|Shimano "RSX" triple road crank re-purposed into a wide-range compact double.|
I also tried converting an old Shimano STX 94mm BCD MTB crank. This wasn't entirely successful because the shape of the inner face of the spider pushed the chainrings too far outboard.
I have been reading about Sugino's recent super compact cranks, the OX801d and OX601d. These double cranks feature a 74mm BCD and a 110 BCD chainring. This would provide the gearing I'm looking for in a more modern crankarm shape, allowing me to use a normal Front Derailleur and my 135mm 8 speed rear wheel. Unfortunately, these cranks are also staggeringly expensive! $490 for the 601!
Looking over the pictures of these cranks, it seems to me that you could build your own with an ordinary 110/74 triple crank. I have a couple lying around so I thought I'd try it.
My test subject was a Shimano RSX triple I bought from Nashbar years ago. This is a decent enough crank which had a ramped and pinned chainwheel on the outer side, the inboard granny gear was a simple, plain chainwheel.
|A look at the inner side, the large ring was reversed to allow the chainwheel bolts to be recessed a bit.|
|Another view. One advantage with reversing the ring is that I can use the unworn side of the teeth!|
I had to install a longer bottom bracket in order to have the chainrings clear the chainstays. A 118mm Shimano UN55 did the trick. The bolts fixing the small chainring come fairly close to the chainstay, but they do clear it and a miss is as good as a mile!
|A 118mm bottom bracket provided just enough clearance.|
The one issue which remains is that the chain contacts the little pins on the reversed big ring when the chain is on the small ring and the 5 outer rear sprockets. It clears the pins fine in the 3 inboard positions, which will probably be the only ones I use with the granny anyway. At any rate, I'm not too concerned because I will eventually replace the large ring with an un-pinned and un-ramped one which should eliminate the problem. I expect there may be some interference in the outermost sprocket positions even then, but I would never use those combinations.
My initial test ride was a success! Shifting was good and the gearing is more to my liking. The 46 tooth outer ring gives me a nice selection of gears for almost 90% of my riding, (I use all 8 sprockets with the big ring), and the 28 tooth granny offers a "27-inch" low gear which is good enough even for touring loads. Shifting was reliable and quick.
|A view of the drivetrain.|
In the future, I will replace at the outer ring with a plain Sugino chainring that has no ramps or pins. Eventually, I will try to find a nice set of new arms to build a brand new crankset with this gearing.
Update: I have replaced this crank with a similarly modified Sugino Messenger: See: Another stab at a subcompact crank.