All of this comes into play when you put shorter springs into your BMW. For every inch you lower the car using shorter springs, you loose an equivalent amount of suspension travel. I've seen cars with springs so short they are actually sitting on the bump stops! Most BMWs with off the shelf "lowering springs" using factory strut mounts and bump stops typically have only around 1" to 1.5" of suspension travel. Not a good situation!

So what happens when a lowered car runs out of bump travel and contacts the bump stops?

1. Very rough ride characteristics. With the reduced suspension bump travel, even small bumps will use up all of the available travel. We've all seen cars driving around (typically manufactured in Japan) that are lowered to the point that every imperfection in the pavement upsets the entire chassis. This is a result of little to no bump travel.
2. Extreme steady-state understeer or oversteer. Under hard cornering conditions, the chassis rolls on the suspension. The outside suspension compresses, and the inside suspension rebounds. A frequent problem is that the chassis roll causes the front or rear suspension to run out of travel and contact the bump stops. If this happens in the front suspension, a severe mid corner understeer problem will occur. If the rear suspension bottoms, a severe mid corner oversteer will be the result. This is a real problem with a lot of BMWs that I've seen, typically in the front suspension causing understeer.
3. Instability during abrupt direction changes. A pretty common problem typically seen in autocrosses. The car handles fine through steady state corners, but either oversteers or understeers through quick transitions. This problem can have many causes, one is lack of bump travel combined with soft shocks and struts. The car will corner in one direction and the chassis will roll from the cornering forces. When the driver quickly changes direction, the chassis will roll in the opposite direction and receive a 'boost' from the inside springs that are still compressed. This will in some cases temporarily put the outside suspension on the bumpstops. This effect can really wreak havoc through a slalom!
4. This is a variation on number two. The car will roll on the suspension during hard cornering, but not quite to the point of contacting the bump stops. Then the slightest bump will use up the remaining bump travel and induce sudden and extreme understeer or oversteer. This type of oversteer can be particularly dangerous! Another symptom of this is a car that is very unsettled driving over FIA curbing at the track.

There are two solutions:

1. Increase the Spring Rates (and possibly sway bar rates). While not increasing the amount of bump travel, the stiffer spring will help keep the suspension from bottoming.
2. Increase the amount of available bump travel.

There are 5 ways to increase the available bump travel.

1. Raise the ride height. Since we are interested in lowering the ride height for performance reasons, we won't consider this to be an option.
2. Modify the suspension pickup points by cutting and rewelding. This is a popular solution in an all out race car, but illegal in most racing classes and beyond the scope of 99% of the BMW enthusiasts out there. We'll ignore this as an option.
3. Substitute the factory upper strut mount for an alternate adjustable camber plate with a shorter stack up height. Note that all aftermarket camber plates are not created equal! Some didn't even consider stack up height when they were designed and are actually worse than the factory setup! Vorshlag camber plates have some of the shortest stack up heights in the BMW aftermarket industry.
4. Substitute a shorter bump stop. Either an aftermarket bumpstop or trimming the factory bump stop is an option. The can cause trouble if not done carefully. This will be the subject of a future tech article. DO NOT be tempted to completely remove the bump stop!! You will most likely damage the strut, shock, car and/or yourself!! Don't do it!
5. Substitute custom shortened strut housing. Several companies sell shorter strut housings, done properly this is a good option. But like most aftermarket parts, all are unfortunately not done properly. We'll have a future tech article about this, along with a full line of shortened Koni shocks and struts.