This is my effort to combine a 3-spd hub gear with a 3-cog front derailleur using bits I already have. Fitting the hub gear was straightforward; the front derailleur was already there. The difficult parts were the chain tensioner, and narrowing the Sturmey sprocket to fit a 3/32” chain.
To narrow the sprocket I made a jig so that I could hold the sprocket while it spun freely, and ground it freehand on a bench grinder. This resulted in a uniform width reduction. See sketch.
The chain tensioner is a Huret hanger of unknown vintage, combined with a Shimano cage. Modifications to the hanger were to straighten out some of the offset (annealing the areas to be bent because the steel is hard and brittle); and cutting the brass knurled spindle thingy to suit the length of the Shimano stub. The Shimano stub is a perfect fit in the Huret spindle hole and is held in with the Shimano circlip. The spindle thingy is already threaded and has a lock nut, which allows for adjusting the chain line of the cage. The spring that provides tension is a stretch type, hooked on the cable stop on the Huret at one end, and the tip of the cage at the other. I welded an extension to the end of the cage to get a bit more leverage and clearance for the spring. This setup has the bonus of providing a constant spring tension throughout the long arc of travel – as the spring stretches the leverage reduces.
I am pleased that it all works, the front gears change easily, and the hub gears are just Sturmey Archer! My riding mode is to keep it in 2nd and use the front cogs most of the time, only changing to 3rd or 1st when I need extra high or extra low. It is solid and low maintenance.
Will I go on to pretty it up and use it for anything special? No, I couldn’t resist trying this setup, but the drawbacks that others have mentioned are real:
- Extra weight and friction.
- Total gear range not as good as a rear derailleur.
- The long ratchet take up in the Sturmey freewheel.
- Bending the Sturmey axle will be a big problem, not so with a derailleur axle
- It really is only a 5spd because of overlap in ratios, but is still better than a 3spd
A well set up derailleur system is definitely better, but when it needs a cleaning and starts playing up, this old 9spd (5spd) beater will always be ready to go.
Thanks to others who have posted their similar experiments, and to Sheldon Brown for wealth of imformation on his web site - an inspiration for all budding cycle mechanics and tinkerers like myself.