How to Calculate Wheel Cylinder Piston Travel for Drum Brakes?

[Indi Monk]

Hello.
I am in the process of calculating the brake fluid volume in the reservoir. I have a front disc and rear drum brake system. While the piston travel is clear for the disc brake, my question is related to the drum brake wheel cylinder piston travel. Assuming that the liner wear is 8 mm, should I consider the wheel cylinder piston travel to be 8 mm or is it higher considering that the wheel cylinder is usually at the top and might have to travel more in order to compensate? If it is higher, how to calculate the exact travel?

 

[jack action]

If the top of the lining is at the wheel cylinder level, then 8 mm of piston displacement will be 8 mm of lining displacement, no matter what.

Usually there is an adjuster at the bottom to compensate for wear. So 8 mm of wear at the top should be 8 mm everywhere.

Some have the adjuster on top, below the wheel cylinder (the lining self center by sliding down). In this case, there is a fixed pivot point and so there is a rotation where d = θr. But as long as the top of the lining is at the wheel cylinder level, the lining displacement should be the same as the piston.