bulgie 

Ah, what to call that tube? Santana calls it a lateral tube but I believe that is wrong, lateral meaning "to the side" so I use laterals to refer to those smaller tubes that are used in pairs spaced out to the side, like traditional Mixte frames. Usually used as "twin laterals". Santana says no, their laterals are "internal laterals"; internal meaning "in the plane of the bike", with lateral meaning more like "going from side to side". But they don't go side to side, IMHO they go front to back (sort of). Their original design, based on one of the designs Jack Taylor used to make, had the "lateral" from the head tube to the rear dropouts (with mid-stays); this was called a Marathon (named by the Taylors I believe). Later they (Santana) changed to the one with the bracing tube from the head to the R BB, which they dubbed the "Direct Lateral" and which they claimed to have invented, though it was used decades earlier. (Dunno if they knew that).

Anyway I just call them bracing tubes, though that's not really meaningful to most people. I don't think there's a generally-accepted term.

Now, do bracing tubes (of any config) do much? Debateable, but we know they aren't needed, because there have been lots of excellent tandems with the "open frame" config = zero extra bracing tubes. That style requires the outer tubes like TT, DT, keel tube etc to be large diameter. I'm pretty sure that if you remove the bracing tubes and use that same amount of steel to make those outer tubes larger diameter, the effect will be a stiffer frame, in the ways that matter (lateral and twisting stiffness). Diameter trumps all. There's no internal bracing scheme that can beat that.

With no bracing tubes, the rear parallelogram* consisting of both seat tubes, the R TT and the keel, depends of the stiffness of the joints to keep it from "parallelogramming". (Hint: imagine if the joints were hinges...) But the vertical compliance this gives is seldom considered a bad thing. (*yes I know it's not really a parallelogram unless the TT and keel are parallel and both seat tubes are parallel, but for the purposes of this armchair engineering exercize, let's pretend it's a parallelogram for simplicity.)

Note, everything I just wrote is considered controversial by some people. Of course I just think it's 100% correct...

I don't consider that a large keel tube, in fact it's a bit paltry (though I like Burleys in general, great value bikes). Earlier Burley frames like this used a single bent tube as the DT and the keel, so the size is a compromise. If it were big enough to be a proper keel, it would be too big (for steel) as a DT. Later Burleys switched to a more normal design with separate DT and keel, the latter being much larger. They also ditched the "loop stays", switching to separate chainstays and seatstays, a pretty good improvement.

Yes, I have a Burley that I was given for free because the cap'n ST is rusted completely through at the BB. A shame, because the bike doesn't seem to have been ridden much. I think rain got in while they were carrying it on the car. It's one of the newer designs I prefer, separate keel, and separate rear stays, with the chainstays being nicely oversized. So I do think I will repair it one of these days.

But as discussed already, tandems are hard to sell, so if I repair it I won't be able to sell it for much, and it's not my size. Plus I have 2 other tandems already that I like a lot.

Mark B