The Ultimate Guide To All Things Aero In Cycling
Cyclists love aero stuff: aero bikes, aero wheels, aero helmets – heck, even aero bike computers. But do we really know how all this wind-cheating stuff works or what it really means to be “aero?” Do those aerodynamic wheels guarantee easy speed? Does aero matter when you’re not riding super fast? Can you be too aero? We tapped the top aerodynamic experts in the industry to tackle these burning questions and more. Here’s what we learned.
The Definition of Aerodynamic
Of all the forces you have to overcome on your bike, the two greatest watt-sappers are: air resistance and (when the road tilts up) gravity. You can avoid the latter by staying on flat ground. But barring a trip to the moon, it’s impossible to avoid air. Even on a perfectly windless day, you create a lot of wind as a cyclist, and the faster you go, the harder it blows. At speeds over 14km/h, it’s the dominant force of resistance.
By the time you hit about 48km/h, 90 percent of your power goes into overcoming air resistance, or what scientists call aerodynamic drag. While aerodynamics is the study of the properties of moving air and the interaction between air and solids moving through it, cyclists should also understand drag reduction.
Here’s a quick refresher on the two major types of drag you face: pressure and skin friction.
Pressure drag: As you ride, you slam into air particles, which get compressed when you hit them and then become spaced out after they flow over you. The difference in air pressure from your front to your back creates a drag force. Aerodynamic shapes reduce this pressure drag by minimising that difference in pressure and allowing the air to flow more smoothly over your front and reduce the low-pressure wake behind you.
Skin friction drag: There is friction between your body and the air particles moving over you, as well as friction between the layers of air around you. The air over your body is stationary; the air passing over you is fast moving and free. The transition between those areas creates friction that creates drag. Skin friction drag can be manipulated to reduce overall drag, as you see with dimples on a golf ball or texturised materials on the shoulders of a skinsuit.
“Surface roughness increases the skin friction by making the air more turbulent near the surface,” says Nathan Barry, a Ph.D. graduate in applied aerodynamics from Monash University, Australia and Cannondale Design Engineer. “The benefit is that a turbulent boundary layer has better energy transfer, and this allows the flow to remain attached longer over the round surface, thus actually reducing pressure drag.”
Who Benefits from Being Aero
There’s a misconception that aero only matters if you’re going fast. “People will say, ‘I’m not fast enough to need aerodynamic equipment,’” Barry says. “But good aerodynamics provides greater time savings to slower riders than faster ones.”
It’s true that the faster you go, the more aerodynamic drag consumes your total power. Doubling your speed from 31-64km/h creates not double the resistance, but closer to eight times the resistance. But even at relatively slow speeds, the majority of your power goes toward overcoming air resistance, Barry says.
“At speeds near 48km/h, 90 percent of your power goes into overcoming air resistance.”
At 16km/h, half of your power is going to overcome air resistance, Barry says. “The slower you go, the more time aerodynamics will help you save, because you’re spending more total time on the road. If you’re out there doing 29km/h, you can still save a big proportion of time by reducing your aerodynamic drag.”
Reducing drag can even help you climb faster. “A bike that is designed to reduce drag will climb faster than a lightweight bike up to about a 6 percent gradient, or a 7 percent gradient for stronger, elite level riders,” Barry says. Above that gradient, you get some additional savings from weight.
What to Know About Aero Gear
When every watt counts, good aerodynamics can equal easy speed and energy savings. Your gear can go a long way for reducing drag, so you can go further and faster with less effort. Here’s a look at what matters most.
Your bike accounts for up to 30 percent of your total drag, which is significant, especially for riders who don’t have aerodynamic postures on the bike, says Len Brownlie, Ph.D., aerosports research consultant at Simon Fraser University in British Columbia. “If you’re not particularly flexible or aren’t built in a way that lets you assume an aerodynamic position for very long, having a bike that reduces drag can help make up for that,” he says. “No matter what you’re doing, the bike is always working for you.”
