot] brake bias / balance
FOT;
I deliberately didn't say what car 'cause I didn't want to unfavorably
bias the responses <G>. It's a 67 AH 3000 to which we've fitted a brake
conversion (4-pot calipers, vented rotors) on the front and discs on the
rear. The question isn't so much about this car, but rather a generic
question about how to establish what the balance is on any car. The
specific car becomes an issue when trying to determine how to change the
balance.
David Talbott wrote:
>
> I've seen people use gauges to define the pressures, and they are
probably very useful to be in the ballpark when you roll off of the
trailer.
Pressure measurements would be a starting point, but pad material ie
friction coefficient, unless the same front and rear, would negate
anything you learn by measuring pressures. The seat-of-the-pants method
seems to be the consensus. We were looking for something
measurable/repeatable.
Tony Drews wrote:
> Since part of the setup depends on how much friction your particular
front and rear pads / shoes have, I don't think there's way other than
seat-of-the-pants.
We have installed softer pads in the rear (semi-metallic in the
front) for exactly that reason; to give the rears more bite. We're also
very aware of the difference tires make, since that's where all the
actual work occurs. An interesting note is that stickier tires push the
bias forward due to increased weight transfer under braking. What we're
looking for is a measurable, ie repeatable way of determining where the
balance is now and when we get it right. So far all the response
advocate seat-of-the-pants adjustments.
Doing seat-of-the-pants in a race car would be lot easier than
pushing a car on the street at the 7/10 or more needed to show a
problem. On this particular car, in a straight line the fronts lock
first. Does that mean we've got 51% front and 49% rear or 90% front and
10% rear? By measuring the temps we surmise that it's more like 60% or
70% front. We'll install a proportioning valve and measure some more
temps and do lots more driving and adjusting.
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