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BB Chainline question

Old 02-17-11, 05:37 PM
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gulfislander
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BB Chainline question

Hi

I am installing a mountain bike crankset on a road bike (Cannondale T2000). I understand that the chainline is 45mm and that most mountain bike cranksets are 50mm. I will probably buy a Shimano m-590 crankset and BB. The 590 BB has enough spacers to move inboard or outboard 5mm but I am not sure if their starting point is 50mm, or if offsetting it enough to give me a 45mm chainline will cause other problems. Put another way, will i be able to make this crankset and BB work OK or do I need to buy a different BB and crankset with a shorter (113mm) spline?

Last edited by gulfislander; 02-17-11 at 05:38 PM. Reason: additions
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Old 02-17-11, 05:51 PM
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deacon mark
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I am not sure but most road bike chainline at least on 10 spd I believe are 43.5
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Old 02-17-11, 05:57 PM
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skilsaw
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I don't know anything about chainline.
I replaced the 105 triple crank (50/39/30) on my Cannondale t1 with an LX triple crank (48/36/26) and it worked fine.
I put the spacers in the same position they were in on the 105
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Old 02-17-11, 06:38 PM
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gulfislander
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BB Chainline question

Thanks for the comments. By the way it's a 9 speed for what thats worth. Also, I think the LX with the chainrings mentioned is a trekking crankset, not a mountain bike crankset. I want to end up with a 22 granny as opposed to your LX 26.

Thanks again.
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Old 02-17-11, 06:50 PM
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deacon mark
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Here it is read this see if it helps. http://www.sheldonbrown.com/chainline.html
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Old 02-17-11, 07:09 PM
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doesnt matter that much on derailleur bikes; the chainline is constantly changing, and is "right" only 2 or 3 times out of 18, or 27 etc
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Old 02-17-11, 07:21 PM
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Your T2000 is a touring bike. Without looking it up, I'm almost certain the rear over locknut spacing is 135mm. For a 9-speed cluster, that makes the natural rear chainline 45mm (as you know).

My Soma Saga 9-speed rear has the same dimensions. I recently switched to the FC-M590 crank (the 48-36-26 version) to take advantage of the bottom end stiffness of the outboard bearings. My new crank came with three 2.5mm spacers. For the standard MTB setup with a 68mm shell, two of the spacers go on the drive side and one on the non-drive side for a 50mm chainline.

Where you put the spacers in your case depends on what front derailleur and shifter you plan to use. If you want to use a MTB front derailleur, I'd recommend putting one of the spacers on the drive side and two on the non-drive side, giving you a 47.5mm chainline. Some MTB front derailleurs won't shift down to the nearest/smallest chainring with a 45mm chainline since they are really intended to be used with a 50mm chainline. They'll usually work at 47.5mm, though. That's really not a bad chainline if you tend to use the middle chainring for most of your cruising around. At 36 or 34 teeth up front, you'll often find yourself on the outer half of your rear cluster anyway.

If you use a MTB front derailleur, you'll have to friction shift it or switch to MTB indexed shifters (and probably swap your handlebars to MTB diameter ones). The MTB front derailleurs require a slightly different amount of cable pull for indexing than the Shimano road front derailleurs. Many (most?) touring bikes use bar end shifters with the rear indexed and the front friction shifted. If that's the case with you, all is well.

A road front derailleur will work with the FC-M590, but you may want to move the crank over another 2.5mm by placing all three spacers on the non-drive side for a 45mm chainline. Most road front derailleurs have trouble reaching out far enough to shift into the big chainring at a 50mm chainline. The IRD Alpina-d will do it just barely at 47.5mm, but it's easier to adjust at 45mm. I use the M590 crank, Alpina derailleur, and Tiagra STI shifters. It shifts well.

Last edited by xyzzy834; 02-17-11 at 07:24 PM.
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Old 02-17-11, 07:35 PM
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It seems like the mtb crankset would be wider than the road crankset to get the 50mm chainline so adding spacers as needed should make the crankset fit. Then the issue becomes when you space the drive side to clear the chainstay will your FD how enough throw to reach the big ring. It probably depends on the chainstay geometry, chain ring size, etc. I'd guess it will work but I'm not the one paying for the parts.
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Old 02-17-11, 07:38 PM
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gulfislander
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That's great info - exactly what I was trying to find out! FWIW I have a 105 derailleur and Campy brifters. I just wasn't sure if using all 3 spacers on the drive side would result in a 45 mm chainline and if that would result in the cranks being REALLY offset.

Thanks again!
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Old 02-17-11, 07:44 PM
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Originally Posted by gulfislander View Post
That's great info - exactly what I was trying to find out! FWIW I have a 105 derailleur and Campy brifters. I just wasn't sure if using all 3 spacers on the drive side would result in a 45 mm chainline and if that would result in the cranks being REALLY offset.

Thanks again!
I run mine at 47.5mm chainline because I can adjust the Alpina to shift into the big ring ok and it's a little less asymmetric. You can see the asymmetry, but I can't tell when riding. The Q factor snobs ( smile you you say that) would scoff at my setup. I'm not sensitive to a fairly wide Q factor.
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Old 02-17-11, 08:00 PM
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Rule of thumb thing, ..
Cog in center of the cassette lines up with the center ,
the middle chainring, or space between, if a double.

long straight edge ..
or measure from the bike center line on both ends.

That reduces the chain angle as much as practical
part of why cross chain makes things run odd ,
out side on one end to inside of the other..
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Old 02-17-11, 11:41 PM
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Probably worth checking

Without getting into a discussion on chainline or Q-factor, just thought I`d mention that those adapters that hold the external bearings can have threaded sections of different lengths.

In fact, the more high end you go, the more likely you`ll be to find more thread on the drive side because thats where more spacers are expected. So since you`l have to torque those goodies down to between 300 and 400 in-lbs, it might be worth confirming that you`ll still have enough threads left on the non-drive side to take that kind of torque after stacking three spacers on there. The people that designed this stuff were expecting one, max.
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