joseff

Hmm... never thought of that before. At first I thought since the axle-road leverage is also reduced, they cancel each other out.

Let's see...

For a 700x23c:
* axle-road distance = 311mm + 23mm = 334mm
* axle-brake distance = 311mm - 5mm = 306mm
50N braking force => 46N decelerating force

For a 20x1.5:
* axle-road distance = 203mm + 38mm = 241mm
* axle-brake distance = 203mm - 5mm = 198mm
50N braking force => 41N decelerating force (11% less)

For a hypothetical 23-406:
* axle-road distance = 203 + 23mm = 226mm
* axle-brake distance = 198mm
50N braking force => 44N delerating force (4% less)

So yes smaller wheels = less effective braking, but not because of the size of the wheel per se, it's due to tyre thickness becoming a significant fraction of wheel diameter.