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Old 08-11-10, 11:56 PM   #1
dnomel
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bending chainstays for crank clearance?

Has anyone done this? I have an old mtb frame that I'd like to use as a commuter, but it's chainstays are a lot wider than they need to be and I'm used to road bikes with more than an inch less width between the cranks. I'm not too worried about ideal chainline, I'd like to change the bb and probably use road cranks. From the looks of factory dents often on the drive side chainstays I'm thinking a diy job wouldn't weaken things too much, but I'm worried about things getting out of alignment. Thanks for any advice.
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Old 08-12-10, 12:12 AM   #2
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Just get the right BB to use with the road cranks. Nothng good will come from bending things. The dropout faces won't be parallel anymore for starters unless you start making more changes. All in all it's not worth it. Just get the BB you need to use with the road cranks.
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Old 08-12-10, 12:23 AM   #3
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The problem is with the proper (shorter) BB-spindle for road-cranks along with the larger diameter of the road chainrings, this will cause the inner chainring to rub on the chainstays. Denting the chainstays is what is done on a lot of MTBs, even road-bikes. I've done this quite a few times, here's the procedure:

1. install proper-length BB

2. slip on right crankarm with chainrings (make sure chainrings are true)

3. mark on chainstay where the tips of the chainring-teeth end

4. remove crankarm and BB

5. heat spot on crankarm with oxy-acetylene torche until an area the size of a quarter glows deep cherry red

6. hold a flat metal bar over the glowing spot, line up the edge of the bar with the mark for the tip of the chainring-teeth

7. make sure metal-bar is parallel to the plane of the frame and chainrings (parallel)

8. tap the bar with hammer, depending upon thickness of chainstay and weight of hammer, tap harder

9. re-install BB and crankarm and inspect clearance. I prefer to have about 2mm. Repeat from step #5
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Old 08-12-10, 12:45 AM   #4
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Quote:
Originally Posted by DannoXYZ View Post
The problem is with the proper (shorter) BB-spindle for road-cranks along with the larger diameter of the road chainrings, this will cause the inner chainring to rub on the chainstays. Denting the chainstays is what is done on a lot of MTBs, even road-bikes. I've done this quite a few times, here's the procedure:

1. install proper-length BB

2. slip on right crankarm with chainrings (make sure chainrings are true)

3. mark on chainstay where the tips of the chainring-teeth end

4. remove crankarm and BB

5. heat spot on crankarm with oxy-acetylene torche until an area the size of a quarter glows deep cherry red

6. hold a flat metal bar over the glowing spot, line up the edge of the bar with the mark for the tip of the chainring-teeth

7. make sure metal-bar is parallel to the plane of the frame and chainrings (parallel)

8. tap the bar with hammer, depending upon thickness of chainstay and weight of hammer, tap harder

9. re-install BB and crankarm and inspect clearance. I prefer to have about 2mm. Repeat from step #5

Thanks, is getting the spot red hot better for the tubing? I was thinking of denting them but without the torch, and looking at the clearances a dent for the end of the cranks might also be needed. What I'd really like to do is actually bend both the chainstays inward in the middle of there length since there is a lot of extra space in there without the fat knobby tires. My main desire is to have a narrower space between my feet like on road bikes.
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Old 08-12-10, 01:05 AM   #5
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Not red-hot as that would hurt the temper of the metal. I'm not sure what kind of bike you have or the materials it's made of. Most low-end and top-end bikes are made with seamed tubing (for cost on the low-end and high-strength materials are too hard for a floating-mandrel mill). Without softening the metal via heating to deep cherry red, you risk cracking the tubing or splitting the seam if you tried to dent it in the cold state. If you look closely at old low-end bikes where the dent is overdone so that the surface is actually concave (dented in), you can see cracks starting at the bottom of the dent.

What you are talking about is making S-bends in the chainstays. Much, much easier done before the frame is assembled. You'll need to squeeze in the chainstays in the middle and bend them back out back at the dropouts. Then re-align the dropouts so they are parallel. That's A LOT of bending and I don't even like doing it that much with bare tubing bent over a mandrel. It may be easier to just scrap the frame and get a road-bike frame.

Last edited by DannoXYZ; 08-12-10 at 11:40 AM.
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