Suspension & Weight Transfer (by KNIGHT)

Questions and answers on steering, suspension, bush replacement, wheels, tyres and much more

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Suspension & Weight Transfer (by KNIGHT)

Post by Russ » Fri Aug 13, 2004 7:57 am

A vehicle's suspension needs to do a number of things, primaraly absob shocks, and maintain tyre contact with the road surface. Ideally, the vehicle with the most grip would be one with ridgid suspension, but as road and track sufaces are uneven, shocks could lead to tyres jumping off the surface and loosing grip, and possibly control of the vehicle, not to mention the ride quality being abysmal. for this reason, track cars have minimum hard suspension,and road cars will have be given more comfort.
So accepting that all vehicles will need suspension, next you will have to minimise the negative effects of this
suspension will control the cars movements under two conditions, bump and roll, where bump is the movement of one or more wheels relative to the chassis, and roll, where the chassis leans with cornering forces on
a level surface. Bump and roll can of course be combined in uneven corners, where the suspension has to deal with roll and movement of individual wheels.
Double wishbone suspension does the best job of keeping the tyres perpendicualy to the surface of the road, but there are a number of different types.

Equal length, parallel wishbones.
the first is equal length parallel wishbones, where the body is connected to each wheel by two parallel bars of equal length.
imagine this as 2 square cardboard tubes, joined together by one side of each, like a squared figure of eight on its side. the two walls would represent the body, and the two other walls would represent the tyres, under bump comditions, one wheel would rise, remaining perpendicular to the road (ie square on tyre contact), but the track would change (the "track" being the distance between two wheels on the same axle, ie the right to left wheel distance) and this decrease pulling the tyres will force the tyres to slip to their new positions, and result in a decrease in grip.
under roll conditions though, as the body rolls 5 degrees, due to eqaul length wishbones, both wheels will also roll 5 degrees (on the box model, imagine sliding the top one direction, and the bottom the opposite, and the centre two walls of the box show the angle the car body is leaning)
this will cause a change in wheel camber, and result in a less effective grip on the surface, and induce non neutral steering (ie over steer or understeer) and although the track distance remains the same, each wheel will be forced towards the outside of the corner as a result of the car leaning, which again forces the wheels to slip to their new positions
and decrease grip.

Unequal length parallel wishbones
to remedy most of these problems, unequal length wishbones were designed.
this can be imagined as the body being wider at the top, and narrower at the bottom, so the connections to each wheel would be shorter on the upper wishbone, and longer on the bottom wishbone.
under roll conditions the outer wheel (which takes more weight due to weight transfer) will roll less, and remain more incontact with the road surface, and the change in track distance is minimal. (but the inner wheel will infact roll more than the body, but as this is taking less weight, no effect in minimised)
Under bump conditions, there is a slight change in track on the wheel that has lifted, as it pivots in towards the body, and a slight trade off as the unequal length wishbone casuse a slight change in camber too.)

unequal length, non parallel wishbones.
In this set up, the wishbones are the same unequal length, as described above, but now, they are not parallel, and are closer together on the internal chassis side.
imagine the chassis as a V shape, with 2 "V" 's coming out from each side, and the wider sections attatching to the wheels.
This will almost cut out all camber changes of the loaded (outside) wheel under roll conditions, but in a bump, the camber changes dramatically.
this setup would be ideal for track cars where cornering and body roll needs to be controlled, and the outside loaded (taking the weight transfered over) wheel remains close to perpendicular to the surface, and dealing with bumps is minimised.
so now that the wheels can be kept under control, you need to take into account the transfer of weight.

weight transfer

as mentioned above, when taking a corner, weight will be transfered to the outside wheel, and away from the inside wheel. this is done by 2 methods.
the first is by the centrifugal force, and can be calculated by this formula.

Weight Transfer = (Weight of vehicle x height of the CG x lateral acceleration) / track

CG is the centre of gravity of the car, and lateral acceleration is the g force felt during cornering.

to give an example take a car with a track of 1.6m, a centre of gravity 0.6m high, and a weigh of 1300kg, cornering with a lateral acceleration of 0.7g.

(1300 x 0.6 x 0.7) / 1.6m = 341.25 kg

so, taking a 50/50 weight distribution, each wheel would carry 325kg when static, but in this example, the outside wheel would carry
325kg + (0.5x341.25) = 495.6 kg,
and the inside wheel would carry
325 - (0.5x341.25) = 154kg.
therefore, you can see that the outside wheel is doing far more work, and hence needs to be kept in contact with the road more, hence the unequal nonparallel is beneficial in high speed cornering.
weight transfer can also be caused by body roll. as the vehicle rolls, it rolls around the Roll Centre, which can be assumed to be a point on the roads surface, and as it does this, the centre of gravity is also rolled over, away from the centre of the vehicle, towards the outside wheel.

to calculate this, you need to calculate how far the centre of gravity has moved over.
taking underneath the centre of the vehicle to be the roll centre, and using the vehicle data above, for a 5 degree roll, the distance "d" the Cg has moved can be calculated by
600mm x sin5 = 52.3mm
the outside load is calculated by
(1300kg x (800mm + 52.3mm))/ 1600 = 692kg for both outside wheels, and therefore
(1300 - 692) = 608 kg for both inside wheels.
giving a total of 84kg weight transfer.

As a tyre gets loaded (transfer of weight on to it) the grip it has increases, but at a declining rate, so as more and more weight gets transfered, the grip increases, but at a slower and slower rate.

this means that as the weight is transfered, the outside tyre gains grip, but the inside tyre looses more, and total grip decreases.
more weight transfer, less total grip.
so to increase cornering grip, you must minimise weight transfer, by lowering the centre of gravity, reducing the weight of the car, or by makeing the track wider, and by making the car more resistant to roll.

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