What is Coefficient of Drag—And Why Should You Care?

This article originally appeared on Triathlete

When we think about riding--and riding fast--we often think about power or fancy carbon frames with expensive wheels, and while these things are all important, the top cycling coaches in the sport have their eyes on something that sounds a little less sexy: drag coefficient. In the simplest terms, drag coefficient (CdA) quantifies the drag or resistance of an object in a fluid environment, such as a triathlete riding a bike along a road. The lower your drag number, the less energy you will lose while riding.

It is something that top bike coaches, such as Matt Bottrill, look at with all of the athletes they work with. Bottrill has coached the likes of Lucy Charles-Barclay, Tim O’Donnell, Tim Don, Susie Cheetham, and Matt Hanson, and he said, regardless of whether he’s working with pros or age groupers, drag coefficient is an important part of improvement for every rider on his roster.

“The key thing to understand is if you can lower your drag number, you will lose less energy while racing. It’s not always about increasing your power; you’ve got to think about speed. Speed is the number one factor you have to think about; if you can reduce your watts for the same speed, that’s a good sign you are improving,” he said.


Bottrill said the CdA for most top pros is typically below 0.2000, adding: “We see some of our athletes down in the 0.185 range, but it’ll depend on the course profile and, of course, how committed they are to training and dialing this in. For age groupers, the average CdA we see is around 0.24-025. If you can get below 0.21 then you’re well on your way to victory.”

It will ultimately come down to the individual and depends on the flexibility of the athlete, as well as their budget, Bottrill said. He said it’s common for most age-group athletes to make gains of 30-40 watts relatively easily when first starting this work if you’ve not done any “aero optimization” before. “Even top pros can normally find gains of 15-20 watts when they first start out on this,” he said.

Lowering your drag coefficient will also dramatically reduce the effect of the wind on your speed, Bottrill said. And when you think about some of the bike courses on the 70.3 or Ironman circuit--especially Kona--this becomes all the more interesting and important.

Once you have gathered your data (which is typically done in a wind tunnel session, but there is an at-home method, plus Bottrill lists some handy resources in his tips below), you can then start to look at ways to improve it, as well as looking at the course profile(s) of your key race(s).

Of course, your bike fit and position play a major role in this, with a massive 80% of the drag coming from the rider. “Bike fit is key,” Bottrill said. “This can be improved by first looking at the biomechanics of the athlete and seeing what limitations they have. You can then work on changes with pole angle, pad positions, height and stack, and even having the right saddle can alter this. You’d then look at clothing and helmet choices. But really the key factor is then making sure the athlete is training in this position.”

Bottrill has his athletes doing aero drills during bike workouts that are specific to them, in much the same way you might work on swim drills to address weaknesses in your swim stroke. “It’s here that a good bike fitter comes into play; it is unique to each individual and isn’t something you’ll find on YouTube,” he said.

RELATED: Fitter & Faster Podcast: Cracking the Bike Fit Code

As bike fitter Ivan O’Gorman talked about on the above Fitter & Faster podcast, bike fit is important, but you don’t want to take it to extremes. Bottrill strongly advises against seeking out a bike position that might seem extremely aerodynamic, but ultimately could impact your ability to run well off the bike. He said: “Knowing your drag coefficient is a big advantage, but it’s about taking the data you find in aero testing and bike fitting and then bringing it to the real world.”

And his top tips for doing this?

  • Get a bike fit with somebody who knows what they are doing

  • You can keep spend a fortune on the top aero bike frames, but the margins are so small--focus on getting the right pads and bars

  • Aero testing can be expensive, but there are an increasing number of ways to do your own aero testing and analysis with things like the Chung method, Notio Connect, Best Bike Split, and My WindSock

  • You have to train in the aero position to race well in it, so be sure to do this and, where applicable, practice doing the right drills for you

Bottrill added: “Just remember, aero really is everything--but you have to train it. The days of just riding your bike are going, but get more efficient, get aero, and you will ride faster and run faster off the bike.”

RELATED: The Five Keys to Finding the Right Bike Fitter for You

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