09 Aug How Aero is Your Aero
If you’ve spent any time with triathletes – especially the gear loving, data obsessed, analysis junkies like me – you’ll have heard a lot said about aerodynamics. Triathlon discussion boards are laden with often heated discussion on what is aero and what is not, how many seconds an alien-head helmet will shave off your 40k bike, or how many grams of drag are shed with a deep carbon wheelset – that costs more than your bike, by the way. I’m going to try to streamline the discussion somewhat, with an emphasis on the most important question of all: should you even care?
To answer that question – or perhaps to refine it to: how much should you care – we need to look at the net forces involved in cycling with an emphasis on aerodynamic drag and how it is calculated.
As you dig deep and crank the pedals to drive yourself and your bike forward, four forces are holding you back.
- Tire rolling resistance. This is the energy expended in deforming your tires as they make contact with the pavement and flatten to form the contact patch. Rolling resistance merits its own write-up, so I won’t dwell on it here. Suffice it to say that while it is not negligible, its magnitude, and therefore importance, is much lower than that of aerodynamic drag.
- Mechanical resistance. This is the energy expended in overcoming friction losses in your bike’s moving parts: wheel, crank, pedal, and derailleur pulley bearings, chain pins, chain-to-cog contact, etc. Spending silly dollars on ceramic bearings is one way to reduce this resistance, but keeping your bike well maintained and lubricated is the most important factor. Just as with rolling resistance, mechanical resistance has nowhere near the impact of drag.
- Gravity. There’s no getting around this one. Climbing is hard, sweaty work. In fact, at a certain grade, gravity overtakes drag as the dominant source of resistance to forward progress. There’s a great discussion on the Cervelo site about weight (the determining component in gravity) versus aerodynamics on hilly courses. You can read about it here:
http://www.cervelo.com/en/engineering/thinking-and-processes/weight-vs-aero.html
- Aerodynamic drag. This is the biggie in all non drafting cycling as it makes up the overwhelming majority of the force resisting forward motion on all but the hilliest of courses. To put another way: nearly all of the force you transmit to the pedals on a flat course is used to overcome drag. Needless to say, reducing drag should be priority!
Next, let’s look at how drag is calculated:
F_{d} = C_{dA} x A_{f} x V^{2}
- Where
- F_{d} is resistive force due to aerodynamic drag
- C_{dA} is the specific coefficient of aerodynamic drag
- A_{f} is the frontal area in meters of bike + rider
- V is the velocity in meters per second
Reducing velocity is really not in anyone’s interest is it? However, the importance of velocity is still worth discussing. Since velocity is squared (V^{2}) in the drag formula, its impact on drag is huge. The takeaway? The faster you get, the more drag matters.
Here’s an example: take a cyclist riding at 40kph versus the same cyclist riding at 30kph. For a speed increase of 33%, the rider will have to exert 78% more force to overcome drag. To put it another way, if riding at 30kph requires 170W of power, riding at 40kph will require 303W. Anyone training with power can appreciate that difference!
We want to minimize F_{d} without touching velocity. That leaves C_{dA} and A_{f}. C_{dA} is a complex coefficient. It changes any time the aerodynamic properties of the object – bike plus rider plus components – change. This change could be a new helmet, a flashy new set of wheels, or a new bike fit. There is no reliable way to calculate C_{dA}. It must be measured. The most exact means for doing so is in a wind tunnel, but since wind tunnel testing is beyond the means of the average age grouper, we can employ an accurate field test instead.
A_{f} is the frontal area of the bike and rider. This is the surface that is exposed to the air coming at the rider. It’s a little difficult to measure. Most of the non-wind tunnel options involve taking a photograph of the rider and literally counting pixels. Slowtwitch recently advertised an app for android phones that will do that tedious work for you if you’re keen. Otherwise, it’s possible to estimate A_{f} by knowing rider height and weight.
Armed with all of this technical information, let’s talk about the really important subject: how do you reduce your aero drag. Here, then, are the steps to take in order of greatest to least importance, including my best estimate of percent effect on drag, and average cost impact for each.
- Buy a tri bike. There is no getting around this one. A triathlon specific bike will get you in a more aerodynamic position than a road bike. This of course assumes that you are flexible enough to fit that position (see point 3 below). Drag reduction: triathlon bikes will on average put the rider in a 20% to 25% more aerodynamic position than a road bike. Note that fit is much more important than the aerodynamic properties of the frame itself. Cost: $1,500 to over $12,000
- Have your bike professionally fitted. Something like 85% of drag is caused by the rider and not the bike. This makes total sense given how much larger the frontal area (A_{f}) of even the smallest riders is compared to his or her bike. Having a perfect fit will, among many other benefits, ensure the most aerodynamic position reasonable for the rider. Drag reduction: difficult to estimate, but the impact on drag between a poor fit and a good one can be as high as 20%. Cost $100 to $200.
- Reduce your frontal area (A_{f}) by improving hip flexibility and overall strength. The number one determining factor in achieving that superstar, flat back, high saddle / low bar, aggressive position is hamstring flexibility. That forward fold must come from hip range of motion not the lumbar spine. Curvature in the lumbar region for long periods in aero is a recipe for lower back strain. If you feel tightness in that region, then I’m talking to you. Correct and meaningful strength and flexibility training in the offseason / early season, combined with a maintenance program in season is the path to hip flexibility – not to mention the increase in power from gains in strength. Drag reduction: again difficult to estimate. Clearly, those with reduced hip mobility will see the greatest gains. Cost $0 to $2,500 (for one off-season).
- Buy an aero helmet. Data varies, but there are many wind tunnel tests out there that claim that an aero helmet will reduce drag as much as an expensive set of aero wheels for almost one tenth of the price. Drag impact: 2% – 4%. Cost $150 – $400.
- Buy aero wheels and tires to suit. Aero wheels are aero. How aero is a matter of debate. Most manufacturers test their own wheels as well as those of their competition. Interestingly, most find their wheels to be the more aerodynamic than all others. Regardless, a decent set of wind-cheating hoops will break the bank for only modest gains. Drag reduction: typically less than 3%. Cost $500 (for off-shore knock-offs) – $4,500
- Buy tight race-day clothing. A tri suit that fits snugly with minimal folds and creases results in a smoother airflow over the rider’s body and a subsequent decrease in drag. Drag reduction: difficult to estimate, but likely under 2%. Cost $100 – $300
- Choose aerodynamic hydration options. Even water bottle locations have a small effect on drag. A study done by Cervelo suggests that a standard bike bottle mounted horizontally between the bars is the most aerodynamic option. Behind-the-saddle mounts are another solid choice. The body of the rider serves to shroud the bottles and cages from the oncoming airflow, reducing drag. Drag reduction: around 2% for the horizontally mounted bottle between the bars. Rear hydration systems are still a contentious matter, although some studies are showing substantial reductions. Cost impact: $20 – $70 for bar mounted systems, $80 – $300 for rear systems.
If it doesn’t get measured, it won’t be improved. All of the aforementioned analysis is based on averages and manufacturer testing. Since drag is so heavily influenced by position and bike fit, your own numbers will most certainly vary. I am organizing a test to determine cycling CdA on the 17^{th} of August. This time trial will use a power meter to get an accurate estimate of your drag coefficient. Better still, you can run it multiple times using different components (wheels, helmets) to gauge the impact of your gizmos for yourself.
Check out the x3training Facebook page for more information and to register.
Helping you get where you’re going: in the water, on two wheels, on the pavement
Coach M
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