PREVENTION OF SHIMMY BY HYDRAULIC STEERING-CONTROL 250048
Experiments with hydraulic steering-control with the object of preventing or reducing shimmying and tramping were made by the author, who asserts that the elimination of backlash by doing away with mechanical joints and by holding the front wheels as rigid as the rear wheels has been amply proved by the results to be a long step in the right direction. With a Marmon car fitted with an hydraulic steering-system and driven over the roughest roads it was impossible to discern any front-wheel wabble as the car approached and passed the observer. The car could be driven unbelievable distances with the hands removed from the steering-wheel and driven over curbs without manual control; the tendency to tramp was greatly diminished, the radiator vibration was noticeably reduced, the sense of stability of the front end of the car was comparable with that at the rear end, the front wheels would straighten out of a turn with the slightest pressure on the steering-wheel, and no vibration of the hand-wheel due to road roughness was noticed.
Six differences between the front and the rear ends of a car that may be responsible for shimmying and tramping are listed and analyzed briefly, the most important being the 8 or 10 mechanical joints between the front wheels and the steering-wheel. Based on this analysis, it is asserted that if the front wheels can be held rigid the problems will be solved. This is accomplished by the hydraulic system, which acts as a dashpot between the wheels and the axle and dispenses with mechanical joints that have a certain amount of elasticity, which permits of the development of backlash and increases with use.
Means by which hydraulic steering is effected are described briefly, since they are experimental and it is probable that better means will be developed. The major elements are a sliding cylinder working transversely on a stationary piston clamped by the piston-rods to the front axle; a divided cross tie-rod secured at the inner ends to the movable cylinder and at the outer ends to the steering-knuckle arms; flexible tubes from the ends of the hollow piston-rods to rigid tubes on the car frame that connect with an oil-pump at the foot of the steering-column; and an oil supply-tank on the dash, with a check-valve below, from which the system is automatically kept full. The geometry of the steering action with the divided cross tie-rod is much nearer theoretically correct than that with the customary single tie-rod.