Drum Rear brakes normal operation
Drum rear brakes are normally operated by a mechanical linkage
shown at the top of the figure below. A few are operated by
cable, these avoid the effect I am talking of here. If the rear
brake is applied as the suspension moves up and down the rear
brake plates. This is caused by the geometry of the components.
If the length of rod 4 is the same as the swing arm the effect
can be minimised, unfortunately the rods length changes as the
brake wares (it is used to compensate for ware). An alternative
arrangement below shows that the pivot point of the swing arm can
be aligned with the pivot point of the rod 4 and brake leaver 2.
Now when the suspension moves up and down no effect will take
place. This does depend on the pivots being aligned when the
brake is pressed! As the movement of the brake leaver is small at
this point good alignment can be obtained, small discrepancies do
lead to minor effects.
Geometric considerations of the lengths
Measure lengths 1 and 2 of the original system. Using the
original position of the foot contact onto the brake pedal
measure the distance back to some point under the swing arm pivot
point. This is approximately the new length 1a. Calculate the new
length 2a by multiplying length 1a by length 2 and dividing by
length 1. Now measure from the length 2a down from the swing arm
pivot point, then remeasure the new length 1a from here to the
foot contact point. You then repeat until happy with the
result.
Geometric considerations of the angles
Keep the change to the angles small and these changes will have little effect on the over all ratio.
Construction
I'd make this of mild steel, it will be flexible and not brake
in a fall, be able to be bent back into shape after a fall and
you can continue on. Other materials may be lighter but less
durable. A mild steel plate about 2 mm thick is made into an L
shape. The longer part of the L shape is curled into the foot
pedal shape similar to the foot pegs but smaller. The junction of
the L has a pivot tube punched into it. Both arms are then curved
in cross section to give some strength. The top of the small arm
of the L has a hole drilled into it and the brake rod is then
fasten to the hole.
If you are good the brake pedal can be constructed such that it
travels behind the foot peg and has it's pivot point at the foot
peg leading to a natural ankle action?
You
can e-mail me at Yahoo. Back to my home page Dated March
2002