Aerodynamics of Frame Tubes
April 14, '00
(www.slwotwitch.com) by John Cobb
Does having an aero downtube actually matter on a bicycle frame? There's been a good deal of debate about that over the last few years with two pretty distinct views on the subject.
One group, mostly frame manufacturers, says it doesn't matter because the downtubes are drafting behind other tubes or are drafting behind the front wheel. There's also the story about a round tube becoming aero shaped if you view it diagonally (the way wind would pass over it). Then there's the theory that a really big tube is more aero because the air has time to stick to it—flow around it—and flow off the back of it.
There is another group that believes aero tubes are better, and they (the manufacturers) go off on tangents about how their particular tube shape came about because of a sign from God Almighty, or someone even higher up. You, the end consumer, have to read ads and ask questions, but you really haven't been given many facts.
Now, as a retailer in the bike business I have manufacturers call me and tell me all about aero this and that. The big equalizer, in my case, is that I go test these bikes in a wind tunnel to help me make better decisions. Does that mean I only sell the most aero designs on the planet? No. I still think bikes have to be functional, and look pretty, so sometimes I sell bikes that have different qualities than simply aerodynamics, like ride quality or handling characteristics. What I don't like, though, are claims that aren't backed up in any way, so I go testing and try to educate myself and you, the racer.
Should you buy a particular frame just because of the aerodynamics? No. Look for positioning potential, road shock absorption and the other things I mention above. Ride the bike before you buy it, even if you have to travel a little. Then, if you're down to a couple of choices, go with the aero one.
Back to the question, do those flattened tubes matter? I went down to the Texas A&M wind tunnel and set up a test to get those answers. I built a test stand that held a 21-inch-long tube at 45 degrees (about where a downtube would normally be positioned). This is an average length and angle for a 53-57 size frame.
I took four tubes with me. I got them by cutting them out of frames here at the shop. The first tube was a standard steel 1-inch diameter downtube with shifter bosses. Number 2 was an oversized aluminum/carbon tube off of a leading manufacturer’s frame, 1-1/2 inches in diameter. Number 3 was an Easton 7000 series aero tube, a tube shape very popular with custom builders and a few manufacturers. For Number 4, I got a piece of aircraft wing-strut tubing eight centimeters wide. It had an extruded, perfectly shaped foil section, and airplane people are supposed to know about airflow, so I thought that would be good for comparison. Number 5 was the same piece of Easton tubing turned around backwards just to test everyone's theory on airfoil shapes. We cranked up the big prop and watched for the outcome.
I've broken the results down to a 40K time-trial distance to help make the numbers more realistic. What this means is that if everything is exactly the same—wind, weather, rider output, etc.—then these would be the differences in the tubing.
#1 - 58:03
#2 - 58:08
#3 - 57:29
#4 - 57:25
#5 - 57:25
OK, I'm opening up a big box of snakes now, but I go test things to learn how and/or do certain things work. Everything looked good and about how I thought it should except that the Easton tubing did better when reversed at higher yaw angles. I'm going to list the drag data at each point to help with this. Please realize that these are very small aero drag differences:
|Degrees of yaw
To help give you a reference on these numbers, a single 32-hole, 700C standard wheel has a drag of .500, an aluminum deep V aero rim would be about .300 and a good carbon aero wheel is .200.
Does this mean everyone is welding their tubes backward? I hope not, but I'm going to do more testing and try to learn some more. I'm pretty sure some bike manufacturers ought to join me to learn some more about it.
What is pretty certain is that while there are a lot of things we can learn from traditional aerodynamic testing in "higher" or "more advanced" industries, like aircraft manufacturing, bicycles operate under very different conditions. The bikes you ride are tied to the ground by the two points your tires contact the pavement. Wind is often, perhaps even usually, not hitting these tubes straight on. So "ideal" tube shapes often don't get to see ideal wind conditions. The only way you truly get to know how a tube is going to work is to put it in the wind tunnel and test it.
The bigger picture here is that round tubing is not ideal, big round tubing is just plain slow, and such bikes shouldn't have the word aero attached to them anywhere. There are some exceptions. We've tested bikes with smaller round tubes and with round seat tubes that curved in front of the rear wheel. Those bikes tested very nicely. But the testing I've just completed at least convinces me that, all other things being equal, a flatter downtube of some sort is the better way to go.