Jim,
Are you lucky or what?! I just spent the last week figuring this all
out! I had my physics books, and automotive repair books all laid out. I
even had help from the physics genius at UCSD! SO, here is the $64,000
answer.
Think in terms of a simple system. i.e., a master and a slave, with an
area of 1" square face on the piston. If 100 pounds of pressure is placed on
the master, that will transmit exactly 100 pounds of force through the
slave. Now, increase the diameter of the slave cylinder to 2" squared, so
that it is twice as big as the master. If 100 pounds is placed on the
master, there will be 200 PSI through the slave.
Now, rather than have 1 slave twice as big, just put two slaves, each
with a face of 1 square inch. The pressure in the system is still the same,
no matter how many slaves there are. Therefore you will have two slave
generating 100 pounds, for a total of 200 pounds. In a car, with 4 wheel
cylinders, you would have 400 pounds of force for the one master at 100.
Going back to the simple system, you can see that decreasing the size
of the master or increasing the size of the slaves, creates the best braking
for the car. So, the ideal situation, is to have wheel cylinders that are 6"
in diameter, and a brake master that is 1/4". OF course, that sounds absurd
because there is no way that the brake master could supply enough fluid to
power 6" wheel cylinders. It would bottom out before the pistons moved
enough to make contact with the drum.
This is where the trade off comes. Here is a simple rule for you to
remember. Smaller master equals MORE pedal travel, and LESS felt pedal
pressure. Larger master means LESS pedal travel, but STRONGER quadriceps.
You want to increase braking power, yet still have enough brake fluid so
that your pedal doesn't bottom out. And, you would like to have a margin of
error, so that the pedal still could go down farther if there was a little
air in the system.
Lets talk about the early cars. They did not have a servo, so braking
power was related to the quads of the driver. The series I & II used a .700"
master. It escapes me right now, but I believe they used a 7/8" (.875") rear
slave. The Series 3, which was boosted, used a .750", and the 4,5, and
Tigers used a .875", and of course, they were boosted. However, on the rear,
they used a 3/4" (.750) slave wheel cylinder.
So, you can see that on the early cars, to get the felt pedal pressure
down, they used bigger slaves in the rear, and a smaller master cylinder.
On the later cars, since felt pedal pressure was reduced by the servo
(when it wasn't sucking up the fluid or locking up), they used a bigger
master for safety, with smaller slaves.
All that aside, I drove my tiger without servo for 8 years every day
with no problem. Now that I am servo spoiled, it would be a problem!
There are 2 other things that affect brakes that you must think about.
The radius of the braking force at the wheel will affect how much pressure
you need. If the wheels are ten feet in diameter, that is like a 5' prybar
putting torque on the brakes. So, decreasing the size of the tires DECREASES
the amount of braking force required. And INCREASING the sizes of the tires,
requires trips to Gold's Gym and the quad master.
The last one is the diameter of the brakes. A brake drum of 10'
requires less force than the brake drum of 5', again, it is the prybar
effect.
The real "last" thing is pad composition, but that is a matter of
choice and race conditions/requirements.
So, all that said, you can't do much about your brake drum, or rotor
size, so we look at them as constants. You will probably put smaller tires
on, so this will reduce the amount of felt pedal force. You will also reduce
the master cylinder bore, so again, less pedal force. If you have a series
3,4,5 or tiger, you "can" get a set of rear slaves from a series I or II,
and this will give you better braking force in the rear. You would have to
experiment with this since you don't want these locking up on you before the
fronts take over.
For safety purposes, I am going with a 13/16 bore, dual master. It is
from a 82-84 Honda Accord. That way, if one system fails, I can still stop
the car. To clear the Weber carbs, I moved the brake master to the clutch
hole, and then welded in the gear for a clutch in the firewall, and used
Tiger pedals and Tiger clutch master. I have strong quads! Lou
----- Original Message -----
From: <ellis838@concentric.net>
To: <alpines@autox.team.net>
Sent: Thursday, February 15, 2001 6:13 AM
Subject: Wilwood M/C
> Hello,
> Question, is the Wilwood master cylinder with the 3/4 inch bore big
> enough to replace the stock 7/8 inch unit and not cause any braking
> problems due to the different bore size. I want to use the Wilwood to
> clear a set of side draft webers. Thanks Jim
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