An Introduction to Aerodynamics for Triathlon

Aerodynamic testing and optimization is a process through which we try to find the best position and equipment a triathlete can achieve to get the highest speed for the fewest amount of watts—always remembering important considerations like the athlete’s ability to produce power and be comfortable enough to get off the bike and run well. There are a lot of technical terms around this process, but here we’re going to keep it as simple as possible.

The science of aerodynamics

Let’s start with a brief description on the science behind aerodynamics before we dig into the good stuff down below. Some really smart people in the late 1990s and early 2000s published articles and mathematical formulas that show the relationship between power and speed, when taking into consideration things such as weight, wind, weather conditions, road surface, and many others. During an aero testing session, we measure the items we can and use formulas to calculate the variables we cannot.

The power a cyclist generates will be used in four different ways: The first is the power to overcome changes in altitude. On a flat course, this will be zero. If you’re on a steep hill, this will be a very significant percentage of the watts. However, on a downhill, this will be negative.

The second category is aerodynamic drag—this is the power to overcome the resistance of the air. As an example, drive in a car with your hand out the window. The faster you drive, the more force you will feel against your hand. A cyclist on a bike has the exact same phenomenon. In the car, close your hand to make a fist and you will feel less force from the wind. This is what aero optimization attempts to do: optimize the size and shape of the cyclist’s body, seen by the wind.

A third category of watts goes towards rolling resistance that occurs literally where the rubber of the tire meets the road. Road surface, tire type, and tire pressure will all impact this.

Finally in the last category, we have acceleration. Once we’ve subtracted altitude watts, aero watts, and rolling watts from the watts produced, if any is left we will accelerate. If there is a deficit, we will decelerate.

An important topic in aerodynamics is what is known as CDA. CDA is shorthand for coefficient of aerodynamic drag. It takes in consideration the size of the object and its shape. The lower the CDA, the more aerodynamic it is.

Imagine your hand outside the car window again. When open, it will have a large CDA; in a closed fist, it will have a small CDA.

Aero optimization wants to quantify and reduce that CDA, and the result of a successful aero optimization session is a smaller CDA in a position that is both comfortable and powerful. The very best cyclists in the world, like Belgian ProTour cyclist ​​Remco Evenepoe, may have a CDA between 0.160 and 0.170. A well-optimized, average-sized age grouper may be around 0.200 to 0.230. If I take pictures from an Ironman event and use my eyeball wind tunnel (which is very inaccurate), I often see something that looks more like 0.270.

Real-world examples of aerodynamics

Let’s imagine a fictitious rider named Fred. Fred weighs 75 kilograms (this is the simplest measurement to use for standard aerodynamic calculations). He has an entry-level tri bike, entry-level wheels, average tires, and he came to the aero optimization session with his regular cycling jersey, shorts, and road helmet. He had a bike fit that put him in a comfortable position that allows him to produce good watts for his fitness level. Let’s say he has a 0.275 CDA.

Coming back to our mathematical formula, if the rider produces 200 watts, with his equipment choices and position, he will achieve a speed of 36km/h (or a 2:30 half-iron distance bike split on a flat course). As discussed, on a flat course 0 watts will go towards elevation change, 162 watts will go towards overcoming air resistance, and 38 watts towards rolling resistance.

If we can drop his CDA by 10%, those 162 watts spent overcoming air resistance will go down 10% (or 16 watts). This is where we see just how important optimizing aerodynamics is.

I recently analyzed all the aero tests done by Brian Stover, an Arizona-based aerodynamics tester, and the average savings across 25 tests was 22 watts—and this was without tire optimization. When we consider that very often people who even book aero testing sessions are already conscious of its importance and have likely taken steps to optimize, this is considerable. To bring Fred from 0.275 to 0.225 would save him approximately 30 watts on his initial 200 watts (at 36km/h); with the same amount of watts, he would go 38.3 km/h rather than 36. This would be a massive success, and it would take nine minutes off his half-iron distance time.

But how does that happen? We would take Fred through a few steps: First, we would optimize his position, because this is where the biggest savings are; then we would help him select a helmet, as the head is the biggest feature the wind sees; we would then make clothing choices. Finally we’d fix his bottle setup and add the cherry on the “aerodynamic sundae” with tires and optimal tire pressure, and get him from a 2:30 split down to 2:19 half-iron bike split.

Below, I’ll break down what we look at for each of the five steps.

