In this case, the braking force is to be determined on the effective diameter. This can be assumed as being the center-point of the brake block (disc-brake) or that of the brake-pad (drum-brake). It depends on the static friction (blocking wheel) or the dynamic friction (rolling wheel) between the pad/block and the drum/disc. In the formula it is assumed, that one tension force for each pad/block, thus two tension forces are effective. Otherwise the factor '2' in the formula must be accordingly altered.