Sunday, October 13, 2013

Bicycle Aerodynamics. Spending Dollars To Save Cents.

Illustration courtesy of CD-adapco

While the “Aero” craze is alive and well in the bicycling world, there are actually two sides to this slippery story.  It is kind of like Yin & Yang, actually.  On one side is the possibility of free speed & on the other side is nothing but what I call “Orgasmic Marketing.”  Think of it this way: Spending resources on an already aerodynamic shape makes sense.  Doing the same on a very un-aerodynamic shape, like the human body, is futile and very expensive at best.  Before anyone begins yelling at their computer screen, allow me to explain my rationale on the subject.
The inspiration for this article was a growing amount of dissent on various on-line cycling sites bemoaning the drag produced by anything “New” on a bicycle, like disc brakes, any exposed cables, taller hoods (think SRAM hydraulic), even wider tires and wider bars that properly fit the rider.  I hate to break it to these people, but in the bicycling world, drag really is not all that huge a consideration worthy of throwing millions of Research and Development (R&D) dollars at.  Sure, it may matter to a rider like World Time Trial Champion (WTTC) Tony Martin, but the odds are, all else being equal, he would still be WTTC, even if everyone rode mountain bikes. 

Not yet convinced?  Please, peruse the following. 
One does not need to be a Certified Aerodynamicist (I certainly am not) to understand that the induced and parasitic drag on a rider and bicycle are indeed quite large.  Let’s face it – Trying to hide a human on a bicycle frame is fantasy at best, and pure marketing hype at its most sublime.  You can “Aero” every conceivable part of a bicycle, except the largest part – The Human. It is similar to rounding the edges of a huge, square rock.  Sure the rock would be a tiny bit more aero, however, it is still a rock.  It is just one with pretty, rounded edges now. 

First some aerodynamic basics: 

Try as we might, the rider will always be the biggest "Drag."
Nature truly knows its business when it comes to what constitutes the best in “Aero.”  There is a reason a rain drop is, well, shaped like a rain drop/tear drop.  However, the Union Cycliste Internationale (UCI) has out-right banned anything actually, and usefully “Aerodynamic,” so the notion of the tear-drop is out.  This useless, bureaucratic decree has thus given birth to a host of names and acronyms to describe frame shapes not violating the UCI’s arbitrary 3:1 ratio rule.  And, the bicycle manufacturers have indeed done their homework, plus they have happily passed on the huge R&D costs – To us.

Second, think of the analogy of moving through water, and since air is indeed a fluid, thus, we and our bicycles are bound by its properties.  So, the more aerodynamic a shape, the smoother it will travel through (with less work) a given medium, i.e. air or water, for example. 

Some basics on drag: 

According to Webster’s Dictionary, drag is defined as: “The resistance caused by a gas (or fluid) to the motion of a solid body moving through it.”  The formula for drag is defined as F_D\, =\, \tfrac12\, \rho\, v^2\, C_D\, A .  Thus, by plugging in the numbers, we can arrive at a known Coefficient of Drag (Cd).  This is where the marketing department jumps on the “Aero” bandwagon.  Why?  Well, the shape of an object influences its aerodynamics, and thus, the amount of air resistance or drag it experiences.  In a nutshell, low Cd good, high Cd bad. 

Example of aerodynamic properties of basic shapes.  Smooth = Good.  Blunt = Bad.
And, here is where the nomenclature of drag boils down to for us cyclists: Induced Drag, the drag produced by physically moving through the air, and Parasitic Drag, the drag produced by the body’s actual shape (there are additional forms of aerodynamic drag, but we are just not going fast enough to worry about those).  Remember, air has mass and volume, so it takes force to push it out of the way.  This is the force (wind resistance) we all feel when pedaling.  And, thanks to Daniel Bernoulli’s hydrodynamic principle’s, we know that drag increases proportionally to the square of the speed (formula R  v2).  In English, the faster you go, the more drag you will produce. 

So, when does aerodynamics really matter?       

In reality, all of the time, just not so much to the everyday, recreational rider (the bread-and-butter of the bicycling industry).  We could do this rider a better service by just having them get into the drops versus spending BIG bucks on an aero bike.  In addition, a larger body, a heavier bike, and a non-aero frame are actually better for this kind of rider from an exercise standpoint.
Here is a relational example. 

I recently rode with a friend of mine whom just bought one of them new-fangled aero-bikes.  In the summertime, he usually rode very early in the morning to avoid the heat and constant afternoon winds normal to our area.  Well, the ride we took was in the late afternoon, in the heat and wind, and he was complaining all the way.  I explained to him that riding at different times of the day, in different conditions, would make him a better cyclist.  Still giving me the “I don’t understand look,” I put to him this way: By only riding at the most comfortable, calmest part of the day, he was actually cheating himself out of getting a better workout.  I related it all to resistance-weight training.  Riding at only one time of day was like only lifting a five-pound weight, all day, everyday – It feels good and it is very easy.  Riding at other times of the day, such as in the wind and heat, was the equivalent of lifting progressively heavier weights – It would make him stronger, and thusly, a better cyclist.

