Calibration of the Wattbike
The Wattbike is factory calibrated and does not need recalibrating.E
Each Wattbike leaves the factory calibrated to within +/- 2% (>100 W). Our own internal test using an independent HBM Torque Transducer (accurate to within 0.5%) indicate general accuracy of +/- 1.5% (>100W).
The Wattbike measures absolute mechanical power in Watts. The amount of power produced is measured from the sum of all the forces applied to the chain through the cranks.
Despite there being only one load cell the sequencing of the applied force is according to each leg position. The double action of the legs - push with left and pull with right - push with right and pull with left gives the total force applied to chain.
Quantification of the force by each leg and differentiation of the action of every muscle group is possible with two dimensional load cells on each pedal but the cost is beyond the retail price that would be sustainable to get this information.
Magnet sensors
In addition to the load cell on the chain there are two magnet sensors fixed on the cranks to detect top left and right leg position.
The time for 180 degree half turn is measured. The pedal motion has nearly constant velocity, so we can calculate the position in degrees with the time i.e. 180 for 0.5 sec, 1 degree for x seconds etc.
Influence of the air and magnetic resistance
The bike uses a combined magnetic and air resistance to provide a resistance or inertia that the rider works against. Both braking systems increase the inertia on the flywheel axle.
As the power on the chain is measured from the rider input, and all the resistance is applied to the same axle of the bike, the power achieved is not influenced by the creation of resistance from the magnetic climb lever, airbrake gear lever or by environmental conditions.
Influence of the chain
The chain itself can have no influence on measurement as long as it's not rusty and stiff. 76 links 1/2 -1/8 chain is simple to replace (open chain clip). On our test Wattbikes (running with a motor) after 1000 hrs there is no sign of chain wear.
Calibration is the final process before the covers are attached to complete the Wattbike construction.
A very precise process is used to calibrate each Wattbike individually to determine square polynomial coefficients using weights in 10 kg increments up to 100 kg. The production chainring/sprocket/crank set is an integral part of the calibration process.
Each Wattbike has a unique Sensor ID – all calibrated Wattbikes have this Sensor ID number stored in the OneWire memory. This number can be read in the Wattbike Performance Computer together with the individual coefficients (from Main menu go to Setup then Properties).
A calibration file for each Wattbike is saved in a factory central database and the coefficients loaded into the OneWire memory of the Wattbike.
When calibration is complete, the circuit board potentiometers are sealed with an acrylic solution to prevent previously set values changing.
If, for any reason there is a need to reload the calibration coefficients Wattbike will provide the calibration file and instructions of how to reload the coefficients (email info@wattbike.com and quote the Sensor ID number of the Wattbike).
The force measured by the Wattbike is the upward force exerted by the chain on a sensor. In order to elicit this force the chain does not run in a straight line between the chainring and the sprocket, but is passed over the sensor.
As force is applied to the pedals in a clockwise rotation, the chain tightens over the sensor applying an upward force. It is this upward force that is measured.
From the force measured, knowledge of the number of teeth on the chainring and sprocket and the length of the crank, the force exerted at the pedals can be calculated by taking into account the turning moment of the pedal around the centre of the bottom bracket.
During the calibration process weights are ‘hung’ from the Wattbike’s production cranks. The calibration process allows for any deviation between crank length and crank stiffness from Wattbike to Wattbike. The Wattbike cranks are specially designed with limited flex to allow accurate measurement throughout the lifetime of the Wattbike.
By changing any of the crank length, crank stiffness, number of teeth on the chainring or sprocket the calculation of force at the pedal will be affected and, as a consequence results from the Wattbike Performance Computer will be inaccurate.
To test and compare the accuracy of the Wattbike it is important not to replace any of the production parts used in the original calibration process. It is not possible to fit a SRM crank set, or any other crank lengths without factory recalibration of the Wattbike and update of the Wattbike Performance Computer firmware.
With the correct equipment and test protocol the accuracy of the Wattbike can be tested using dynamic calibration rigs or by attaching a torque sensor (such as HBM or Vacumed Calibrator), without affecting any of the production parts.
This may involve considerable cost for test equipment but it is important to apply scientific methodology and rigour to ensure test accuracy. Wattbike are very happy to provide any assistance is required, particularly to coaches and sports scientist that may wish to validate the accuracy of the Wattbike power output and to assist in development of adapters for calibration equipment.
Please see the instructions on returning your Wattbike Performance Computer to Zero State to restore Zero State to your monitor.
To see a comparison of the Wattbike and SRM values please see below:
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The Wattbike is constructed and calibrated by experienced engineers, tampering with the Wattbike invalidates all warranties and Wattbike cannot guarantee accuracy if the Wattbike is taken apart or any of its production parts are replaced with non Wattbike approved materials. |
