Oscillation and Driving Comfort


1. Sprung weight and unsprung weight

The body is supported by springs. The weight of the body, etc., which is supported by springs is called the sprung weight. On the other hand, the wheels and axles, and other parts of the automobile which are not supported by springs, make up the unsprung weight. It is generally said that the greater the sprung weight of an automobile, the better the riding comfort becomes, because, as the sprung weight is made larger, the tendency of the body to be jolted decreases. Conversely, if the unsprung weight is large, it is easy for the body to be jolted. Oscillation and jolting of the sprung parts of the vehicle -especially the body- have a particularly great effect on riding comfort.

2. Oscillation of sprung weight

Oscillation of sprung weight can be classified as follows:

(1)Pitching Pitching is the up-and-down oscillation, in relation to the vehicle’s center of gravity, of the front and back of the vehicle. This happens especially when the vehicle goes over large ruts or bumps in the road or when driving over an unpaved road which is rough and full of potholes. Also, pitching occurs more easily in vehicles with softer (easily compressed) springs than in those with harder springs.

(2) Rolling When turning or when driving on a bumpy road, the springs on one side of the vehicle expand, while those on the other side contract. This results in body rolling in the lateral (side-to-side) direction.

(3) Bouncing Bouncing is the up-and-down movement of the auto body as a whole. When a vehicle is running at high speeds on an undulating surface, bouncing is likely to occur. Also, it occurs easily when the springs are soft.

(4) Yawing Yawing is the movement of the vehicle’s longitudinal centerline to the right and left, in relation to the vehicle’s center of gravity. On roads where pitching occurs, yawing is also likely to occur.

3. Oscillation of unsprung weight

Oscillation of unsprung weight can be classified as follows:

(1) Hopping Hopping is the up-and-down bouncing of the wheels which usually occurs on corrugated roads while driving at medium and high speeds.

(2) Tramping Tramping is the up-and-down oscillation in opposite directions of the left and right wheels, causing the wheels to skip over the road surface. This occurs most easily in vehicles with rigid axle suspension. (3) Wind-up Wind-up is the phenomenon in which acceleration or braking torque acting on the leaf springs attempts to wind the leaf springs around the axle. Wind-up vibration has an adverse influence on riding comfort.


Measures to prevent wind-up:

Asymmetrical leaf springs Wind-up is reduced by offsetting the rear axle so that it is located slightly forward of the center of the leaf spring. This also serves to reduce the up-down motion of the body during acceleration and deceleration.

Shock absorber location Wind-up can be reduced by mounting the shock absorbers away from the center of wind-up and by bias-mounting them. That is, by mounting one in front of and one behind the axle.

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