The reason for butted tubing, as with anything else, is to make it stronger near the joint. That's where the forces are greatest, so that's where it needs to be stronger. The lug is supposed to do this, to some degree, as well as making a stronger joint than you would get by just butting and brazing or welding. In reality, though, lugs on our bikes are more decorative than functional.
Ideally, the tube thickness would be tapered, but that's not practical, so stepping the thickness is next best. Things like triple butting, on the old Miyatas (and, actually some of the newer ones, too) is a closer approximation to a taper. Theoretically better, but I doubt it makes any practical difference.
You can see this principle applied in all kinds of places. Right now I'm looking at the power cord for my printer, which has a tapered region where the cable emerges from the plug, to prevent a hard kink. Streetlight supports are wider near the base and taper towards the top; this isn't just decorative.
By the way, an old-time bike mechanic once told me that a common point for frame cracking is in the down tube, under the head lug, especially if the lug is pointed. That focuses forces at that point and it eventually fatigues. Most frames I've seen have the underside of this lug rounded, perhaps for that reason.
Steve Maas Long Beach, California
Harvey Sachs wrote:
> In a generally spot-on message from Don Gillies, I have doubts about two
> statements, so I'm going to <snip> the rest to comment on them:
>
> <snip>
>
> Don: The main purpose of butting is that (a) it's designed to allow thin
> and springy tubes but also (b) it's designed to be thick at the ends
> for the joinery of the tubes. So, the thick ends of the tubes should
> have a long enough butt (typically 75-100mm or 3-4") so that the lug
> points do not dig into the ultra-thin part of the tubing. So an
> italian long-point lugged frame should probably have longer butts than
> a bocoma pro (short-point) lug set.
>
> Harvey: I've never heard the assertion about lug points digging into
> thin tubing; it's all very soft at temperature. I think the real point,
> which you allude to, is that the thicker butt was supposed to increase
> the heat capacity and thus limit the local temperature rise when doing
> hand-held (or hearth) brazing.
>
> Don: The benefits of quad-butting are imho doubtful. On some bikes like
> Miyata, I believe they put the downtube shifter bosses after the first
> (but not second) butt. Tearing at the shifter bosses is a very common
> downtube failure mode. So I can perhaps understand triple butting on
> a downtube, but I seriously doubt if it's needed when you could use
> low-temperature silver-solder for installing downtube shifter bosses,
> bottle bosses, and pump pegs, etc., to minimize heating and weakening
> of the tubes at the thinnest points.
>
> Harvey: Agreed on the silver solder for tiddly bits, but I've never seen
> a frame with tearing at the shifter bosses. Accidents? Real monster
> abuse like kicking the thing? I have seen frames that failed with
> transverse cracks behind the head tube, as though heat had been too
> localized (but could have had other causes), and there was some talk
> that Nishiki Competition frames tended to suffer downtube failures.
> Below the shifter bosses, as I recall, but this is veyr fuzzy.
>
> Harvey sachs
> mcLean va