5th IOC World Congress on Sport Sciences
with the Annual Conference of Science and Medicine in Sport 1999

Sydney 31 October -5 November 1999

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Validation of SRM power cranks using dynamic calibration

E.W. Lawton*, D.T. Martin & H. Lee
Australian Institute of Sport, Canberra, Australia

The SRM (Schoberer Rad Messtechnik) Training Crank (Jülich – Welldorf, Germany) was developed in 1986 to quantify power output in the field using a cyclist’s own bicycle. Torque applied to the crank (detected by 4 strain gauges) is multiplied by angular velocity (cadence) to calculate power. Factory calibration using an eddy current brake has shown that the accuracy of power measurement for the professional road version is ± 2.5%. The purpose of this investigation was to evaluate the accuracy of the professional road version SRM Training Crank (SRM) using a dynamic calibration rig (CALRIG) designed and built by Bio-Med Electronic Services. Nineteen SRM cranks were tested over a one-year period in a laboratory under standardised conditions (19-23° C, 695-710 mm Hg). The SRM cranks were mounted on the bottom bracket of a bicycle attached to a stationary wind trainer. The bicycle was attached to the CALRIG for determination of the true power at 100 rpm. After a warm-up protocol and prior to testing, the zero offset for the SRM was set according to manufacture’s instructions. The CALRIG load cell was calibrated with precision masses (1-10kg) and any dynamic system losses were measured to establish no-load zero condition prior to each testing session. All testing sessions were performed at 100 rpm and involved shifting through the full range of 9 gears in the rear cluster using both the large and small front chain rings. After a 15-30 second stabilisation period in each gear, power and frequency output (Hz) from the SRM and the CALRIG were recorded over the range of 50-900 W. For each of the 18 gears selected a relative error term and a calibration factor (slope) were calculated. These data were used to calculate the average (%ERRMEAN=2.5± 5.0%,), minimum (%ERRMIN= -0.7± 5.0%), and maximum (%ERRMAX=5.6± 5.0%) error terms for each SRM crank. These data fall within the specifications of the manufacturer. However the %ERRMEAN was only <2.5% in 9 and <5.0% in 12 of the 19 SRM cranks tested. In four SRM cranks %ERRMEAN was 9-10%. Calibration data from the CALRIG were used to adjust the slope, resulting in a %ERRMEAN <2.5% for all SRM cranks. In summary, the SRM crank is reliable for quantifying cycling power output but accuracy depends on the particular crank purchased and can range from 0-10% measurement error.


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