Correct brake bias for hard street driving? Options

[Bob Wolcott]

What is the correct brake bias (%F and R) for a 914-6 for street/ autocross use? With one master cylinder (and my current brake setup, not stock) I am at about 60/40 which seems like too much in the rear. I was told that the CG height for a stock 914-6 was 16" which would make the correct bias around 80/20. Does this sound about right? I am considering designing a dual master cylinder setup for tyhis if I am too far off.

Thanks,
Bob

[Joe Bob]

Big brakes in front... (IMG:http://www.914world.com/bbs2/html/emoticons/cool_shades.gif)

[Bob Wolcott]

I currently have Brembo calipers up front with two pairs of pistons (1.33" and 1.25"). With the stock rear caliper I believe there will be too much rear bias. The rear caliper's piston is 1.491" in diameter and given those dimensions and one master cylinder, the bias will be roughly 60/40. Anyone know what the stock front caliper piston diameter is for a 914-6?

Thanks,
Bob

[ArtechnikA]

QUOTE

With one master cylinder (and my current brake setup, not stock) I am at about 60/40 which seems like too much in the rear.

Brake Info Site

the factory proportioning is pretty conservatively front-biased, because they were concerned somebody would fill the tank, put a load of anvils in the front trunk, leave the back empty, and spin in a panic stop. and they added the proportioner valve, one of the first road cars i know of to have done this. (do you still have your proportioner valve ?)

partly it will depend on your style - if you can brake smoothly you'll have less transfer and can use more rear bias. if your car is lowered, you'll have less transfer, and can use more rear bias. same thing in the rain...

some adjustability is good, especially to account for those asymmetrical trunk loading and rain things. dual MC's is definitely the ultimate approach but it may be a lot of work for not much payoff - your CG doesn't change that much over the course of an AX like it does with the endurance cars with big, ever-changing fuel loads.

setting up a system with a pronounced bias in one direction and using a GOOD hydraulic proportioner (not like the factory antique, there are good modern ones...) so you can dial some of that bias back out may be the hot setup. but it's your car, do what you want...

[lapuwali]

QUOTE (Bob Wolcott @ Jun 17 2005, 11:25 PM)

I currently have Brembo calipers up front with two pairs of pistons (1.33" and 1.25"). With the stock rear caliper I believe there will be too much rear bias. The rear caliper's piston is 1.491" in diameter and given those dimensions and one master cylinder, the bias will be roughly 60/40. Anyone know what the stock front caliper piston diameter is for a 914-6? Thanks, Bob

Lessee:

5.23 sq.in for each front caliper
3.49 sq.in for each rear caliper

This gives you a front to rear ratio of 1.5:1, or 50% more brake in front than in back.

The stock front 914 calipers had 42mm pistons, giving 4.29 sq. in of area for each caliper, or a front/rear ratio of 1.2:1 (but that's with a prop valve, and I have no idea what the limiting is there).

So, you have much more front bias with your setup than stock.