biker jk wrote:Alex Simmons/RST wrote:It's reasonable to criticise marketing departments (and hence Brands) for manipulating or misrepresenting wind tunnel data.
But I wouldn't be so quick to criticise the wind tunnel methodologies used to ascertain the data. Obviously one would like information on the tunnel, process and personnel involved but wind tunnels have been and are excellent tools to assess aerodynamic properties of cycling equipment and cyclists and have been demonstrated to reliably predict actual performance in "the real world". That's because the physics or air flow doesn't magically change just because you make measurements in a tunnel.
Hambini Engineering disagrees.
Speed tests are often conducted at 40km/h (50km/h in the latest bikeradar test), they do this because if they did this at a lower speed, there would be insufficient difference between the best wheel and the worst wheel to justify the horrendous price difference. The average "good" rider will probably be going at 35km/h (air density will affect this), most club riders are at 28km/h. At these "mortal" speeds a NACA aerofoil is quite effective because it's side to side vortex shedding is limited. A toroidal aerofoil is really only good in completely steady state conditions such as a wind tunnel. Out on the road which is where it matters, there are various bits of road furniture that upset the airflow coming onto the rider - technically this is called transient analysis.
Items such as lamp posts, a passing car, a hedge all affect the airflow. Another factor which nobody seems to have commented on is when a rider is going full gas, their bike tends to rock slightly from left to right, only by a few degrees but this upsets the oncoming airflow. An aerofoil which sheds fewer vortices during this process will be more aerodynamic.
The point about air speed is reasonable, which is why I much prefer to provide aerodynamic comparisons as differences in CdA since that is independent of air speed* (across the range of typical cycling speeds / Reynolds numbers).
Yet when we attain analysis from both wind tunnel and field testing, we find close agreement between them. This is not news, it goes back to at least the 1990s in published work which ultimately resulted in a validation of the mathematical model of road cycling power. Indeed the real world results were so good, it was recognised that when field testing with a quality power meter (and good process and analytical technique) it was possible to attain resolution as good as quality wind tunnels.
That's probably because all of the small transient factors have a tendency to average out. It's rare that such things create a bias of any significance in the results.
You can do CFD analysis as way to model various scenarios as well.
The aerodynamics testing I do is with the rider actually riding their bike.
As an example, theoretical models, CFD analysis, wind tunnel testing and field testing of riders riding in close proximity to each other result in the same aero outcomes for both leading and trailing riders.
* there are some interesting specific cases where this might not be the case, e.g. strategic use of trips on an object of a particular size and shape (like an upper arm) at cycling speeds where CdA may not be independent of Re. In any case, the same impacts are measurable in the field and in a tunnel.
lol
