I recently bought some cheap clamp-on aero extension bars.
I fit them on Tuesday, taped them on Wednesday and then tried them out on my turbo trainer. I used a FaceTime connection between my iPad 2 and my wife’s iPhone 4S to see myself from the side as I was pedalling: I wanted to see what effect the various hand positions had on my pedalling action and approximate aerodynamics.
Unsurprisingly, moving from the tops to the hoods meant I got lower; and moving from the hoods to the drops made me lower still. It did surprise me that moving from the drops to the aero extensions left my spine position unchanged. The aerodynamic advantage comes from moving the arms inboard: it is as though your torso is drafting your arms, cancelling the drag from your arms almost entirely; the drag of your torso is also reduced because your arms form a convex shape which helps to guide the air around you, rather than the concave shape formed in the drops or on the hoods. On that point it would not surprise me too much to learn that the tops-to-drops transition doesn’t give you an aerodynamic benefit, because although you are lower you are more concave.
As expected, I had no major problems pedalling with my arms in the aero position on the turbo trainer; since the rest of me was in basically the same position as when I’m in the drops. It is perhaps a little more difficult to pedal, but the difference is fairly minimal.
This morning I tried them in the real world. Getting into the aero extensions was fairly straightforward, and subjectively (warning: confirmation bias) I did feel like there was less air resistance. I can’t see the display of my cycle computer in the aero position; happily it records peak speed so I know that at some point on a slightly-downhill (looked flat, but I was noticeably faster in one direction over the other with zero wind) section I got up to 39.8km per hour (24.73 miles per hour) in a 50×17 (approx) gear. Had I been able to reach the shifter I reckon I could have pushed 50×13 at the same cadence quite easily which would have given me a new PB of around 52km per hour. Bearing in mind my previous best of 45km per hour was set on a long straight 8% downhill in as tucked a position as I know how to get (bum between saddle and bars, head over front axle) the aero-bar numbers are very impressive.
The speed (and its aerodynamic source) is really brought home when you try to get out of the aero position: moving either arm creates a lateral drag differential on the bike-you system, which feels very destabilising. I will feel a lot more confident once I get used to that effect. It took me three goes to get out of the extensions at the end of my fast run this morning, which puts me off using them on the road.
Still, the effect is large enough that the skill seems worth learning.