So, by bowing to the alter of All-Things-Aero, he was cheating himself out of quantitative improvement, plus that aero frame was really not making him any better on its own.  OK, I take that back.  It made him Feel better.  Score one for the marketing department.

“Yeah, but what about the Racers?”  

For a racer, this is truly as Aero as it gets. Body position matters more than all parts of the bike put together.

Aero matters to racers, because they are paid to make sure it matters.  To the professional racer aerodynamics can produce a slight advantage, however, those tiny little bits of many things must add up to a real drag-reduction-whole.  And, it indeed matters more so to the Time Trail (TT) riders and Triathletes, where fractions of a second can indeed make the difference.  However, before anyone can claim that current WTTC Tony Martin needs every aero advantage he can get to win, I previously pointed out that he would most likely best all-comers, regardless of the type of bike he rode, or the bikes’ tube shapes. 

See, any aero advantage one rider has can easily be matched by another riders aero, or even overcome by another rider of superior strength and skills, all else being equal.  So, how much do those magic fractions of a second an aero bike supposedly provides actually matter in the empirical sense?   

But wait, there is always the proverbial “More.”  Another rider with an aero advantage, plus superior riding skills, plus good Performance Enhancing Drugs (PED’s) will wipe the former two off the map. 

The latter case is what we call a Tour de France winner.

So, what does this all mean? 

It looks good and sounds good, we just are not going fast enough for it to make a real difference.

So, the bottom line is this: No matter how hard we try, moving a large, un-faired object through a fluid is nothing but pure drag.  Once an object begins to move, our old friends, Induced and Parasitic Drag, join the party no matter how tight your skinsuit is, how much you tuck in, whether you have an “Aero” frame or not, and certainly it will not matter if you have integrated brake caliper, and sexy, deep wheels or not. 

The money spent to save a gram of drag here and there, save for a focused professional, is not money well spent, and keep in mind, those research costs are passed on to all of us when we buy a new bicycle, the sexiest, deep carbon wheels, or the latest “Aero” component.

A Boeing Commercial Aircraft engineer was once very upset when the hinge fairing for the vertical stabilizer trim tab on the 777 ended up being much larger than were originally planned.  He had a right to be upset.  A small amount of parasitic drag on an already aerodynamic shape would cause a significant drop in fuel economy over time.  Now, on an object like a rider and bicycle, which to the wind looks like a phone booth (remember those?), a minute reduction in drag makes no overall, significant difference for the everyday rider, and a tiny difference, at best, for the professional.

Can true, significant, aerodynamic gains actually be made?  Of course they can.  That is not the point of this article, though.  The 800-pound gorilla in the room no one admits to is while these multiple, minuscule gains may indeed add up (I repeat, MAY), it is being served up at an extremely high cost.  It is similar to throwing thousands of soldiers into battle and having them slaughtered to just to gain a few inches of ground. 

The major difference to us regular, everyday cyclists is that the sacrifices we are taking are not human beings, but those soldiers dying on the hill are our hard-earned dollars seeking watts and seconds which really do not matter in the grand scheme of things.


  1. The first thing I wonder is whether you truly believe that you can only get *minuscule* aerodynamic gains from optimising your position, kit, wheels, and frame?

    It's not uncommon to see people who have never been to a wind tunnel improve their drag area (CdA) by 10% with positioning changes alone. That's 25W out of 250W. Many people would spend a lot of money trying to get that kind of extra wattage out of their bodies.

    If you're only aiming your shots at aero equipment purchases, then consider that the difference between a bad road helmet and a good one is about 3% of total body CdA. And that the gains in going to an aero helmet are another 2% on top of that. And that a skin suit, in some circumstances, can net you another 2-3%.

    All of the equipment aero gains above are due to manufacturers spending the millions on R&D. And you'd have to spend a lot of money on coaching to get the same gains via training.

    If speed matters to you, the aero matters. If speed doesn't, then sure... do anything you want.

  2. Excellent tips. Really useful stuff .Never had an idea about this, will look for more of such informative posts from your side... Good job...Keep it up. carbon fiber bicycle frame

  3. LAVA magazine in June 2014 issue published an article on helmets. Analyzing the benefits relative to weight, aerodynamics and ventilation. The conclusion if you race and train where its hot get the helmet with the best ventilation - cooling capacity. You're going to "blow up" faster from over heating than from carrying a bit of extra weight or a few extra grams of drag