Bent frame. How do I check if the frame is bent?

[glore2002]

Hello friends!

When it isn't obvious. How can I check if my bicycle frame or any mtb frame (old or new) is or not bent?

Is there an easy method I can use to check this at home?

I think this will be very useful not only for me. Well, I hope so...

Thanks in advance,

German.-

 

[kdelong]

There is no reliable method for checking it at home if it is not obvious. You need to take it to your LBS and let them check it. They will have special tools to check for the slightest bow in the frame.

 

[CAMPYBOB]

???

Buy a few tools and you're off to a flying start.

http://www.cyclingnews.com/tech/fix/?id=frame

Back when bike were steel, I Used to do all my own layout and bending on a 4' x 6' granite plate, but most steel tables are ground plenty flat for bicycle alignment standards.

I'ld tell you how to use monofilament fishing line and an accurate steel scale to obtain the answer you seek, but the scientists would throw a hissy fit and demand that only ruby lasers or autocollimators can be used in conjunction with differential calculus and little trig to yield a valid result which will be orders of magnitude below the detection threshold of the human brain as educated by the America public school system.

 

[glore2002]

Thanks to both of you. Yes Campibob. I would like to learn your fishing line method. I will really appreciate if you could tell me more about it. I don't want to buy tools to check if my frame is ok.

Thanks again,
German.-

 

[CAMPYBOB]

Well...Ok...at great personal risk from the 'scientists' and one moron from oz I'll let you in on the basics. In return, please send me any Un-used Campy of the approximate level of shimaNO 105...I'm trying to build da ultimate fast bicycle.

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

These instructions are just a start. Threr's more on the web and yet more in the older repair manuals. With some common sense (in very short supply on this forum) and a complete disregard for the known laws of the Newtonian universe you can put darn near any chunk of steel right. A few precision tools can expand the areas in need of measurement too. See Harbor Freight's website for inexpensive, but very serviceable measuring tools.

Be advised that some aluminum frames and all the carbon frames that I'm aware of cannot be cold set. Good luck and always remember it's your ass on that re-aligned frame.

 

[threaded]

Hello friends!

When it isn't obvious. How can I check if my bicycle frame or any mtb frame (old or new) is or not bent?

Is there an easy method I can use to check this at home?

I think this will be very useful not only for me. Well, I hope so...

Thanks in advance,

German.-

Erm, I use a ball of string. Easy, cheap, and more effective than any LBS I've found.

 

[glore2002]

Erm, I use a ball of string. Easy, cheap, and more effective than any LBS I've found.

Great! and How you use it?

Thanks,

German.-

 

[threaded]

Measure similar distances with the string. Say from top of the head tube to the bottom bracket. If one side is shorter than the other, then it is bent. You may find that the bottom of the head tube gives an opposite result for a twist in the frame.

Pull the string taught along a tube. If the tube has a bow WRT string, which is real easy to see, then it is bent.

Sit the frame on its stays and use the ball as a plumb-line. Head tube and seat tube, if they're not vertical, then it is bent.

On tubes that are meant to be round, is the diameter the same in both front/back and side/side, especially you get such ovalised head tubes in crashed MTB.

 

[oldbobcat]

Erm, I use a ball of string. Easy, cheap, and more effective than any LBS I've found.

In the 70s and 80s the best bike mechanic in the Northeast (the one the national teams used when they were up there) used a ball of plain cotton string, too.