Brake Bias/ brake cylinder ratio

[andy robinson]

car : 1450 kg front engine rwd ford falcon.
Car has adjustable brake bias & even with bias to full front it still locks rear wheels first & yes ive checked bias is going the correct way & also really wears the rear pads quickly

Brakes cylinder's have 21 stamped on front & 19 on rear, im guessing this is cylinder diameter in mm & so with the same length of stroke a larger cylinder diameter will flow more fluid than are smaller one & hence the larger on the front.
However somebody has told me the smaller cylinder should feed the front as it more effieicent


??


cheers andy

[Tim Falce]

The somebody was correct, with the smaller one you will need a longer stroke to move the fluid but it gives you more pressure that's why you are locking and wearing the rears with the set-up you have.
I'm sure someone such as Phoenix will be along soon to give you a better explanation and more precise figures

[andy robinson]

cheers Tim thats wanted I wanted to hear, no wonder ive struggled on the brakes since I bought this car 2 yr ago, not the sort of thing you expect to be looking for, this has cost me numerous race wins over the last 2 seasons with various spins whilst trying to stop the thing

[phoenix]

I can't really be more precise than Tim without a lot more information, but as plumbed (if you are correct about the meaning of the '21' and the '19') you have higher pressure going to the rear brakes than the front, which is most likely the cause of your problem(s).

Switch them round and things should improve - it certainly won't make things worse. Might be worth comparing setup with another Falcon owner though?

[Tim Falce]

If 19 and 21 are the sizes in mm they seem quite large, does it have a servo?

[andy robinson]

no servo, brakes are really heavy, like everthing else on the car

[Tim Falce]

Originally Posted by andy robinson

no servo, brakes are really heavy, like everthing else on the car

Swap them round and try it, it will be better. You may be able to make the pedal feel lighter by reducing the master cylinder size (or changing the pedal ratio) but this will of course depend on wheel piston and or cylinder size.

[phoenix]

Caliper piston size is of course important (and it's impossible to calculate the overall brake effort without knowing piston size and brake pedal ratio) but knowing no other details, swapping the master cylinders front/rear will reduce pedal effort by at least 18% of current effort (19^2 / 21^2).

Trying to stop a car on what is effectively rear brakes only takes a LOT of pedal effort!

[GORDON STREETER]

Andy, just think of the ratios on master to slave cylinders as gear ratios. Small to big = more powe (less movement)r. Big to small = less power (more movement)

[phoenix]

OK, I've done a bit of digging.......

I have had to make a couple of significant guesstimates, but the ideal brake ratio for a Falcon weighing 1450kg with a 60% front, 40% rear weight distribution should be in the order of 68% front and 32% rear on road tyres and at standard ride height.

What you have at the moment is probably in the order of 46.5% front 53.5% rear brake distribution. NOTE: this does depend heavily on my guess as to the efficiency of the existing rear brakes (are they drums or discs?) To achieve a balance with the master cylinders as they are, you would need in the order of 1.42 times the current front brake line pressure and 0.58 times the current rear brake pressure. This would require the balance bar to be adjusted 71% front and 29% rear - which is probably outside the adjustment range of the bar - and lots of leg effort.

Swapping master cylinders will swap this F/R ratio to 53.5% front and 46.5% rear. The efficiency of the rear brakes with respect to the fronts is, once again, very important in this guesstimate. Even with the swap, my guess is that you could still expect the rears to lock first under heavy braking with the bias bar set in the middle. HOWEVER the rear brakes may be less efficient than I am assuming.

If all my assumptions are correct, by adjusting the balance bar you should be able to get the fronts to lock just before the rears. If you can't, a smaller front master cylinder would be the best option, as this would reduce leg effort some more, though at the expense of more pedal stroke.

[andy robinson]

cheers everyone
phoenix, I should have gave you some more infor on car, it not a 60's yank falcon, but a 1999 auz V8 supercar falcon, 300/680 R18 slicks allround, 5.5 degrees camber on front & 2.25 on rear, spool diff can use a fair bit of rear brake as obviously you cannot lock just one rear wheel
Guessing I think 380 ish 6 pots harrops on front & 350 ish 4 pot ap's on rear

[andy robinson]

I think these brake cylinder's are the correct sizes for this car & sometime in the past, somebodys just mixed them up.
Car still brakes quite well, but in the past what make up on the straights with big power I loose more on brakes against the like's of Joss Ronchetti's sunbeam which I've been bumper to bumper with all season

[phoenix]

Originally Posted by andy robinson

spool diff can use a fair bit of rear brake as obviously you cannot lock just one rear wheel

Maybe, but if your braking is rear-biased it will tend to get unstable under braking. More importantly, you are throwing away the grip available from the front tyres for much, much harder braking and therefore you are losing out on shorter braking distances.

[andy robinson]

sorry didnt mean it can carry lots of rear brake just a bit more than a similar car without a spool, going to swap cylinders around tomorrow
.............and wait..............and wait until testing starts next season

might have a quick trip down the road with trade plates on though

cheers again for your inputs