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How much does a wheel's weight, rotating inertia, and drag affect performance? Topics on this page: Wheels are a small portion of the forces on a bike and rider. The differences between wheels are small. Ordinarily power can be measured to what is normally a small tolerance, say plus or minus 3%, maybe plus or minus 1% under ideal conditions. As the table below shows, a typical difference between wheels may be 1% or less. Clearly, another approach, other than direct power measurement, is needed. The table below gives typical values for the forces on two riders. The Standard Rider is on 32 hole standard wheels and the Test Rider is on Specialized tri-spokes. Forces are in grams of force since such forces are often quoted that way.
A paper by D. I. Greenwell, et. al., entitled "Aerodynamic Characteristics of Low-Drag Bicycle Wheels", Aeronautical J., Vol. 99, No. 983, Mar. 1995, pp.109-120, has a good discussion of the aerodynamics of bicycle wheels. Conclusions by Greenwell et al:
Please note that for the coefficients given in the above table, the conventional wheel is significantly different from the deep-section wheels, and deep-section wheels are significantly different from the disk wheels. However, there is no significant difference between the deep-section wheels or between the disk wheels. It's easy to calculate a wheel's rotational inertia using a kitchen scale, a stopwatch, and a tape measure. The general approach is to measure the time period for a wheel swinging at the end of a pendulum. See Figure 1. From the time period of a swing, one can calculate the rotational inertia of the wheel about the point of rotation of the pendulum. The rotational inertia about the point of rotation of the pendulum can be transformed into the rotational inertia about the center of gravity of the wheel.
 Most of the error of the method comes from measuring the period. Timing 100 swings and dividing by 100 gives a good estimate. This minimizes the error of starting and stopping the stopwatch by hand. A pendulum has the property that its period is constant as it slows down. Take care that the wheel swings in the same plane at all times. The method will be invalid if it does not. Go to Calculation of Inertia to calculate rotational inertial for your own wheels. Data on some wheels is shown in the following table. Wheels were complete, meaning they had tires, tubes, rim strips, rims, spokes, hub, skewers, free wheels, just like they would be ridden. As individual components, rims lend themselves to calculation of rotational inertias; tires and tubes don't. There is a large variation between advertised weights and actual weights as manufactured. More real-world, meaningful results come, in my opinion, from measuring wheels in an "as ridden" state. Hence the values here are for fully rideable wheels, just like the ones handed to you from your support vehicle.
Wheel weight and wheel rotational inertia matter when a rider and bike are accelerating. Drag matters whenever a rider and bike are moving. It is not enough to estimate rider and bike performance under constant conditions. Differential equations are used to describe motion under transient conditions. Such equations let us evaluate the combined effect of wheel weight, rotational inertia, and drag. The following differential equation, with an appropriate starting point and initial speed, describes the position, speed, and acceleration of a rider over time. Using this equation, a comparison can be made between a "Standard Rider" and a "Test Rider" to see the effect of various alternatives. This is the equations that is evaluated in each of the case studies presented here.
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