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My Bike Pages |
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A.Webster Feb 2005 |
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Websters on the Web |
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Brompton Twin Rear Sprockets If you have looked at my other pages you will know that I have already extended the range of my gearing to encompass 22 to 86 gear inches over 6 gears (52/24 at the front and 13T sprocket AW3 hub gear at the back). The next logical step was to fill in the "gaps" between these gears by adding an extra sprocket at the back and swapping the chain tensioner for a derailleur. Parts List:
Note that you will need an axle-fitting derailleur unless you want to braze on a hanger (well outside my capabilities). Fitting the sprockets All went OK, apart from the fact that the sprockets would only fit if I removed the Sturmey Archer dust-cap. I hope this won't cause long term problems. Wheel Fixing Nuts I needed to replace the Brompton wheel nut with a standard one as the Brompton nut is designed to go on top of an ordinary hex nut as the main wheel securing nut. The thickness of the derailleur mount meant omitting the hex nut. The Brompton style Sturmey Archer nut has only a couple of complete threads before the "hole" to view the indicator chain. As I have broken these nuts in the past I didn't want to risk it. The more standard Sturmey Archer nut has a decent threaded part before the hole. I planned to use the derailleur limit screws to give a simple, two position change. This worked at the top (small sprocket) end of the range, but not at the low end. The original limit screw just wouldn't go in far enough and a longer replacement waggled around too much. The solution was to fit an indexed lever at the front, using the indexing to limit movement at the "low gear" end. This setup worked as described, but I still ended up overchanging occasionally when moving to the larger sprocket I needed to re-instate the aluminium disk originally there for the purpose of stopping the chain coming off on the inside. I needed to enlarge the hole in the fluted aluminium disk (using tin snips and a file). This then fitted onto the hub-shell inside the sprockets in the right place to stop the chain coming off on the inside. I can now overchange enough to get a positive shift without risking the chain coming off the top of the large sprocket. Having a 13/15T rear setup gives evenly spaced gearing across the whole range and margnally lowers bottom gear. You might think the double changes (Hub and derailleur) to move through the gears in sequence would be a problem, but in practice, you soon get used to it. Disadvantages are in folding; the derailleur can't take up all the slack so more care is needed when folding and unfolding (now a 30s task, not 15s). March 2005 |
