Originally Posted by
FastJake
Seriously?
It's a simple mechanical advantage problem. Disc brakes take a LOT more force to stop a given weight bike, but they use a completely different design to accomplish it. That's why you'll see disc brake equipped bikes with 230mm rotors in front and 180mm in back (front brake is more important, requires more force to lock the wheel than in the rear.) Rim brakes take advantage of a huge "disc" - the rim of the wheel.
Try this: spin any wheel attached to a bike. Stop it by grabbing the spokes near the rim (don't do this with bladed spokes!) Now spin it again at the same speed but now stop it by grabbing the spokes near the hub. You can easily stop the wheel near the rim, but grabbing it near the hub is much more difficult and you may get your fingers pinched a little. But it shows the point.
Edit I was responding to the suggestion from
Kimmo that the brake position relative to the outside radius makes a difference in braking as in putting on a larger tire affected braking in a detrimental manner. I was not implying that it takes less force to impart the same torque regardless of radius from the center.
When you put a bigger tire on, the brakes still hit the rims in the same place, right? And, as I pointed out before, it is the total weight of the bike that determines how hard it is to stop the bike.
So, do we agree that by putting on bigger wheels, we don't makes braking harder?
Charles