Figure 2
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Lowering |
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Too low can increase roll couple |
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The figure below shows why it might
be bad to lower an E30 M3 too much. This applies to the front suspension
which consists of MacPherson struts. In fact, it applies to any car which
employs Mac Pherson Strut suspension - and that includes the front of most all BMW's
The figure shows how one determines the roll center on a strut setup. This is somewhat idealized, since the instantaneous roll center is really computed by considering the left and right side suspension simultaneously. However, just looking at one side will suffice for this example. |
Figure 2
| When cornering, the centripetal force which acts through the CG will act to rotate the vehicle about the roll center. The distance between the roll center and the CG determines the lever arm with which the centripetal force cause the car to roll. This is called the "roll couple". The longer this lever arm is, the more the car will roll for a given G-loading. |
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The key point is that as a MacPherson strut suspension is lowered, the length of
this lever arm increases. Just compare A1 to A2 in the figure
above. Thus even though the car is lower,
it will tend to roll more! This is not what many people expect
when they lower their cars.
There are benefits to lowering your car - reduced weight transfer and less "jacking". And there are also disadvantages - increased roll couple, a negative effect on camber curves and possibly increased bump-steer. Thus, lowering your car involves a trade off. Lower is often better, but too low is not good. If you stiffen the front springs at the same time that you lower your car (this is usually the case) then the stiffer springs offer an increased roll rate to counter-act the increased roll couple. In this case you get reduced weight transfer without necesarilly increasing roll. And often the reduction in weight transfer will offset the reduced camber curve and bump steer effects enough that the car will ulitmately be faster around a track. However, you should not lower your car without at least being aware of the possible negative consequences. |
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