In repeated testing, wheels with very good transient performance work best for the average rider. This limit of separation is affected by a variable known as Reynolds number (A combination of Speed, density, profile of the shape and viscosity)Īerodynamic design is always a compromise, increasing the separation point at high yaw angles will always negatively impact drag at very low (<5 degree) yaw angles. A more blunt (toroidal) cross section might get to 15 but that’s really the limit. The effectiveness of their marketing is remarkable as many posters on the Internet also believe this.ĭue to the laws of physics, for an average rider, the maximum yaw angle before complete separation occurs is around 12 degrees. Wheel manufacturers tout their wheels as having fantastic drag at varying yaw angles. This allows a much more realistic estimate of drag to be estimated as it simulates road conditions.
This requires a wind tunnel with Horizontal and Vertical Louvres to add Swirl to the air before it hits the bike and rider. Bare in mind they can adjust their weighting to make their wheels look better!Ī superior method of analysis is to carry out a transient analysis in a wind tunnel. Wheel manufacturers are now using a weighted analysis of yaw angles and speeds to give an overall rating for their wheels. Most wind tunnels are not geared up to carry out transient analysis. It is technically more difficult to carry out transient analysis both in CFD and in a wind tunnel. Modelling this type of situation is called transient analysis.
The reality is on the open road, wind does not come in from a perfect angle, it’s speed changes and things like street furniture (hedges, kerbs, passing cars, rider rocking from left to right) upset the airflow over the rider. In the real world, very few riders have the ability to maintain a speed of 50km/h for a length of time as they are simply not fit enough. A steady state analysis assumes the wheels, bike and rider are in a nice environment where air is hitting them at a perfect speed and perfect angle. The testing that has been carried out is usually steady state. A comparative analogy would be fuel consumption for a car driving along a perfectly smooth glass like road surface with no wind and no change in speed – it is totally unrealistic. Journalists on the other hand tend to visit their local university and ask some clever boffin to conduct their testing for them and give them the results or go to their local velodrome, hold a speed and see how much power the wheels consume.īoth of the above testing methodologies are not representative of the real world. The reality is this type of test is not impartial. This is usually achieved by a combination of adjusting speeds and angles. Wheel manufacturers will usually adjust tests to make their particular wheels look more favourable than their competitors in testing. In the bike industry, wheel aerodynamic testing has generally been conducted by two groups of people – Wheel manufacturers and journalists. The effective air speed at the top of a wheel/tyre is double the indicated speed of the bike. The first is they hit the air first as they are the most forward part of the bike and second because they are rotating. The drag caused by wheels is significant because of two fundamental reasons. In terms of drag caused by a bicycle rider, the biggest loss is caused by the rider themselves followed by the wheels and frame.
The effect of Tyre width on Aerodynamic performance.
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