Brake maths check...

[HRG]

GreenV8S said:

OJ said:

GreenV8S said:

OJ said:

Oh and to add, if you're that worried about it, just fit a brake bias valve. Job's a goodun.

Sorry to contradict again but no, no it isn't. You need to set the static and dynamic brake balance and a bias valve will not do both.

Can you clarify that for me?

The theoretically ideal brake balance requires a curved relationship between front and rear braking, corresponding to the weight transfer that occurs under braking. The goal is to produce a brake system that is reasonable close to that ideal curve under all conditions from light to very hard braking.

At low levels of braking the weight transfer is negligible and the balance is determined purely by the hydralic/mechanical leverages through the system. This is termed the static brake balance. You can tweak this by changing caliper sizes, moving calipers towards/away from the hub, with twin m/c setups you can alter the m/c sizes or use a balance bar to vary the mechanical relationship between the two m/cs.

As the amount of braking is increased the linear relationship defined by the leverages needs to be modified to produce a curve. Usually this is done very crudely by putting a regulator valve in the rear circuit that allows the rear brakes to see full line pressure up to a certain limit (1:1) and then restricts the rear flow above that limit (1:3 is typical). If you plot the front versus rear line pressures you get a line with a kink in it. As long as this kinked line stays fairly close to the theoretically ideal curve, the brakes will be more or less balanced OK. To achieve that you need to set the mechanical leverages up to give the right distribution under light braking, and then use a bias valve to introduce the 3:1 kink at the right point.

Edited by GreenV8S on Thursday 9th October 14:49

I believe you are very familiar with the AxB/C theory as it came from The Bible

Rear caliper is the same, I'm just relocating it further from the centreline of the hub.

According to my (possibly very suspect) maths

The original ratio when it left the factory was 949.459 to 1999 or approx 2.1054:1 front bias
I believe my current ratio with four pots is 949.459 to 2654 or 2.79527:1 front bias

If I upgrade the rear it will give a 2.559:1 front bias.

It's a Griff, so I don't really want to restore the original bias...

[Mr POD]

I have a contact at Lucas Car Braking Girling Fen End Braking Development section.

No wait. That was 15 years ago and he was ready for retirement then, and Fen End was sold to Pro Drive.

He lives in Barnt Green named Dave Walepole. He was a regular in the pub.

Nice Chap. Had a Beard

[OJ]

HRG said:

I believe my current ratio with four pots is 949.459 to 2654 or 2.79527:1 front bias

If I upgrade the rear it will give a 2.559:1 front bias.

As many an engineer has said... Close enough

[GreenV8S]

OJ said:

So basically, I should have stopped at the end of my first post

But surely at light pressures its only really a road driving brake and pedal feel issue though, rather than one of actual balance, and that the relatively light forces involved you'd have to make some pretty big changes to make an appreciable difference?

I would have thought for safety purposes only the top end of the pressure spectrum really matters? I suppose you could argue that if you really stamp on the brakes then you'd be more likely to lock the front wheels if the static bias was towards the front

Not so. When you're hooning round a damp roundabout and need to hit the brakes you could be right on the limit of grip and still only braking gently. The brake balance needs to be about right under all conditions from very light to very heavy braking. If there are any points where the balance is significantly out then under those conditions the car will be out of balance.