Re: Seat tube distortion...
Originally Posted by
shand
There's was Bike Tech article from the 80's written by some dude called Keith Butranger or something that discusses this in more detail. Basically looking at temps of brazing vs Tig and also time spent at temps and how it affects strength. Interesting reading, I'll see if I can dig a copy out. (where's Truls when you need him?!)
Steven
Thanks Steve. It's great to read that again .... and several very interesting and relevant things stood out to me this time ... "What this really shows is that the HAZ strength drop seems to be directly related to heat cycle time."
As an aside, my search for "butt", "gauge" and "wall" in the KB article did not find a use of those terms. KB's gussets are mentioned and so I now read the article as meaning the gussets are intended for plain-gauge TIG-welded tubing. So IMO (as I keep banging on about) the primary reason to use butted tubing is that it is reinforcing the tube near the welded end and so takes the load off the weld/braze. (This makes me more convinced that KB gussets would be unnecessary for butted tubing since the butt takes the role of the gusset and so gussets are an odd thing to use on butted tubing).
So the ends of a butted tube, being reinforced, lessen the impact of the "heat cycle time" somewhat for a fillet-brazed joint. Especially since the drops in Rockwell/strength numbers appear to be only of the order of 5-10%. Have I described that clearly? It's quite a complex thing.
Similarly, a single-butted seat tube has no place at a seat cluster unless it is reinforced with a decent seat lug, (which is what it was developed to be used for ), or a brazed/soldered collar. You might be able to braze or TIG it, or put a silly little seatpost-clamp lug on top of it, but it will break prematurely if it is 0.6mm wall and it aint reinforced. It might break at the hole at the end of the seatpost clamp slot. It'll certainly be more difficult to weld/braze without distortion too.
Ewen Gellie
Melbourne Australia
full-time framebuilder, Mechanical Engineer, (Bach. of Eng., University of Melbourne)
[url]www.gelliecustombikeframes.com.au[/url]
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