Jim Jackson wrote:
[...]
>I know from automotive studies that the braking
> power gets better from a larger diameter rotor, more caliper pistons (simple
> hydraulics) and/or from a larger total surface area of rotor contact.
There is more to it than that.
Bigger diameter is almost always better. Our brakes work with mechanical
disadvantage, as in the rotor being a smaller radius than the tyre.
By increasing the rotor diameter you change the ratio of radii between
the tyre and rotor and reduce the mechanical disadvantage.
If you kept the same brake and changed to front wheel to a 17 inch
(in motard style) then you also reduce the mechanical disadvantage
and improve the braking.
More caliper pistons does not necessarily help things on their own
but usually are a conseqence of other design changes.
> Ideally we could mount a same-sized rotor with wider surface area and
> appropriately bigger caliper and not have to worry about it. The larger the
> diameter the greater the torque forces on the caliper (exponentially?).
Not exponentially, linearly (if all else equal).
A 12-inch radius rotor will give twice the braking torque as a 6-inch
radius rotor (all else equal, of course).
> Also, more fluid moving around in the caliper means more travel in the brake
> lever right?
Not necessarily. It's more a matter of piston area.
If you have 2 small pistons with the same area as 1 big piston then
travel should be the same.
The hyraulic ratio can be tricky to get right when converting to
different
calipers.
[...]
> Approach one will mount to the old holes with the caliper further out from
> rotor centerline, same spot otherwise. This places extreme forces on the
> mounting bolts in ways it twern't intended to be applied so we'd have to
> have some sort of bracket that distributes that force to the fork tube
> itself. Not exactly pretty. Can the fork handle that?
Haven't seen it done, myself.
> Second approach is better and that is to engineer the bracket to mount the
> caliper at an angle from the original bracket such that the braking torque
> hits evenly on the original bolts holes.
The further away from the fork leg you mount the caliper the more you
change the forces on the mounts. Add a disc of much bigger radius and
things can easily go wrong.
> If the current brakes can lock up
> the front tire (done that) then the mounting holes are strong enough to
> handle this. I'm just not nearly good enough at geometry to fig'r out where
> the caliper should go.
Pretty simple where to put the caliper. If the caliper and disc are
a matching set from some other bike, then the caliper only really
fits properly over the disc at a certain distance from the axle.
Once you find this distance then you figure out how to mount the
caliper as close as possible to your original mounts.
Luckily, our bikes have brakes small enough to be able draw them
in real size on a sheet of paper, maybe A3 size.
A real pro conversion would entail welding new mounts onto the fork
which is not as simple as it sounds.
Which is why it can be easier to swap to forks from another
similar but better-braked bike.
Mister_T
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