Motorcycles: Adjustable Rear Spring Rate

Motorcycles need adjustable spring rates on their rear suspensions much more than a car would. This was not likely to happen because most people still think (wrongly) that the usual preload adjustment actually was "stiffening" or "softening" the rear spring. So I was very happy to see that a motorcycle with a true adjustable spring rate has been introduced.

Not surprisingly, this technical innovation appeared on the German BMW K1300GT/S models. Since the late 80's BMW motorcycles have shown a remarkable no-nonsense approach to motorcycling innovations. Harley Davidson has resolutely stuck to it's old V-twin designs, and it's greatest suspension changes have come in the form imitating the look of an old fashioned hard tail suspension or of the old springer forks. The engineers at BMW have more often erred on the side of doing something really new, which nobody ever expected, and which nobody even knew they needed until BMW produced it. One example would be BMW's adjustable windshield, which was at first mocked, and then furiously imitated, because it really works, and we really need it.

A word about the K1300 models first. When BMW motorcycles first went to water cooling in the eighties, there was a strong backlash against this heretical form of keeping cylinders at a constant temperature. In order to keep their faithful customer base happy, BMW had to backtrack and come up with a new two cylinder motorcycle that did not use water based coolant. In other words, like they used to be, for customers who were not ready to move into the modern world. From that time forward, BMW has kept two roughly equivalent models, one for the traditionalists, and the other for the technologically inclined. The K1300 is the bike that carries the technology banner for BMW. And if there were such a person as "The most interesting man in the world" I'm pretty sure he would ride this bike, and not the air cooled R1200 which is the traditionalist model. Here is a link to a video of what it's like to ride the water cooled K1200GT model with about 150 horsepower.

http://www.youtube.com/watch?v=4ds2SHwfJlI

OK enough BMW insiders propaganda for now, and back to the actual suspension.

The adjustable spring rate was accomplished by adding a cushion ring on top of the spring, which is actually the only adjustable part. This cushion absorbs the smaller bumps, and when it is fully compressed, then the main steel coil spring begins to to its job. Here is a link to show the details. It is called ESA 2 for electrically adjusted suspension 2. The first ESA did not adjust spring rate.

http://www.i-bmw.com/showthread.php?t=18298

I was hoping for a rear suspension using a crank lever similar to my Vulcan 900, except with an electrically adjustable extra link. But I don't know how much more expensive it would be, or even if it can be done at all. The only disadvantage would be you could only put it on the back, while this BMW cushion system can work on any coil spring.

Until I become the most interesting man in the world, I can approximate the BMW adjustable spring rate, and save my rear end, by strapping an old air mattress to the top of my motorcycle seat.