Q I am particularly interested in the forces that cause a
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vehicle to turn over when it is negotiating a roundabout, notably an artic. I appreciate that the overturning couple is the product of the lateral force acting through the centre of gravity and the height of the centre of gravity. But what is a secondary roll couple?
And why is independent suspension an advantage with regard to roll resistance?
A If a vehicle had no suspension, a
secondary roll couple would not be produced. With any type of suspension, the spring mass tends to move outwards when a lateral force is applied to it on a corner or during a swerve and the force required to bring the mass to rest is added to the overturning force produced by the primary roll couple.
Roll stiffness is proportional to the square of the spring base. In the case of an independent suspension system, the effective spring base is equal to the track and the gain in roll stiffness may be as much as four times the stiffness obtainable with a conventional beam-axle layout. With some types of independent suspension, the roll centre is raised above its normal location and this also increases resistance to roll.