Questions seem to come up fairly regularly about using one size
master cylinder with another size slave cylinder on a clutch or brake
system. Here's some math to help determine what will happen. Three
master cylinder diameters are given in the first column - 5/8", 3/4"
and 7/8". Column 2 gives 3 slave sizes for each master size. (all
measured in inches) Column 3 shows what happens to the slave when the
master is moved 1/4". For example when the 3/4" MASTER moves 1/4", a
5/8" slave will move 0.360". The slave moves more because the larger
diameter master is pushing fluid into a smaller diameter slave. The
size of the clutch line or brake line makes no difference in the
distance the slave moves. Important parameters are the diameters of
the Master and Slave.
The equations used are the area of the MASTER Cylinder times the
stroke distance = the area of SLAVE Cylinder times the stroke. You
can calculate the areas because you know both the MASTER and SLAVE
diameters. You assume a stroke for the MASTER and calculate the stoke
for the slave. ((@PI*(master diameter^2)/4)*master
stroke)/(@PI*(slave diameter^2)/4) = slave stroke.
However, since @Pi (3.14159) and the number 4 show up on the top and
bottom of the equation, you can cancel them and simplify the equation
to; Master diameter squared * master stroke divided by slave diameter
squared = slave stroke.
Just keep your units consistent (don't mix English and metric units)
and you'll be OK.
Master Slave 1/4 stroke-slave moves
0.6250.6250.25
0.7500.174
0.8750.128
0.7500.6250.360 (example)
0.7500.250
0.8750.184
0.8750.6250.490 <---- ~2 times
0.7500.340
0.8750.250
This math pertains purely to a hydraulic system with no leaks or soft
hoses. The rest of the system, pedals, linkages, soft hoses etc
can/will effect the calculation of the distance the slave moves. The
assumed 1/4" movement of the MASTER cylinder is measured AT the
master cylinder and no someplace else in the system. Same for the
SLAVE - measured at the slave.
Notice how using a 7/8" dia MASTER will move a 5/8" slave about 2
times further than the master moves.
Now my head hurts. . . LOL
Tim Collins
http://www.flickr.com/photos/12702006@N07/
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