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This question at aviation.se got me calculating the acceleration of a bike strapped to the outside of a plane if the straps broke. As you do. I couldn't find a figure for CdA of a bike without rider anywhere, and it seems to me that the rider's body will dominate the system CdA.

A more realistic situation for most of us would be consdering the forces on a bike on top of a car, again from the point of view of secure attachment (e.g. tryiong to fit an unusual bike with home-made brackets).

So are there published figures for the CdA of bikes (any type) without riders?

  • I think the Cd of a bike would be different when it is strapped atop a plane – user74671 Dec 21 '17 at 0:56
  • @user74671 there will be small differences because it's moving with the surface it's on, rather than worth respect to that surface, but did you have more in mind than that? – Chris H Dec 21 '17 at 7:13
  • I think the air would be disrupted by the nose of the airplane too but i agree that it would be a good approximation – user74671 Dec 21 '17 at 16:38
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There are, but measurements typically will be protocol-dependent (and vary slightly between different wind tunnels).

For example:

Here is a link to a comparison done at the A2 Wind Tunnel in North Carolina almost a decade ago, on bicycles standardized to frame and wheels, but without saddle or handlebars. Other protocols may include saddles, handlebars, and the standard wheels and tires provided with the bike.

Here is a link to a comparison done in 2012-2013 by Trek on their Madone in the San Diego Low Speed Wind Tunnel. Note the differences in protocol.

Here is a link to a Trek whitepaper on their Speed Concept.

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  • Thanks. That's really nice, though I wish they'd state the speed at which those drag measurements (stated in grams) were taken (one of the 3 sources gave it explicitly as 30mph). This gives a CdA of very rough 0.06, or just under 1/10 of a bike with a rider on aero bars. That seems plausible. – Chris H Dec 19 '17 at 16:47
  • Ah, yes. I should have pointed that out as part of the warning about protocols. For odd historical reasons, in the US the wind tunnel speed is 30mph. Elsewhere, the air speed is often 50 km/h. The CdA of a combined rider+bike can range from ~.18 m^2 (on a TT bike) to above .7 m^2. – R. Chung Dec 19 '17 at 16:56
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    Cd is, of course, dependent on the Reynolds number so bikes are typically tested in the range of 30mph/50kph, where the Cd is relatively stable. – R. Chung Dec 19 '17 at 17:01
  • 30mph=50km/h to within my error margins. TBH 30mph=**3**0km/h to within the margins I've used in commenting on the linked Q. – Chris H Dec 19 '17 at 17:01
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Just to add another reference / data point, this image is from an aerodynamics course presentation by Bert Blocken of Eindhoven University of Technology. In it he references wind tunnel test results of a rider in three different positions and provides data for the bike + rider, and the rider alone:

enter image description here

In this case the bicycle used in these tests had a separately measured CdA of 0.077m^2.

For the positions and rider tested this represents approximately one-quarter to one-third of the total aerodynamic drag of the bike + rider. Obviously the relative proportion of aero drag for each of the bike and rider will vary depending on the rider's position, morphology, clothing and helmet choices, as well as the bicycle set up and equipment (frame, wheels, bars, cabling etc).

Just looking at the images of the positions tested, that rider has quite some aerodynamics gains to be made with improved positioning, which will push the proportion of the bike's CdA to total CdA higher again.

This is the study abstract (2010) for reference: http://www.sciencedirect.com/science/article/pii/S002192901000059X

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