Vehicle dynamics basic principles

Vehicle dynamic basic principles:

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

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

3, What do you think would happen if roll center was

• At the CG Height.
• At the ground level.
• Below the ground level(Think in terms of loading and unloading).

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

5, Can you prevent rear spring from extension during braking? (atleast theoretically).If yes,Explain how would you setup suspension to eliminate compression.

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

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

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

9, Why is negative caster undesirable?

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

Answers:

1. Four degree of restraints – as it have two degree of freedom (vertical motion+roll)
2. To keep the Tyre perpendicular to the ground during Cornering which increase the contact patch area which increases the grip which results in higher cornering force generated
3. Rc at CG Height-

As distance between CG and Rc is zero so the rolling moment which is between the center of gravity and roll center will be zero because of no arm and there will be no elastic weight transfer.which means there will be no body roll as there is no moment which has to be restricted by springs and dampers and the car will act as a rigid body

But, the lateral force will result in another moment which is the overturning moment acting between the roll center and the ground which will act on suspension links trying to overturn the vehicle due to geometric(inelastic) weight transfer

Rc at Ground Level -

All the lateral force will be turned in a rolling moment So, the Lateral Load is transferred directly from chassis/body to tyre through springs/dampers as there is no overturning moment resisted by suspension links.

Rc below Ground Level –

Same as first case ,but the inner wheel will be loaded and outer wheel will be unloaded, as the overturning moment direction will reverse and the body roll will increase because of the larger arm but the links will resist rolling because of the overturning moment will be acting in a reverse direction compared to the rolling moment

4. Mcpherson strut can only provide little negative camber gain only at the beginning of jounce travel and then reverses into positive camber gain at high jounce amounts while SLA double wishbone provide increasing negative camber gain all the way to full jounce travel.
5. Limiting rear spring extension means preventing dive in front suspension.This can be done by ensuring we satisfy this relation:- [ e/d = h/L ]

e= distance of IC in side view from ground

d= distance of IC is side view from Wheel-ground contact point

h= CG height

L= Wheelbase of vehicle

6. Wheel Rate = Amount of Force at the wheel that gives particular amount of Travel at the wheel. Kw=ks (a/b)^2

Where “a” is distance between spring and pivot point

And “b” is the distance between wheel and pivot point

Spring Rate = Amount of Force needed to displace 1 unit of spring/damper e.g. 100N/mm, it means 100 N required to move springs by 1 mm.

7. outer wheels will turn more
8. 67.5 % Ackermann
9. The restoring force and moment is acting in the direction of steer which will result in loss of steering stability. As the steering will be very amplify the human torque. Also loss of straight line stability can happen.
10. Revoknuckle reduces torque steer by reducing the scrub radius which reduces the Torque Steer Moment (difference between moments each one=Longitudinal Driving Force * scrub radius) .