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E46 M3 Performance
How Lowering a Car Affects Weight Transfer - Page 2
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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 tend to roll for a given G-loading.
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 have a greater tendency towards roll. This is not what many people expect when they lower a car.

There are obvious benefits to lowering a car in terms of reduced weight transfer. Also, the lower roll center (which results from lowering a strut suspension) implies low instant centers for the two front wheels. And this means less lateral scrub of the front tires as they move up and down through their travel range (lateral scrub is not good as it can cause the car to wander left and right as the suspension encounters one-wheel bumps). Having instant centers (and thus a roll center) close to ground level will also reduce "jacking" forces. Jacking forces occur when the lateral force at the tire contact patch causes a torque (moment) around the roll center in such a way that the car raised up when cornering. This is generally considered a bad handling trait.

But there is rarely a free lunch in anything, and this is no exception. There are possible disadvantages to lowering a strut based front suspension: increased roll couple, a possible negative effect on camber curves and also possible increase in bump-steer. Note that by lowering the car one has not changed the geometry of the front suspension. Thus bump steer and camber curves are not really changed per se. It's just that we have moved our static condition to a different point on the original factory bump steer and camber gain curves. We may not longer be on the "optimum" point on the curves therefore. This is an often misunderstood distinction.

In conclusion, lowering your car involves a trade off.  Lower is often better, but too low is not necessarilly good either. 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 possible 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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