For the bike itself, even on bikes that are mighty fast to begin with, tube shaping can shave significant amounts of time. For example, when Cannondale engineers tested the new SystemSix, the brand’s first and radically designed aero road bike, against its Evo race bike, they found a rider could save over two minutes over the course of a 40K time trial, assuming both bikes used the same wheels. That savings jumps closer to three minutes if you compare the aero bike to the traditional race bike with low profile wheels. The right cockpit design has a big influence, Barry says. “Going from a non-round handlebar is really important for reducing drag.”
Interestingly, disc brakes offer more than just improved ability to slow or stop; they also allow for a more aerodynamic frame construction, Barry says. “If you can take the rim brake off the front of the bike, there’s a lot of freedom of design. You’re going to see a lot of improvement in drag reduction there.”
Wearing a classic aero TT helmet rather than a standard road lid can save you more than a minute over a 40K course, assuming a speed of 50km/h, according to Brownlie’s research. But TT helmets are notoriously uncomfortable and restrictive. Fortunately, today’s aero road helmets are better and more comfortable than ever. “Modern aero road helmets such as the Giro Vanquish and the Specialized S-Works Evade II are very close in terms of aerodynamics to the traditional full aero lid, but they’re more comfortable and don’t turn into a sail if you look down,” Brownlie says. “If you’re going to pick just one helmet for all around performance, I would pick one of those.”
A rider wearing a state-of-the-art skinsuit can buy more than two minutes over a 40K TT compared to a counterpart wearing an ill-fitting club kit flapping in the breeze, and the technology is only improving as engineers turn to clothing as a hot frontier for major drag reduction, says Brownlie, who has worked with Nike to develop vortex generators (which are kind of like a series of tiny wings) for competitive athletic wear.
For track-running sprinters, they can reduce drag by about 10 percent, which equals a one percent improvement in performance over 100 meters, Brownlie says. “There are a couple of cycling apparel manufacturers who have developed similar concepts for time trial suits, placing vortex generator-like textured fabrics on the shoulders, upper arms down to the elbows, and on the thighs, which should help ‘roughen’ up the air as it flows over your limbs and keep it closer to the body to create less drag.”
There’s a definite point of diminishing returns, however, Brownlie notes. “Our recent wind tunnel tests of time trial suits have revealed that some of these suit designs may have gone overboard with too many textured panels,” he says. “The lowest drag suits we have tested confine the textures to the shoulders and upper arms and rely upon seam designs that are aligned with the airflow and fabrics that are very smooth and tight fitting. Suits manufactured by Castelli, Sugoi and Bioracer provided the lowest drag in our tests.”
Wheels are complicated because they’re paired with tyres, which can dramatically change drag, Brownlie says. “Several years ago, we tested the same aero front wheel with tyres manufactured by five different manufacturers. We were astounded to learn that wheel drag could vary by up to nine percent depending upon tyre selection.” But generally speaking, you’ll get the greatest reduction in drag from a fairly deep 60mm to 90mm rim, Brownlie says.
The time savings for good aero wheels are significant, according to Barry’s research on the SystemSix and its new Knot64 aero wheels. Even on an aero road bike, the addition of aero wheels saved 48 seconds over a 40K TT at 300 watts compared to a typical 30mm-depth alloy training wheel set.
How Aero Can You Go?
Establish a baseline: Find a nice flat section of road that you can ride uninterrupted. Cover up your computer so you can’t see your numbers. Then, using your normal position, do two out-and-back runs at an RPE of 6 on a scale of 1 to 10, with 10 being as hard as you can go. Check and record your average speed.
Test yourself: Lower your torso a few degrees. Again, without looking at your numbers, repeat the course at the same RPE. Check and record your average speed. Repeat the test (without wearing yourself out) in incrementally lower positions until your average speed slows down.
Stretch and train: Keep your position right at that breaking point for three weeks, riding at least three times a week, including one long ride on the weekend. During this time, stretch your hamstrings, glutes, calves, quads, and hip flexors daily.
Retest: After three weeks, go back out and retest yourself to see if you can now ride faster in that position. If yes, lower a bit more and see if you can go faster even lower. If not, bump your position back up to where you clocked your fastest average speed.