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This calculator plots the mechanical advantage of cantilever brakes as a function of yoke height. In addition to yoke height, you can change four constants. The major constants are arm length (PA) and cantilever angle (α). The minor constants are the vertical and horizontal distance of the pivot studs from the center of the rim (OD and OP respectively). These last two are specific to your bicycle as they are positioned by the frame builder, although in the case of centerpulls, they are dimensions of the brakes themselves.
To calculate mechanical advantage for your own brakes, make sure that you take measurements while the brake pad is in contact with the rim.
The figures for the example brakes are just approximations, as any specific brake has a significant range of possible cantilever angles determined by a number of factors such as brake boss position, pad position, pad thickness, rim thickness etc.
Additionally, it is important to realize that performance of brakes is dependent on more then just theoretical mechanical advantage. Losses due to cable friction and flex in the system, whether in the brakes or levers, or due to poorly installed housing, can have a significant impact on your brake's performance. Please use this as a guide to understanding the principles of the geometry of brakes.