Help Determining MC sizing for dual brake setup. (long)

[Jim Smolka]

Problem,

Dual MC setup is spongy, has too much throw, and the rear brakes lock up. Here is a picture of the setup



The brakes have been bled with pressure system and buddy system.

If the brakes are pumped up, then the pedal is OK. But let it sit, then about 1/2 travel to get them to start to engage.

One problem is the MC are below the calipers. This can be solved with 2 LBS line pressure retainers.

Next is the biasing. Some say a 1.6 to one is best. Here is the math of what is used.

Pedal Effort (Lbs) 100
Brake Pedal Ratio 7.00

[MC Bore Diameter (Inch)] ["MC Bore Area (In^2)"] [MC O/P Pressure (psi)] )
0.625 0.307 2282
0.700 0.385 1819
0.750 0.442 1584

Since the 4 rear piston sizes are 1.375 "
and the fronts are 2", the area on one side of the calipers are 1.485 inch^2 and 3.142 inch^2 respectively

For the existing system, this is the math to determine the bias

With 100 Lbs of brake pedal pressue and then assuming a 7 to one ratio and a rear piston of size of 1.375" and a front piston size of 2" the pressure is:

Rear Pressure (Lbs) 3388
Front Pressure (Lbs) 5714
Front to Rear Bias 1.687


With 100 Lbs of Brake Pressure and assuming a 7 to one ratio with the same pistons and a 0.75" diameter MC's for both front and rear the pressure are:

Rear Pressure (Lbs) 2353
Front Pressure (Lbs) 4978
Front to Rear Bias 2.116

The third option is with a 0.7" MC for the rear and a 0.75" MC for the rear. The math works out to be

Rear Pressure (Lbs) 2701
Front Pressure (Lbs) 4978
Front to Rear Bias 1.843

Assuming the math is correct (email me for the spreadsheet), what MC size would one use?

[Bill Verburg]

4x 2" pistons in each front caliper?
4x 1.375" pistons in the rear?

= radius rotors f/r?

[Jim Smolka]

Bill,

Thanks,

Yes, 4 x 2" for the front, 4 x 1.375" for the rear


Rotor diameters, I will measure tonight, but they are big.

Jim

[Bill Verburg]

2 observations
those are awfully big front pistons
the ratio between f/r is unusually large

[Bill Verburg]

Probably something like 7/8 front 3/4 rear. If non boosted you would need a healthy leg.

[Jim Smolka]

Bill,

Yes, the brakes are not boosted.

You suggest 7/8 & 3/4" MC's. How does one determine MC size? The math part is easy to determine pressures and ratios, but after the math, what criteria is used?

FWIW
With 100 Lbs of Brake Pressure and assuming a 7 to one ratio with the same pistons and a 7/8" diameter MC for the front and 0.75" MC rear the pressure are:
Rear Pressure (Lbs) 2353
Front Pressure (Lbs) 3657
Front to Rear Bias 1.554

The 7/8" and 3/4" ratio would decrease and thereby increasing the likely hood that the rear brakes would lock up?

[rcwaldo]

When using a balance bar assembly, much like the one pictured, proper setup is rather important.
-ensure brake pedal height is adjusted such that pressure builds before the pedal passes vertical of the pivot point. If pressure builds after that, you'll lose the mechanical advantage of the action by going over center
-adjust the balance bar so that its square to the pedal arm (90degrees when looking down on it) at the point pressure builds--some bias or angularity is normal when at rest
-ensure the m/c pushrods are parallel to each other
-if pressure is lost on one braking circuit, check that the bar will bind such that you can still build pressure with the -good- circuit--no need to lose the function of both circuits
-when adjusting dynamic brake bias, only adjust the balance bar 1/2 turn at a time

Typically, the bias bar clevis width is adjusted such that there is minimal lateral plunge. In efforts to maintain proper m/c pushrod geometry, or concentric to the m/c bore, the pedal tube that houses the bias bar spherical bearing may need to be widened, based on the picture above.

[Jim Smolka]

Chris,

Thanks for the balance bar notes. Obviously, the picture was taken before the dual MC setup was installed. And yes, the MC piston rods are not aligned since they were not mounted to pedal assembly.

[Bill Verburg]

Jim, assuming that this is for your 914. The pedal ratio and chasis setup is similar to a 911 so sizing will be similer.

When the caliper slave cylider is increased(keeping the master constant) it moves bias to the front. Keeping the slave constant but incresing m/c size has the opposite effect.

again assuming equal diameter rotors f/r, and the caliper piston s mentioned you are front biased ~2:1, on a 911 you would want to reduce this to ~ 1.6:1. I'm unsure as to what you might need on a 914.

To reduce the front bias you will want a sligthly larger front m/c.

Pedal feel is the combination of many factors, some like mount stiffness are out of your control, the main thing you can control is slave to master ratio.


Bigger m/c reduce total hyd. ratio gving less brake torque for a given amount of leg. Unbosted ratio of ~32:1 is close to the limit and I would certainly want a boost at that level