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Old 02-22-09, 01:20 AM   #1
Glenn1234
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How to Check a frame

Are there any tips or guides on how to check out a frame on whether it is bent or not? Or how to correct it if it is?
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Old 02-22-09, 01:28 AM   #2
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Are there any tips or guides on how to check out a frame on whether it is bent or not? Or how to correct it if it is?
The first test I do for frame alignment is to check if the wheels are in alignment which is the important outcome you want to achieve with a straight frame anyway.

Turn the bike upside down on its saddle and bars and sight along the wheels to see if they are in alignment. Spinning the wheels while you do this is usually a good idea. If they are in alignment then all is good. If they are out its not the end of the world. You just have to check as to why its out. The rear wheel may not be in straight or the wheels may not have been built with the right dish.

Anyway I always start by checking wheel alignment.

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Old 02-22-09, 02:12 AM   #3
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The first test I do for frame alignment is to check if the wheels are in alignment which is the important outcome you want to achieve with a straight frame anyway.
The wheel alignment seems good (of course I know how eyesight can deceive, too). The wheel spins basically upright and vertical in the frame. The question actually is more to this point. I suspect one of the rear "forks" (for lack of a better term - maybe "dropout"?) is bent, but no real way to see that and I wouldn't even know where to start to straighten it....
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Old 02-22-09, 06:20 AM   #4
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The wheel alignment seems good (of course I know how eyesight can deceive, too). The wheel spins basically upright and vertical in the frame. The question actually is more to this point. I suspect one of the rear "forks" (for lack of a better term - maybe "dropout"?) is bent, but no real way to see that and I wouldn't even know where to start to straighten it....
Yes the bit of metal that the axle bolts to is called a dropout. I've found looking at dropouts for alignment to be deceptive. If the wheel is going in straight and the locknuts are flat against the dropouts then its fine. If they are bent and its a steel frame then you can just bend them back. If its an aluminium or carbon frame then don't touch them. You will do more harm than good.

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Old 02-22-09, 07:27 AM   #5
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Bike shops have alignment tools that can measure dropout, derailleur hanger and major frame tubealignment. If the bike has been in an accident or you have some reason to suspect the frame has been damaged, it would be worth it to have it professionally checked.
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Old 02-22-09, 07:28 AM   #6
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Run a piece of string in a circular fashion from the headpost to the end of the stays; tie the string once tight.
Now using the seat tube, where the string crosses, measure the distance between the strings, find the middle point, should be easy to see if it is "shorter" on one side.
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Old 02-22-09, 07:58 AM   #7
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To check frame alignment, put the bike in a stand and remove the rear wheel. Tie one end of a long piece of white string to the inside of the rear dropout where the wheel locknut rests. Run the rest of the string forward to the *other* side of the head tube, bring it around the head tube and run it back to the inside of the other dropout. Pull it tight and tie or wrap it in place. (The two runs of string will cross somewhere ahead of the seat tube.) Make sure the knots are arranged so that the string lines up with the *inside* of the dropouts, or have a friend pinch them that way. Now measure the distance from the edge of the seat tube to the string on each side. (That's the reason you cross the string: to make it come closer to the tube which allows more accurate measurement.) If the seat tube is not centred between the strings, your frame is bent somewhere.

To check the fork, remove the front wheel and de-install the fork from the frame. Tie a plumb line into the hole in the crown that is drilled for the brake bolt. Now prepare a true level surface: lay a carpenter's spirit level on edge on your bench and shim up one end as necessary to bring the level, well...level. Have a friend hold the fork standing vertically on the level, resting on its dropouts with the plumb line hanging from the brake hole, the bob suspended between the dropouts. Measure the distance from the bob to the inside of each dropout. This only works if the dropouts are identical, since you're levelling against the tips resting on the spirit level, not against what really matters: the faces where the locknuts rest.

A shortcut that doesn't require de-installing the fork is to clamp a wheel that you know to be "true" (i.e., properly centred and no wobbles) into the dropouts. Make sure the wheel is fully engaged into the full depth of both dropouts before you lock it up. Measure the gap between rim and fork blade on both sides with calipers. This is actually a better real-life test because it takes into account any imperfection in the dropouts, but it's only as good as the goodness of your wheel.

Bent dropouts don't ususally cause alignment problems per se. Instead they create bending stresses on the axle which ultimately breaks the axle in the hub. Of course if they are grossly bent, you won't be able to get the wheel in at all.
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Old 02-22-09, 08:00 AM   #8
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Sorry for repeating xfimpg's string method. I type slowly and didn't see his before I posted!
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Old 02-22-09, 08:03 AM   #9
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A shortcut that doesn't require de-installing the fork is to clamp a wheel that you know to be "true" (i.e., properly centred and no wobbles) into the dropouts. Make sure the wheel is fully engaged into the full depth of both dropouts before you lock it up. Measure the gap between rim and fork blade on both sides with calipers. This is actually a better real-life test because it takes into account any imperfection in the dropouts, but it's only as good as the goodness of your wheel.
If you can't be certain the wheel is perfectly centered on it's hub, you could do the measurements to both fork blades, then reverse the wheel in the dropouts and measure it again. If the distance error is the same but reversed, the fork is straight and symetrical.
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Old 02-22-09, 08:53 AM   #10
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Are there any tips or guides on how to check out a frame on whether it is bent or not? Or how to correct it if it is?
For info on checking the alignment of the rear triangle with the front triangle, click this link and scroll down to the section titled "frame symmetry." If it's out of alignment, you can follow the directions in this same article to take care of it, if it's a steel-framed bike:

http://sheldonbrown.com/frame-spacing.html

Last edited by well biked; 02-22-09 at 09:17 AM.
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Old 02-22-09, 03:20 PM   #11
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Sorry for repeating xfimpg's string method. I type slowly and didn't see his before I posted!
No worries, I liked your explanation better!
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