At 10:16 AM 1/23/99 -0800, you wrote:
...
>The most recent example is with regard to
>the clutch master cylinder, which has a 5/8" bore for a stock Tiger and
>3/4" for the Alpine. The difference in applied (foot) force between these
>two units goes as the area of the bore or the square of the diameter, which
>is a factor of 36/25 or 44% more pedal force needed to operate the 3/4"
>unit. While needing this much more pedal force, the bigger unit has a like
>reduction in the travel needed to accomplish the job, so the total work
>done by your foot with each unit is the same. This is just the hydraulic
>equivalent of the mechanical advantage tradeoff with a longer or shorter
>lever arm. Oh yes, and the result of switching these units in my car was as
>expected, much harder to operate the 3/4" unit, although honestly I did not
>quantitatively verify the 44% harder factor. Nor did it even occur to me
>that this effect needed verification. ...
It is important to recognize that the "brake hydraulic system" consists
of
both levers and cylinders. There are ratios for both at both ends which
end up actually transferring or applying the mechanical force applied by
your foot to the brake friction material. The stock mechanical ratio of
the brake and clutch pedals are approximately 4.47:1. This means that for
every 10 lbs you apply with your foot, about 44.7 lbs are applied to the
piston in the cylinder. The hydraulic ratios take it from there. Remember
also that essentially no system pressure is produced until the receiving
cylinders move and the friction material contacts the discs and or drums.
At this point the system develops a uniform hydraulic pressure (ignoring
proportioning valves and similar devices). All of these ratios apply all
the time to the total system dynamics. The booster is simply a device
which provides linear amplification of the system pressure.
All of these components are potential variables but it is important to
know the change that will occur with substitution. Bob wanted to raise the
clutch pedal so that it operated in a narrower range. His chance reduced
the hydraulic ratio resulting in more slave movement at the price of higher
pedal pressure. He could have also reduced the distance between the pedal
pivot and the connection to the master cylinder with the same result, as
long as the mechanical components stayed within appropriate operating ranges.
Typical brake systems for race car applications are designed with a 6:1
mechanical ratio so you can see why the stock 4.47 ratio typically
requires boosting. Changing components is intriguing but it frequently
alters the total system dynamics such as front rear brake bias so be
prepared for some unexpected results until you've done some field testing.
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