Something about Virtual Suspension (4)

Aug 26
19:30

2013

Olivia Tong

Olivia Tong

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There’s one final concept that ties these ideas together. A car’s centre of gravity is another ‘virtual’ point. If you imagine all the car’s mass concentrated at one point, you have the idea of the centre of gravity.

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Roll Centre and Centre of Gravity

There’s one final concept that ties these ideas together. A car’s centre of gravity is another ‘virtual’ point. If you imagine all the car’s mass concentrated at one point,Something about Virtual Suspension (4)    Articles you have the idea of the centre of gravity. 

In different cars the centre of gravity can be at different heights above the ground (if the heavy objects in a car are low, so will be the centre of gravity), and it can be closer to the front or to the rear of the car. (For example, a rear engine car will have a centre of gravity that is often rearwards.) In most cars, the centre of gravity is aligned on the central longitudinal axis of the car.

 

So what has the centre of gravity got to do with suspension? If the centre of gravity is higher than the roll centre, the car will lean outwards when being cornered. In fact, the higher the centre of gravity is compared with the roll centre, the greater will be the lean for a given cornering acceleration.  Therefore, lowering the roll centre increases body roll! If the roll centre is at the same height as the centre of gravity, no roll will occur. If the roll centre is above the centre of gravity, the car will actually lean into the corner.

 

Let’s have a look at why these characteristics occur. We know that the cornering forces all get fed through the roll centre, and that the centre of gravity can be thought of as being the point where all the car’s weight is concentrated. If in this diagram there’s a force pushing towards the left at the roll centre, the car body will roll to the right. That’s because the vertical distance between the roll centre and the centre of gravity creates a virtual lever arm (red).

 

If the roll centre and the centre of gravity are at the same point, no lever arm is developed and so no roll occurs. 

Finally, if the centre of gravity is below the roll centre, a lever arm will develop that causes the car to lean the opposite way – into the corner. Now of course this sounds very attractive, but it comes at the cost of having a high roll centre – which in turn causes the jacking (and other) problems we referred to earlier. 

Conclusion

It’s very easy to get lost in suspension design, especially when obscure terms are thrown about freely. However, by starting with simple suspension systems, where the ‘virtual’ ingredients can often be actually seen, understanding what the ideas mean becomes a much simpler process. 

 

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