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Compact frame geometry and aero seat posts
10.22.01, Q&A between Dan Empfield and John Cobb
(www.slowtwitch.com)

PUBLISHER'S NOTE: I discuss compact frames below with John Cobb, the aerodynamic expert for Lance Armstrong and the U.S. Postal Service Team. So that you understand what compact geometry is, we've given you a place to look on both Giant Bicycles' website and on the Specialized website. Both these pages show bikes with top tubes that slope down as they travel back toward the seat tube. This is the essence of compact geometry.

The photos you'll see also show the bikes with aero seat posts coming out of round seat tubes. Both of these features––compact geometry and aero posts––are discussed below.

Keep in mind that this is one set of tests, and perhaps there are other tests, or other opinions, that are equally valid and should be noted. We're happy to hear from anybody who's got a dissenting view. Also, realize that if it appears that compact geometry is not the aerodynamic blessing that some people would like it to be, there is a rationale for this geometry that has nothing to do with aerodynamics, and we're not discussing attributes of compact geometry that extend beyond its aero quality.

DAN EMPFIELD:
So you just got back from the tunnel.

JOHN COBB: Yes. For over 15 years I've been going to wind tunnels to learn about the aerodynamic traits of bicycles, the parts you put on them, and the positions of those riding on top of them. There is no book called "Everything about Bicycle Aerodynamics," and you won’t find one written by me because every time I go to the tunnel it seems that practically everything I thought was right turns out to be wrong. Aero speed comes from strange places. I found that out again this last visit, when I tested compact frames and aero seat posts.

DAN EMPFIELD: Tell us about the aerodynamic quality of compact frames.

JOHN COBB: I was recently performing some tests to specifically compare traditional frames against compact frames. We tested the bikes with and without riders, with and without wheels, with different wheel combinations and with several rider/wheel combinations. It was a lot of data––over 600 data points––which included yaw angles out to 30 degrees. As with all testing, there is one surprise after another. The compact frames with comparable tube shapes were never faster than the traditional frames, but compact frames with a tall, thin aero seat post were about 10 percent better with no rider on board. That made sense to me.

DAN EMPFIELD: What about with a rider aboard?

JOHN COBB: As usual, the rider-aboard test didn't relate to the non-rider results. At this particular test, though, I had something new on my side. I had time. Wind tunnel time costs about \$500 per hour, and the tunnel engineers don't care if you’re eating doughnuts or testing pieces—they get \$500 each hour. For this test, the really good people at Texas A&M donated me three days of free time. This was partly to help them do a study on low-speed calibrations and better ways to interpret and express data.

The information they gathered and the methods of presentation will soon help others more easily understand aero data. It gave me a chance to test very specific areas about which I've had suspicions. I was testing a compact frame/rider with an aero seat post and the numbers didn't add up to the bare frame differences. I started using smoke and high-speed cameras to better visualize where the air was going. Then I did some wake-flow studies to see more about the air behind the rider. The net results of my most recent test suggest that compact frames offer no aero advantage.

DAN EMPFIELD: How about aero posts? It seems to me that an aero post on a compact frame ought to be a pretty good idea.

JOHN COBB: For many years I have recommended aero seat posts to many racers. Practically all of us well-intentioned retailers have sought after, stocked and sold some types of aero seat posts. Almost every rider who has gone out to set either a personal record or some world record has had their advisor or mechanic eye their seat post to help shave off those last few seconds of speed. It seems to make perfect sense to have some thin, willowy post between your legs so that air can easily flow out the back of the bike and give more speed. But what seems to make intuitive sense doesn’t always test out that way.

Using tall aero seat posts on a compact frame––though it seems like it ought to be faster––seems to actually cost time. How much slower is it? My results suggest that it might be about 45 seconds slower over 25 miles. Aero posts on traditional frames aren't as bad but still aren't as good as round posts.

DAN EMPFIELD: So what works?

JOHN COBB: A tall seat tube with an integrated post that has an aero shape but which tends to fill in the area between the rider's legs. In other words, a skinnier post does not seem to be necessarily better. But it can’t just be a fat post—it has to have a trailing edge that reconnects and directs the air behind the rider. So if it's a small, thin post, a round shape is fine. Best is a well-integrated seat tube and post where it's big enough to take up some space, while also having a good aero shape.

DAN EMPFIELD: It sounds to me like it's a good thing to keep the wind out of the area between the rider's legs. This brings to mind a trait that I've noticed with good timed racers—that their knees are always close to the top tube all the way around the pedal circle. They ride sort of knock-kneed. It seems to me that this is not only a good habit as far as aerodynamics are concerned, but it also may help isolate one's vastus medialis––that muscle just above the kneecap that good cyclists seem to develop.

JOHN COBB: Yes, we've tested different leg positions during the pedal stroke. This makes a big difference. You know, it's taken as a given that a rider represents 80 percent of the drag, and the equipment is the other 20 percent. But I think we can get a rider's position such that it may be only 50 percent of the drag. That way you describe the knee position, that's the sort of technique that makes a rider's drag disappear. Once we can accomplish that, then the importance of his equipment starts to again become the chief concern.