This is the leverage ratio of tyre tread to axle over pad to axle. This is different to below.bosvit wrote:If this is the case how can a 140 place more stress on the hub than a 160? According to the physics I am aware of the bigger the disc (attached to the same hubs with the same bolt positions) the higher the torsional stress on the hubs for the same amount of braking pressure on the hand brake.
The distance of the pad from the axle matters as the wheel becomes a lever with the pivot point at the pad (when braking). This is the leverage ratio of tyre tread to pad (radially) over pad to axle. As the disc gets smaller, the pad (pivot) gets closer to the axle and the leverage on the axle gets bigger. Not only are you increasing the distance from tyre to pad but at the same time you are reducing the distance from pad to axle. So the difference in leverage between 185 and 140 is significant. How this affects the quick release is it provides a huge amount of sudden force which moves the quick release when braking, then the quick release relaxes after braking and moves back. This causes it to slowly unwind.
Assume diameter of wheel with tyre is 68cm, then:
For 185mm disc
(34 - 9.25) / 9.25 = 2.68
For 140mm disc
(34 - 7) / 7 = 3.86
3.86 / 2.68 X 100 = 144%
So my basic calculations show the force at the axle is 44% higher with a 140 than a 185.