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Old 02-19-06, 11:42 PM   #1
grinderbob
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chain touch inside of large ring when in small ring

I have a problem. I just built up my bike with 2x9. When I have the chain on the smaller crank chainring,
it is rubbing the inner edge of the large chainring, but only at the entry and exit points of the
chainring. It would rub if the chain is on the last 2 rear cogs. It seems like the
crankset is bolted down too much. But it is not, even if I back off the crank bolt quite a bit, it still rubs.

What can I check? In the smallest rear cog, the two pulleys are aligned with the smallest cog.
Should I tighten or loosen the derailleur cable?

Please share your experience. thanks.
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Old 02-20-06, 01:03 AM   #2
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First off, the crank fixing bolt always needs to be tight. Whether you have square taper cranks or some splined variety, they all rely on the fixing bolt to be tight otherwise your cranks will come loose as you ride and you will damage the splines or tapers. Cranks fir onto a bottom bracket spindle a certain distance and are then firmly wedged into place. tightening or loosening the bolt will move the cranks in and out, but only by a few thousandths of an inch.

It sounds like you are crossing the chain. When using the smaller ring, you should not be using the smallest cogs in your cluster because of the extreme chain angle it creates. Likewise, going from largest to largest is a bad idea. If you use all but the smallest two cogs does the problem go away?

You might need to space the rings apart slightly. I don't know personally but I imagine products are made to do this for weird situations.

Keep those crank bolts tight!


edit: I thought of another possibility. Are you certain that you have a nine-speed chain? If you are using a wider chain designed for fewer rear gears with a nine-speed-specific crankset, I bet the extra width of the chain could cause this to happen.
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Old 02-20-06, 08:12 AM   #3
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That's not uncommon.

One possibility is to move your whole crank to the right by putting a 2mm spacer under your bottom bracket.

OR - you could train yourself to think of the chain rubbing as an audible indicator that it's time to shift into a different gear combination.
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Old 02-20-06, 09:05 AM   #4
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2x9 and 2x10 setups are not designed to be used in the extreme cross-chain combinations. Putting any torque on the pedals in the small-small combination will rapidly wear out your smallest freehub cog. Using the large-large combination will grind down the teeth of your large chainring. Your "18-speed" bike is really a 16-speed, or perhaps a 14-speed.
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Old 02-20-06, 09:10 AM   #5
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I have the same issue with my 2x7. Most of the rubbing occurs in the max cross chained situation, though I still get some in the next gear up. Mine is worse than it should be because I have a BB that's about 2mm too short on the drive side. If you have the correct BB for your crank, this isn't your problem. As others have said - you shouldn't be cross chaining like that for more than a few moments at most. Teach yourself to use the right combos for the gearing you want/need.
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Old 02-20-06, 09:51 AM   #6
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What chain rings do you have?

My D-A 2 X 9 rubs the 53 big ring slightly if fully cross-chained small to small (39/12).
Bottom bracket spacers are available in 1, 1.5, and 2mm thicknesses. This type of spacer moves the bottom bracket spindle and complete crankset out. If you want to be able to use the inside chain ring and the next to smallest cog then I suggest that you try a 1.5 mm spacer, but that will increase the angle for the big to big combination, not something you should use anyway.

I would not separate the rings with spacers.

Al

Last edited by Al1943; 02-20-06 at 11:44 AM.
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Old 02-20-06, 09:54 AM   #7
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Check your chainline. If that is set right, then you can't use the combinations of chainring/cogs that rub.
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Old 02-20-06, 10:26 AM   #8
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Quote:
Originally Posted by MacG
You might need to space the rings apart slightly. I don't know personally but I imagine products are made to do this for weird situations.

Don't do this; it will allow the chain to slip between the chain rings (and get stuck, of course).
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Old 02-20-06, 12:44 PM   #9
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All right! thanks for all the tips. I wasn't going to loosen the crank bolt for good. It was just an experiment.
I'll try getting the BB spacer and put it on the drive side. It should push the right side of the BB out enough to avoid
that rubbing. I think small-small is more common than large-large, so I should be able to live with the consequence.

Thanks again.
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Old 02-20-06, 03:36 PM   #10
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Quote:
Originally Posted by rmfnla
Quote:
Originally Posted by MacG
You might need to space the rings apart slightly. I don't know personally but I imagine products are made to do this for weird situations.
Don't do this; it will allow the chain to slip between the chain rings (and get stuck, of course).
Not to mention it'll mess up your indexing on the FD. Draw up a shifting-diagram of your gearing and plot a shift-pattern through it:



Notice that the two smallest cogs combined with the small-chainring is duplicated anyway. Don't use those gears, they'll wear quickly and you're not as efficient due to the crossed-chain. Given two equivalent gears, the combination with the big-ring will be more efficient with less friction because it bends the chain and rotates the links less, resulting in less friction between the plates.
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