Challenge

Vehicle Ride

DDevaiah I S8 Nov 2021

1. Why is wheel rate different from spring rate?

Wheel rate is effectively the spring rate when measured at the wheel as opposed to at the spring. It is basically spring rate multiplied by the motion ratio squared. Where, the motion ratio is the ratio between how much the spring is compressed compared to how much the wheel is actually moved.

2. Why is ride rate different from wheel rate?

Ride rate is the vertical force per unit vertical displacement of the tire ground contact with respect to chasis. Whereas wheel rate is the measure of the effective stiffness of the suspension system measured at the wheel centre.

3. Can the same amount of damping be used for,

Low and high frequency regions?
Compression and rebound?

It is not possible to use the same amount of damping because,

High damping is required at low frequencies and vice versa.
During rebound, if the same amount of damping is used for compression then it would slam the wheel back to the ground. Therefore higher damping is required.

4. Why does the rear ride frequency need to be higher than the front?

A time delay occurs between the front and rear wheels when they hit a bump. To avoid pitch and have a flat ride, the ride frequency of the rear wheels is higher than that of the front wheels to ensure that the frequencies are in phase.

5. For improved vibration in the wheel hop region, which is preferrable,

Higher or lower unsprung mass?
Higher or lower damping?

6. From a ride frequency standpoint, why are air suspensions beneficial in heavy vehicles?

7. Calculate the sprung and unsprung mass natural frequencies for a vehicle with 2000 Kg sprung mass, 50:50 weight distribution, 50 Kg unsprung mass at each corner, spring stiffness of 25 N/mm and tire stiffness of 250 N/mm. Assume acceleration due to gravity to be 10 m/s^2.