Friday, December 16, 2016

Improving the Royal Enfield four-speed shift linkage

The left-shift four-speed linkage is held on by this pincher mechanism.
But notice that it hangs off the end of the shaft. Shifting action soon loosens it.
But notice, too, that the shaft insde the pincher is threaded for a grease fitting. A ha!
Part 3

The Royal Enfield Bullet four-speed transmission shifts on the left in the United States, thanks to a linkage that runs behind the motor (the gearbox itself remains on the right).

It was required by U.S. law for officially imported motorcycles. No doubt Americans do like to shift on the left.

But the linkage introduces slop. Shifting precision (Ha! As if there was any of that) suffers.

This is the third in a series of posts about how I tried to improve shifting by tightening up the linkage shafts and arms on my 1999 Royal Enfield Bullet.

Here I once again confront the fact that the linkage is a bodge, crudely made.

A pincher bolt attaches the linkage to the shaft that carries the shifting motion across the bike to the gearbox on the right. On my Bullet, this pincher mechanism hangs off the end of the shaft.

Only half the length of the teeth inside the pincher engage with the shaft. Perhaps it is made this way because the linkage arms couldn't make it down to the gearshift if the pincher was farther to the right. The linkage arms are trapped right where they are between the motorcycle frame and the primary drive.

This means that every time you shift gears the linkage tries to push or pull the pincher mechanism off center. Keep in mind that this mechanism lives in a nice warm environment behind the motor, bathed in vibration and oil thrown off by the chain.

Inevitably it loosens and wobbles, introducing motion into the shifting process and ruining the teeth the pincher bolt relies on to keep things attached.

Would this help? A bolt, washer and lock washer.
It finally dawned on me that the shaft that carries the shifting across the motorcycle is tapped at the linkage end, to accept a grease fitting. There is no grease nipple fitted, but the threads are there and I decided to make use of them.

I used a fine-thread 8.8 bolt with a 10mm head and 20mm of thread. With a washer and lock washer it went right in.

It is my hope that this will help hold the pincher mechanism on and combat wobble.

The bolt and washers should help hold the mechanism in place.
Compare this to the photo at the top of this post.
So I went for a test ride.

Results? Shifting is still bad. Clunky is a kind word for it, particularly when attempting to get into first gear.

My suspicion is that in tightening up the linkage at every point I have introduced some interference at some hidden point between the frame and the primary drive. If so, that should leave marks that will help me make further adjustments next time shifting becomes unbearable.

Part 1: Improving the gearshift action.

Part 2: Improving the linkage bushings.

Part 4: Improving the linkage fit.

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