Purpose Differential

Rectilinear motion.

Both rear wheels are equal to the frequency of the differential gear turns. Pinion bevel gear differential rotate uniformly.

Stir in the corners.

Wheel extending outer radius extends a greater distance than the inner. Variable speed is aligned wheels, otherwise one of them would be permanently stalled in turns.

This provides a tapered transmission: faster rotation of the outer bevel gear is transmitted through the intermediate gear to the external gear, and it rotates slowly.