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Powertap Garmin calibration check

At the request of Cyclepowermeters the new Garmin firmware updates for the Edge 500 and Edge 800 have changed the powertap calibration screen to show the amount of torque measured by the hub on the screen. This allows you to check the calibration of your hub.

In the Garmin 500 go to

Menu -> Settings -> Bike Settings -> Bike 1 -> ANT+ Power -> Calibrate

In the Garmin 800 go to

Menu -> Spanner -> Bike Settings -> Bike profile -> Bike 1 -> ANT+ Power -> Calibrate

or

Click the power button -> Press the dumbell power icon -> Calibrate

Quick check of your hub

Once you have connected the powertap hub to your garmin (powermeter detected) spin the wheel and check that the display shows speed. Speed reed switch is working.
Now go to the appropriate calibrate page for your garmin detailed above.
***Wait for it to come back with the 'calibration successful' message***
Twist the freehub body or apply load to the pedals if the wheel in still on your bike. Does the Torque value increase as you apply more and more load? If yes then your powertap is probably working correctly. If you are unsure about it's calibration carry out the checks below.

Powertap Calibration

Your powertap will need to be fitted to your bike in order to carry out this calibration check.

Equipment required:

ANT+ Enabled Powertap wheel
Bicycle
Known weight which can be hung from the pedal (at least 10kg preferably 30-40). This needs to be accurately weighed, perhaps take it to your local post office as they should have accurate scales capable of weighing 10- 30kg
Spirit/Bubble level
Garmin 500 or Garmin 800 with the latest firmware
Something to hold your bike while you apply the load

Method

Put the bike into the small chainring and the largest sprocket - this will apply the maximum torque from a given weight to the rear wheel.
Put the bike on a solid work stand or on a turbo between 2 desks/tables/trolleys - somewhere where you can put weights on it safely without it collapsing.
Spin the wheel to switch it on and make sure it is connected to your Garmin.

Lock the rear brake solid - tie a rope or big zip tie around the rear brake lever. Be very careful putting it on the first time in case the brake isnt tight enough and the wheel moves.

Go to the calibrate page on your Garmin as above.
Wait until you get to the message 'calibration successful'
Use the spirit/bubble level to check the crank arm is horizontal then place the weight onto the pedal.
Wait for the Torque reading to stabalise and then write down the following information:

Torque Reading:
Weight applied to the pedal: (in Kg)
Chainring size:
Sprocket size:
Crank Length: (in mm)
The enter all that information into the calculator below to check your hubs calibration.

Weight Applied (kg):
Crank Length (mm):
Chainring Size:
Sprocket Size:
Garmin Torque Reading:

Javascript adapted from David M Osborne davidmosborne@yahoo.com www.dophysio.co.uk

Calculated Torque (nM):  this is based on the information entered above.
Accuracy (%): % This is calculated from  (Garmins torque reading - Calculated Torque)/Calculated Torque * 100.
Powertap hubs should be accurate to +/- 1.5% so you would expect your accuracy to be within these limits.
The precision of this method of checking accuracy is limited primarily by the precision of the powertap hub. It reports torque in units of inch pounds of torque so the highest precision you can get on any calibration check is +/- 2inlb of torque (+/-1 for the unloaded value and +/-1 for the loaded value). If you used a low weight say 5kg and used the 53/11 gear combination the applied torque would be quite low and the precision would be approximately 6% applied load so an accuracy within 7.5% would still be within specification. This is why you should use a LARGE weight and the SMALLEST CHAINRING and LARGEST SPROCKET to maximise the torque reading so the precision has little effect on the calibration check.
Precision (kg):  this is the smallest weight the powertap can measure given the gear ratio and crank length used
Precision as a Percent of the appled load %. Add this percentage to the 1.5% expected accuracy of the powertap, If the accuracy reading above is within this amount then your powertap is working within specification.