This is not a full fledged treatise but a general outline as to how the items come together.
There are basically two types of shocks, rebuildable and not.
It has been a well documented fact that BMW shocks, specially the rear can loose their effectiveness around 30K miles. This item needs to be qualified on the basis of how the bike is being used and the skill level of the rider.
The Boxer is fitted with a style usually referred as coil over, where the shock and the coil spring are incorporated in one unit.
Taking the shock as an entity and removing compression and rebound adjustments rom the equation, the critical dimension is the distance (center to center) of the mounting points.
Critical measurements that affect the coil springs are the number of free coils (complete coils) and the OD of the coil, as those define the compressibility of the spring, usually referred to as the K constant, the units being pounds/inch compression. (kg/mm)
Let us now look at what are the issues
Shock center to center distance
As long as the mounting points are the same, hole in bushing or clevis, as the original fitment, the center to center distance is within 1/2" (to be safe), the bike swing arm can accommodate the increase/decrease in swing without any interference with the chassis/body of the bike the shocks will probably fit. The best way is to visually compare or request the above measurements from the seller. Let me explain:
Our Boxers have about 4+ inches of travel at the rear, if you install a 1/2" longer shock (with the same K constant coil spring) the rear of the bike will go up the amount of the length increase times the length of the acting arm (that would be the length from the center of the shock lower attachment point to the center of the rear wheel), exactly the opposite will happen if the shock is 1/2" shorter (we are also assuming that the preload, is out of the equation)
Basically the same will also take place at the front. Let me add another little twist, let us assume that your replacement is 1/2" short than original, everything else being equal and you want to retain the original front height, assuming the shock has enough thread at the top stem one can shim the distance (between the shock and the mount) the required amount to attain the desired result.
Coil springs
These are selected at the factory based on the weight of an "average" rider and adding, sufficient preload capacity, in our case, to accommodate two up riding and luggage. Generally a weight around 170 pounds has been used to size the spring. Marketing size riders, like yours truly, will find the basic setting too soft and will generally increase the preload to 80% to 90% of max and so will those riders that will track the bike, but I digress.
OEM vs. aftermarket shocks
Generally speaking it's all in the eye and pockets of the beholder and since our training is based on getting the best for the least amount it really becomes a personal choice. However, in this case the common denominators are:
OEM - Expensive and not rebuildable
Aftermarket - Typically 1.5 more expensive but they are better quality, last longer and are rebuildable
Another obscure solution - There is an outfit in Canada, the name escapes me, which will rebuild any non rebuildable shock for a couple of hundred dollars. This outfit has its niche in the antique and collectable market. Essentially do is mount the shock on a lathe and cut the tube at the bottom, they clean the parts, replace the seals, add oil and weld it shut.
Coil spring replacement - Normally this will take place under the following conditions A) It breaks ,rare B) The rider's avoirdupois is well beyond the original fitment and C) The bike will be used under a very specific set of conditions.
Hopefully this will help those that will be venturing into the "shock zone" in the near future......
) and will post piccies sometime this weekend. I'd say it's roughly 1/2" shorter, at minimum preload.
(Given that the Rockster's main failing in terms of a street-fighterstyle bike is it's power output).
