Results of the Measurements
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Drag Measurement on HPVs

Results of the Measurements

A coastdown measurement with a statistics of 6 to 10 coastdowns each was applied to

The rack shipped by BikeE mounts under the seat. This lets the panniers hang into the free space under the rider. As an improvement a standard rack for uprights was mounted above the rear wheel, lifting the panniers by about 10 inches. A further attempt to improve the aerodynamics was to mount the standard rack behind the seat. Unfortunately, this sets the center of gravity too far to the rear and the bike becomes unstable.

The goal of these measurements was to test the data logger hardware and to develop an evaluation software, not to achieve greatest possible accuracy. All measurements were done at night (because that's the time with the lowest wind and traffic) with dynamo light turned on (except on the Trice). The additional drag should be constant (not dependent on v) and thus go into the rolling resistance. For an easier comparison the first four F(v) curves have been plotted into one diagram and the error bars have been removed:

The data is also available as a set of ASCII-tables, also containig all the other bike's data. The raw data (hexadecimal tables of pulse counts) is available on request.

A curve fit with

leads to the coefficients (with statistical errors):

HPV runs a / N err b / N/(m/s)2 err
BikeE 6 7.1889 4.4% 0.2088 3.9%
BikeE with panniers 10 7.5177 6.7% 0.2840 4.6%
Specialized 10 6.4090 4.5% 0.2189 5.3%
Specialized with panniers 10 7.9456 3.6% 0.2260 2.9%
BikeE with panniers on standard rack 8 8.4341 4.6% 0.2611 1.7%
BikeE with panniers behind the seat 6 8.5511 3.7% 0.2161 3.4%
Kingcycle with full fairing 6 11.9857 0.5% 0.1638 4.2%
Kingcycle with rear fairing 6 12.4726 1.2% 0.1900 5.5%
Trice with front fairing 10 6.5073 8.6% 0.1707 5.6%

At an estimated air density of 1.167 kg/m3 we obtain the drag coefficients

HPV m / kg CrerrCdA / m2 err
BikeE 85 0.0086 4.4% 0.3578 3.9%
BikeE with panniers 100 0.0077 6.7% 0.4867 4.6%
Specialized 85 0.0077 4.5% 0.3751 5.3%
Specialized with panniers 100 0.0081 3.6% 0.3873 2.9%
BikeE with panniers on standard rack 100 0.0086 4.6% 0.4474 1.7%
BikeE with panniers behind the seat 100 0.0087 3.7% 0.3704 3.4%
Kingcycle with full fairing 84.5 0.0145 0.5% 0.2808 4.2%
Kingcycle with rear fairing 82 0.0155 1.2% 0.3257 5.5%
Trice with front fairing 103 0.0064 8.6% 0.2926 5.6%

The statistical errors should not be taken too seriously, because they were derived from only 3 to 5 samples. A good error margin is usually two to three times the statistical error, which sets us into the 10% to 15% accuracy range.

Interpretation regarding Pannier Mount

As expected from a frontal area estimate, the naked BikeE has similar aerodynamics as a comfortable upright. However, if panniers are mounted, the aerodynamics becomes significantly worse on the BikeE (by 36%), while it stays approximately the same on the upright. This can be understood, because the panniers are hidden behind the rider's legs on the upright and don't add to the frontal area, while they hang out under the rider on the BikeE. Mounting the panniers behind the seat on the BikeE improves the aerodynamics significatly, but shifts the center of gravity too far to the rear.

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Author: Christian Starkjohann <cs@hal.kph.tuwien.ac.at>