Shortening and lowering BMW Boxer and K-model forks
Why shorten forks: The original reason for shortening the forks of BMW 2-valve models was to improve handling. Classic BMW 2V Boxers are originally designed to be rather rear-heavy. As a result, the feel for the front wheel and handling are not very good, and at higher speeds, the front end becomes light and the BMW starts to wobble.
The first measure was to install new, length-adjustable shock absorbers. The original components were inherently too soft, and their spring force naturally decreased further over 30 years. Additionally, these old shock absorbers did not have a well-functioning damping system. Replacing the shock absorbers was a significant improvement, but we at SE Concept Bike were not yet satisfied.
The second measure was to lower the front. The handling basically improved with every millimeter we lowered the forks. In the -50mm range, the feel for the front wheel was good and handling was precise. Riding comfort did not significantly worsen.
What happens if I shorten more? If the fork is shortened by more than 50 mm, there is no significant improvement in handling. However, comfort decreases with every millimeter. In extreme cases, if forks are shortened by 120mm, no residual spring travel remains. A Monolever has a total spring travel of 175mm. If you subtract the negative spring travel of approx. 40mm, 135mm remain. If you shorten the fork by 120mm, you only have an effective spring travel of 15mm.
Shortening forks, but how?
There are two ways to shorten BMW two-valve forks. I'll start with the professional solution favored by SE Concept Bike:
The upper fork tubes (stanchions) are shortened on a lathe by the amount the fork is to be lowered. The corresponding layout of the fork tubes from the machined area, i.e. threads or grooves, is transferred back to the shortened fork tubes, i.e. re-machined. Since the upper parts of the fork legs are now shorter, correspondingly shorter fork springs are also required. The advantage of this measure is that you basically only get the positive effects, as the suspension travel is largely maintained.
The second option is to reduce the suspension travel of the original damping system (lower part of the original fork) with spacers. Here, however, with every millimeter the fork is lowered, the suspension travel is also shortened. 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.
A very important note at this point: It is strongly advised against shortening the original fork springs. Shortening a spring changes the spring rate. Moreover, 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, and on uneven roads, it is even dangerous.
How do I proceed if I want to lower my forks?
The matter depends on whether only the removed and cleaned stanchions are supplied or alternatively the complete fork.
If individual stanchions are supplied: The fork stanchions are shortened by us; for the assembly, which you then carry out yourself, you will need a suitable spring set.
If the complete fork is supplied: In this case, in addition to "shortening fork stanchions" and the appropriate spring set, you will also need a fork overhaul. In this case, you will receive a ready-to-install fork back.
Can the fork lowering on BMW two-valve models be registered?
Registration within the technically tolerable range (50-80mm) can be carried out by individual approval. We have not created a parts certificate for this, as the demand for it is simply too low. Especially since such a parts certificate is quite costly and would therefore make the product more expensive.
Customers usually do not register the lowering. If the shortening is in the range of 50-80 mm, it is not really recognizable from the outside. To answer your question: If you want to do it correctly, the lowering/fork shortening must be registered. However, this is usually not how it is handled.
Why shorten forks: The original reason for shortening the forks of BMW 2-valve models was to improve handling. Classic BMW 2V Boxers are originally designed to be rather rear-heavy. As a result, the feel for the front wheel and handling are not very good, and at higher speeds, the front end becomes light and the BMW starts to wobble.
The first measure was to install new, length-adjustable shock absorbers. The original components were inherently too soft, and their spring force naturally decreased further over 30 years. Additionally, these old shock absorbers did not have a well-functioning damping system. Replacing the shock absorbers was a significant improvement, but we at SE Concept Bike were not yet satisfied.
The second measure was to lower the front. The handling basically improved with every millimeter we lowered the forks. In the -50mm range, the feel for the front wheel was good and handling was precise. Riding comfort did not significantly worsen.
What happens if I shorten more? If the fork is shortened by more than 50 mm, there is no significant improvement in handling. However, comfort decreases with every millimeter. In extreme cases, if forks are shortened by 120mm, no residual spring travel remains. A Monolever has a total spring travel of 175mm. If you subtract the negative spring travel of approx. 40mm, 135mm remain. If you shorten the fork by 120mm, you only have an effective spring travel of 15mm.
Shortening forks, but how?
There are two ways to shorten BMW two-valve forks. I'll start with the professional solution favored by SE Concept Bike:
The upper fork tubes (stanchions) are shortened on a lathe by the amount the fork is to be lowered. The corresponding layout of the fork tubes from the machined area, i.e. threads or grooves, is transferred back to the shortened fork tubes, i.e. re-machined. Since the upper parts of the fork legs are now shorter, correspondingly shorter fork springs are also required. The advantage of this measure is that you basically only get the positive effects, as the suspension travel is largely maintained.
The second option is to reduce the suspension travel of the original damping system (lower part of the original fork) with spacers. Here, however, with every millimeter the fork is lowered, the suspension travel is also shortened. 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.
A very important note at this point: It is strongly advised against shortening the original fork springs. Shortening a spring changes the spring rate. Moreover, 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, and on uneven roads, it is even dangerous.
How do I proceed if I want to lower my forks?
The matter depends on whether only the removed and cleaned stanchions are supplied or alternatively the complete fork.
If individual stanchions are supplied: The fork stanchions are shortened by us; for the assembly, which you then carry out yourself, you will need a suitable spring set.
If the complete fork is supplied: In this case, in addition to "shortening fork stanchions" and the appropriate spring set, you will also need a fork overhaul. In this case, you will receive a ready-to-install fork back.
Can the fork lowering on BMW two-valve models be registered?
Registration within the technically tolerable range (50-80mm) can be carried out by individual approval. We have not created a parts certificate for this, as the demand for it is simply too low. Especially since such a parts certificate is quite costly and would therefore make the product more expensive.
Customers usually do not register the lowering. If the shortening is in the range of 50-80 mm, it is not really recognizable from the outside. To answer your question: If you want to do it correctly, the lowering/fork shortening must be registered. However, this is usually not how it is handled.