Attemping to build a frankin frame from two old frames with top tube damage...
01-10-12, 04:23 PM
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Attemping to build a frankin frame from two old frames with top tube damage...
So I have a metals class at school and I have a idea for a project. I have never attemped to fabricate any thing bicycle frame related. So I'm looking for input, opions, and general guidance as to wether this is a realalistic idea.
What I'm working with is a 56cm Colnago sprint or something and thr top tube cracked half way round the front internal cable routing, otherwise in great shape.
The other frame is a 60-61cm Gios professional with a rusted through top tube, otherwise its not bad.
So the 56cm colnago is my size and I would like to use this as the frame in which will be used in the end. I am prepared to unbraze the Gios and use this for donor tubes/lugs...
I dont want to just throw a top tube in, but since this is a art class I wanted to do something similar to a flying gate. I a big fan of the 70's english tt bikes with crazy short wheelbase...Bottom line is I would like to have a unique yet classic and functional design when done. Please help, I can take pictures and include tubing lengths and dimensions.
To start I will replace the top tube on the colnago, unless the crack is repairable (doubtful). Then plan on cutting the seattube from bottom of seatpost and unbrazing from the bb. Adding a donor tube from the Gios as new full length seat tube. Then adding the two new seat stays, possably from the gios seatstay. I'm most concernened about the angle of the new seat tube coming from the BBshell. The angle is going to change quite abit, will the shell conform to this change if so how? Also I'm assuming I will need to take the rear in a bit by shortening the chainstays. Not planning on taking apart anything on the colnago that doesnt need be...So am I out of my mind or is this doable for a first timer...Thank you for your help!
What I'm working with is a 56cm Colnago sprint or something and thr top tube cracked half way round the front internal cable routing, otherwise in great shape.
The other frame is a 60-61cm Gios professional with a rusted through top tube, otherwise its not bad.
So the 56cm colnago is my size and I would like to use this as the frame in which will be used in the end. I am prepared to unbraze the Gios and use this for donor tubes/lugs...
I dont want to just throw a top tube in, but since this is a art class I wanted to do something similar to a flying gate. I a big fan of the 70's english tt bikes with crazy short wheelbase...Bottom line is I would like to have a unique yet classic and functional design when done. Please help, I can take pictures and include tubing lengths and dimensions.
To start I will replace the top tube on the colnago, unless the crack is repairable (doubtful). Then plan on cutting the seattube from bottom of seatpost and unbrazing from the bb. Adding a donor tube from the Gios as new full length seat tube. Then adding the two new seat stays, possably from the gios seatstay. I'm most concernened about the angle of the new seat tube coming from the BBshell. The angle is going to change quite abit, will the shell conform to this change if so how? Also I'm assuming I will need to take the rear in a bit by shortening the chainstays. Not planning on taking apart anything on the colnago that doesnt need be...So am I out of my mind or is this doable for a first timer...Thank you for your help!
01-10-12, 04:38 PM
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you can't change a bb shell more than a couple of degrees.
Do either of these bikes have chrome? If so, you have to remove it or heating it up will make you sick. Un-brazing tubes from lugs is not an easy task, especially since these were almost surely made with bronze.
Do either of these bikes have chrome? If so, you have to remove it or heating it up will make you sick. Un-brazing tubes from lugs is not an easy task, especially since these were almost surely made with bronze.
01-10-12, 05:24 PM
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So something like this? https://www.classiclightweights.co.uk...-track-rb.html
"harvesting" and re-using tubes from a donor frame is not a great idea. The heat required to free the tube will weaken and distort them enough that it should not be re-used. CrMo frame tubes are really fairly inexpensive, just purchase the replacement tubes you need;
https://www.cycle-frames.com/bicycle-...g/TUBES-STEEL/
I would say that this is a fairly ambitious project, probably at least as difficult (if not more-so) than just building same bike from scratch. Having the new verticle seattube terminate into thin butted section of the top tube seems like a bad idea, joints should be made at thicker end sections. If short chainstays are your goal, a split seatube design like a Rigi might be easier to pull off, less complication with the single seattube lug in the Bottom Bracket being at wrong angle.
"harvesting" and re-using tubes from a donor frame is not a great idea. The heat required to free the tube will weaken and distort them enough that it should not be re-used. CrMo frame tubes are really fairly inexpensive, just purchase the replacement tubes you need;
https://www.cycle-frames.com/bicycle-...g/TUBES-STEEL/
I would say that this is a fairly ambitious project, probably at least as difficult (if not more-so) than just building same bike from scratch. Having the new verticle seattube terminate into thin butted section of the top tube seems like a bad idea, joints should be made at thicker end sections. If short chainstays are your goal, a split seatube design like a Rigi might be easier to pull off, less complication with the single seattube lug in the Bottom Bracket being at wrong angle.
01-10-12, 10:41 PM
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Yeah that's about it the flying gate would be cool, but the BB shell may pose too many problems, unless I can fabricate a sleeve at the correct angle. Then to shorten the chain stays, would I have to unbraze the seat stays also? It seems like too much work especially if I have to replace the tt. Can I get away with repairing the crack and filling in the internal cable holes?
