Question from a n00b with no framebuilding experience:

I see a lot of track bikes with non round seat tubes that extend out over (a bit) the rear wheel, with a cutout for the rear wheel. How is this made? Is it just elliptical tubing brazed over a regular seat tube and a piece of sheetmetal is cut and bent to fill the gaps?

What is the advantage of a design like this?

I have only seen that with carbon bikes. You could use some sort of hydro-forming with a ductile metal, most typically aluminum. I'm sure some people have brazed together tubes for that effect, but it isn't that great of an idea.

Like this?

Easton Elite Aero Tubing. Nova also has AL 7005 Shaped Tubes. Builders use one the aero down tubes and cut the wheel arch. Then they put in a receiver for the seatpost.

On a custom frame, the shaped tube could be extended all the way up and capped with a saddle clamp topper.

I first assumed that that's how they did it, but then it didn't make sense to me because when you insert the seatpost receiver, you can only braze at the top...so how does it stay secure in the downtube?

What does cutting out the actual down tube do, structurally speaking? Does it compromise the strength of the frame? Basically I really like the track look, and if I'm going to build my own frame (first timer...doing it for fun, I don't actually race or anything), I might as well make it look what I want it to look like. But I bought my first fixie last year and have been getting into basic (read: poser) fixie tricks. I need something that will withstand any beating I might give it.

Can someone recommend materials? Is it useless to go for the track look for a fixie that will bear some beating?