Minimum length of tubing

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  • Hi framebuilders!
    A very noob question: say I want to build a frame with the following geometry (really just as an example)
    TT (c-c): 520
    ST (c-c): 480
    DT: 580
    What should be the minimum length of the raw tubes for each of the above dimensions? I suspect there is no maximum length really, because I can always buy longer tubes and cut them, but what's the minimum length?

  • Depends to on the diameter of the tubes and the angles as that will determine how deep the mitres on the end of the tubes go and the diameter of the bottom bracket. As a general rule of thumb I'd say you'd want at least 20mm to play with at each end you're going to mitre - no need to cut the top of the seat tube presumably - possibly more for the DT as the mitres will be deeper due to the angles.

    If you're using butted tubes there will also be a maximum length which will depend on how long the butts on the ends of the tubes are, as you don't want to make the butted sections too short so you can't just hack the tubes down to whatever length suits.

  • Good point about the maximum length, because the tubes I am after are indeed butted (Columbus Cento). See https://www.framebuilding.com/Columbus%2­0CENTO%20catalogue.pdf
    The thick sections at the end are 50 mm long, so how long of the butted sections do I need to keep at least?

  • (Please abstain from the comment "why would a noob waste Columbus Cento tubes to build his first bike?" -- I can explain :D)

  • The thick sections at the end are 50 mm long, so how long of the butted sections do I need to keep at least?

    I think Columbus recommend 35mm of butted section as a minimum

  • Interesting, this means I can cut maximum 15mm on each side in this case

  • The thick sections at the end are 50 mm long, so how long of the butted sections do I need to keep at least?

    Are you going to be using TIG or fillet brazing it? Obviously you won't be using lugs with a 44mm DT. The HAZ is smaller with TIG so you can get away with shorter butts, and larger with fillet brazing.

  • fillet brazing

  • do you have a reference for that?

  • fillet brazing

    In that case, I definitely wouldn't want to take too much off. It's difficult to say how much you can take off, as all the ends of the tubes will be mitred apart from the top of the seat tube, so the tubes won't have a single length. Given that Cento comes in 25mm increments for the DT and 30mm increments for the TT, you shouldn't need to take much off anyway. Unless you're building a very small frame, in which case I really wouldn't be using Cento in the first place.

    There are two ways to really work out how much tube you'd need to remove and which size tubeset you need. The first is to use a program like RattleCAD or BikeCAD which will give you the tube lengths. The second is to draw it all out and then work out the depths of the mitres and add them to the resulting tube lengths.

  • I was half remembering this doc

    Which has a specific max length to cut for each tube. Mostly leaving around 30mm

    Specifically for Life though

  • This is awesome, THANK YOU!

  • Back at this, I want to make a decision, so that I can buy the tubeset (FOMO because end of the year is approaching). Let me make my question more pragmatic.

    Let's say I want to have a TT of 520 c-c on a bike with classic geometry (horizontal top tube), is it enough to have a raw TT from the Cento tubeset of 530 to start with? or is 560 preferred? What would you do at my place?

  • What would you do at my place?

    Not be looking at using a £500+ tubeset*.

    The document posted by @PhilDAS seems to suggest 30mm of full thickness should be left intact. If we assume Cento is the same then the minimum I'd want between the deepest parts of the mitres (ie the outsides of the abutting tubes) on the 560mm tube would be 520mm so this tube would not be suitable for a 520mm c2c toptube frame.

    I do note that the wall thicknesses on the Cento tubes are a bit lighter than on the Life ones so I'd actually want to err on the side of caution and leave a bit more than 30mm of full thickness.

    *I know you said to abstain from this kind of comment but it does sound like you are planning on building with this tubeset yourself and that you have no/very little prior framebuilding experience so I can't help but wonder if you understand that tubes with this kind of wall thickness are not going to be easy to work with? Are you planning on buying a Cento tubeset and holding onto it to build with once you have more experience?

  • Thanks for the reply!

    Are you planning on buying a Cento tubeset and holding onto it to build with once you have more experience?

    You got it exactly right! I saw the Cinelli Nemo Zagato and fell in love with that tubeset. And as I am a big fan of Columbus/Cinelli, I have a fear of missing out on this limited edition, so I want to buy the tubeset now and "freeze" it. My full plan is: full course at the bike academy in March 2020, then a few builds for friends with cheaper tubesets, and only after that, the Cento!

    I will go for Measure A then, with 530 TT. Thanks a lot!

  • I think someone else on here was thinking about buying a tubeset and stashing it away for a while.

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Minimum length of tubing

Posted by Avatar for steelest @steelest

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