S91F is a semi-metallic compound developed for rear use in rally cars. It gives a moderately fast friction build-up so as to not cause lockups during hard braking. It can also be used for the rear in racing applications on light cars for drivers with a “light braking foot” that are not using high brake pressures. It can also be used for both front and rear on single seater open wheel racing cars up to the Formula 3-level.