Editor's note: This article was written in 2000. Since then tubeshapes have changed in certain brands, most notably in those tri bikes made by Cannondale. Tube shapes in its bikes starting in 2003, when C'dale became Ironman's bike licensee, became notably skinnier, and its minor diameter of main tubes shrunk down below 30mm, that is, in the same neighborhood as those used by Cervelo, Yaqui, and the like. Therefore, read the article in this light, and remember that the point of the article is to urge you to consider each tube shape individually, not simply to see whether it's teardrop or "aero."

I'll leave it to John Cobb, Steve Hed, and other smarty pantses when it comes to what shapes bike tubes should be, and the aerodynamic value of such. Generally, I've found that those who look at tubes and pontificate on their aerodynamic qualities are those who haven't spent much time in the wind tunnel. Those who have spent the time, like Cobb, are generally reticent to say anything about a bike, or a tube, unless the thing itself is tested in the tunnel. The grizzled veterans of bike aerodynamics have been surprised too many times.

That said I, of course, will, off the cuff, make statements from the seat of my pants; shall, like James T. Kirk, blow smoke where no smoke has been blown before; and, with no supporting data, I'll make unsubstantiated guesses on mere whims. I shall jump to conclusions. Because a tube is fatter, or longer, or skinner, or shorter, I will make assumptions as to its aerodynamic worth.

That said, I'm just making guesses, and I don't purport them to be more than that. For example, I'll give you my armchair guess that, if I was a betting man, I'd take longer and skinnier tubes over shorter and fatter ones (but if I was betting, I wouldn't bet very much).

I do think it is interesting that, at the cash register, it seems that any frame built with "aero" tubes is perceived as more desirable, and worth more, than any frame built with round tubes. Yet consistently round-tubed bikes "beat" one-third to one-half the "aero" tubed bikes in the wind tunnel.

I thought it might be illustrative to point out one or two reasons why that might be. I present a spreadsheet below detailing the dimensions of six bikes, or more precisely, the dimensions of their down tubes. There are all kinds of problems with this: I'm not discussing the shape of these bikes' seat tubes, seat stays, seat posts, forks, cable routing, and so forth. This means a lot of variables are not mentioned here. But I get the impression that a lot of people in Consumerland don't know that there are a wide range of dimensions in the shaped tubes manufacturers use on bikes they term "aero."

Below is a graphic which illustrates the smallest box each of the aero bikes made by the following companies would fit in. In other words, if you were to take the major diameter (how long the down tube is) and the minor diameter (how wide it is) and make a box out of these dimensions, these are the dimensions of the boxes. Notice that Cannondale's box is quite large. Their "aero" down tube is 45mm wide. Meanwhile, Cervelo's P2k has a down tube that is only 28.5mm wide. But, these tubes are about the same length. While I do not know how aerodynamic Cannondale's bikes are, I do know that Cervelo's are quite aero, and one might surmise that the reason is the attention Cervelo pays to getting its minor diameter very thin.

Cervelo uses an extruded tube. Imagine a cake decorator, where the pressure of a squeezing hand forces icing out a small aperture (that might be in the shape of a flower). Cervelo's down tube is made that way, and both the outside and the inside diameters are controlled by the shape of a die. The sides of Cervelo's tubes -- where the tubes are most narrow -- are thicker, so as to bear the greater torsional and twisting loads that make a down tube flex. This is why Cervelo can make their tubes thinner while still achieving a nice ride.

Notice that one of the bikes is "my road bike." This bike is made out of Easton's Scandium necked-down tubing, and although my road bike is aluminum, and quite laterally stiff, the total area of the tube is quite small. While the total area of my road bikes' down tube is less than any of the other bikes, its frontal area (determined by the size of the minor diameter) is larger than either Trek, Yaqui, or Cervelo, and it does not have a gradual, trailing edge that is the hallmark of all aero tubes. At some point, though, the beneficial shape of a tube is negated by its sheer bulk, and one might speculate that a round tube is not so bad compared to an aero tube, if the frontal aero of the round tube is considerably less.

The graphic below is a one-third size view of two of the above tubes, one place over the other.

The green tube is the down tube shape of a Yaqui Mariola, the purple is the down tube profile of a Quintana Roo PR7. QR uses standard #7005 aluminum, the smaller tube is Easton's Scandium tubing. Easton used to make the smaller tube in its #7005 alloy, but many people complained about its lack of lateral rigidity (the bike was flexy, whippy). It is very rare to see a tube this narrow in a simple #7005 format. QR is probably wise to use the dimensions it is using, as this is probably the smallest #7005 tube you could safely use if you wanted the bike to be stiff enough. But the Scandium double-butted tube Yaqui is using is quite a bit stiffer than #7005, and can therefore be made in the smaller dimensions and still be adequately stiff.

Interestingly -- and I certainly hope I'm not opening up a can o' worms here on the eve of the Tour, we called some Trek retailers we know and asked them to take out their calipers and measure their Trek Hilo down tubes. We got a range of 80.5mm of minor diameter, down to 79.5mm. It is our understanding that 80mm is the UCI limit for legality (Rule# 1.3.020). Trek's product designers made a big deal about how their Hilo frames were essentially exact duplicates of their Tour TT frames, with the exception of certain engineering issues they've yet to solve for production (such as an aero seat tube). I take that to mean they are using the same down tube in the Hilo that their time-trialers use on the Postal Service team. Of course some of the bikes might have been mis-measured by the dealers, or might have simply had a thick coat of paint. I'm sure Trek's engineers, smart people that they are, have taken pains to make sure their teams' bikes have tubes which are inside the 80mm legal limit.

On paper, it would seem like Cervelo has a very nice downtube shape, with almost the narrowest tube, and the longest aspect ratio. The Yaqui has a slightly smaller frontal area, but they'll never get to the aspect ratio of a Cervelo, because extruding a tube -- as Cervelo does -- will always yield a more radical aspect ration than forming a round tube (as Easton does). But, Easton can butt the ends of their tubes, and you can't do this when you use extruded tubes. So, there are weight and ride quality issues Easton feels they achieve via their process that no extruded tube can match. Also, it is extremely difficult to extrude some of the more exotic alloys, and certainly Easton's Scandium is just such an example.

When you see a bike with aero tubes -- alone, not next to other aero bikes -- it is hard to know how such a bike and its tubes will compare to another. It takes having various bikes, or their tubes, side by side to see the radical variations. One might not realize how a lot of bikes all purporting to be "aero" can generate a wide range of drag numbers, but viewing the big differences in down tube shapes above, perhaps those drag numbers are now a little more understandable.