How and why is the fork shortened on the two-valve BMW Boxer models to lower the forks

BMW Boxer shorten fork
Why shorten fork:
The original reason to shorten the forks of the BMW 2-Ventiler models was to improve the handling. The classic BMW 2V boxers are originally designed rather rear-heavy. This makes the front wheel feel and handling not really good, at faster speeds the stem becomes light and the BMW starts to lurch. 
The first measure was to install new and length adjustable shocks. The original components were inherently too soft, the spring force has naturally decreased further in 30 years. In addition, these old shock absorbers did not have a well-functioning damping system. Replacing the shocks was a significant improvement, but we were not yet satisfied with SE Concept Bike. 
The second measure was to lower the front end. The handling basically improved with every millimeter we lowered the fork. In the -50mm range, the feel of the front wheel was good and the handling precise. Ride comfort did not deteriorate significantly.
What happens if I shorten more? If the fork is shortened more than 50 mm, you no longer achieve any significant improvement in handling. However, comfort decreases with every millimeter. In extreme cases, when forks are shortened 120mm, there is no remaining travel. A monolever has a total travel of 175mm. If you subtract the negative travel of about 40mm, you still have 135mm left. If you shorten the fork by 120mm you have an effective travel of only 15mm. 
Shortening the fork, but how?
There are two ways to shorten the BMW two-valve forks. I'll start with the professional solution favored by SE Concept Bike: 
Here, the upper fork tubes (standpipe) are shortened on a lathe by the amount by which the fork is to be lowered. The corresponding layout of the fork tubes from the cut-off area, i.e. threads or grooves, is transferred back to the shortened fork tubes, i.e. reworked. Since the upper parts of the fork legs are now shorter, this also requires correspondingly shorter fork springs. The advantage of this measure is that you basically only take the positive effects, since the spring travel is largely retained. 
The second option is to reduce the travel of the original damping system (lower section of the original fork) with spacer sleeves. Here, however, it is the case that with every millimeter by which you lower the fork, you also shorten the spring travel. Other fork springs are also required for this measure. For this reason, we favor lowering BMW forks by shortening the stanchions in conjunction with new spring sets.
At this point a very important note: It is strongly discouraged to shorten the original fork springs. Shortening a spring changes the spring rate. On top of that, the original springs are progressive. Where do I cut the spring? At the top, in the soft area, or at the bottom, in the hard area? We have often driven vehicles with shortened springs. The handling is poor, even dangerous on rough roads.
How do I proceed if I want to lower my fork?
The matter depends on whether only the removed and cleaned stanchions are delivered or alternatively the complete fork. 
In case of delivery of the single stanchions: The fork stanchions will be shortened by us, for the assembly, which you will then carry out yourself, you will still need a suitable spring set.  
In case of delivery of the complete fork: In this case you will need a fork revision in addition to the items "shorten fork stanchions" and the suitable spring set. In this case, you will receive a ready-to-install fork. 
Can the lowering of the forks on BMW two-valve models be registered?
A registration in the technically tolerable range (50-80mmm) can be carried out by individual acceptance. We have not created a parts certificate for this, the demand for it is simply too low. Especially since such a parts certificate is quite cost-intensive and would thus increase the price of the product.
As a rule, customers do not register the lowering. If the shortening is in the range of 50-80 mm, it is not really visible from the outside. To answer your question: If you want to do it correctly, the lowering / fork shortening must be registered. However, it is usually not handled in this way.