By Jon Johnson
“Aerodynamics are for people who can’t build engines.”-Enzo Ferrari
My sincerest apologies for the following sentence:
Not only is aerodynamic resistance a drag, it sucks and it blows.
I think we can all agree there was no excuse for that. My only defense, such as it is, is that those aren’t just puerile metaphors; they are actually reasonable descriptions of the two basic forms of aerodynamic resistance acting on a cyclist, namely “friction drag” and “form drag.” Friction, the resistance a cyclist encounters from air scraping speed while dragging across the body and bike, is the lesser of the two evils, but by no means trivial. It’s the reason skin suits made of all manner of space age fabrics are becoming popular, and an ostensible reason for shaving our legs.
For cyclists, unstreamlined lumps of meat that we are, form drag is what’s most important, and it comes from the high pressure area created by air “blowing” on the front of a moving cyclist and the low pressure area in the wind eddy behind the cyclist, “sucking” him backward.
Just how all of this plays out is really complex, so much so that trying to explain it in a blog post would be futile. The good news is that the basics are pretty simple, determined by cyclist and wind speed, air density, the frontal area of the cyclist and cycle, and the “coefficient of drag,” a semi-mysterious value I’ll have more to say about shortly.
Remember, except on noticeable climbs, which is to say most of the time, aerodynamic drag is far and away the most important source of drag, somewhere around 90% when riding at speed, and velocity is the most important input. Velocity is a factor in two ways:
- The other aerodynamic variables get multiplied by simple ground speed (Vg).
- The other aerodynamic variables get multiplied by the square of apparent wind speed (Va2), which is ground speed plus (or if a tailwind, minus) ambient wind speed.
On a windless day, drag increases as a function of the cube of velocity, and it’s even worse riding into a wind. If a medium sized rider (154 lbs) needs 307 watts to maintain 25 mph with no wind, he needs 425W into a barely noticeable 5 mph wind, but only 207W riding with a 5 mph tailwind. (More details at ovto.net.)
Reducing velocity is not an option for racers, and since everything else gets multiplied by the enormous number that is velocity3, it makes sense to reduce those other factors where possible. There’s not a lot you can do about air density while on the bike, it’s worth knowing a little about. If you’ve ever wondered why you’re slower on cold days than warm days, air density plays a part (beyond constraints from being wrapped up like Ralphie in A Christmas Story). Cold air is denser than warm air. Mr. Hypothetical Rider above needs 30 watts less to maintain 25 mph at 90°F than at 32°F. Similarly, air becomes less dense with increasing altitude. Mr. HR needs 65 fewer watts in Mexico City (7,400′) than at sea level, which is why Merckx, Obree and others made their hour attempts down south. (For Ozarkies who’ve ridden Hwy 71, you need 12W less atop Mt Gaylor than in Mountainburg.) Weather systems even play a role, making a 10-20W difference on low vs. high pressure days. On the whole, if you’re attempting the hour record, you’d choose a hot high altitude venue–assuming your cardiovascular and heat dissipation systems are up to it–on a day with low atmospheric pressure.
This brings us to frontal area (A) and the coefficient of drag (Cd). They are often included into a single multiplicative “CdA” term, but it’s worth considering each separately. A is, at least conceptually, easy. It’s the size of the hole a rider and bike punch through the air as they move forward. Smaller A means less drag. Reducing A has long been an obsession of time trialists. You Google up methods for calculating your A using a digital camera and digital image processing programs, but you can probably just as effectively imagine ways to do it—think arms to the front, flat back, head down, hunched shoulders… Atrophied torso… Pencil thin arms…
You pretty quickly exhaust ways to reduce A, but there a lots of ways to lower Cd, the coefficient of drag. Unfortunately, this can be a pretty mysterious process requiring elaborate rituals carried out by white coated mystics who put supplicants through various forms of contortionist torture carried out in windy tubes. But it can pay big wattage dividends.
