The first FN motorcycles were lightweight single-cylinder machines with belt drive. But with the advent of the four-cylinder shaft-drive model in 1905, these early singles with their high frames were beginning to look very dated. For 1907, a new single-cylinder machine was introduced, with a frame design inspired by the four-cylinder. I have a motor from one of these machines, which sports a rather interesting internally-geared pulley.
From a period catalogue, we find the following specifications for the model:
Removing the large nut, and another (left hand thread) one underneath allows the pulley to be removed and the internal gearing revealed. The ratio is around 3 1/2:1. Just how the belt-tensionner is attached to the tankside lever I'm not sure: it seems to be a rack-and-pinion affair. Can anyone fill me in? (Thanks Jacques - see below.)
The lightweight model had only a brief life, replaced in 1909 by the shaft-drive single which was to last into the early 1920's. I have no idea how many of the lightweights were produced, but there is quite a bit of surviving junk from them in Australia. In addition to a complete bike which exists in Western Australia, I have seen three motors in recent years (one of which can be seen in the FN Photo Gallery), and a frame which has sadly been taken to with a hacksaw.
Thanks to Jacques Maertens for the following photo of a lightweight under restoration in Belgium. Looking at the photo, we see that the "rack" to adjust the belt tension is driven by a worm gear. Just twirl the lever if the belt begins to slip!