gearing by fitting a rear derailleur to the bike and finding a suitable method of attaching more than one sprocket to the hub driver. For the 'three spline' driver, an additional sprocket may be added in place of the spacers so long as care is taken to ensure the gap between the sprockets is not so great that the chain will drop into it instead of moving between sprockets. Two sprocket blocks have been produced for these hubs in the past too, though these are now obsolete and hard to find. For the 12-spline driver of late 1930s and 1940s sporting hubs, similar arrangements may be made as for a 3-spline driver, however component sourcing is likely to be a bigger problem in line with the general scarcity of these drivers and the sprockets which fit them. For the threaded drivers which were fitted to touring hubs before 1951, a standard thread-on freewheel may be fitted, although care must be taken to ensure there is sufficient clearance in the frame and sufficient thread engagement to ensure the setup is safe. More than three sprockets is unlikely to yield successful results. Some Sturmey-Archer hubs, by their design, are fundamentally incompatible with hybrid gearing and hybrid setups should not be attempted with them. These are: AF, FM, FC, and AC because of the secondary planet train which is used to advance the sun gear in Low and High gears, this causes the internals to turn as the bike freewheels and applies tension to the bottom of the chain, this tension will damage a derailleur setup. For the same reason, it is normal for bikes with these hubs to spin their pedals when they are pushed forwards, where on other Sturmey-Archer gear hubs this would indicate an overtightened left hand bearing cone. ASC Three Speed fixed gear, in addition to the reason set out above, any fixed gear hub is incompatible with derailleur gearing because of the need to apply chain tension in both directions when slowing down on a fixed gear bike. Dynohub The Dynohub was Sturmey-Archer's hub dynamo (generator) for bicycles. The initial GH12 12-volt model was introduced in 1936 and followed two years later by the GH8. This 8-volt unit was discontinued in 1941 and replaced in 1945 by the lighter-weight GH6 6-volt version, which remained in production until 1984. The term "dynohub" is sometimes applied generically to bicycle hub