brake bias

[T77911S]

as you know I have been having brake issues in the race car. late model circle jerk car.

last time on track had bad issue of rear end very very lose on turn one under braking. (too much rear bias).
I have a NEAL dual MC brake pedal but I don't quite get the bias adjustment on it.
looking at the tilton pedal on line it says the center bearing moves changing the leverage to the 2 MC's. THAT I do understand, but my center pivot, or bearing does not move. maybe its stuck. I tried banging on the end of the rod but nothing.
anyway, on to my real question.

the tilton video mentioned the SMALLER MC for the front brakes. I had mine connected with the bigger MC to the fronts as I am 99% sure that is how it was when I replaced the 2 MC's. BTW, one is 1 inch and the other is 7/8.
so I got to reading about brakes, again.
just to make sure I understand.

the smaller dia MC provides more FORCE so it goes to the fronts.

[sc_rufctr]

I would have thought a larger diameter MC moves more fluid so it would have more force but thinking about it, a smaller one has more mechanical advantage... So even though a smaller one moves less fluid it has a higher pressure for a given pedal force. (I hope that makes sense)

Anyways... On a lightened race car, would it be better to have almost equal force front to rear with a slight front bias?
Considering weight shift when braking.

[wdfifteen]

Given the same pressure on both pistons the smaller one will experience greater psi.

[vonsmog]

What you need is one of these to adjust the pressure, front to rear. I got this to use on the Unimogs. Used it once, now it just collects dust.

[John Rogers]

Maybe this will help: https://www.joesracing.com/master-cylinder-math/

[Zeke]

When you use a bigger MC you have less pedal travel. Maybe that's another way to understand it. The difference between 7/8 and 1 inch is significant.

Braking is but one of the driving techniques, throttle and steering being 2 more. How are you doing with respect to tossing the car into the slide and throttle steering?

[T77911S]

Originally Posted by wdfifteen

Given the same pressure on both pistons the smaller one will experience greater psi.

that is the way I understand it.
the larger MC moves more fluid but at less pressure.

I have different calipers front to rear. I have dual piston HOWE calipers up front and single piston (but very large at 3+ inches) in the rear.
but they both use the same size pad.

here is a site I read.

https://enderw88.wordpress.com/automotive-theory/brake-system-theory/

pressure= Force on MC/ area of MC.

so if area gets bigger, pressure goes down.


I swapped lines going to the MC's so the smaller is now on the front.

[smadsen]

I'd try the smaller diameter M/C on the front, larger on the rear. Put whatever adjuster you're using in the center of it's travel. Go drive the car at operating temps and see which end locks first. Then adjust accordingly until the fronts lock momentarily before the rear. At least that's what we did on symmetrically set up road racing cars.

Maybe on a dirt oval car it's advantageous to be able to turn the car with a locking rear brake. That makes panic stops an adventure however.

To adjust in the garage we'd lift the car, have someone modulate the brake, and then give the wheel a bear hug until the fronts would just be locked, then go to the rear and the rear should drag, but still turn slightly. That would indicate more bias to the front.

Then paint/chalk vertical marks on the tire sidewall, put the car on the track and have an assistant watch what locks first and adjust accordingly.

[spuggy]

Originally Posted by T77911S

the tilton video mentioned the SMALLER MC for the front brakes. I had mine connected with the bigger MC to the fronts as I am 99% sure that is how it was when I replaced the 2 MC's. BTW, one is 1 inch and the other is 7/8.
so I got to reading about brakes, again.
just to make sure I understand.

the smaller dia MC provides more FORCE so it goes to the fronts.

Hard to generalize whether you need the larger M/C on the front or rear circuit, because it depends on both caliper piston area(s) and rotor diameter(s) front & rear - and what target bias is "right" for the car and how it'll be used.

The piston area of the smaller diameter M/C generates higher line pressures for a given amount of movement - but moves less fluid. More/larger caliper pistons have more area, thus need more fluid moved. Larger diameter rotors make more torque. The ratio between the master cylinder area(s) and the caliper pistons determine how much pedal movement results - and the size of the master cylinder(s) determine how much force is needed to generate the required line pressure to stop.

See, it's easy, right?

Most conventional setups would use a smaller M/C for the front. 911's pre 3.2 had caliper piston sizes that provided a natural balance front/rear - so used tandem M/Cs with same size for both circuits, and would want equal-sized M/C in a dual-master setup.

Some very specific scenarios want the exact opposite to "normal" - like fitting 996 brakes to a G-body and correcting for the resulting excessive front-bias. Then you might use 7/10" front 5/8" rear for the N/A variants, or 3/4" front 5/8" rear for the TTs. YMMV.

1" M/C is pretty big; for example, would result in little pedal movement/high pedal effort on 911's, unless using 6 pot Cayenne/GT3 calipers or something. SCRS used 930 brakes and a pair of 5/8" M/Cs.

On a race car which could have literally almost anything fitted? Probably should do the math/research what bias ratio etc is the ballpark.

But just based on the rears locking up and how "most" conventional setups would use smaller M/C on the fronts, it does sound like you might want them hooked up the other way round...