The Under-Explored "Side" of Aero
Tuesday, September 25, 2012 at 10:37AM 
We've all been exposed to magazine articles touting the advantages of aero bikes, wheels and helmets. Watching bike races and events we see disc wheels, aero suits and slippery bikes whisking by barely disturbing the air.
When we review articles by major bike and wheel manufacturers we see super sophisticated CFD (Computational Fluid Dynamics) illustrations with flow density color renderings of boundary layers and turbulence. Without getting super technical, it's "drag" or air resistance that impedes the forward propulsion of the bike and rider. This drag vector points directly along the bike’s top tube. We are educated that being "slippery" in this direction is better. Sitting at the control desk of the FASTER wind tunnel, I get to see a multitude of wheels, bikes, and riders under the scientific microscope of grams of drag. Most companies utilize the longitudinal drag numbers or apparent wind angle of drag analysis. However, our wind tunnels analyses have shown that lateral forces from left to right that act perpendicular (90 degrees) off the direction of bike travel can be very important as well.
As we rotate the athlete in the FASTER wind tunnel, we record these lateral forces too. I took the liberty to review the magnitude of these forces for a randomly selected group of athletes that have visited the FASTER wind tunnel over the last few months. At wind angles as small as 20 degrees off a direct headwind, the magnitude of these forces can vary from 4.7 kg to 9.5 kg with a 30 mph wind!
Let’s translate that to common speak: if the speed of your bike and wind equates to 30 mph and the apparent wind angle is about 20 degrees off pure front, then the side forces on you and the bike approach 20 pounds of pure side push! The athlete needs to counteract those forces or be pushed off the road. In multiple-hour events, that corrective force amounts to significant energy. Aero bikes and disc wheels have substantial side area. Greater side area equals greater side forces by an off-angle wind. Keep in mind that these side forces are very real. As you add water bottles, nutrition, CO2 inflators, etc., your side profile grows rapidly. The control forces grow proportionate to side area. If an athlete is light, these side forces can exceed their capability to counteract them. That means that you can get pushed off your desired line of travel or blown off the road!
The takeaway? It's important to not only aerodynamically optimize an athlete's frontal profile, but their side profile as well. (The FASTER wind tunnel, with 350 degrees of remotely controlled yaw rotation, allows us to do that). One example of this is looking at the difference between a shallow rim profile and medium to deep rim profile. Looking at some real data from our wind tunnel for a shallow and medium rim depth wheel (45mm and 58mm) design from the same company on a Cervelo S5 test bike shows a 38 gram difference in frontal drag grams (medium rim less drag) and a 473 gram difference in side force (medium rim greater side force) at a wind angle of 20 degrees. For reference, 38 grams is a pocket full of dimes while 473 grams is a pound! That's just the wheel delta effect for 13 millimeters of rim depth! Now add your body and bike cross-section, or a super deep rim like an 81mm deep wheel, and you have significant impact. Something to think about.
In short, control forces are very important, so don't disregard them as you will pay with slower times and control efforts draining your power reserves. The "end game" is not simply to reduce head on drag; consider your side profile as well. Getting blown off the road by your competition or a gust of unexpected wind are equally bad things.
Jay White is the FASTER wind tunnel engineer, a triathlete and a mountaineer who has climbed Everest. Jay specializes in electronic analysis of complex shapes and rapid movements for tailored product designs. For more information on the FASTER wind tunnel, visit: www.ride-faster.com.
Reader Comments (4)
How does riding the draft at 20 mph or so effect lateral wind forces? Would suspect a reduction of significant degree, right?
Thanks.
Robert, great question. There is only one wind tunnel in the world studying the effects of multiple riders as they are all lined up in the draft. The real question is how does the wind flow from lateral or off line of the riders get disturbed and adjusted from rider to rider. I do not have data to quantify that effect. Any lateral forces that MAY occur from off axis wind will have an effect proportional to the side area the bike and rider 'presents'. That said if there is a side force magnitude experienced in the second-third or fourth rider the more side area the greater the effect. Understanding the randomness of cycling one can make the personal decision on equipment decison and position in the peleton.
Jay
So what you're saying is that the there could be significant energy expended, particularly by lighter riders, in maintaining a straight path because of constantly having to work against that side force, correct? It isn't a matter of that side force causing aerodynamic drag (vector along the top tube as you point out) but there could be a rreal cost to the rider in terms of fatigue from battling the side forces. How would you go about quantifying the cost/benefit of reducing drag vs increasing the energy cost of side forces? Obviously this would be specific to the individual rider - a very light rider will have to work harder to maintain that straight line compared to a heavier rider so the lighter rider may be better off overall with shallower wheels for example, even though they may have greater aero drag. I guess you'd need some way to measure the energy expended counteracting those side forces but even then, that cost doesn't really get manifested until you are off the bike and into the run - since we are really interested in optimizing the entire S/B/R combination (for triathletes anyway).
AHHH not so fast Dr. Watson.
Work done by a variable force is ∆W=F∆x or loosely in words work = force x differential distance.
The Force exerted to side is EQUAL regardless of WEIGHT of rider. It’s the side area that determines side force. The DISPLACEMENT of the rider across the road could be greater for the lighter rider.
P=W/t now that is power = work / time . that is the time involved to do the work. The more work done the larger the POWER or conversely the shorter the time period of W the greater the power.
Force = mass x acceleration … Work = ma∆x… Power = ma∆x/∆t… Force – Work – Power are not the same at all. And that Energy would be best = ½ mv^2. That is kinetic Energy is ½ mass times velocity squared, is a different beast altogether….
Resisting any side force will definitely require both Work and Power proportional to the side load and the time- distance element. Suffice it to say the Power required to resist a 1 kilogram side force for an hour
•1 Watt = 0.1 Kg(force) X m/s
No mixing metaphors in Physics!
Jay