Black Cat Spoke Tension Calibration Jig
Handsling Bikes wheel builder, Ian Lynch, explains what his new Black Cat jig does.
Our wheels are built near to where the National metre, Kg and atomic clock are housed. These are standards by which all other instruments that measure length, mass(weight) and time can be calibrated. In other words we can only estimate the accuracy of our measuring tools by checking them against something more accurate than they are. At the top of the scale of accuracies are the National Standards.
Spoke tensions in our wheels are measured using a respected (and expensive) digital gauge from Wheelfanatyk. Manufacturers supply such gauges with a table of readings that indicate tensions in Kgf for a range of spoke thicknesses and materials.
Examples above are gauges from Wheelfanatyk, Sapim and DT Swiss, but they all do the same thing. The tension in a spoke is estimated by how much it can be deflected (bent) when loaded with a known force. This known force is applied by a spring in the gauge that is calibrated by the manufacturer.
When a reading is obtained using the gauge, it is cross-referenced on a chart created by the gauges maker to provide a tension in Kgf.
But such tables are necessarily just “snapshots” taken across a range of representative spokes at some point in history.
These tables are good for comparing relative tensions between spokes but translation into absolute tension values in Kgf or Newtons is subject to a number of possible errors. Since readings are dependant upon the force applied by the spring, the tabled values recorded at the time of manufacture may not be the same after the gauge has been used for some time or not well looked after.
Secondly the spoke samples used to create the table of values are “generic” in terms of their materials and dimensions. Different manufacturers may use different materials for the wire stock used to make their spokes and there are dimensional tolerances involved too, factors which can each lead to different deflection readings.
For these reasons our tension gauges are recalibrated regularly and specifically for each new batch of spokes using the gorgeous (and yes, expensive) 6kg of the Black Cat spoke tension meter calibration jig.
At the heart of its chassis is an industrial grade strain gauge mounted on a sliding carriage. The spoke to be measured is attached between the carriage and a fixed anchorage. Rather than taking a deflection reading and seeing what tension a chart says it should be, a range of tensions in 10Kgf increments are applied to the spoke and their resulting deflection value noted. These values are then graphed to interpolate the intermediate values.
If we are building with say Sapim CX-Ray spokes, then it’s a CX-Ray from the stock we are going to use that is measured. If it’s a Pillar Wing20 then it’s a Pillar Wing20 from the current batch that is measured.
We don’t expect, and indeed don’t often see significant variances between batches so why do we do it? The answer is so that we know. We only use quality brands of spokes, but we still don’t presume that each new batch of spokes is going to come from the same factory or is made from exactly the same wire stock. We don’t presume that tooling never wears out, tooling that affects the final dimensions of the spokes after the brutal pounding they receive during manufacture. We don’t like these grey areas. We like to know.
We know and record the deflection readings of every spoke in every wheel we build. We therefore know the actual tension in each spoke of every wheel as it leaves our hands.
It’s good to know.