Cyclist
Are on-bike aero monitors set to become the new power meters?
Back as flat as I can make it, shoulders rounded, hands together, head down, eyes on a fluctuating number on the Garmin screen. 0.226… 0.231… 0.229…
The number is my CdA – effectively describing how aerodynamic I am – and as I pedal round Reading’s outdoor velodrome it’s being measured by Body Rocket, a commercially available live drag measurement system. When my first ten laps are complete, I pull up to the stands and ask what a good number is.
‘The really fast guys, the Hour record guys, are maybe 0.15, 0.16,’ says Eric DeGolier. I’ll not be challenging Filippo Ganna anytime soon, but that’s not the point.
DeGolier is Body Rocket’s founder and CEO and has his sights set ‘on bringing aerodynamic testing to the outdoors for elite and everyday athletes, because if you can work out how to reduce your drag, you can go faster’. Which is where I come in.
Having completed a baseline run of ten laps (each lap is 459m, data then averaged) I’ll trial four different setups suggested by Body Rocket’s AI software, followed by an all-out 4km effort. The idea is that ‘aerodynamics is personal’, so in being able to see my live CdA I can see how different setups change how aerodynamic I am.
I’m doing all this on a BMC time-trial bike that has been provided for today’s test, although the Body Rocket system promises to be effective for road cycling as well. The question is: how does it work and is it accurate?
Behind the numbers
CdA refers to your coefficient of drag (how slippery you are as a body) multiplied by your frontal area (the amount of you that hits the wind). Get more ‘slippery’, say with a smooth skinsuit or an aero helmet, and CdA decreases; get ‘smaller’ by adopting a different ride position and CdA also decreases. So all things being equal, the lower your CdA, the faster you’ll go.
To calculate CdA in the simplest terms you need to know air speed, air density and drag force (CdA = 2x drag force / air density x air speed2). For live drag systems, measuring the first two is easy in the sense that on-board instruments already exist to do so accurately. Measuring drag force away from a wind-tunnel, however, is harder, and this is Body Rocket’s USP.
‘The best way to describe it is if we put you on a bike with a Body Rocket system in a wind-tunnel and I walked up and pushed you on the shoulder, I’d be putting a force into you that you have to resist [or else you’d fall off]. That’s the same principle as drag force,’ says DeGolier.
‘In a wind-tunnel, drag force is measured by a sensor under the floor that the bike is bolted to; in our system, load sensors are placed under the four touchpoints of the bike – bars, saddle and pedals. So if I pushed your shoulder with five Newtons of force, the wind-tunnel sensor and our sensors would both measure five Newtons. Now replace me pushing you with wind being blown on you. Both systems are measuring the force of the wind, the drag force.’
Unlike a wind-tunnel, however, Body Rocket is only measuring the drag of the rider, not the rider-bike system. That might sound like a limitation, but DeGolier sees it as necessary, and in a way, entirely satisfactory.
‘To measure the CdA of rider-bike you need to know the other forces resisting you as well. In the real world that’s rolling resistance and drivetrain losses [gravity also comes into play, but this can be accurately measured by Body Rocket and its rivals]. Because there’s nothing out there that measures them in real-time, they need to be estimated, which means systems claiming to measure rider-bike CdA are full of pitfalls.’
The truth of that is debatable – certainly live-drag measuring rivals such as Aerosensor and Notio claim to be highly accurate – but from a logical standpoint DeGolier makes sense.
The more you can measure directly, the more accurate your results, and it’s true that rival systems don’t measure drag force like Body Rocket does. But is just knowing a rider’s CdA enough? After all, Ganna didn’t ride 56.792km in an hour by only optimising his position.
‘A rider is 75-80% of the overall drag, so we’re just concentrating on measuring the biggest element of drag as accurately as possible,’ says DeGolier. ‘It’s not like there isn’t a relationship between bike and rider, but generally a wheel or a frame that’s fast for one person will be fast for another person, and anyway, most riders just ride the bike they have.’
Moreover, because Body Rocket analyses the rider, it can measure the impact of the kit they’re wearing.
Interpreting the data
The results of my testing are a mixed bag. My baseline CdA wearing a Trek Velocis road helmet is measured at 0.228. A high-hand position on the bar extensions that is predicted to make me more aero actually increases my CdA to 0.236, which Body Rocket’s software equates to a 9.6W loss. Stretching my position from baseline – 20mm forward at the bars, 15mm back at the seat – sees a 3.6W saving.
The same position swapping to a Giro Aerohead TT helmet elicits a 14.3W saving, then stuffing my pockets with jacket, tools, tubes, food and phone (my idea, not the AI software’s) sees a disastrous reversal, a 9.6W loss. My final all-out run, stretched position but back in my road helmet, sees a 6W saving.
Afterwards Body Rocket analyst Jason Fitchew looks over my data, which extends into all manner of areas, from estimated hydration loss (the load sensors measure rider mass too, and I appear to have lost around 1.5kg over two hours of testing) to pedalling dynamics. Weight distribution has the potential to be particularly insightful, with the load sensor on the saddle able to help determine how fore or aft I’m sitting and, crucially, how that changes during a run.
‘You can see that early on in runs your weight distribution is more consistent, then it gets choppy as you go on, which happens in line with your CdA getting worse. Likely that’s down to fatigue, meaning you’re reverting to more familiar riding positions.’
It’s true to say that the stretched-out position – my fastest – was an entirely new position to me on the day, and it seems obvious to anyone that key to being aero is being able to sustain an aero position, rather than simply just being shown ‘this is how you should ride to get faster’. So where does this leave Body Rocket?
In terms of a system, it’s impressive. For most riders, such as me, the data Body Rocket produces has to be taken at face value, but the method behind the concept appears sound. But does an everyday rider need it?
At £2,900 for the kit it’s expensive, and Fitchew explains that most riders, save for the most tech-savvy, will need Body Rocket’s help to install it. Then there are all the running changes to bike setup needed to ascertain the fastest position, interpreting yet more data along the way, and then training yourself to sustain that position.
Body Rocket won’t be for everyone, but the same was said for power meters once upon a time, which were seen as the preserve of the pros and coaches, and look where we’re at now – so many of us obsessing over one, neat number. So while there’s surely a long way to go, it’s not beyond the bounds of possibility that CdA might one day sit alongside power and heart rate as one of cycling’s golden metrics.
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