Your Ultimate Guide to on-the-bike Aerodynamics: Where to find hidden speed.
Whether you are looking for every advantage ahead of your next race or just need to free up some expended energy to stay with your weekend group ride, we'll help you target the main areas for lowering your aero drag coefficient.
Understanding drag coefficient
Every pedal stroke you take when cycling is a fight against air resistance and gravity. The battle against the latter is a futile effort: either stick to the flats or accept the inevitable reality of it's forces increasing exponentially as the gradient pitches up. Air resistance is a constant companion as well and only increases with speed.
At 10 miles per hour, nearly half of your power is going towards overcoming air resistance. Bump the speeds up to 30 mph and that figure jumps to 90%. The pressure an individual rider experiences from air resistance is referred to as aerodynamic drag. By increasing your efficiency of dealing with this drag, you lower your Aerodynamic Drag Coefficient (CdA).
CdA is influenced by size, shape, and surface texture and can be measured for an individual component, complete bike, or rider. Frontal area is a major factor in CdA calculation, but not the entire picture since overall shape can significantly alter drag. As you'll see, lowering your overall CdA is a combination of optimized equipment choices and your on-the-bike position.
Fortunately there are several techniques and setups you can exploit to lower your CdA which will save energy and increase speed!
Top 5 ways to get aero
Each topic below dives deeper into the main categories for aero optimization:
Positioning Yourself for Speed
Wheels & Tires that Work Together
Positioning yourself for speed
When you’re looking to reduce drag, the first place to look is in the mirror. The rider accounts for over 70% of the frontal drag when positioned on the bike. Therefore, optimizing your position will have the biggest impact on total CdA.
For a rider pushing 300 watts on a 40km or 25 mi TT, just bending the elbows and lowering the torso could save nearly 3 minutes!
A study conducted at Monash University helps quantify the watts required to ride in different positions at race speed. At 28 mph, a rider in a full upright position had to pump out a staggering 430 watts to maintain speed. Moving to the drops while keeping arms straight saved a respectable 13 watts. Simply bending the elbows while in the drops netted a massive 45 watt savings. Yet the fastest position tested was hands on the hoods with elbows bent and forearms parallel to the ground. This optimal position was 58 watts more efficient than full upright! That 13.4% reduction in required power output makes rider position the quickest and easiest way to lower overall CdA.
However, there can be such a thing as "too aero". For most riders, there is an inverse relation of lower torso angle to power output. You'll likely find a tipping point where the reduction in drag from an aggressive position becomes outweighed by the reduced power, leading to a slower overall speed.
A few extreme positions, such as the "super tuck" and "puppy paws" push the envelope of safety and have been banned in pro and amateur racing by the UCI and local governing bodies. Should you adopt these positions in your local weekend group ride in the name of getting to the county line first? Probably not. No amount of gain is worth risking your own safety (or of those around you)!
Consider working with your coach to find that perfect balanced position for the lowest CdA, most power output, and optimal control.
Wheels & Tires that work together
It's been nearly two decades since deep section carbon wheels began showing up on the fastest bikes. If you've ever switched from a standard box-section aluminum rim to an aero-optimized carbon wheel, you'll have felt the immediate advantage firsthand. The aero gains and watts saved by these deep-section wheels have been shown through lab and real world testing many times over. While that data isn't new, what has come to the forefront of aerodynamic research is the relationship between the rim and tire that's mounted to it.
"A rim must be at least 105% the width of the tire if you have any chance of re-capturing airflow from the tire and smoothing it."
The Rule of 105 has been the standard for pairing the most aerodynamically efficient tire and rim combos. In order to realize any aero gain from your deep section rim, the measured tire width when mounted* should be slightly narrower than external rim width. *(Labeled tire widths are not to be trusted as actual measured width once mounted is highly variable based on internal rim width and variances between manufacturers.) The deeper the rim, the more air flow that can be recaptured and smoothed out, reducing turbulence and drag.
