A
Alvin
Guest
Most of us lower are cars by installing shorter springs. This is the easiest way to achieve lowering. An alternative, more expensive way of lowering a car is to relocate the hubs and spindles upwards. This is ideal since it lowers the car without changing any of the suspension geometry. Indeed this is how Super Touring cars achieve their extremely low ride heights. Designers in that series are not allowed to change the suspension pickup points by more than 20 mm and they must keep the factory "type" of suspension (i.e. a strut must remain a strut, a trailing arm must remain a trailing arm etc.). But they most certainly do re-manufacture the struts and suspension components. This allows them to relocate the hubs farther upwards which in turn gets their cars very low. That's great, but for the majority of performance enthusiasts using shorter springs is the only viable way to lower a car. What are the disadvantages of lowering a car by shortening the springs? The primary disadvantage of using shorter springs to lower a car is that the suspension travel is reduced. This is shown in Figure 1 above which displays a schematic of a car with front strut suspension. Caster is set to zero to aid visualization. Note the reduction in suspension travel that occurs when the car is lowered (shown by the red circle). This happens because using a shorter spring brings the upper strut bearing (or camber plate) closer to the top of the strut housing. This pushes the bump stop down and since travel is defined as the distance between the bump stop and the top of the strut housing, travel is reduced. The only way around this is to either use a shortened strut housing or to cut the bump stop. Shorter strut housings (and correspondingly shorter struts) have been available for some cars in the past, but are not available for BMW's to my knowledge. Cutting the bump stops is therefore the only alternative. Cutting the bump stops may seem like a bad idea at first. After all, the bump stop is meant to cushion the impact when the limit of travel is reached due to a particularly large bump. But the fact is, most of the shorter springs that people install to lower their cars are also stiffer. Thus although a lower car has less suspension travel, it also needs less travel since a stiffer spring does not compress as far as a softer spring under identical conditions. So if you can achieve the proper combination of travel and spring rate, then your suspension is less likely to even reach the bump stops. The interaction of the bump stop with the spring is part of a system analysis that will require a separate write up. Suffice it to say that the degree to which you can trim your bump stops is linked to how stiff your springs are. If you lower your car and do not trim the bump stops sufficiently then you will be contacting the stops frequently which will add a highly progressive effect to your overall spring rate. On the other hand, if you trim the stops too much then, when your suspension does hit the stops, the impact will be rather severe. Ultimately, matching bump stops to spring rate and ride height is a trial-and-error process. Figure 1.
