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Old 03-28-08, 06:11 PM   #1
Joshua A.C. New
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Calculating (?) chainline (and pics of this build)

OK, so, I'm building this Cannondale. I've written elsewhere about how this frame is like hooking up with a highschool crush twenty years on, so I'll post this in-process photo:



I'm really how pleased with how hot she is, and I'm proud of the bike, too.

Anyway, the chainline is farkakte. The high gear is right up against the chainstay and the chain is least noisy in sixth gear.

Since 8th or 9th is most likely to be cruising gear, I'd like that one to be smoothest, as I've got a single ring on the front.



So, it looks like I can move a spacer over to the right and redish. But that'll only get me as far as seventh. This looks to me like the BB should be wider, but how do I figure out how long it should be? Should I have had a way to figure this out using the power of numbers, instead of the power of trial and error? Cuz finding bottom brackets by trial and error seems expensive.
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Old 03-29-08, 10:10 AM   #2
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With a 1x9, your chainline should be as close to middle of the cassettte as possible. Or, if you know you won't spend much time on the biggest cogs, then over a bit is probably OK if you can still get it up on the big cog if you need to.

If sixth gear is the quietest, then that's probably the chainline too.

A quick way to check your chainline is to take the chain off and drape it over the chainring while hanging the bike by its front wheel. Kind of like a plum bob. To get more accuracy, put something on the left crank to balance the weight. You get the picture. Right?

I can't tell from the picture, but is your chainring bolted inboard or outboard on the crankset? If it's bolted on the inboard side, move it to the outboard side. That will give you a few millimeters without spending a penny or impacting the Q factor of your crankset. The only reason that chainrings are designated as inboard or outboard is for people with front derailleurs.
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Old 03-29-08, 11:43 AM   #3
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If your two smallest cogs are going to be used the most in the current setup doesn't that indicate you should have a larger front chainring? Would make sense to me to have your middle cogs be the most used. If you set your chainline to the smallest two you'll also get some really wonky chainline when you shift to the largest cogs
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Old 03-29-08, 11:50 AM   #4
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Yeah, if you're going to be preferring a higher gear generally and all other things are the same I'd think a larger chainring and a shorter bottom bracket would be best.
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Old 03-29-08, 08:24 PM   #5
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The front is a 52. The wee ring in the back is a 12.

What the crap am I worried about? That's 117 gear inches. 27 mph at 90 rpm. If I can sustain that long enough to get serious chain wear, then I'm Mr. Princess Captain Awesome.

You're right. Near the center should give me a good sweet spot. Top and bottom are for extremes.
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Old 03-29-08, 10:06 PM   #6
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How come I don't see bottom bracket spacer or other spindle length adjustments as a solution to problems like this?
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Old 03-30-08, 08:02 AM   #7
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Quote:
Originally Posted by Joshua A.C. New View Post
OK, so, I'm building this Cannondale. I've written elsewhere about how this frame is like hooking up with a highschool crush twenty years on, so I'll post this in-process photo:



I'm really how pleased with how hot she is, and I'm proud of the bike, too.

Anyway, the chainline is farkakte. The high gear is right up against the chainstay and the chain is least noisy in sixth gear.

Since 8th or 9th is most likely to be cruising gear, I'd like that one to be smoothest, as I've got a single ring on the front.



So, it looks like I can move a spacer over to the right and redish. But that'll only get me as far as seventh. This looks to me like the BB should be wider, but how do I figure out how long it should be? Should I have had a way to figure this out using the power of numbers, instead of the power of trial and error? Cuz finding bottom brackets by trial and error seems expensive.
I think there may be two issues. The chain seems really close to the chainstay in the small cog. If I'm seeing this right, you have to get clearance before the chainline (alignment between the cogset and the chainwheel) matters. You might get clearance with a smaller number of cogs, by adding a spacer on the cogset side of the rear axle, or with a narrower chain if one is available.

You can check the chainline mathematically. If the frame is aligned and the wheel is as the designers expected, the center of the chainwheels (a plane thru the center of a single ring, or thru the midpoint between dual rings) should line up with the center of the cogset. Shimano and Campy design for this plane to be 43.5 mm from the center of the BB. You can approximate teh center of the BB by the center of the downtube.

Zinn has us measure from the left side of the downtube to the outside of the large chainring, and from the right side of the downtube to the inside of the inner chainring. The average of these two measurements is the front chainline, 43.5 mm per Shimano for a double. I think a single should also have this value. He also says fastest shifting and queitest running are around 42 mm.

Based on this you can see how much chainline correction you need and get a new BB that is suitably shorter.

One thing to check first is that the crank is torqued onto the BB with the right torque. It moves a lot as you bring ti up to spec.

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Old 03-30-08, 09:46 AM   #8
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Based on this you can see how much chainline correction you need and get a new BB that is suitably shorter.
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I stand corrected. BB spindle length does have a possible place in this discussion.
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Old 03-30-08, 10:15 AM   #9
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Excellent. Thanks, everyone!
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Old 03-30-08, 01:41 PM   #10
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If you have Zinn's Road Bike book, he goes into more depth on choosing the best target front chainline, and on measuring the rear chainline. You really do want them to match up withing reason.

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Old 03-30-08, 04:00 PM   #11
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I don't have it, but one of my LBSes has it. I'll pick it up and add it to The Bicycle Wheel, The Big Blue Book, the Bicycling Magazine Book of Bicycle Maintenance & Repair. I'm happy to get more concrete information in my head and library.
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