Too wide of a tire and the air flow is pushed beyond the rim surface, negating any aero benefit from your high dollar wheels (other than at very high angle cross wind scenarios). For example, an aero rim with a 28 mm external width would be optimized for a 25 or 26 mm tire (measured when mounted). Here's where you need to take note of internal rim width: a rim with a 28 mm external width is often designed around a 21 mm internal measurement. When installing many commonly labeled 25 mm tires, you'll likely find the actual measured tire width when mounted balloons up to 28mm...no longer adhering to the Rule of 105.
Due to the research in tire rolling resistance showing a wide, high volume tire at lower pressure can be significantly quicker than a narrow offering at high pressure, many manufacturers are increasing the widths of their rims to accommodate these setups and maintain aero optimization.
At current, the widest tire width that is supported as still being "aero" is 28mm on the likes of Enve's SES 4.5AR or Bontrager's Aeolus RSL 51 TLR. These modern wheelsets sport 30+ mm external widths allowing a mounted 28 mm tire to adhere to the Rule of 105 and give you what is for now the best of both worlds: low aero drag and rolling resistance.
But what about the larger frontal cross section of these wider setups? Surely that can't be as fast as a narrow tire and rim combo... In a head on, zero degree yaw comparison, there is a very slight penalty for the wider combos. That being said, the lower rolling resistance of the higher volume, lower pressure tire overcomes that penalty and result in a faster overall setup.
Remember to think of wheels and tires as a total system that should work together. One of our coaches can help you in optimizing your equipment choices for maximum efficiency!
Clothing for the win(d)
We've already established that CdA is influenced by size, shape, and surface texture.... And 70% or more of aero drag comes from the rider's body. So it's no surprise that clothing can have a significant impact on aero drag. With that in mind, let's take a closer look at how we can get faster with what we wear on the bike.
Take your jersey for example. You may have seen different terms like "club fit" or "second skin" used to refer to how tightly the material hugs the torso. There can be quite a bit of savings by reducing the amount of loose jersey flapping in the wind. Trading in a club jersey for an ultra-fast skinsuit can save you 15w at just 25mph. If you are racing at 30mph that savings could double!
There are even free watts hidden in your sock choice. Going for a pair of aero socks over a standard knit option can save 4-8w! Who would have thought? But wind tunnel tests have confirmed that a good aero sock is an easy way to shed drag and pick up speed.
Another part of your kit that has a major influence on CdA is the helmet. Upgrading from a standard road helmet to an aero option can save you another 15 watts. While some aero helmets do get a little warmer at slower speeds like when climbing, the overall efficiency gain and increased speed over the full course of your ride can be worth it.
Similar to the dimples on a golf ball, an aero base layer is designed to manipulate the boundary layer of airflow around parts of the rider’s body. Striped fabric ridges bulge through the fabric of your skinsuit or jersey and ‘trip’ the airflow from laminar (smooth) to turbulent (messy). In turn, this reduces your aerodynamic drag. At 25mph, an aero base layer could be saving you up to a suprising 7 watts!
Bars, bags, and bottles...oh my!
The leading edge of your bike is extremely important when it comes to aerodynamics because it is the first thing the wind touches. Cleaning up this frontal cross section can lower your bike's aero drag in a big way!
There's a good chance your bike came with a standard round drop handlebar. A simple upgrade to an aero handlebar can save an easy 6.5 watts. To take it a step further, an integrated bar/stem combo can net you up to a 10 watt savings over a standard setup.
Even optimizing something simple like your out-front computer mount can reveal hidden speed. An aero upgrade there can give you another 3 to 6 watt gain, making an aero-optimized cockpit one of the best bang for the buck upgrades.
Certain disciplines allow for the use of aerobar extensions which let the rider adopt a more aerodynamic position while maintaining stability. Whether part of a complete system setup like on triathlon and TT bikes, or simply a set up "clip-ons", the use of these extension bars can lower rider CdA and in some cases save you up to 30 watts.
