Brake balance

Vince's car at Willow Springs on Friday 12/2/2011.

It has a 7/8" master cylinder for the front brakes, 3/4" cylinder for the rear. No proportioning valve. When we set the balance bar all the way in the direction favoring front brake brake bias, the car still locks up the rear brakes when testing at low speed (10-15 MPH) in the paddock area. I'm concerned that changing the rear cylinder to a smaller, or front cylinder to a larger may result in the balance bar contacting the outer tubular housing of the balancer assembly. It is possible that the cure for the compromise is to install an adjustable proportioning valve in the line going to the rear.

I found this Tech Tip cheat sheet online from Wilwood. It offers a few band aids to adjust around concerns, but it doesn't really address the problem.

http://www.wilwood.com/TechTip/TechPedalTip.aspx

Here are a few graphics of the assembly.



Attachment 288

Attachment 289

Attachment 290

An old acquaintance, Steve Ruiz wrote this up and it's worth the read for those who need help and direction.

http://dev.stoptech.com/docs/media-c...ster-guide.pdf

I would contact Todd over at TCE in Tempe, AZ. He's good, but so is Jack Hadley at M&M.

Todd Cook and I worked together at Bondurant many, many years ago.

I'm kind of sort of thinking that if we can identify exactly what parts that Vince is using, things will become apparent. We still need to know his front caliper piston diameter. Wow this new solar powered keyboard from Logitech is neat. It even works in the dark!!!

Have we figured out which brembo is in use in the front yet? What about at the rear? Cobra? GT? SVO? What flavor are the pads?

MM should be able to provide the piston diameters. Iirc the rear calipers are stock SVO.

The front calipers are 2000 Cobra R Brembo calipers and have 36mm and 40mm pistons, according to Vince.

Response from MM. Please note that the F/R bias target according to MM is ~61% to 66% front biased.
======================
Mr Rinner,

1) Currently your car has 50% too much rear brake bias. That is a lot. Even with
your bias bar adjusted all of the way to the front,
it probably still has 25% too much rear bias which limits its deceleration to
about 1.3g. When your rear tires lock, the brake pedal
force should be about 133lbs.

2) If you swap your m/cs so that the 0.75" unit is in the front and the 0.875"
unit is in the rear, things are fairly close. To get
the brakes balanced you would still need to adjust the balance bar about 20%
towards the front brakes. This would allow the car to
stop at around 1.5g. The brake pedal force should be about 140lbs.

3) If you put the 0.75" unit in the front and install a 1.0" unit in the rear:
To get the brakes balanced you would still need to
adjust the balance bar about 5% towards the front brakes. This would allow the
car to stop at around 1.5g. The brake pedal force
should be about 155lbs.

The absolute accuracy of the brake pedal force calculations isn't that good as
they are a function of the available tire grip and
the brake pad coefficient of friction. The same is true of the deceleration
level as it is a function of the tire grip. However, the
relative differences between the three situations will be very close. If you go
from situation #1 to #3, the brake pedal force to
create brake lockup will be 16% higher (1-(155/133)). The same goes for the
deceleration calculations.

If the required brake pedal force was marginal or too high in situation #1, both
m/cs could be made smaller to drop the overall
effort and increase the travel. The Wilwood m/cs are a bit of a problem as they
don't come in every 1/16" increment, so you can't
always get the exact size you need. A 0.625" front, 0.875" rear balances
perfectly, but the overall effort drops to 115lbs, so the
resulting travel will be 15% greater than situation #1 which I think will be too
much. It is always better to make the pedal a
little on the stiff side.

When he gets it re-plumbed properly, one method to do an initial set up on the balance bar is to do the following:

Car on jackstands

Takes two people, one in the seat to push on the pedal, one to grab the tires.
Tire grabber at front, turning the tire, guy in the seat pushes on pedal until front tire can't be turned.
Guy on the pedal holds it steady, while tire grabber runs to the back and tries to see if he can turn the rears.
Adjust the bar until the fronts stay, and the rears can just barely be turned.

When on the track, in a nice steady state turn, touch the brakes, if the balance bar is set properly it should not change the attitude of the car.