Choosing Bike Wheels— Coast-Down Tests Provide a Quantitative Measure

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					                         Choosing Bike Wheels—
              Coast-Down Tests Provide a Quantitative Measure

                                Stephen J. Derezinski, Ph.D.

Wheels are arguably the most important single component of a bicycle. The right wheels
even on a mediocre frame (however, one that does fit) will make the most dramatic
difference in performance. Every serious biker, whether racer or recreational, should first
pursue the best wheels when deciding to modify his bike to improve performance.

So, what is the best bike wheel? This, of course, will depend upon the type of riding or
competition for which it is used; 1) road race, 2) critirium, 3) time trial, and 4) short or
long touring or serious recreational use. Wheels have several physical characteristics that
suit them to these specific uses. Here, the physical characteristics are defined as they
relate to usage, and measurements are used to evaluate, compare, and recommend the
best wheels. The variety in selection is based primarily on the number of spokes, their
size and shape, and their lacing pattern.

                               Performance Characteristics

The performance of a wheel depends on the energy consumed by the wheel. This will
include the aerodynamics of the wheel, its weight, its inertia, and the friction of the
bearings. Rolling friction of the tires is important, too, but it is not part of this study. Of
the factors considered here, aerodynamics is the over-riding factor for wheel energy, and
it becomes increasingly important at higher speeds (speeds above 25 km/hr). High speed
will be developed by high bike speed and/or by the addition of head wind.

Aerodynamics. The time trialer needs to obtain the most aerodynamic wheel. Weight
and inertia are secondary. Paradoxically, some advantage from added inertia could be
obtained for a time trial course with wind gusts or slightly rolling terrain. That is, the
wheel inertia will store energy in the wheels (acting as flywheels) that will help maintain
a constant speed for a short period of time when wind gust or short upgrade is
encountered. Maintaining a constant speed is a key factor in posting a good time trial.

Weight would become more of a factor for a very hilly time trial course, in which case it
would be have second priority. For a very hilly time-trial course, wheel weight would
gain equal importance to aerodynamics. However, typical time trial courses are relatively
flat, and aerodynamics is paramount.

Rotational Inertia. The need for low inertia wheels would be most desirable for sprinters,
as in a road race or critirium. Also, air drag in a road race or critirium is less of a factor
because drafting in a pack of riders greatly diminishes the contribution of air drag losses.
The frequent acceleration and maneuvering typically in a road races and critiriums makes
wheel inertia for these applications most important. The fraction of a meter advantage
that low inertia wheels would provide in the final sprint often could win the race.
Weight. For the recreational rider and for touring, overall weight is the most significant
issue (after durability and comfort). Low inertia will accompany low weight wheels, but
not necessarily, if the weight is distributed in the tires. This may often be the case for a
good recreational wheel because added tire weight means a larger tire (more comfort) and
a thicker tire and tube for more reliability. Good aerodynamics would be advisable, too,
for bucking head winds, but not a paramount factor, especially for pure recreational use.

Bearing Friction. Bearing friction is a small factor, but applies to all applications. It is
typically so small that it has minimal impact as compared to the other factors. Rolling
friction of the tires is another issue, but it is not part of this study.

Therefore, four primary factors are used here to evaluate wheels: 1) air resistance, 2)
inertia, 3) weight, and 4) bearing friction. Table 1 suggests the priority that each has for
the different applications. It is clear from the table that each application will best be
served by a wheel with different characteristics.

                                            Table 1
                                  Priority of Wheel Factors

                                    Air         Inertia       Weight     Bearing
                                 Resistance                              Friction
            Recreation and           2             3            1           4
             Road Racing              2            1            3            4
               Critirium              3            1            2            4
            Time Trial (flat)         1            3            2            4

A system of evaluating bike wheels has been developed, which evaluates their
performance based on the loss in energy during spinning. It requires their measured
weight, rotational inertia, and speed lost during coast-down. Coast-down is the decrease
in wheel speed when it is spun in a stationary test stand.

Weight. The total weight of the wheel is measured with the tire, but without the skewer.
Skewer weight is not included because in does not turn, and front wheels are primarily

Inertia. The inertia of the wheel depends on its mass and the radial distribution of the
mass. Mass near the rim creates the most inertia, and the mass at the very center has
minimum inertia. The effect of mass distribution on inertia is disproportionate to its
radial location. A gram of mass at the rim will increase the inertia by a factor of 4 over
the same gram of mass at ½ of the wheel radius. Therefore, tire weight is critical to
inertia of the wheel.
For the tests here, the inertia is accurately calculated from the mass of the wheel and its
period of rotation. Mass was first obtained by weighing the wheel and tire without the
skewer. The period of rotation is obtained by hanging the wheel in a horizontal position
with three strings of accurately known length, as shown in Figure 1. The wheel is then
made to oscillate freely about its axis (hub), and the period of oscillation in seconds is
timed. The mass of the wheel, kg, and the period are then used to accurately calculate the
inertia, kg-m2. No knowledge or calculation of the distribution of mass is needed by
determining the inertia with this method.

                      String attached
                      to ceiling


Figure 1. Setup to measure the rotational inertia of a bicycle wheel. Three strings are
attached to the ceiling and support the wheel. The wheel is slightly turned and released.
The period of its oscillation is used to calculate the rotational moment of inertia of the

Coast Down Energy. The bike wheel is mounted in a stationary fork, and it is spun to a
speed equivalent to about 50 km/hr bike speed or about 400 rpm. It is then allowed to
coast down, and the speed of the wheel is recorded as it slows to stopping. At any
moment in time, the speed can be used to accurately calculate the energy in the wheel,
because the energy of the wheel is proportional to the product of the speed and inertia of
the wheel.

Coast Down Power. The rate of change of the energy of the wheel as the wheel slows is
numerically calculated from the data of the coast-down test. The rate of change of energy
is a result of the power lost by the wheel (Watts) as it slows. The loss in power is then
given as a function of wheel speed converted to equivalent bicycle speed, km/hr. The
resulting curves of lost power in Wattage versus bicycle speed can then be used to
compare different wheels. Figure 2 shows the results for several different wheels.
Wheel Bearing Friction. Bearing friction is most easily determined by the energy loss as
the wheel approaches stopping by assuming that air resistance is negligible at this point.
It can then be made proportional to the speed and subtracted from the total power at the
speed to obtain the net air-drag power of the wheel. This was done for the results in
Figure 2.

                                       Wheels Tested

The wheels are described in terms of spokes and rims. The number, length, diameter, and
shape of the spokes and the size and type of rim are included. Materials used are also
noted. The overall diameter is that of a 700 c wheel, and wheels considered are primarily
front wheels. Size and type of hub are not considered. A compilation of the factors is
given in Table 2.

                            Power Consumption
                          Air Drag on Spinning Wheels
                               No Forward Motion
                   Zipp, 18 Aero Spokes
           5       Spinergy, 8 Blades
                   Rolf, 14 Aero Spokes
           4       Radial, 32 Flat Spokes

                   Radial, 16 Flat Spokes
                   Std. 3x 36 Front Wheel
                   Std. 4x 36 w/cassette


               0       10         20        30         40        50
                             Spinning Speed, km/hr
Figure 2. The air drag power consumption of eight wheels as a function of bicycle speed.
Test done in still air, so results are for comparison purposes only. Rotational speed is
given in terms of bicycle speed for the applicable wheel diameter.

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