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Old 03-23-11, 03:32 PM   #1
JimiJams
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crankarm too tight

Hey, I installed the crankarm and bottom bracket from my old bike onto my new one. The trouble is the chainline is too close to the frame of the bike and I can't get the derailleur to move over far enough even with the L screw loosened all the way.

Do you think by overtightening the crankarm, and in the process narrowing the chainline, damaged the taper of the spindle on my BB or opened the square hole on the crankarm too big or both?

I'm asking because I'd like to replace either the crankarm or BB to fix this problem and am just wondering which is more likely to get damaged. Seems like the spindle is made of more durable metal than the crankarm.
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Old 03-23-11, 03:40 PM   #2
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I doubt you were able to overtighten it enough to cause your chainline problems. In any case if the chainrings clear the frame it should be close to correct chainline. To confirm measure from the outer ring to a frame tube, add 1/2 the diameter of the tube, and subtract the distance from the outer ring to the center ring, or center of the gap on a double and see if that's close to spec.

If so, check your FD. Disconnect the cable and see if it moves in enough or what, if anything, is blocking it.
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Old 03-23-11, 03:47 PM   #3
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If you did over-tighten the crank and anything was damaged, it would almost certainly be the relatively-soft aluminum crank that was damaged, not the hard steel BB spindle.
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Old 03-23-11, 04:50 PM   #4
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I did measure the chainline a couple times. I have a three ring crank so I measufred from the middle of the middle ring to the center of the seat tube and it was like 42 or 43 mm. So very narrow.

If you're certain I'm unable to overtighten the crank arm then maybe the diameter of the seat tube on my new frame is larger than my old bike and the derailleur is therefore closer to the chain.
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Old 03-23-11, 05:23 PM   #5
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Shimano specifies a 45mm chainline (at the center of middle ring) for triple crank.

Is the FD square (parallel to the chainrings)?
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Old 03-23-11, 05:29 PM   #6
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Did you measure the BB housing? You may need a longer BB to move the crank out to maintain clearance.
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Old 03-23-11, 05:55 PM   #7
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Did you measure the BB housing? You may need a longer BB to move the crank out to maintain clearance.
By BB housing, you mean BB shell, right?
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Old 03-23-11, 07:55 PM   #8
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45mm? I usually see 47.5mm or more commonly now 50mm. Anyway I unbolted the crank and moved it so the chainline is about 47.6mm and the inner plate is still against the chain. So I don't know what's up.

I guess I could use a spacer between the BB and the BB shell or get a new FD...
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Old 03-23-11, 09:14 PM   #9
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45mm? I usually see 47.5mm or more commonly now 50mm. Anyway I unbolted the crank and moved it so the chainline is about 47.6mm and the inner plate is still against the chain. So I don't know what's up.
A road triple is usually set up for a 45mm chainline to the center chainring. A MTB triple is usually 50mm.
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Old 03-24-11, 01:26 AM   #10
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Just realized I was assuming a road bike. Did you by chance take a road crank and install it on a frame with a MTB FD?
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Old 03-24-11, 08:55 AM   #11
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Are you using the proper spindle?
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Old 03-24-11, 07:52 PM   #12
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Johnd: I took the BB and crankset from my previous bike. So the spindle and crankarm are completely compatible. I think I may have tightened the crank on too much and widened the crankarm hole.

But the other possibility is that the seat tube has a bigger diameter on my new bike than my old one and is pushing the FD closer to the chain.
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Old 03-24-11, 08:07 PM   #13
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Originally Posted by JimiJams View Post
Johnd: I took the BB and crankset from my previous bike. So the spindle and crankarm are completely compatible. I think I may have tightened the crank on too much and widened the crankarm hole.

But the other possibility is that the seat tube has a bigger diameter on my new bike than my old one and is pushing the FD closer to the chain.
Not trying to be a smarta$$, but have you checked that yet? While it's possible to over tighten a crank arm, it's highly unlikely unless it has been removed and installed numerous times previously. You didn't put grease on the spindle before you installed the crank did you?
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Old 03-24-11, 08:52 PM   #14
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Originally Posted by JimiJams View Post
Johnd: I took the BB and crankset from my previous bike. So the spindle and crankarm are completely compatible. I think I may have tightened the crank on too much and widened the crankarm hole.

But the other possibility is that the seat tube has a bigger diameter on my new bike than my old one and is pushing the FD closer to the chain.
Is the BB shell width the same? Perhaps you could place a spacer under the fixed cup to move it out a millimeter or two?
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Old 03-25-11, 01:40 PM   #15
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Just checked the seat tube diameters. My new bike is about 1-1.5mm smaller. So now it's really perplexing.

Like I said earlier too, I unbolted the crank bolt and slid the arm out so the chainline was 47.5mm and still with the low screw all the way loose the inner plate was against the chain.

Spacers is what I'm thinking right now for a solution. Eventually I'll I have to go the external BB way as there aren't a whole lot of options for square taper brackets anymore.
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