From: Donald Gillies <gillies@cs.ubc.ca>
"I have a new bike ( Clean 1973 Raleigh Gransport, $75 shipped, whoo-hoo!) with a Stronglight 93 crankset. A previous owner chucked the outer chainguard, moved the middle ring to outer position, added a 48T middle ring, to get a triple (42 - 48 - 52)
The chain looks like an early 1990's sedisport.
Shifting from middle ring to large ring, the chain frequently jams between the chainrings. These are original 1970's stronglight rings which have a shoulder on one side only. The shoulder on the side where the countersunk hole for the chainring bolts is. In its current configuration, the shoulder from inner-middle-outer when looking down while riding in the saddle are on the left, left, right. This keeps the middle and outer rings as close as possible together, flat-side of these rings against the spider.
When i shift onto the middle ring, the chain appears to be centered between the inner and outer rings.
Is this crankset unfriendly to narrow chains ??"
---- Hi Don,
I rode a Gran Sport for 14 years, and I quickly converted the Stronglight 93 from the stock config to a normal double ring. With 42-52 rings, it should work fine with a semi-skinny chain (something similar to the ultra-6 chains).
With the funky half-step arrangement that you have right now, I'm not surprised that you have trouble with the chain jamming. My commuting bike is set up with a Sugino AT triple, which was designed for wide chains back in the days of normal width 6 speed stuff (I believe). It's set up for half-step plus granny, with 26-46-49 rings. The step from the 49T down to the 46T ring is so slight that it's easy for the chain to hang up halfway between them. If I happen to be shifting under load at the same time, the chain (a SRAM PC48) will severely jam! If I take care to pedal lightly, and make a decisive shift onto the 46T ring, I don't have any problems.
However, the same Sugino AT configured with cross-over gearing of 26-39-49 shifts just fine between the 39T and the 49T. Must be something to do with the big jump between the two large rings. For your situation, I would suggest either switching to a 42-52 double, or finding some nice old wide chains. I notice that Nashbar sells some wide 6 speed chains... maybe your LBS can get them too? Perhaps they will work well with your current half-step arrangement? If they work, please let me know! :-)
Steve Kurt
Peoria, IL