Originally Posted by
aixaix
I see I didn't make my point very well.
A larger rotor will be more efficient than a smaller one, not because the force required to slow it down (given the same size brake pad), but because it has more surface area to give off heat. For a given input (100psi at the pads, e.g.), the initial *********** will be the same. The value of a larger disc is twofold. First, being larger means it can radiate more heat. (For our purposes, this is an advantage, although there are some applications where this is not.) Second, the swept area can be made larger still by increasing its width. My point was that the increase in swept area doesn't give a mechanical advantage in terms of leverage. It absolutely does for other reasons (heat transfer, e.g.).
I'm sorry I didn't make that clear the first time around. By the way, what usually limits the size of a rotor on a car is interference with the rim or other component, just what the original poster was having trouble with.
The theoretical basis for designing brakes, the physics of changing motion into heat, has not changed in any fundamental way since 1986. Technology, practice, engineering & materials have all come a long way, however. Puhn's book is probably out of print, but I'm sure copies are still available at Alibris, Powell's or Amazon.
I just wanted to posit that this post is wrong. I'll let a mechanical engineer in the forum make the appropriate arguments. Something I'm not interested in doing. However, using larger rotors isn't just effective merely to dissipate heat. They simply work better as a brake, do to the mechanical advantage of the larger rotor size. Kind of similar to how a small cog is your go fast gear, but the large cog provides better mechanical advantage (for a given rider's wattage) to get up the big hill. To say there is no mechanical advantage, or variance, between the small and large cog is absurd. Just as it is to say there is no mechanical advantage between a small and large rotor. It ain't all about the heat.