Longitudinal tire forces on four‐wheel drive cars, both with and without a differential gear between the axles, are compared with those on two‐wheel drive cars; tests were made on a curved trajectory.

A “critical speed” may be defined which depends solely on characteristics of the car and test tires; this is usually of the order of a few tens of kilometers per hour. Above that speed, for the lateral accelerations normally reached, the additional longitudinal forces caused by lack of an intermediate differential do not increase the relative tread wear rate relative to that on a two‐wheel drive car. Cars almost always operate above this critical speed. Below it, however, the additional longitudinal forces caused by lack of a differential are much higher and obviously would lead to higher rates of tread wear.

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