Q Am I correct in assuming that raising the roll
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centre improves the roll stability of a vehicle and if so is this because it reduces the distance between the roll centre and the centre of gravity? Secondly, how can roll centre be determined and could you explain how it is possible to calculate the centre of gravity?
AYou are correct in assuming that raising
the roll centre of a suspension increases roll stability, that is it reduces weight transfer when cornering, given that the spring base is not reduced. In a particular case, employing independent suspension could reduce the roll centre height but the increase in spring base obtained would more than offset the greater tendency to roll. In practice a roll axis is plotted by joining the roll centres of the front and rear axles, and the distance between the centre of gravity and the axis is measured. The greater this distance the greater is the weight transfer when cornering.
In the case of a typical leaf-spring suspension the roll centre is the centre of a line drawn between the centres of two lines connecting the shackle pins of the springs and normally lies on a point considerably above the axle. In the case of a swing type of independent suspension (which has recently been applied to semi-trailers) the roll centre is determined by drawing a line between the tyre contact point and the pivot point and continuing the line until it crosses the vertical centre line of the vehicle. The line drawn through the points mentioned on the other side of the suspension should meet the first line at the roll centre.
The roll centre of an independent suspension based on wishbones varies in some cases according to deflection. To determine the roll centre, lines are drawn through the pivot points of each link and continued until they meet, and a line connecting this link centre is continued until it meets the centre line of the vehicle, which is the roll centre. If the suspension is of the parallel link type with wishbones of equal length (which is now rarely used) the lines meet at infinity and the roll centre is at ground leveL In modern wishbone suspensions, a longer lower link is normally employed with a shorter upper link, and the links are parallel when the vehicle is unladen and stationary. With an increase in load the rollcentre height may be initially increased and therefore reduced (possibly to below ground level) so that under. critical conditions, roll resistance decreases with weight transfer.
The centre of gravity of a vehicle is determined empirically by separating the total mass into a series of component masses. Taking two masses at a time, it is possible to determine the centre of each weight system by the normal method of moments, and this is repeated until the centre of the total mass has been obtained. In practice, the centre of gravity of the whole vehicle, laden or unladen. can be ascertained by tilting the structure in a rig until it is on the overturning point. This is rarely feasible with a laden vehicle, but calculation of the centre of gravity is fairly straightforward (on the lines indicated) if the load is evenly distributed.