Cd is a number that means, more or less, how “draggy” a particular thing in a particular circumstance is. It includes both friction and form drag created by the whole cyclist system—the rider and the machine—and is an attempt to capture into a single number a mind numbingly complex set of fluid dynamics phenomena, the entirety of which is not completely understood.
Surprisingly, Cd does a reasonably good job of this, IF you hold things like wind direction, body position, frame geometry and shape, clothing tightness and fabric (and seam placement), helmet shape and material, head tilt, wheels, position relative to other riders, and any of several other factors constant. Change any of these things and Cd changes—maybe just a little, maybe a lot. Cd is a prime reason road and time trial bikes and bike equipment cost so much money. It’s also why time trialists pay hundreds of dollars an hour to the aforementioned sadistic mystics. Little tweaks unique to particular cyclists can make big differences.
Since there is no way to come up with precise general values of Cd, the best we can do is come up with good empirical estimates collected on riders in different configurations of position and bike. Data is most easily collected on the combined CdA term, and estimates range all over the map. Generally, though, they vary between .20 (optimized aeroposition on tricked out aero-time trial bike) and .40 (standing). (Again, more details at ovto.net.)
What does it mean? The graph below shows the speeds Mr. Hypothetical would attain putting just 250W into his bike. Standing, he gets to about 21 mph. In the drops, he gets to just shy of 24 mph. On an optimized time trial bike, almost 27 mph.
You’ll notice the “Kyle Fairing” value way to the left, which gets to 36 mph at 250W. It’s a contraption built by a research in the 1970s that looks like this:
It’s a standard road bike with an air-foil shaped fairing. Note that in order to cover the cyclist and bike, the fairing’s frontal area must be larger than that of the uncovered cyclist and rider, yet its overall CdA term is considerably less than even a tricked out time trial rig. Folks, I would be the world time trial champion if I could, somehow, convince the UCI to let me, and only me, ride with one of those things. (Recent news has shown that the UCI may be open to certain kinds of persuasion…)
Bottom line: The coefficient of drag is very, very important. Doing what you can to gently separate and gently replace the air you displace is the trick. Perhaps in a few thousand years, we cyclists will evolve in interesting ways.
Four basic takeaways:
- Humans really are incredibly unaerodynamic blobs. The UCI takes a dim view of making cyclists more aerodynamic.
- Every little bit helps, especially if it improves Cd. Aerodynamic equipment counts, but the rider’s position counts far more.
- Because aerodynamics are so chaotic (literally), even small adjustments to position and equipment can make a big difference in Cd. Moving a centimeter forward/backward or up/down can make a big difference.
- There’s a big difference in CdA between positions. My hunch is there’s a lot of advantage to be had from tweaking positions on standard road frames.
The last obvious thing you can do about aerodynamic drag is draft, a topic of overwhelming strategic importance. That’s next.
Jon is utterly fascinated with aerodynamics & racing strategy. He is a billy goat on the bike and a professor at the University of Arkansas off the bike. – OCA
Thanks for this. Are you aware of http://aeroweenie.com/? I follow the guy on FB. Very interesting and educational.
That’s a good resource, as is http://analyticcycling.com and http://www.cyclingpowerlab.com. New sites come up from time to time, a veritable geek feast. The best resource for the analyticals out there continues to be _Bicycling Science_ (an old school book) by Wilson (IMHO).
Good stuff. As you pointed out, how much is the Cd effected when drafting, or put in other terms, does aero matter as much when sitting behind Kirk. We all know it does when trying to draft off the likes of Alvarado.
I’m always looking for Michael Kirk’s wheel. Strong, smooth, big wake… I’ll take up the subject of drafting next. Stay tuned.
Looking forward to the drafting piece! I have my favorite “locomotives” to draft behind, and weighing in at just over a buck 40, have now lost count of how many comments I’ve had from people on my wheel that say something to the effect, “Hey! This ain’t no draft back here!” or “You skinny guys on aero bikes…” etc. Ha ha.
Great read Jon. Thank You for sharing your wisdom.