Your chassis choice will depend on either your personal driving style or the grip condition of the track or of the car.
A Soft chassis (blue) is used in a LOW GRIP condition, such as a slippery track, dusty track or with low grip tires. The Soft chassis is suggested for track like Carrera or Ninco when there is no tire rubber on the track surface.
A Medium chassis (black) is used in a MEDIUM GRIP condition, such as track or tires with good grip (i.e., NSR Ultra-grip tires). The Medium chassis is suggested for track like Carrera or Ninco when there is a small amount of tire rubber on the track surface.
A Hard chassis (white) is used in a HIGH GRIP condition, such as track or tires with a high grip (i.e., NSR Extreme tires). The Hard chassis is suggested for track like Ninco when there is a lot of tire rubber on the track surface or for Wooden Track that uses light/medium traction compounds.
An Extrahard chassis (green) is used in a VERY HIGH GRIP condition, such as track or tires with a very high grip (i.e., NSR sponge tires). The Extrahard chassis is suggested for Wooden Track that uses medium/heavy traction compounds.
