the caliper depends on the wheel... more than likely you'll need something in a single action piston type with the fixed inner brake pad... like the stock one... because dual, opposing piston, type calipers might be too bulky off the back-side and 'tag' or bind up on the spokes - the more narrow the wheel, the more upright the spokes are, the tighter the space between the caliper 'inside' and the spokes - especially on a narrow on a 21" wheel - understand? - visual doodle - if you're using a mag wheel, however, this might not be a problem, like an invader or an avenger, with centered 'spokes'... clear of the caliper.

Also, keep in mind, the bracket holding the caliper has to pivot up and down on a brass or bronze sleeve bushing off the axle... with the brake 'steady' arm attached to the bracket and the rear leg by pivoting mounts... like heim joints, or clevis's - so the whole rig can travel up and down with the springer 'spring action' and not bind up during travel...  

and HERE is an example of a whole pivoting brake bracket rig - but i've got a wide springer and a wide hub, so I was able to use a four piston caliper (opposing piston) on a 21"... it just barely clears tho...  and the steady arm is curved because it has to travel over the top of the rotor itself just to clear the inside of the front leg...

I like my Women like I like my coffee... Cheap, Hot, and served to me by another Woman.

Ding Dang Motorpickle Project @#*!!...
Last Edited By: Curb Mutt 04/25/10 7:47 PM. Edited 2 times.