Originally Posted by Flinstone

You had a right idea but lost it (next time read the whole post). Yes forces transmit around the rim, but the force that the rim transmits to the brake is FORWARD. Spin the front wheel forward and then grab it with your hand just as the brake does. Does the wheel tug forward or backward on your hand? Forward of course. How can a forward pull on the brake/boss/frame slow the bike? Real world enough for you?

Where your idea was right is the brakes can and MUST resist the TORQUE on the wheel from the ground, and just a bit more to slow the wheel itself, and this can get passed around the rim (and/or through non radial spokes, but mostly the rim). The net force though (not torque) from the wheel to the frame WILL be m*a until you loose traction, and whatever the difference is between the force at the boss and m*a, MUST be transferred through the spokes to achieve that net. Or put the other way, the deceleration a, times m, WILL be whatever that NET force is. Either way you say it, the total force on the frame F adds up as F=m*a.

In the case of top mounted brakes, you start with approximately m*a in the wrong direction at the bosses and the spokes must actually take twice the force to resist the bike's forward motion AND counter the forward force at the brakes.