Why Does a Car “Pivot”? 270022

REMARKING the difficulty of explaining logically the strange phenomenon known as the “pivoting” of a car, the author, after presenting citations of actual experiences with various combinations of front-wheel and rear-wheel braking and their tendencies to cause a car to pivot, analyzes pivoting and explains its causes under (a) “dry-roadway” and (b) “skiddy-roadway” conditions.
Concerning (a), the author states that when two-thirds of the braking force of a four-wheel-brake system is distributed to the rear wheels, the preponderance of the stopping or braking force will remain active to the rear of the center of gravity of the car, causing a so-called “drag-anchor” effect to counterbalance what is termed the “spin effect,” and no dry-roadway pivot can occur. Since the friction available between the roadway and the tires is comparatively small on a skiddy roadway, the retarding forces at the two sets of wheels should be utilized to their utmost. Owing to the change of so-called “effective weight” from the rear wheels to the front wheels when the brakes are applied, the brakes on the latter will resist much greater pressure from the foot-pedal before the wheels lock than will the brakes on the rear wheels before the rear wheels lock. Consequently, a greater braking-force should be distributed to the front wheels than is supplied to the rear wheels. If two-thirds of the braking force from the foot-pedal is distributed to the front wheels for skiddy-roadway conditions, the author says that the front wheels always will lock momentarily before the rear wheels lock and that no pivoting can occur. He therefore concludes that for the two conditions, (a) and (b), under which a car is to be stopped, it is necessary to design a four-wheel-brake system that combines two different methods of appropriate distribution of the braking force because the requirements for proper functioning under these two conditions are so different.


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