The smaller the spring, the softer it is and the earlier in the RPM range the clutch will engage
Available in 6 different spring hardness:
B8.5: 0.85mm springs for very slippery tracks.
B9: 0.9mm for slippery track.
B9.5: 0.95mm for grippy tracks.
B10: 1.0mm. for very grippy tracks.
B11: 1.1mm for very high traction or muddy conditions.
B12: 1.2mm for Rally Game class.