March 10, 2016
If your tensiometer grabs a spoke but the spoke thickness is not what’s expected, what happens? No surprise, the tension reading is WRONG. The news is how huge these errors can be.
Why?
All commercial spoke tensiometers read spoke thickness as well as deflection. The ONLY exception is the Wheel Fanatyk (Jobst Brandt) design. For all others (DT, Sapim, Park, Wheelsmith, Pillar, CN, Icetoolz, Hozan, Union, Centrimaster, etc.) spoke thickness is a major part of the reading.
Drawn wire is not as precise as one might assume. 14 gauge, for example, varies commonly from 1.96 – 2.02mm. Swaging (for butting) and forging (for blades) is also imprecise for dimension. Sapim publishes the CX-Ray thickness as 0.9mm. Many would admit this is often 0.95mm. Such variation is within normal spec but wreaks havoc on tensiometer accuracy.
© DTSwiss
Painted spokes are popular. Paint thickness varies all over the map, even within small batches. The chart below shows some careful (but limited quantity) measurements and the reading errors they induce.
For white CX-Rays, batch 1 and 2 were far apart but similar within the batch. Perhaps batch 2 was double coated? This would make for an extra rugged finish, hardly a spoke company error. But attempts to read tension go haywire.
Do not over-analyze this chart
These specific predictions are not the point. It is the principle that needs addressing. Spoke thickness (at the location the tensiometer is working) can be all over the map these days. To avoid huge tension measurement errors you have limited options.
Three options
(1) Rely on your trained sense of appropriate tension (comes with practice) to avoid big mistakes. This is especially effective if you have years of experience, have made many learning mistakes, and are familiar with the components in hand.
(2) Measure the spokes in your wheel before you use your tensiometer. The best way to determine thickness is not a basic vernier caliper. You want 0.01mm accuracy which can be consistently delivered by a micrometer. You need to know average thickness and approximate range.
Inferring correct tension from a measured thickness discrepancy is not so simple. Common sense leads in the right direction but tension vs reading is not linear and knowing the shape of the underlying curve requires a fair bit of math. A simple engineer’s task but daunting for so many tools, spokes, paint, and for each wheel. Certainly limited insight is better than blindness!
(3) Switch to Jobst’s design where NONE of this has ANY effect. Thirty years ago he wanted tension reading free of spoke thickness. With this priority in mind, many possible tensiometer designs are useless. The position of a deflection load, points of contact, and indicator location make a design vulnerable or immune to spoke thickness variation.
It is not a stretch to assume tensiometers of the future, whatever their appearance, will take care to isolate the deflection from thickness. Errors like builders are now suffering are entirely avoidable. There is no associated cost to a correctly designed tension measuring tool.
Anecdote
During NAHBS (2016) I was fortunate to learn about wheel building at Edco in Switzerland from Randy Kilgerman and Rob van Hoek. They use Hozan tools and painted spokes in some wheels and I worried about the possible tension errors as Hozan is vulnerable (like nearly all others) to spoke micro thickness variation.
They assured me, good engineers that they are, this is well understood and they calibrate their tensiometers to the spokes used. Their own calibration “chart” accounts for the painted spokes additional thickness (assuming it is consistent).
How many of you make your own calibration charts for particular examples of painted (and bladed) spokes? Spoke tension is not the end-all in wheel building but it is beyond silly to spend time measuring or discussing spoke tension if accuracy is absent!
