Aerodynamics - The Wheel Story

Two new independent studies on wheels, one in Australia and the other in England, have provided a comprehensive analysis of the aerodynamic performance of most wheels on the market. Their conclusions can be taken with some degree of confidence as they came up with essentially the same result.

The British study tested six wheels - 2 full discs, 2 three spoked composite wheels and 2 deep rimmed spoked wheels and compared them against a standard 36 round spoked wheel with a box shaped rim. In these tests all the aerodynamic wheels had similar drag when riding with no wind or into a head wind (zero yaw angle).

As the wind moved more to the side in a cross wind (the yaw angle increases) the disc wheels showed the lowest drag. This is because the disc shape produces lift and a component of the lift force acts to reduce the wheel drag. However, associated with a large lift is a large side force which is detrimental to control and stability of the bicycle. Consequently disc wheels have the highest side force and the largest yawing moment (see box) which affects the bicycles stability.

M = moment about steering axis
F = sideforce on wheel
d = distance of point of sideforce from steering axis
The British Researchers concluded "in terms of axial force (ie the wind drag on the wheel) the disc wheels offer superior performance particularly in a cross wind. However this assessment must be coupled with the side forces and the yawing moment characteristics and their impact on stability."

The researchers also found that some wheels tested , the three spoked wheels and the Hed CX, had a sudden reduction in yawing moment at one point in the angle of the cross wind followed by a sudden increase. This means that the force needed on the handlebars to hold the front wheel straight when riding in a cross wind is changing in an unpredictable way. They concluded that this characteristic would have a serious effect on stability in gusty conditions. They did not find this to be the case with the Campagnolo Shamal. They also found the Shamal to be more stable than any other aerodynamic wheel at low yaw angles. These wind tunnel tests were corroborated by road tests that found that the Shamal was relatively unaffected by crosswinds. Their final conclusion was that the composite spoked wheels and the full discs offered no advantage over the deep rimmed spoked wheels.

Of the two spoked wheels tested they concluded that the Hed Cx would give a marginally better performance in still conditions while the Campagnolo Shamal was better in a variety of conditions.

In a much more comprehensive study carried out at the Department of Aerospace Engineering at RMIT in Melbourne, Paul Merlo tested the aerodynamic characteristics of 23 different wheel options. These included the standard 3 cross, 36 spoked wheel with a box rim and a variety of wheels with the same overall configuration but different numbers and shape of spokes. As well as these conventional wheels, aerodynamic wheels with deep rims, 3 and 4 spoked composite wheels and full discs were also tested.

Paul Merlo's results indicated that the box shaped rim is very inefficient aerodynamically and it is hard to improve the characteristics of this type of wheel by changing the spoking pattern. Using aerodynamic spokes and reducing their number will decrease the drag on this type of wheel slightly, but using a standard aerodynamic rim such as the Campagnolo Omega or the Velocity aero has a greater influence on reducing drag. For instance, a wheel with 32 round spokes in a Campagnolo Omega rim had less drag than a wheel with 16 round spokes in a box shaped rim.

A big reduction in wheel drag was then apparent between these wheels and the true aerodynamic wheels ( ie. The deep rimmed wheels, the 3 and 4 spoked composite wheels and the discs). Of these aerodynamic wheels, and there were 13 tested including the majority of European and American wheels, there was little difference in the drag characteristics at zero yaw angle, with one exception. Clearly the lowest drag with a 10% reduction in drag over the next best performed wheel (the HED CX) was the 700c Campagnolo Shamal.

Better again the 26" Shamal had a further 14% reduction in drag compared to the 700c Shamal. The Campagnolo Shamal was the only 26" wheel tested and it gives it an unfair advantage over the other wheels because reducing the diameter of any wheel will reduce the drag. However for the athlete the results are clear, less drag = more speed.

When the wheels were tested in a side wind the drag characteristics changed in a manner similar to that found in the British tests. As expected the discs performed better in reducing drag but had the associated instability problems. Comparing the Shamal and the HED CX, as the angle of the sidewind increased the HED CX had less drag than the Shamal. This was similar to the finding of the British study.

Direct comparison between the two studies cannot be easily made because there were different methods used in each study. For example the British study tested all rear wheels while the RMIT study tested mostly front wheels. There were also different wheel configurations between the studies. The British study had a 24 spoke HED CX and a 16 spoke Shamal, while in the Australian study both types of wheels were 18 spokes. However although the values measured in the two studies varied a little, clear trends emerged from both the studies and these were very similar. This can give us a lot of confidence in the overall conclusions.

Both studies indicated that Campagnolo and HED have got it right with their deep rimmed wheels. The results of these tests suggest that for the variety of conditions encountered by the triathlete in racing, these two brands of wheels give better all round aerodynamic performance than any others.