I found this quote somewhere around here.

Not only was it unchallenged but the next poster gave it a +1. Other comments I've seen have pointed at similar misconceptions and it seems worth pointing out here in mechanics. A related misconception is probably that disk brakes transmit stopping force through the spokes and rim brakes do not.

Both notions are completely false of course or even somewhat upside down. In short momentum conservation dictates that ALL force to decelerate the forward motion of a bike MUST come through the ground (except for a little through the air), transferring momentum to the ground. For top mounted rim brakes (like all front rim brakes) this force is essentially transmitted through the spokes to the frame. (a bike hurling through a vacuum can never stop itself.. you have to push from the ground)

As the brakes make the wheel try to roll slower it is resisted by the ground since the weight of the frame and rider still want to continue over the ground at the same initial speed. Ignoring air resistance, the force at the ground is the full rearward acceleration (foward deceleration) force, m*a.

That force to stop the bike is then transmitted from the wheel to the hub through the rearward spokes which pull backwards and through the forward spokes which push backwards. This is true on radial spoked wheels essentially just as on cross wheels. What disc brakes do that rim brakes don't is disk brakes put TORQUE on the rim through the spokes, but only to slow down the wheel itself. Both put stopping force through the spokes radially because again, stopping force MUST come from the ground, not the brakes.

There is a force on the brake boss, a big one, but if the boss is on top (as usual for front wheels), the wheel is moving forward there and pulling the brakes, the boss, and the bike FORWARD! As it turns out the force on the brake boss IS equal in size to the force on the ground (for weightless wheels) in order to balance toque on the wheel (if the wheels have weight, SOME net torque is required to decelerate the rim, and then this force is even bigger). But if the brakes are top dead center, that force pulls the bike forward with m*a. That means to place net m*a on the frame, the force transmitted through the spokes is actually 2m*a, backwards!

It seems that saying the braking force goes through the bosses is at least misleading here. For chain stay mounted rear brakes the situation is a little different, and the partially rearward force on the bosses does contribute to frame deceleration, reducing the load on the spokes instead of increasing it.