Chapter 13

What I figured out regarding a common brake setup when contemplating disks….
I wanted to use the “Anson” style swinging pedal setup sold here in Minnesota by a local fabricator. It uses a somewhat unusual master cylinder from a 61′ Chevy C20 truck. This master is actually two masters side by side in the same casting, sharing a common reservoir. The valving needs to be swapped side to side, as the C20 used the driver’s side master for the brake in the original configuration, opposite to what my pedal set-up dictates. This master was originally used on cars with only drum brakes. Its appeal to me is the clean nature of the install, and the opportunity to use a hydraulic clutch.

The pedal ratio is 6:1
Cylinder bores are 1.125″
Master cylinder volume is 8 ounces
Master cylinder throw is 1.75″
Pedal assembly throw is 1.625″
Typical foot pressure is between 100 and 175#
Rule of thumb is that to be safe in a hard stop, you need 1000 to 1200 psi from the pedal/master setup, at least for disk brakes.

To calculate your effective master cylinder pressure, first find the area of the MC piston

((1.125″)/2)x((1.125″)/2)x(3.14)= 0.9935 square inches

Then multiply the pedal pressure (150 lbs say) x pedal ratio

(150 lbs)x(6)= 900 lbs

Divide the effective pedal pressure by the area of the piston

(900)/(0.9935)= 906 psi

175 lbs of foot pressure would give 1057 psi

2” longer pedals (pedal ratio 7:1) would give 1057psi for 150 lbs foot pressure and 1233 psi for 175 lbs of foot pressure.
The MC volume must be adequate to have some fluid still in the MC if the pads have worn down. To calculate how much fluid, you use the pad thickness and radius of the caliper piston. pi times the radius squared, and then times the pad thickness, adding up the volume for each caliper. The front caliper pistons are bigger than the back ones. Got to remember that with the MC that is needed for your pedal assembly, with a common reservoir, there may be situations where the clutch is depressed and moving fluid out of the reservoir at the same time as braking is occurring.

The two front caliper pistons are 2.934” diameter, the single rear piston is 2.375” diameter.

Combined areas of the pistons are:

(((2.934/2)x(2.934/2))x3.14) + (((2.934/2)x(2.934/2))x3.14) + (((2.375/2)x(2.375/2))x3.14) = 17.9 square inches

The swept volume of spent pads is no more than 0.60”, so the total displaced volume would be

0.60x 17.9= 10.76 cubic inches             1 ounce = 1.8047 cubic inches so:

10.76/1.8947= 5.96 oz.

The setup delivers 906 psi with a pedal pressure of 150# or 1057psi with a pedal pressure of 175.
That means that for disks, the pedal/master is just adequate for panic stopping, in terms of pressure and might be ok. The volume of the reservoir is too small for most 4 wheel applications, but for me, using only three old style GM disks (which are small volume by today’s standards) with a total of only 3 pistons and the clutch slave I will be fine, with almost 2 oz. extra in the reservoir . Calculating the volume needed by the clutch, I needed only an additional 0.367 oz when depressed 1.5”. So worst case scenario is 6.33 oz. or 1.67 oz. surplus (about two shot glasses of red-eye poured by a stingy bartender).

I am thinking about extending the pedal assembly 2 more inches to give a 7:1 pedal ratio. As noted above, that will give 1057psi with 150# pedal pressure and 1233psi with 175# pedal pressure which is further within the recommended parameters for harsh stops.

Since I have only done calcs, I can’t confirm that the numbers are correct, and I didn’t account for what they call frictional losses. I can’t vouch for the “feel” of the brakes in this scenario either.

Armed with these calculations, I summoned up the courage to drill 9 holes in my firewall and mount the master/hanging pedal.

To do this required a fairly high piece of landscape on the firewall, normally occupied by the 37′ Stude hood hinge. This is a parallelogram type of hinge, normally motivated by a pair of rather stout springs. Since I would undoubtedly be opening the hood a lot on this thing to show the unorthodox power train, I was reluctant to dispense with the spring. I came to a compromise, which allowed the majority of the hinge to remain in place, minus the lower part which anchors the springs. This mod would only be necessary on the driver’s side, so there would still be some springing in the circuit. Probably, I’ll be fastening the hood down with leather belts and maybe hood pins anyway. The debate still rages on whether the Tribaker will have hood sides or front fenders. This will be a rather fair-weather friend, having dispensed with the top, side windows etc. Even the deco-design heater will likely not be a heater, but a disguise for electrical switches hidden under the heater doors.

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