ELEMENTARY DYNAMICS OF VEHICLE SPRING-SUSPENSION
Riding-discomfort from road inequalities can be divided into two general classes, of which the first, direct discomfort, includes jolts, jars and unpleasant forces that occur during, and as the immediate result of, passage over the inequality. The extent of these discomforts depends chiefly upon the magnitude of force exerted against the passenger and the rate at which this force is applied. The second type of discomfort may be called potential and includes such motions of the car, following, and resulting from, passage over the road inequality, as lead to “not holding the road,” extreme pitching motion, or throwing the passengers off the seat and the like. This potential discomfort is more or less proportional to the amplitude of spring motion and the extent to which this motion interferes with the uniform straight-forward progression of the car.
The springs of a vehicle supported at the front and the rear seldom operate individually. Vertical acceleration of the front axle develops a moment around the center of gravity, and a pitching motion is set up whose rapidity and amplitude depend upon the relative spring flexibilities and weight distribution.
Analysis of a single spring operating individually yields the conclusions that (a) direct riding-discomfort is greater with less-flexible springs and with a greater amount of friction resisting spring-deflection, (b) through the higher range of speeds, the force transmitted to the passengers increases but slightly with the speed, (c) amplitude of swing is slightly greater as the spring flexibility is increased, and (d) energy stored in the spring is greater with the less-flexible springs.
Experimental observation of pitching action shows that rapidity and amplitude can be reduced by concentrating the weight as much as possible over or near the axles and increasing the flexibility of the front springs, sometimes with reduction of flexibility of the rear springs. With more-flexible front-springs, careful attention must be paid to steering linkage, and adequate means must be employed for absorbing spring-recoil energy.
Internal or inter-leaf spring friction cannot be made great enough to absorb recoil adequately without materially increasing the direct discomfort on bumps. Recoil-checks or shock-absorbers should absorb the recoil energy but should not oppose yield of the spring on bumps or apply an appreciable constant counter-force tending to tie the axle to the frame.