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Old 05-04-05, 11:31 AM   #1
keevohn
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Intentionally denting a chainstay

Hey gang -

A quick question: on my fixed, I need to move my drive-side crankarm in by 4 to 5 mm in order to get good chainline. But, my early 80's steel Trek frame hasn't got the indentations in the chainstay that allow the crank to get close enough to the frame. As it stands, I'm using a 111mm BB with a 144bcd crankarm, and the arm clears by 2 mm. I'm intending to buy a 102mm BB to give me good chainline, but need a few more millimeters to get the crank to clear the frame.

Is it possible and/or safe to dent my chainstay to allow me the extra 3-5 mm necessary to move the crankarm in? If so, what's the best way to go about doing that? Do I run the risk of messing up my frame alignment?
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Old 05-04-05, 11:36 AM   #2
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Quote:
Originally Posted by keevohn
Hey gang -

A quick question: on my fixed, I need to move my drive-side crankarm in by 4 to 5 mm in order to get good chainline. But, my early 80's steel Trek frame hasn't got the indentations in the chainstay that allow the crank to get close enough to the frame. As it stands, I'm using a 111mm BB with a 144bcd crankarm, and the arm clears by 2 mm. I'm intending to buy a 102mm BB to give me good chainline, but need a few more millimeters to get the crank to clear the frame.

Is it possible and/or safe to dent my chainstay to allow me the extra 3-5 mm necessary to move the crankarm in? If so, what's the best way to go about doing that? Do I run the risk of messing up my frame alignment?
Wouldn't a different crank be a better option?
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Old 05-04-05, 11:38 AM   #3
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or a different bb?

Enjoy
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Old 05-04-05, 11:42 AM   #4
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How about re-spacing and dishing the rear wheel?
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Old 05-04-05, 11:45 AM   #5
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Originally Posted by powers2b
or a different bb?

Enjoy
How does that work? The one he needs for proper chainline causes the clearance problems. Cranks with a different spider design are the way around the problem.
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Old 05-04-05, 11:46 AM   #6
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I once added a small ding to the chainstay on my Ibis Scorcher so the inner spider arms would clear the chainstay using a shorter BB to achieve a good chainline.
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Old 05-04-05, 12:19 PM   #7
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Quote:
Originally Posted by The Fixer
I once added a small ding to the chainstay on my Ibis Scorcher so the inner spider arms would clear the chainstay using a shorter BB to achieve a good chainline.
That's what I'm interested in doing. What'd you use to ding it?

As for a different crank, I'm cheap, and I like my old crank. I suppose I could get a 110bcd crank to replace my current 144bcd crank, which I imagine would allow me to get the spider arms closer to the frame.

As for respacing/redishing... it's a fixed gear, with a flip/flop hub, so I'd like to keep the hub equally spaced.

Another option would be to get a longer BB than I currently have, somewhere around 120mm, and move the ring to the inner side of the crankarm (it's a double crank) to get my chainline. But that increases my Q factor, and frankly I don't like the aesthetic of fixed gears with rings mounted on the inside of the crank. I'm shallow like that

So, you can see my desire to dent my chainstay.
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Old 05-04-05, 12:27 PM   #8
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Quote:
Originally Posted by keevohn
That's what I'm interested in doing. What'd you use to ding it?
I used a snub end punch and hammer.
And yes, this enables you to mount your ring on the outer spider.
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Old 05-04-05, 12:30 PM   #9
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Deforming heat treated metal is never a good idea.

Enjoy
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Old 05-04-05, 01:42 PM   #10
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Steel isn't heat treated. Aluminum, on the other hand, is. I say go for it, and be careful is all. You might even check a framebuilders forum to see how they do it, they'd probably have some good suggestions for you.
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Old 05-04-05, 01:52 PM   #11
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Quote:
Originally Posted by rykoala
Steel isn't heat treated.
Maybe so on the bikes you ride.
Better check your materials handbook.
http://www.matweb.com/search/GetKeyword.asp

Enjoy
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Old 05-04-05, 02:09 PM   #12
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[QUOTE=Steel isn't heat treated.[/QUOTE]

Quote:
Originally Posted by powers2b
Maybe so on the bikes you ride.
Better check your materials handbook.
http://www.matweb.com/search/GetKeyword.asp

