The Hierarchy of Aerodynamic Improvements for Road Riding

The implications of heavy and ‘aero’ vs. light and ‘dragalicious’
The laws of physics are here to stay, and understanding how they effect your ride isn’t just for the racer.

I’ve been asked the question as to whether it’s more beneficial for the cyclist to add more aero features to their bike, or invest in ways to minimize the weight of the machine. Here in SoCal most cyclists are more concerned with weight, just go to one of our local criterium races and you will hear the racers talking about the weight of their new bike.

Rather than answering the ‘which is better, more aero or less weight?’ question, I will address how the cyclist can become faster and more efficient by concentrating on both elements.

When Does all of this AERO Stuff Matter?

If you are riding at less than 15 mph (24 kph), then you won’t see much of a difference in terms of aerodynamic efficiency. A 180 pound rider with a 0.5 mph headwind on a flat road is only using 42.4 watts of power to push through the wind. As you get faster, you’ll end up riding against an exponentially increasing resistance.

As an example of this difficulty, increasing your speed from 30 mph to 31 mph requires 33.5 additional watts (328 watts to 361.5 watts). For reference, 33.5 watts is appropriately the same total power requirement for (as the example) a 180 pound rider on flat ground, with dead air at 14 mph. Looking at our rider going from 37 mph to 38 mph requires 50.5 additional watts due to the aerodynamic drag working against the rider.

Be mindful there exists two categories to considerations of aerodynamic improvements, the rider, which contributes 75-80% of aerodynamic drag, and the bicycle with the remaining 20-25%. Careful thought of how to make changes to your setup or riding style can provide dramatic improvements to your endurance and average speed.

Body & Positioning

Size does matter and a larger rider will always produce more aerodynamic drag than a smaller rider, so it is more important for a larger rider to get as efficiently positioned on the bicycle as they can. Also, to get more aerodynamic you will want to get as low as possible. Holding the tops of the bars in an upright position generates the largest frontal surface area. Riding in the hoods is a little better and riding in the drops produces the smallest frontal area. A rider who is more flexible can get lower in the front by bending their elbows and rotating the hips forward.

In addition to the body and positioning, consider your cycling clothing. Flapping apparel will slow you down. Even having a jersey partially unzipped produces more drag than a fully zipped-up jersey. Don’t forget about the outer layers. While on your group ride, take off the loose-fitting wind jacket as soon as it starts to warms up.

Beyond this, we get into more specialized provisions specific to this quandary, like aero-helmets and skin-suits; aero booties (shoe covers), going glove-less, shaving your legs, not shaving your legs, dimpled fabrics, smooth fabrics – there is no end to the debate, but the gains are marginal at this point, a single digit watt advantage over 100 miles of distance traveled.

The Machine

The frame, or type of bicycle is next. A bike specific to racing triathlons is the most aerodynamic, but comfort, climb-ability and turning capabilities are typically compromised. If road is your preference, then one of the newer aero road bikes would be worth considering.

Aero road bikes are narrower – forks, frames, seatpost, hidden (internal) cables – are all designed and built to slice through the wind more efficiently than a climbing bike. How this is accomplished varies between maker, but wind tunnel testing should be a requirement for this design process.

Deep Dish aero wheels will be your next consideration because the wheels will make the most difference. Current trends for aero road bikes are borrowed from the triathlon setup of having a larger depth wheel on the rear and a shallower wheel on the front. For example, riders showing 40mm+ front wheels and 60mm+ rear wheels. However, many manufacturers are still providing 60mm+ wheels for both front and back.

Riding in the land of esoterica, the latest debate is aero bars vs round bars. I’m not talking triathlon aero bars (the perches riders lean on in a tuck position) those have been established as having significant benefit. I’m talking an aerodynamic road handlebar that is flattened along the top. If you want the best aerodynamics, leave off the bartape as testing shows that even applying handlebar tape can cost you several watts of added wind drag.

Most aero road bikes will have full internally routed cables. This is something that is designed into the frame by the manufacturers. Some options of road aero bikes have exposed cables routing under the handlebar tape to the brakes and dérailleurs, but bikes with internally routed cables will have less aerodynamic drag than those with exposed cables. It’s 2019, and if a manufacturer is bringing a bike to market under the ‘aero road bike’ category, full internally routed cables should be considered the default.

Aero Matters

If you are riding at less than 15 mph (24 kph), then you won’t see much of a difference in terms of aerodynamic efficiency. In fact, referring to the table below, a 180 pound rider with a 0.5 mph headwind on a flat road is only using 42.4 watts of power to push through the wind. If you ride at these speeds you really won’t need to upgrade to an aero bike, unless you want the latest in design technology. But, the faster you want to go, you will soon be up against an exponentially increasing resistance.

Referring again to the table below, you can easily see why going from 30 mph to 31 mph is so difficult. This 1 mph requires 33.5 additional watts (328 watts to 361.5 watts) which is about the same power as riding along at 14 mph. Going from 37 mph to 38 mph requires 50.5 additional watts.

Summary

The automotive industry figured out a long time ago that aerodynamics matter. These 1000cc MotoGP monsters have an estimated 300HP and achieve speeds of 220mph on the race track. The amount of air these things are pushing out of the way is astonishing.

As vehicles get even faster, they tend to get lower, narrower and longer (e.g. NHRA Top Fuel dragsters, F1 cars) and aerodynamics play an even larger role, primarily to keep the vehicle on the ground. At 300 mph, the front wing produces 700 pounds of downforce while the rear wing produces over 5,000 pounds of downforce. This stuff is fascinating, but perhaps saved for another article.

By going aero on a bicycle, the penalty is (a) slightly heavier bike and (b) a bike that is not quite as stiff in the bottom bracket (compared to a pure climbing bike). But if you want to go faster, more aero rider positioning and component selection far outpace weight reduction efforts.