I've been trying to figure out the friction coefficient of some brake pads. My test fixture only allows me to pull (F2) a brake disc between two brake pads clamped around the disc with a force F1. I then measure with a load cell the maximum force required for the brake disc to slip. My question is simply, will the basic equation F=UxN be valid, or is it correct to assume that F= 2x(UN) in this scenario assuming F1 and U is identical for both brake pads? I've been googling the topic and my impression is that both equations are used.