Enjoy
An 80's era Trek most certainly isn't heat treated. But today's high end steel alloys are heat treated. And it is indeed best to check the actual material than to guess.
And not all aluminum is heat treated. (most 6061 is, most if not all 7005 isn't) Blanket generalizations are rarely accurate!
Denting the chain stay will compromise the lateral rigidity of the frame, but how much is anyone's guess. Given the age of the frame, I'd not worry about it. Although it would be BEST to find a solution that eliminates the need to dent the frame, it would be a nice pet project. You may have some alignment issues after denting, but a string run from the dropouts past the seat tube and around opposite sides of the head tube is an excellent alignment guage. 80's era frames are absurdly easy to align compared with, say, a steel frame made with True Temper OX Platinum.
Remember Cannondale's old ads with a guy contemplating the advantages of Al while crouching on the rear triangle of a bare CDale frame? You couldn't do that with steel back then. But today you can! Today's steel is amazing!!!
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Old 05-04-05, 02:23 PM   #13
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Quote:
Originally Posted by rykoala
Steel isn't heat treated.
Actually it can be.There are plenty of bike tubesets that are. His Trek isn't.
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Old 05-04-05, 02:28 PM   #14
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Quote:
Originally Posted by LV2TNDM
An 80's era Trek most certainly isn't heat treated. But today's high end steel alloys are heat treated. And it is indeed best to check the actual material than to guess.
Trek may not have used Reynolds 753, but it is old school and certainly in use back then.
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Old 05-04-05, 05:11 PM   #15
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Moving the crank further inboard may result in heel clearance problems with the chainstay.

Instead of using a shorter BB, have you considered just moving the chainring inboard? If you are using large chainring, you can usually move from the outboard to inboard side of the spider. This should give you 2-3mm better chainline. If you need to go further, you can get chainring spacers from harriscyclery.com to move in further. You may need to get longer chainring bolts while you're at it. If you are using the small chainring, then just get spacers and longer bolts as necessary.

Watch your chainring to chainstay clearance, especially when using a big chainring.
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Old 05-04-05, 06:35 PM   #16
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Originally Posted by supcom
Instead of using a shorter BB, have you considered just moving the chainring inboard?
Yeah, but that moves the ring in too far. My BB length is just exactly long enough to be useless: outboard the chainring is too far, inboard the chainring rubs the chainstay.

Thanks all for the info and suggestions... I think I've come up with a solution:

Buy a new crank for my road bike!

Logic: my current road crankset (beat-up 105 SC) with Shimano BB gives me 40mm chainline on the inner ring, which would be perfect for my fixed gear. My current fixed gear crankset (old skool Superbe) is mounted to a Campy BB. Swap BB's between bikes, install the 105 SC crank on the fixed gear, buy a new Campagnolo crank for the roadie... it's all good.

Anyone want to buy a Superbe crank?
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Old 05-04-05, 06:44 PM   #17
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Originally Posted by keevohn

Anyone want to buy a Superbe crank?
Sooooo, what's wrong with the superbee on the roadie?
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Old 05-04-05, 06:53 PM   #18
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I'd rather grind a few mm off the crank than dent the frame
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Old 05-04-05, 10:26 PM   #19
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Quote:
Originally Posted by elicheez
I'd rather grind a few mm off the crank than dent the frame
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Old 05-04-05, 11:25 PM   #20
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Originally Posted by elicheez
I'd rather grind a few mm off the crank than dent the frame
Interesting option.
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Old 05-05-05, 06:12 AM   #21
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Quote:
Originally Posted by elicheez
I'd rather grind a few mm off the crank than dent the frame
I had to do both when I put 170mm cranks on my Diamond Back F1 years ago. The frame held up just fine, and the cranks are still doing well.
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Old 05-05-05, 10:19 AM   #22
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Quote:
Originally Posted by keevohn

Anyone want to buy a Superbe crank?
I just sent you a PM. - I might be interested in your crank......

John D.
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Old 05-05-05, 10:29 AM   #23
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Crimping the chainstays is something that framebuilders do using just brute force.
http://www.sonic.net/~maryking/rear_triangle.html
You may want to secure an old axle in the rear dropouts to prevent too much movement of the dropouts.
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