Friction surface material is Chrome leather. Back surface material is Chrome leather. The belt factor is 5 N/mm at 1% elongation for product code MTS 50LL,7.50 N/mm for MTS 75LL, 10 N/mm for MTS 100LL, 15 N/mm for MTS 200LL, 23 N/mm for MTS 300LL, 30.50 N/mm for MTS 400LL.