Lbxpdx

While stripping my new 1990 Novara ponderosa down, I noticed the seat tube tapers to an oval at the bottom bracket shell. What is the reasoning for this?

merziac

Oversize MTB DT needs more room at the BB shell.

bulgie 

I might be quibbling but I wouldn't call that tapered, just ovalized. I say that because there were also tubes that flared (tapered) to a larger diameter below the front derailer, and were also ovalized. Examples include the Davidson Stiletto, and Zunow. Those frames have a seat tube that started out as 1-1/4" (round) at the BB, which tapers down to 1-1/8" by the time you get to the derailer mounting point. But the oversized 1-1/4" round tube is then ovalized, to about 1-3/8" wide by 1-1/8" front to back. Very stiff against side-to-side sway from hard pedaling.

On my tandem, I used a tube that's 1-1/8" at the top but tapers to 1-3/8" round at the bottom (way oversized), which I then also ovalized, to about 1-1/2" wide. Super stiff.

Seat tubes that were ovalized (but not tapered) below the derailer, round from the derailer on up, were seen on frames as far back as the 1930s I think, certainly by the '40s, for example Barra. Tom Ritchey made some frames with that feature when he was a teenager in the early '70s.

Neither Barra nor Ritchey had oversized downtubes, so that's not the reason for their ovalized ST. Ask them for their reasons (OK not Barra who's no longer with us.) As a fan of ovalized seat tubes myself, I can only say why I did them that way, and it's for a bit of "free" stiffness and strength with no extra weight. The bottom of the seat tube sees mostly side-to-side loading (and flex), and the oval shape is better at resisting that than a round section (cylindrical) tube. A round-section tube is better at resisting twisting loads, so that's why I wouldn't ovalize the DT at the BB — only the ST should be oval.

Personally, I think all STs should be ovalized at the BB, unless you want to use a lugged BB shell with a round socket, or if you want to save a little money by skipping the ovalizing step. Round tubes are a little easier to fixture also, for example for mitering. Finally, I think some people may not like the look of the ovalized tube, which is de gustibus.

Mark B in Seattle

T-Mar

+1, it is for increased lateral stiffness in the bottom bracket regaion. Stiffness is a function of the section modulus of a tube. When compard with a round tube having the same perimeter length and metal thickness, a oval tube will be stiffer in the direction of the major axis but less stiff in the direction of the minor axis.

Reynolds 

My '90s KHS Montaņa pro has it too.

Lbxpdx

Thanks for the info.