The locomotive pinions engage the rack and provide all the traction and braking effort; the wheels are free to revolve on the drive axles, to allow for the inevitable difference between the wheel radius and the effective radius of the pinion. The wheels serve only to support and guide the vehicle; if the pinion were missing, handbrake on and locomotive crank locked, the vehicle would still roll down the mountain. (The wheels are not capable of providing useful adhesion on such a gradient, so this is not the disadvantage it might seem.) The two driving axle pinions on a locomotive are mounted with a half a pitch difference between them. Combined with the half pitch difference in the two rack bars, this feature aims to make the transfer of power more continuous, and thus smooth the hauling of the train. In spite of this, the vehicles still suffer from very high levels of vibration.