Tech Talk - Wheel Aerodynamics, How Important Are They?

Bicycle Wheel – Product Selection

In the bicycle industry when there is discussion of wheel aerodynamics, power is used to quantity the amount of energy expended or saved.  Power is defined as how much work is done during a given time period.  Some of the key terms that have an effect on a wheels performance are:

• Aerodynamic drag
• Inertial forces – Front and rear wheel mass
• Rolling resistance

Which is the most important?  Overall the factor that is the most beneficial is the rider pushing the pedals.  A rider than can push the most power for the longest amount of time usually wins the race.  However, a smarter rider who can become more aero will gain an advantage.  A simple study will help back up these facts.

Table 1 shows 3 different tests by the same rider.  In the first column was a long training ride, the second column shows the riders climbing output from column 1 and the third column shows a criterium race.

Table 1- Power for a rider during different bicycle rides

The wheel performance factors from this chart mean that aerodynamics played the most important part on power savings.  Wheel mass was far behind.  Wheel inertia effects in all cases were insignificant.  This is because inertial forces are a function of acceleration.  In bike racing this peak acceleration is about 0.1g’s to 0.2 g’s and is generally from an initial velocity of zero.  Further, the mass of the wheel is so small compared to the rider that any difference in acceleration will be unperceivable.

Now that we see that aerodynamics play an important part in the motion of the bicycle, let’s look at how the spokes can save power.

Table 2 shows a number of different wheels.  The graph is a function of power versus wind angle.  The test wheels were sat in a wind tunnel and constantly spun at 30 mph.  A picture of these can be found in Figure 1.

The worst performance was the top line of the graph.  This was an ordinary rim with 16 round spokes.  The next line down was a Zipp 303 wheel that had 24 blade spokes.  The third line down was a 2001 Zipp 303 wheel with 24 bladed spokes.  Next was a 2004 Zip 303 with 20 CX-Ray spokes.   The final two were an Italian made deep dish wheel with 18 bladed spokes and the best, a 2004 Zipp 404 with 20 CX-Ray spokes.

Table 2 - Wattage versus Wind Angle for different types of wheels

The two wheels with the best aerodynamics were deeper dish and had bladed or ovalized spokes.  Power savings for these wheels is about 3-4 Watts more than the third place wheel.  Overall though they saved about 15 Watts compared to the plain wheel.  Over long distances a good wheel could save the rider a lot of energy.

Now that a more in depth look of how a wheel can save energy, how does it compare to the overall performance of the rider?

Another study was conducted similar to Table 1.  This time, the rider did the same three experiments again, training rider, hill work from the training ride and the criterium but this time different variables were changed.  In the first row a normal wheel is used, this will be used as the reference for all the other studies..  The second row shows if the rider lost 10 pounds.  The third row shows if there was a 50% decrease in the riders drag coefficient.  Next crow shows a 50% reduction in the frame drag.  Finally a 50% decrease in the rolling resistance.

Table 3 - Comparing different variables for a bicycle and measuring there respected wattages

The biggest decrease in power was from the lower rider drag coefficient.  Wheels and the frame showed similar characteristics.  The riders weight however doesn’t play much of a factor.  The steeper the hill however, the more important mass becomes.  The uncertainty for this study was roughly 3% of the overall power unaccounted for.

Overall the wheels account for about 10% of the total power required to race your bike.  This fact is dominated by wheel aerodynamics.  The best wheels in terms of performance are ones that are lightweight and aerodynamic.  These factors are dependant on cost.  The more technology that goes into building a wheel means the more money it is going to cost the user.  Choosing a wheel that is aerodynamic, light, durable and cheap is not easy to find. 

Racing teams in the Tour de France are lucky because they get most of the wheels subsidized to them.  For the common person this means that they would have to buy one set and use it for all different situations.  Hopefully this report provides some helpful insight on choosing the right wheels.