The Myth of Aero Bike Frames: A Case Study in Engineering Jargon and Marketing

Cycling Power Output and Wind Resistance

One of the things I discuss in FASTER is how engineering jargon has made its way into advertising.

It seems every bike demands a research paper full of charts and numbers you don’t understand in order to justify your purchase. Comparing numbers we don’t understand isn’t a very comfortable proposition. Taking an engineer or salesman at his word that his company made the best bike isn’t any more appealing. We sort though those issues in the book and discussed how to use them in a step-by-step vetting process of bikes in earlier posts. Today I’m going to take a look at a specific example of a problematic use of science as a marketing tool for the Cervelo S5, written by Cervelo engineer Damon Rinard.

Full disclosure: I have met Damon Rinard and I own a Cervelo S5. I think he is a fine fellow and I love my bike. That said, it is my opinion that a product testimonial posted on the Cervelo web page commits the same wrongs as many other bike manufacturer pages. I feel the page leaves out key data points that would have added context to the information Rinard provides. The post does not lie or even mislead. It just doesn’t fully equip a typical consumer to assess what’s being said.

The main thrust of the article is that aerodynamics matter a great deal in the “peloton,” or the main pack of cyclists riding close together, of professional bike racing. Most people don’t think that it matters as much because aerodynamic resistance is reduced as you draft behind other cyclists.

The myth of aero bikes, science of triathlon

Rinard says as much and acknowledges that this is correct. His argument is that an aero road bike (in this case, the S5) gives a further advantage to the draft situation, providing a cyclist with perhaps an extra 6 watts of savings. He goes on to suggest that a 6-watt advantage could have made all the difference in David Millar’s victory in Stage 13 of the 2012 Tour de France. (While not terribly significant, Rinard is wrong about David Millar winning Stage 13. He won Stage 12.) It’s important to note that Rinard draws a connection between how those 6 watts helped Millar and how they could help you.

The first snag is the significance of 6 watts. In FASTER, I explain that the margins of error in measuring aerodynamics and power on the bike mean that an athlete must save at least 10 watts for a measured energy savings to be considered legit. Any claimed wattage savings under 10 watts might simply be a mismeasurement.

Literature published by Cervelo itself references the same principle. One could argue that the 6 watts from Millar’s S5 frame, plus another 4 watts from some other equipment item combine to make that “silver bullet” difference, but the book discusses why “aero arithmetic” in the real world on the road doesn’t add up the way it seems to add up in the wind tunnel.

The second problem is that, even if Millar saved 6 watts while drafting throughout that stage, very few amateur cyclists would ever reap the same benefit from their aero bike frame. Why?

First, Millar is one of the very best cyclists in the world. He can push double the amount of watts over a 5-hour ride than most human beings. And he can do it day after day. The speeds at which he rode in that (and every other) stage are simply out of reach of the common amateur athlete.

Second, the power required to overcome aerodynamic drag is exponentially related to speed. In other words, because of wind resistance, it takes 8 times as much power to double your speed. Now look at that relationship from the opposite direction. How much less power do you produce at speeds only a mile or two per hour slower than Millar? What about three or four miles an hour slower? Those 6 watts might matter to Millar, but they sure won’t make a difference for you.

Most wind tunnel tests on bikes set the wind speed to 30 mph. While it is possible that some cyclists can achieve speeds that simulate those conditions (30 mph in calm conditions or 25 mph against a 5 mph headwind), few people can do it for an entire stage in the Tour de France.

Cycling Power Output and Wind Resistance

So here’s the context Rinard leaves out:

Millar rode the 140.4-mile route in 5 hours and 42 minutes, giving him an average speed of 24.6 mph. That would be a respectable bike split on any Ironman course. Now add in the consideration that there were two gigantic mountain climbs on this particular stage and you see just how monstrous Millar’s performance was. The bottom line here is that extremely few people can ride at a level where they will get an “extra” 6 watts benefit from their bike’s aerodynamics in a tight pack situation.

Did Millar save 6 watts by riding an S5 in Stage 12 of the Tour de France? Probably. Did it make a difference? Given that he won by mere seconds in a final mad dash to the line, it certainly didn’t hurt him. Is the S5 going to give you the same “silver bullet” advantage in a race? Only if you’re a pro cyclist.

Final full disclosure: I bought my Cervelo S5 for specific reasons. I liked the way the 51cm frame fit me compared to other brands. I ride in ultra-distance bike races where you can easily go 100 miles on straight flats and then spend the whole next day in a mountain range. I don’t like to switch bikes and I can’t ride in the TT position for 12 hours straight, which makes S5 a versatile choice for me. Most importantly, it was in my price range!

If you’re interested in getting faster, you’ll be fascinated by FASTER: Demystifying the Science of Triathlon Speed. In Faster, astronautical engineer and triathlon journalist Jim Gourley explores the science of triathlon to see what truly makes you faster—and busts the myths and doublespeak that waste your money and slow down your racing. With this knowledge on your side, you can make simple changes that add up to free speed and faster racing.