It's trendy and can be super convenient...we're talking about the fanny packs of bikes - the bar bag. This one accessory can be costing you a 10% or more increase in total drag. Some manufacturers have marketed "aero" bar bags, but in reality, unless you need every inch of space for an ultra-endurance event, there are several more aero locations to pack your gear.
A rear-mounted saddle or tail bag takes advantage of the already turbulent air behind the rider and has been shown to have little to no effect on drag. The next best mounting location is within the main triangle of the bike frame. It is a little harder to access and can make rapid bottle deployment tricky, but squeezing a bag into this space can actually smooth air flow if done right. A little less hidden from the wind is the top tube bag. While this location is very accessible and can make on-the-bike nutrition a breeze, depending on your bike's cockpit setup, adding a bag here can result in a slight CdA penalty.
That leads us to one of the most essential elements of riding our bikes, hydration and where to store it. By far the most common setup is the dual bottle cages: one on the down tube and one on seat tube. But depending on your particular discipline, the ideal storage location can vary quite a bit. For example, in triathlon, many bikes are now equipped with internal bladders, completely hiding the essential fluid from the wind.
In long-duration events where support is limited and stopping can get you left behind, planning for additional fluid storage is critical. Wind tunnel testing has shown a well fitted hydration pack can actually lower your aerodynamic drag. Despite the additional heat generated from a pack pressing against your back, this is usually the best solution.
Another common placement for additional bottles is behind the saddle. This setup was brought to prominence in the early days of IRONMAN. While the method can work well with little to no aero penalty, bottle security does limit use to smoother road applications. Also popular in many triathlon setups is a "between the arms" front-mounted hydration system. Although slightly less aero than behind the saddle (about 1 watt), this allows for secure and easy to use hydration while maintaining an aggressive riding position.
Aero vs traditional frame shapes
Manufacturer's began touting new tubing shapes as a way to cheat the wind in the early 2000's. Over the years these aero profiles have been refined using tools like Computational Fluid Dynamics and Flow Visualization. Now the top aero frames claim up to a respectable 20 watts savings vs a traditional round tube frameset at 25mph. Although, cutting drag at this level doesn't come without a cost. To achieve such low CdA values, concessions are often made in terms of weight and ride quality.
Choosing between an aero-optimized bike vs a less slippery, light-weight option has become an ever increasingly difficult proposition. Fortunately there has been significant testing to help you determine which tool is best for your upcoming event and riding style.
Both types of bikes generally have a few subjective and objective pros/cons.
A well designed light-weight bike is often described as responsive and comfortable. For those spending long hours in the saddle where outright speed isn't a major consideration, the light-weight bike is a solid choice. That being said, it has one major advantage over its speedier counterpart: climbing. At gradients above 6%, a light bike will on average outpace a heavier aero bike by 1 or 2 percent. For that reason, it's no surprise that climbers still reach for the more traditionally shaped, low weight options when the roads tilt upwards.
Now when outright speed is absolutely necessary, there's no denying a modern aero shaped frame will be consistently faster over a wide range of terrain. The most surprising result from testing was the aero bike's low drag made it a faster option even on lower gradient climbs vs a traditional light-weight bike. These frames tend to be quite stiff, so don't expect a plush ride. But for most courses when it's a race against the competition, going with an aero-optimized frame will net you the fastest overall time from start to finish.
To add to the options, some brands are further muddying the waters by combining both aero features and weight savings into a single platform. These "do-it-all" bikes are not just a marketing gimmick to appeal to the masses, but allow riders to truly consolidate resources into a single bike for many different terrains and disciplines. Now before you go out and trade in your fleet for one of the latest high-dollar offerings, compare this with the cost to optimize your setup with the other methods already discussed. You may be surprised at how much free speed is hiding right under your nose!