Re: Off-Beat Steel Tubesets?
For any material, I wouldn't use plain-gauge main tubes, i.e. not butted, especially with thin walls <0.9mm. That's because engineering theory supports good old-school practice of using butted main tubes to make a more robust frame than plain gauge tubing, especially with thin walls where the frame is meant to have some amount of nice flex and be supple. You 'might' claw back some robustness if you add 'Bontrager gussets' to the plain-gauge tubing. Non-tapering bridge structures basically broke at the ends so structural engineers all moved on to stress-distributing tapering designs. https://www.google.com.au/search?q=b...=lnms&tbm=isch
I reckon most novice framebuilders asking questions here wouldn't understand the significance of butting and would benefit enormously by a better understanding. I only worked out the importance of butting a few years ago and before that I thought it was simply to save weight and make the frame feel more lively but those two things are just a bonus. That is why I also prefer to use single butted chainstays, which as well as tapering, are thicker-walled at the BB end.
I'd say it's quite daring to use thin-wall plain-gauge main tubes in a frame because the problem that led to Reynolds' innovation still applies since designing robust yet lightweight bike frames is about designing-out 'stress-risers'. Butting was developed to make plain-gauge tubing more robust, by reducing the stress in the material at the ends, right next to the welds or the lugs. Heavier-duty frames may well use plain-gauge tubing with few premature failures, but I only ride and make lively ride-feel frames.
"In 1895 [Reynolds] began examining a problem which cursed many frame builders of the day: how to join thin, lightweight tubes without weakening the joints at which they are connected.............Reynolds came upon a way to increase the thickness of the walls at the ends of the tubes only (and so reducing the stress that the material experienced) , .........Up until then frame builders had to manually insert a liner into the end of each tube to reinforce the joint or use heavy, thick tubing."
From the second paragraph of Reynolds history
This is my opinion, which I can also argue in more technical, engineering terms. I've also inspected or repaired frames that matched the theory; e.g. a frame that had a heavy frontal collision; the top and down tubes crumpled away from the welded ends, away from the braze-ons, where the tube wall thinned at the butt, (that I measured directly by chopping up the frame to check). The location of the crumple very clearly indicated the location of the most highly stressed material in the tubing, where you'd want it to be, away from the weld or the relatively 'sharp' lugs. That design would also do well in terms of resisting premature fatigue failure because the tubing material that is most highly stressed is 'clean', placed away from 'stress-risers' that would reduce the robustness.
I think we need a separate 'sticky' post for basic good design theory.
Ewen Gellie
Melbourne Australia
full-time framebuilder, Mechanical Engineer, (Bach. of Eng., University of Melbourne)
[url]www.gelliecustombikeframes.com.au[/url]
[URL="http://instagram.com/gellie_custom_bikes"]http://instagram.com/gellie_custom_bikes[/URL]
Bookmarks