The Multi gear is one of the truly inspired devices of the veteran years. Doomed by the advent of the Sturmey Archer CS gearbox and its imitators, but inspired. Stay a moment to figure out how it worked.
Belt drive was cheap, smooth and well accepted by the motorcyclist of the pre-WW1 years in the UK. The major problem was the fixed gear ratio, so that there was always compromise between outright speed on the one hand, and flexibility and hill climbing ability on the other. Most later veteran engine pulleys could be adjusted on the roadside by winding the flanges closer together (to give a "speed" gear as the belt rode higher) or apart for the hills. Of course this change required an adjustment in the belt length to suit the new pulley diameter, and it was not uncommon to carry two belts of different lengths.
Enter the Multi gear. In the model illustrated below, from the 1914 catalogue, the protruding cylinder on the end of the engine shaft houses a clutch. Quite a neat device with around 50 small-diameter metal plates and a single coil spring. A similar clutch could be fitted to the single speed Rudge of the day to give the Free Engine model.
But the clutch is not a necessary part of the Multi gear. Pullin's TT-winning Rudge, for example, has the Multi gear but not the clutch. So what eaxactly is the Multi gear?
The long lever in the tank-side gate is the gear change level. As it pivots about the engine mainshaft, it slides on four ramps on the crankcase and cause the flanges of the front pulley to close up (level forwards) or open (lever back). The pulley halves are splined together. So far, it is not so different from the adjustable pulley. But what about the belt tension problem? This is the inspired bit. If you study the figure above, you will notice a linkage between the front gear and the rear wheel, via a bell crank.
The rear wheel is a testament to the fine wheel builders that Rudge Whitworth were at the time. It is in essence two wheels, built on separate, concentric hubs. Wheel number 1 - the "usual wheel" - carries the rim and tyre together with the inside half of the rear belt pulley. This part of the wheel is fixed relative to the frame in the usual manner, and uses 40 radial spokes. Now imagine a second hub, sliding over the hub of the first wheel in between the spoke flanges. Laced to this second hub by 40 more radial spokes (yes, that means 80 spokes in the rear wheel alone!) is the outer half of the belt pulley.
If you're still with me you will have the plan by now. As the gear lever is pushed forwards, the flanges of the front pulley close up, causing the belt to ride higher. Simultaneously, the outer half of the rear pulley is sliding outwards, causing the belt to fall lower into the rear pulley. Hey presto! the gear ratio becomes taller, and the belt tension remains constant. Infinitely variable gear ratios - at least between 3 1/2 and 7 to 1 - and constant belt tension. Good enough to last well into the twenties.