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Chainline not straight(5 posts)

Chainline not straightkilimanjaro
Jul 1, 2002 10:32 AM
I recently noticed that my chainline is not straight. The chain curve in towards the frame a little from the chainring back to the cogs. I also noticed that there chain wobbles side to side a little when I turn the chainrings, making it a little toughter to ajust the FD. I don't think that this is due to warped chainrings as I just replaced my old biopace rings with new round ones. There seems to be no play in the BB.

Does this mean that my alignment is off?

Thanks for any help,
re: Chainline not straightQuack
Jul 1, 2002 11:38 AM
It's actually more common than not for a crank spider to be off a little. I have DA cranks on a road setup and S-Works cranks on a MTN and they are both off at least a few mm. As far as the chainline not being straight goes, it can really only be straight in two of the possible gear combinations. All others will be off slightly. If your chainrings are as close as they can be to the frame, your chainline should be straight from the small ring to about the 3-4 rear cog and from the large ring to about the 5-6 rear cog. If you have a dial gauge or a decent ruler, you can probably determine which arm of the crank spider is off a little. As far as bending it into alignment again, good luck. I've never had much luck with it.
re: Chainline not straightkilimanjaro
Jul 1, 2002 12:18 PM
Thanks for the reply. It makes a lot more sense now. I don't want to bend it into alingment. I was just wondering if it is in alingment. I forgot to mention that when I checked for a straight chainline it was small chainring big cog or big chainring big cog. It never occurred to me that the straight line will be with one of the other cogs.

re: Chainline not straightgrzy
Jul 1, 2002 3:39 PM
Common sense says that the chainline should fall in the middle - otherwise one side would be at an extreme while the other is not.

If you just replaced biopace chainrings on an original biopace crank then you're dealing with a pretty old and possibly lower end product. No telling what the crank arm and BB have been through over the years or if they were even straight to begin with. My experience is that the recent 105/Ulterga/DA stuff is pretty tight on tollerances and you don't get much wobble. You're probably running a ball and cage BB which will have some wear. It's possible to put in a cartridge type BB with square taper ends for not very much money.

In the end unless something is fairly bent you should be OK.
re: Chainline not straightcurlybike
Jul 1, 2002 5:29 PM
Chainlie is from center of space between double chainrings to center of freewheel cluster. Or center of middle chainring in triple to center of freewheel IE; #5 of 9speed or between 4&5 on 8 speed.