Hi, it's me again, the guy that started the fight last week about Toyota
calipers. I'm still thinking about brakes and some of the things I've read.
In the VTR article written by John Lye, he replaced the rear wheel
cylinders with 7/8" units and claimed an decrease in braking distance. He
also said pedal travel increased. When I read Roger Williams' book (How To
Improve Triumph TR5, 250 and 6) he states: "If you feel you want more
agressive rear brakes then you will need to reduce the size of the slave
[wheel] cylinders. 0.625 would be the smaller size used by most to upgrade
their rears, but if your rears are locking and you want to ease their
application, try the 0.75 in. diameter cylinders."
This got me thinking and wondering, because they can't both be right, so
which is it? If you apply 100 lbs of force against your brake pedal and it
acts on a master cylinder which is 1 sq. inch in area, you produce a
pressure of 100 psi. If this pressure bears against a wheel cylinder which
is 1 sq. inch in area, the force applied to the friction surface is 100
lbs. If you increase the size of the wheel cylinder such that the area of
the cylinder is doubled (now 2 sq. inches), apply the same 100 psi, the
resultant force is 200 lbs. It would appear the Roger Williams is in error,
if he is talking about braking force. Conversely, reducing the size of the
wheel cylinder will make it travel farther with the application of force to
the master cylinder, making them come on sooner. Perhaps this is what he
meant by "agressive."
I know that by increasing the wheel cylinder size, pedal effort will
decrease for a constant clamping force. Pedal travel will increase with
increased wheel cylinder size. If the tires are up to it, increasing
clamping force will lower stopping distances. It seems to me that if your
rears don't lock up at the same time as the fronts, it's a safe bet you can
increase the rear wheel cylinder size.
Just to keep things from getting out of hand, if I'm off base with my
reasoning, please reply to me personally and I'll offer a public apology
later. No sense dragging the whole List into another long-winded discussion.
Phil Barnes (peb3@cornell.edu)
Cortland, NY (nowhere near New York City)
'71 TR6 CC61193L (26 year owner)
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