How to optimize your aerodynamics for triathlon

Step 1: Position

When optimizing position, we need to find the balance between aerodynamics, ability to produce watts, and comfort. This is often a compromise, but for age-group athletes, the ability to produce watts is the critical component for shorter distances, while comfort is a priority for longer race distances.

In aero testing, position affects equipment choices and equipment choices can affect position. For example, I can put a person in a position and then select the best helmet for that position. But if I start with a given helmet, the optimal position may be different. When testing age-group athletes, I prefer to focus on position and adjust equipment accordingly. Pros are different: They have sponsorship obligations. Recently I worked with pros that contractually “had” to wear a specific helmet. So, we adjusted position to make the equipment/position interaction as fast as possible. The biggest misconception in aerodynamics is that going “lower” is always faster. This is often true, especially when the athlete starts from a very high position. But at a certain point, too low is not faster, and it limits power production and comfort. In most sessions we go both up and down to find the sweet spot. Everything must be measured and confirmed. Finding the fastest position, with feedback from the athlete, is how we find the “right” combination.

I often hear an athlete say, “I will give up 3 aero watts to be able to produce 10 more watts.” I also hear, “While I didn’t lose or gain aero watts by coming up 2 centimeters, I feel much more powerful.” Sometimes there are “tricks” to help with the position changes—shorter cranks being one that many fitters use.

Other big changes involve arm pad width and arm tilt. Very often it’s not the change itself that impacts aerodynamics, it’s the consequence of the change. For example, narrow pads are believed to be more aero—by themselves they often are. But sometimes they prevent the head from coming down which costs many more aero watts than gained. There is only one way to find out: to test.

Many athletes also copy what their favorite pro does. This is bad. Often, they copy one attribute of the position, but do not understand the reason behind the initial change. It is extremely rare that an optimal aero position reduces how far down the road you can see. If you can’t see, you are probably not aero optimized and you are going to hurt yourself and possibly others. This is the biggest problem with people “copying” their favorite pro who is riding on a closed road with a car to guide him. Forearm tilt is one of those things people try to mimic, and often give up aero watts in the process.

The biggest misconception in aerodynamics is that going “lower” is always faster.

Step 2: Helmet

Once we have found the optimal position, we need to select a helmet. A 10-watt difference between an aero helmet and regular road helmet is not abnormal. The problem is helmets are quite individual to the rider’s position. So, what works for me, may not work for Fred. Also, a good aero road helmet may be a valid compromise in a very warm race. Maybe I only get half the watts, but not overheating allows me to finish the race. These are all things you can quantify in an aero session.

I believe that people who say they can tell the best helmets from pictures should stick to predicting the future with tarot cards.

Step 3: Clothing

Again, clothing is somewhat individual. There are general rules: Tight is good; too tight is bad in several ways. On top of being constricted, the rider will sometimes lose the aerodynamic benefits of the suit. Again here, 10 watts is not uncommon to find between a reasonable aero suit and a regular cycling jersey and shorts. Sleeves covering the upper arm usually test faster than skin. Complete sleeves are something you would want to test.

Step 4: Hydration

In long-distance triathlon we need to carry hydration, and bottle placement has an impact on CDA, which should be measured. First, we figure out how much hydration we need to carry and test carrying solutions. Many of the front-end hydration systems provide an aero benefit. Bottles between the arms are beneficial or neutral. Round bottles on the tube are usually a penalty. Bottles behind the rider are usually neutral. It is not uncommon to see a .005 CDA impact with hydration changes.

Step 5: Tires

While not a very big aero impact, tires have a big impact on the rolling resistance component we discussed above. The example above had 35 watts going towards rolling resistance. It is not uncommon to knock 20% off that number (seven watts) with the proper tire choice and tire selection.

While we didn’t get into wheels, they can and should be tested. A disc wheel can provide a good aero benefit. I personally find front wheels should be selected based on the athlete’s ability to handle them. Someone I worked with won the world masters cycling time trial for his age with a 85mm-deep wheel, but he couldn’t handle it and had to sit up. We tested him on a 45mm-deep wheel, which he could handle, and we measured it to be about 2 watts slower for him—but he did not need to sit up which made him considerably faster. The decision was simple.

Post-aerodynamic testing

The scenarios above are realistic but rarely do we get them all finalized in a single session. It’s important to try them in race simulations and experiment with things. Follow-up sessions are a good idea, and working with an aero tester who can follow, monitor, and assist in your progression is something you want to be looking for.

This article originally appeared in Dutch in the Netherlands-based Triathlon Inside and has been reprinted here with their permission.