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Old 01-03-10, 11:40 AM   #1
dougtoma01
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Bought Wrong Size Crank - Possible To Modify?

Quick question.

I like to restore old vintage 10-12 speed bikes.

I wanted to spiffy up an old Raleigh Capri and so I bought a new crank and arms.

I didnít notice the crank was for 7,8,9 speed freewheels. Mine is a 6 speed.

Thus, the chain line is not correct. The crank gear side needs to move inboard.

The only option I can think of is to try to file the inside of the crank to slide further onto the spindle?

Any other options?

Best regards,
Doug
Cincinnati, Ohio
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Old 01-03-10, 11:52 AM   #2
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if it is a square taper crank u can get a shorter spindle for the bb
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Old 01-03-10, 12:01 PM   #3
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Yes, it is a square tapered spindle. Sounds reasonable.

I am assuming these are available on the internet or LBS.

I guess the center point between the 2 crank rings should be centered on the freeweheel?

Once I know how far I am currently off, try to find a spindle that much shorter.

Is it correct to assume spindles are equal length from centerline? Thuis if I am 5mm off I need a spindle that is 10mm shorter (5mm each side)?

Thanks, never thought of that one.

Best regards,
Doug
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Old 01-03-10, 01:01 PM   #4
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I would not assume that you have a problem without measuring- assuming youhave a BB that matches the crank. If not, you must start with that - both a spindle length and taper made to fit the crank, not whatever the bike had on it before.

The chainring positions have not changed for a very long time. Even though the the rear dropout spacing changed from 126 to 130mm, the wider cassettes of today spread the extra width in both directions.

The common chainline specification makes little sense to me. It's always listed as 43.5mm to an unmeasureable point between the rings rather than an easily measured point to the tip of a tooth on one or both rings. With the chainring tooth to tooth spacing about 7.8mm, that makes the tip of a tooth on the big ring in the 47-48mm range and the little ring about 40mm.

http://www.sheldonbrown.com/gloss_ch.html#chainline

Last edited by DaveSSS; 01-03-10 at 01:50 PM.
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Old 01-03-10, 01:18 PM   #5
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Originally Posted by dougtoma01 View Post
Is it correct to assume spindles are equal length from centerline? Thuis if I am 5mm off I need a spindle that is 10mm shorter (5mm each side)?
No, spindles can be symmetric or asymmetric. But, I would first use what you have and see if there is any significant chainline problem.
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Old 01-03-10, 01:36 PM   #6
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Assuming you have a 68mm sized BB, the cheapest fix is a Ritchey spindle. They are symmetrical and can be purchased in several lengths from iCycle or on eBay- I paid around $10 shipped for a 120 recently. Harris has assymetrical ones.
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Old 01-03-10, 02:53 PM   #7
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Thanks men.
The spindle is what was in the bike. 68mm bottom bracket.
I bought a new crank and cranks arms to go alloy.
When I put it on, I noticed the chain line looked out.
I went back to website I purchased from and noticed the crank was for 7,8,9 speed and mine is a 6 speed.
Also. the front derailer doesn't look like it can be adjusted to swing out as far as the outer ring on the new crank.
I will take another look and actually put a chain on it and verify I have a significant problem before proceeding.

If I do, then I will proceed with purchasing a spindle to locate the crank where it needs to be.

Appreciate everyone's helpful guidance.

Best regards,
Doug
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Old 01-03-10, 03:04 PM   #8
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Just to clarify a couple of points. A 7-8-9 speed compatible crank works just fine with a 6 speed cluster. I've been using a setup like this for several years. Do NOT whatever you do file the alloy cranks interface with the spindle. Its a precision contact point and you will destroy it if you try. What you need is a shorter BB spindle and its a good idea to measure the chainline you have currently with the alloy crank in order to get an idea of what new length you need to buy.

Anthony
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Old 01-03-10, 06:31 PM   #9
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Thus, the chain line is not correct. The crank gear side needs to move inboard.
How far off from a "perfect" chainline is it? If it's less than 5mm, I wouldn't worry about it because the vast majority of the time, you'll be in a gear that bends the chain sideways anyway.
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Old 01-03-10, 06:42 PM   #10
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I haven't had time to check. I will look at it again this week and see if it's not as bad as I 1st thought. After the input here, it sounds like it should be fine. When I have figured out final verdict, I'll update thread.
Thanks again Gentlemen.

Best regards,
Doug
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Old 01-03-10, 07:56 PM   #11
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I've encountered a similar situation several times when putting mountain bike cranks on touring bikes. Using a shorter BB fixed the chainline problem. Use your old crank to determine the your current chainline distance. You can do some measurements using your current BB and new cranks to determine the axle length you need to get close to the current one. You don't need to measure the exact chainline, just the distance from the center chainring to the down tube. Measure the distance between old and new cranks, multiply by 2, and subtract from length of current BB axle.
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Old 01-04-10, 03:22 PM   #12
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If your BB cups are at all worn, you may want to consider concurrently upgraded to a cartridge BB while getting a shorter spindle. Decent Shimano BB's can be had for <$25 from your LBS, with the added benefit that they'll likely accept an exchange if you get one a little too short.

Also, if you do get one that's too short on the drive side and seems like it could come in a bit on the nondrive side, BB spacers are dirt cheap and come in 1 or 2 mm sizes to really dial in your chainline. Before you get a shorter spindle, double check the amount of clearance between the chainrings and the chainstay.
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Old 01-04-10, 05:14 PM   #13
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Originally Posted by DaveSSS View Post
I would not assume that you have a problem without measuring- assuming youhave a BB that matches the crank. If not, you must start with that - both a spindle length and taper made to fit the crank, not whatever the bike had on it before.

The chainring positions have not changed for a very long time. Even though the the rear dropout spacing changed from 126 to 130mm, the wider cassettes of today spread the extra width in both directions.

The common chainline specification makes little sense to me. It's always listed as 43.5mm to an unmeasureable point between the rings rather than an easily measured point to the tip of a tooth on one or both rings. With the chainring tooth to tooth spacing about 7.8mm, that makes the tip of a tooth on the big ring in the 47-48mm range and the little ring about 40mm.

http://www.sheldonbrown.com/gloss_ch.html#chainline
I measured this morning and tube to inner ring was 40mm.

I might be fine. Just by sighting it looked off.

I'll hang a chain this week and see if I can adjust front DR to work correctly. It's close.

Best regards,
Doug
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Old 01-04-10, 05:29 PM   #14
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As long as that's the center of the tube to the tip of an inner tip tooth, you're good. Measuring from the side of a 28.6mm tube, the would be about 26mm.
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