Suspension Kinematics - Fundamentals

1. How many degrees of restraint does a solid axle suspension have?

- A solid axle has a total of 4 degrees of restraint i.e. Vertical (+ -) and Rolling (+ -) whereas 2 degrees of freedom i.e. Jounce and Roll considering a 3D coordinate space.

2. Why do we need the suspension to gain negative camber in jounce?

- During turning, the maximum load is on the outer wheel as compared to the inner one. It is essential for the suspension to gain negative camber in jounce so as to ensure better contact and traction between the tire and the road surface. It will compensate for the effect due to centrifugal force and restore the tire to the initial position.

3. What do you think would happen if the roll center was:

At the CG Height - The vehicle would not roll at all as there will be no distance for the generation of the moment arm.

At the ground level - The vehicle would roll more as there will be significant distance for the generation of the moment arm.

Below the ground level (Think in terms of loading and unloading) - This creates an inverse jacking effect. The non-rolling moment acts in the opposite sense. The inner wheel is loaded during a turn and the outer wheel is unloaded.

4. With regards to camber control, what is the disadvantage of McPherson strut?

- As there is the absence of an Upper Control Arm (considered as the top mount), there is the lesser liability of choosing the camber angle in a McPherson strut.

5. Can you prevent rear spring from extension during braking? (at least theoretically). If yes, Explain how would you set up the suspension to eliminate compression.

- Yes, we can prevent rear spring from extension during braking with a perfect (100%) Anti Dive. This would ensure that the entire load is acting on the front spring. We can set up the rear suspension so that the swing arm passes through the center fo gravity (CG) of the vehicle.

6. What is the difference between the wheel center rate and spring rate?

- Wheel center rate can be defined as the upward/downward movement of the wheel whereas the spring rate is the extent to which the spring would deform on the application of a force. Both these quantities are related on the basis of motion ratio (installation ratio) as follows;

Wheel center rate = Spring rate * (Motion ratio)^2

7. For reverse Ackermann, does the outer wheel steer more or less than the inner wheel?

- The outer wheel steers more than the inner wheel for a reverse Ackermann condition.

8. If the turn center for front wheels is at a distance of 4000 mm behind the front axle and wheelbase is 2700mm. What is Ackermann?

- Ackermann = Turn center from the front axle/Wheelbase `= frac[2700][4000] = 0.675 = 67.5%`

9. Why is negative caster undesirable?

- A negative caster is undesirable because it affects the stability of the vehicle by creating an unbalanced moment.

10. How does the revoknuckle geometry help reduce torque steer?

- In revoknuckle geometry, the knuckle and strut are separated which tends to lower the distance between the wheel axis and the knuckle axis and hence, the offset i.e. scrub radius. This helps to reduce the torque steer.