There are three locations for these gear-changing problems: 1) external to the engine, 2) inside the clutch cover (which means service does not require engine removal), and 3) inside the gearbox proper, service for which requires removing the engine from the frame and splitting the cases (exception: engines built with a so-called ‘cassette’ gearbox that can be removed from the engine in a few minutes).
In the first category, I always ask the rider if he/she has recently adjusted the height of the gear-change pedal. It can happen that a shifter operating through linkage jams against itself, preventing the mechanism from achieving full stroke. Another kind of problem arises if the change pedal can hit the pavement, in effect shifting itself. And yet another (I’ve only seen this once) is the bike whose change pedal is so heavy that hitting a hard bump can cause the inertia of the pedal to change gear.
Take hold of the pedal and move it slightly up and down against its centring spring. The movement should be easy and fluid, with no perceptible friction. Why would there be friction? Because at some time in the bike’s life it has fallen over on its change pedal, bending the shift shaft, creating heavy friction between the shaft and what supports it. A bent shift shaft can be replaced without engine removal.
The classic kinds of shifting problems are 1) halfshifting, in which you end up in a neutral between the gear you were in and the gear you wanted, 2) over-shifting, in which a shift sometimes puts you in a neutral that is beyond the gear you wanted, 3) refused shifts, in which movement of the shift pedal produces jerky motion from the bike, a grinding feeling, and inability to complete the shift, and 4), jumping out of gear.
The usual cause of half-shifting is that if the driving dogs in the transmission hit face-to-face rather than dropping neatly into engagement, surface roughness on the dog faces can prevent the dogs on one gear from slipping off and into engagement, with the result that the change pedal does not go full stroke and a between-gears neutral is found. This one is engineout-and-split to fix, and involves smoothing the dog faces against a piece of abrasive paper.
Over-shifting is caused by incorrectly set limits of shift-shaft rotation, inside the clutch cover (easy fix). Normally the shift shaft’s rotation is limited by something akin to an eccentric pin secured by a nut. It is desirable that the motion of the shift shaft be stopped by the pin about one millimetre before the shift drum drops into the detented position for the next gear. This is because otherwise, a fast-moving shift-shaft can continue driving the drum all the way to its detented position for the next gear, allowing its inertia to carry it past the detent and into a neutral beyond. Stopping the shift mechanism a millimetre before the detent allows the shift drum to be drawn the last millimetre into its next detent position by the detent spring.
Refused shifts are the worst because they can require replacement of gears and possibly shift forks. Let’s say you are a rider in a hurry, upshifting without the clutch because it’s racy good fun. Depending on how the dogs come together, they can hit face-to-face, they can slip sweetly into engagement, or they can hit corner to corner.
The problem comes with hitting corner-to-corner, because the engaging dogs, with engine torque behind them, will if this is oft repeated hammer what was well-defined dog corner into a rounded shape. And as the dogs become rounded, one dog set begins to glance off the other, producing a powerful wedging force that tries to kick the dog sets apart. This is shift refusal or jumping out. The rider’s foot meanwhile is urging the dogs together, so the hammering becomes intense and great pressure is applied to the shift fork that is trying to push the one gear’s dogs to engage those of the gear adjacent on the shaft. What we then see is not only rounded-off dogs but also a heat-blued and possibly bent shift fork. Replace internal parts. Pay bill.
Because the rpm difference between first and second gear is the greatest in the gearbox, there is the least time available for the dogs to engage normally, so shift refusal is commonest on the first-to-second upshift. All I can recommend is that riders have a degree of compassion for machinery. In the long history of the TT races, it has mainly been those with such compassion who finish.
Wait! If you were about to leaf past this page, expecting it to be another of my tedious social treatises – don’t. My subject this time is pure mechanics: poor gear-changing and its many causes.