R51 has adopted EZ steer for triple tree of front fork to set the sidecar.
It has special structure that more you turn the steering, the trail length decreases.
Normal triple tree shaft is parallel to the fork, but the upper part of R51/2 is a little tilted as can be seen in the picture.
The reason is...
Generally, straight-line stability increases when the trail length increases during straight-running. In the case of the sidecar, it has a characteristic that steering become heavy when turning, so EZ steer is the mechanism to mitigate that.
EZ steer is the earlier system than the motorcycle adopted Earles forks such as R50, R60, R69, which is famous for the sidecar. It seems to be adopted to some trial motorcycles even now.
By the way, Earles forks are named after Ernie Earles who invented them.
BMW has adopted a lot of innovative suspension such as paralever suspension and telelever suspension. Other manufactures should be learned from them.
The last is a picture of the fork spring and the fork.
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