In order to provide pulsating thrust on vehicle half-shafts during excess differential actions such as occur on spin-outs or skids on ice, the side gear teeth are modified to exhibit two different e.g. involutes on the single tooth engagement zone (P.sub.1 -P.sub.2). Their junction line is a convex discontinuity (Q), the modified teeth becoming thicker. The pinions are typically unmodified.The pulsating increases friction, which reduces spin-out tendencies. The friction can be increased more by inserting friction rings between the side gears and their bearings.