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Chainring Alignment
#1
I bought a compact crank (48-36 Velo Orange) for an old 1980 Trek. I found that the old bottom bracket spindle (122mm) was too long. The recommended bottom bracket length for the new compact crank was 110 mm. I bought and installed a sealed 110 x 68 bottom bracket. I thought I would be out pedaling within an hour. However, with the chain on the small chainring and the smallest rear cog (12 tooth) of a 6 speed freewheel, the chain is rubbing and catching on the large chainring. I need to move the crank out, or slightly to the right. I tried to fix the problem by installing two spacers on right side of the bottom bracket, but the chain still rubs. I doubt that installing more spacers is advisable. I can see that my chain angle is too extreme and a longer bottom bracket is needed. I'm resigned to buying a second bottom bracket, but at $30 a pop, I don't want to buy the wrong one. I'm thinking a 113mm or 115mm should work, but I'm guessing. Can someone tell me a simple way of measuring the chain angle or chainrings that will give me the right length for a new bottom bracket? Thanks.
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#2
Look up the spacing of your 6 speed freewheel here: http://sheldonbrown.com/cribsheet-spacing.html

Get rid of the spacers you installed. Not a good idea.

Move the chain to a combination of chainring and rear sprocket that results in the chain being as close to parallel with the centerline of the bike as possible. Decide how far that you need to move the chainrings out to get the performance you desire.

Get a new BB with an axle length that is 110mm + 2 times the distance it needs to move to the right.

I have found that BB length recommendations are not very useful; for example on my GT, the Shimano recommended length was 123mm for the crankset - I am changing to 113mm......
Nigel
  Reply
#3
Thanks Nigel.
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