Introduction: Torque Test / Calibration Using Bicycle Cranks
This is a simple rig for determining the torque applied through the crank bolt. It uses a known weight hanging from the end of the crank which has known length.
It could be used for calibrating a torque wrench or for training your arm to deliver a certain torque by feel.
Step 1: Secure the Left Crank With a Strong Cable/zip Tie
With the bike on its back, tie the left crank to the frame with a strong cable tie, so that the cranks are horizontal.
Step 2: Fill a Bag With a Known Mass
Fill a bag with a known mass. For this example, 4 cartons of almond milk are used with approximate mass 1.05kg each, which includes the tare weight of the container. So 4.2kg total. The plastic bag weight is around 5g which I considered negligible.
Step 3: Hang the Bag From the Right Pedal
Hang the bag from the right pedal and ensure it is not resting on the bike frame. It's okay to hang it off the end of the pedal to get it clear of the frame.
Calculate the torque required to lift the bag hanging at the end of the crank:
crank length (denoted r for radius in the equation below) = 0.17m
mass (denoted m) = 4.2kg
The bag pulls down on the pedal with a force F = ma = 4.2 x 9.8 = 41.16N
This force acts on the length of the crank to give a torque = rF = 0.17 x 41.16 = 7Nm
When the torque wrench turns the crank bolt clockwise, once it reaches 7Nm of torque the crank holding the bag will begin to lift. To test the torque wrench, set it to 7Nm and check that it trips at the moment the crank begins to lift. Calibrate as required.
With a regular 8mm allen key wrench you can train your arm to deliver a certain torque. Try pushing on the allen key at the very end of the handle and then again halfway down the handle, noting the different force required from your arm. Try filling the bag with more weight to increase the torque. Up to 28Nm should be pretty safe for these bolts, provided your cable tie is strong enough tying the left crank to the frame.
The accuracy of the rig depends on the accuracy of measuring the mass. It also depends on any friction which resists turning the cranks. Ideally there would be no chain connected. The bottom bracket bearings need to be spinning freely with negligible resistance, as most sealed units do.
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