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Nother chainline question

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Nother chainline question

Old 11-22-07, 09:38 AM
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soma2x
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Nother chainline question

Installed a shimano un54 68 x 110mm bottom bracket with a Sugino XD500 compact double. That's the BB size Sugino said I need for the cranks. Problem is when I'm on the small chainring and shift to my two smallest cogs, the chain rubs against the teeth on the largest chainring. I't so bad on the smallest cog (8th gear) that I really can't use it at all and I'm afraid that I'll screw up the chain, the large ring or both. I'm a wuss and live on my 36T small ring so I'm not exactly married to the 50T big ring. Would using a smaller (48T or 46T) big chainring help? Or should I just put back my old 113mm bottom bracket (which I really need for son's bike) and try that? Would three mm make that much of a difference?
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Old 11-22-07, 09:48 AM
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Sheldon Brown
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Originally Posted by soma2x View Post
Installed a shimano un54 68 x 110mm bottom bracket with a Sugino XD500 compact double. That's the BB size Sugino said I need for the cranks. Problem is when I'm on the small chainring and shift to my two smallest cogs, the chain rubs against the teeth on the largest chainring. I't so bad on the smallest cog (8th gear) that I really can't use it at all and I'm afraid that I'll screw up the chain, the large ring or both. I'm a wuss and live on my 36T small ring so I'm not exactly married to the 50T big ring. Would using a smaller (48T or 46T) big chainring help? Or should I just put back my old 113mm bottom bracket (which I really need for son's bike) and try that? Would three mm make that much of a difference?
Cross chaining is a Really Bad Thing, and the smaller the chainring, the worse it is. You should never, ever ride in the small/small combination, and the small/second-smallest is generally undesirable too, especially given that you're running a 36 in front.

My best advice is to re-train yourself to use a better shifting pattern.

If you're not going to do that, there are several possible hardware options:
•Use a narrower "9-speed" chain.

•Go to a wider BB.

•Stick a spacer under the right-side BB mounting ring. The traditional item for this is a Sturmey-Archer sprocket spacer, 1/16" (1.6 mm) thick. http://harriscyclery.com/saparts

•Add 0.1 mm spacer washers between the chainrings and spider...if you can find them.

•Buy a new frame with longer chainstays. ;-)
Sheldon "Options" Brown
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Old 11-22-07, 08:27 PM
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+1
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Old 11-23-07, 08:14 AM
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I didn't realize cross-chaining was as much a problem on doubles as it is on triples. If anything, its worse since I have no problem accessing all eight cogs from the middle on my triple. Thanks for the advice. I'm thinking about doing away with the large chainring altogether. Using your gain ratio calculator I realized that if I go with a single 38T ring it would give me all of the gears I like on the 36T with a little extra on each end. Then I could ditch the front derailleur altogether.
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