Brake Bias Valve

[FFRSpec72]

It seems odd that FFR suggests that the brake bias valve is plumbed into the front brakes and not the rear brakes.

[SixStar]

It's to be able to turn down the front brakes when using the booster. Post booster delete there has been some debate. We put ours on the rear circuit within reach of the driver on our 818R so that we can dial down the rears on course. Rear lockup before front lockup is VERY dangerous.

[matteo92065]

We are building two 818's here and we (5 auto enthusiasts) have been just discussing this the other day. We have made convincing arguments for having the valves on the front brakes, but also made just as convincing arguments to have them on the rear. :rolleyes:

I hope someone will post a good explanation why they should be in front (or rear).

[FFRSpec72]

It's to be able to turn down the front brakes when using the booster. Post booster delete there has been some debate. We put ours on the rear circuit within reach of the driver on our 818R so that we can dial down the rears on course. Rear lockup before front lockup is VERY dangerous.

Agree, it's bothered me why it's not on rear with the R no matter what (and it's a lot easier to plumb with my line going down the center console. I just don't see an argument for the front brake bias on the R

[flynntuna]

Check out these threads

http://thefactoryfiveforum.com/showthread.php?15224-Front-or-rear-brake-bias-adjustment

http://thefactoryfiveforum.com/archive/index.php/t-13543.html

[SixStar]

Go ahead and add spring rates to the mix on this discussion...... The way I see it anyone building a car has a 50/50 chance of getting both brake bias and spring rate correct. GOOD LUCK EVERYONE!

[Wayne Presley]

Put them on the front unless you are running Wilwood pedals. The hydraulic pressure bias on the WRX is set toward the front based on caliper piston size vs MC piston size.

[billjr212]

i'm going to add this gem for anybody wanting to think way too much about this: http://en.wikipedia.org/wiki/Weight_transfer

Based on my *rough* math:
...1.0 g braking (that's 60-0 in around 120 ft if I'm doing my math right, which I don't think I am)
...18 in CoG (wild guess - this is what a BRZ is and only production car lower is a ZR1 based on my searches)
...90 in wheelbase
.......result is 20% load transfer off the back and up to the front
.......At 1,800 pounds assuming an initial 40 F/60 R split, the end point is 60F/40R split of the load (the math works out way too neatly with these assumptions).

Some of the unofficial comments from FFR seemed to be that they just left the front proportioning valve all the way open. The math seems to agree that it would be unnecessary to dial the front brakes back much, particularly if you did a mild upgrade to the rear brakes. Although piston size messes this up and all my assumptions go out the window.

[billjr212]

I edited my comment because Wayne has pointed out that my entire assumption and calc gets thrown out the window because it doesn't factor in caliper piston size vs MC piston size. poop.

[wleehendrick]

Based on my *rough* math:
...1.0 g braking (that's 60-0 in around 120 ft if I'm doing my math right, which I don't think I am)
...18 in CoG (wild guess - this is what a BRZ is and only production car lower is a ZR1 based on my searches)
...90 in wheelbase
.......result is 20% load transfer off the back and up to the front
.......At 1,800 pounds assuming an initial 40 F/60 R split, the end point is 60F/40R split of the load (the math works out way too neatly with these assumptions).
.

But you need to do the same analysis for the donor vehicle... That will result in even more front weight under hard braking in the original application, which is presumably what the stock brakes were biased for, right? So clearly, we don't need to dial the fronts down much, and overdoing it could be dangerous, but physics still justifies some bias adjustment.

[billjr212]

You are absolutely right. A similar rough calc on a 2002 WRX gives around an 80/20 F/R split on 1 g of deceleration. That is with 21" CoG (semi reasonable guess given the BRZ result, and the aluminum doors, hood, roof, and trunk lid the WRX had at the time) and 99.4" wheelbase.

I will wonder aloud though - has anybody confirmed if the WRX brake proportioning was done through the master cylinder or through the ABS system? Assuming through the ABS system only, the master cylinder bore sizing would become irrelevant as would the WRX stats and the only thing that would matter is the intial 60/40 split I mentioned and the diameter of the pistons on the f/r calipers.

Geez - i feel like I have completely derailed, somebody stop me.

[billjr212]

Based on pages 39 and onward of this, I'm thinking more and more that no proportioning valve, or upgrading of the rear brakes plus a rear proportioning valve is really the only way to go. Proportioning valve on the front brakes basically means that at some point in the pedal travel, the front brakes will "max out" and then when you push harder, you will send more braking power to the rears while simultaneously removing dynamic load on the rears, potentially causing a sudden lockup of the rear brakes at an inopportune moment.

http://www.fkm.utm.my/~arahim/daimlerchrysler-gritt.pdf

I'm officially cutting myself off now to do actual work. :-)

[FFRSpec72]

Put them on the front unless you are running Wilwood pedals. The hydraulic pressure bias on the WRX is set toward the front based on caliper piston size vs MC piston size.

The WRX master cyl has equal base pressure for front and rear.

[flynntuna]

Not an expert or engineer, but it seems that if you have the same pressure going to calipers with different bore sizes that the larger bore size has the better clamping force. Right? Or am I off base?

[Wayne Presley]

Not an expert or engineer, but it seems that if you have the same pressure going to calipers with different bore sizes that the larger bore size has the better clamping force. Right? Or am I off base?

You are correct and that was what I was pointing out.
Braking force is based on pedal pad pressure input x mechanical leverage of the pedal x hydraulic leverage of the MC vs caliper x friction coefficient.

[D Clary]

I plumbed mine according to FFR I can easily change it. I have used these valves in the past and it is a finite adjustment at best, so I am not thinking it is going to be a drastic change.

[wleehendrick]

You are correct and that was what I was pointing out.
Braking force is based on pedal pad pressure input x mechanical leverage of the pedal x hydraulic leverage of the MC vs caliper x friction coefficient.

and rotor/tire diameter too, as this determines the leverage from the pad surface to the tire's contact patch... there are some pseudo-big-brake kits that use a bracket to mount stock calipers on larger rotors. This type of mod doesn't change the braking force on the rotor (since the MC and calipers are unchanged), but definitely can screw-up bias, as the longer moment arm away from the hub creates more torque, and therefore more force at the tire's contact patch.