AM5’s are excellent town bikes, but their limited gear range makes them only marginally useful as touring bikes. After exploring several of the more obvious options for adding gears, I hit upon a scheme for converting my AM5 to use a dual chainwheel and a front shifter, giving 10 speeds and a gear range of 29-88 ins. This was fitted without any modifications to the frame and without taking the bike off the road at any time during the adaptation.
Transmission system showing the chain tensioning device from Fix-Free Drives
Several more obvious options were ruled out by the dimensions of the AM5’s rear forks – they aren’t wide enough to take any of the recently-announced 7 speed hubs, and an earlier attempt to fit a Dacon converter with 3 derailleur sprockets on the existing 5-speed Sprinter hub failed for similar reasons – the additional sprockets caused the chain to foul the rear triangle.
With a tour of the Scottish highlands and islands in prospect, I was determined to fit more gears without replacing the whole bike. I started by installing a Stronglight 950 triple chainset (52,38,28). This was a good choice because it has offset cranks that enable a short bottom bracket axle to be used. In practice, the inner ring isn’t used in my setup, so I would probably have been better off with the double-ring version of the Stronglight 950.
I had heard via the Moulton Internet mailing list of a “car exhaust bracket” method for mounting a front gear mechanism on AMs. This turned out to be a surprisingly effective scheme. As the above photograph shows, a standard 1.5 in (38 mm) exhaust bracket, available from any car accessory shop, is an almost perfect fit on the AM seat tube, requiring no packing and clamping firmly on the seat tube with no evidence of any damage to the tube.
I fitted a Shimano 600 band-on mechanism of unknown vintage on the exhaust bracket. The mechanism is perfectly positioned relative to the chainwheels. By discarding the 600’s mounting band and cutting away some of the alloy I was able to bolt the mechanism directly to the exhaust bracket using the rear end of the u-bolt to secure it. A Suntour thumb shifter was mounted on the handlebars. Fortunately, my AM5 has some extra cable stops designed for use with the old dual-cable Sturmey Archer hub gears, and I was able to exploit the spare ones to run the gear cable.
A chain tensioning device is made specifically for use in hybrid gearing systems with rear hub gears and front derailleurs by Fix Free Drives (Tel. 01453 873541). I obtained one of these with some aluminium shims to adapt its clamp-on fitting to the straight tubing of the AM rear triangle. When correctly mounted the tensioner does its job well, with little or no tendency to throw the chain. Note also that when the hybrid system is not required, the chain tensioner can be easily removed and a short chain fitted, restoring the AM5 to its standard setup.
The resulting system has been very successful, retaining the convenience of hub gear shifting for town use while making the AM5 a really general-purpose bike, suitable for touring as well as commuting.