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July 05, 2015 5 Comments
Yup, it’s spoke tension time again. I hear too many builders saying “I don’t really know if my tension gauge is accurate but it’s easy to use and we do OK.” Ask yourself, what is a measuring instrument without a scale? Or with a scale that is wrong? Not much. Perhaps a waste of time.
If you use a tensiometer, and not everyone does, be sure of its accuracy. A simple in-shop weight test is better than nothing. Hang a bucket of known weight from a spoke and see what your tools say. Mass doesn’t lie. DT, Park, and we offer calibration tests. There is no specified amount of use that recommends you have a tool checked. Under ideal conditions, a simple articulating, spring loaded tool should go forever. However, the world is full of dirt, accidents, and mischief that puts tools into doubt.
Where did this all start? I suspect tension in textile lines (weaving factories, sailboats) were the first to benefit from monitoring. Then came airplanes whose early structures included many wires for structural stiffness and controls. Even today’s planes have wire controls for many flight mechanisms, if only as backup systems. Their tension must be known and a host of tools serve that market. Just search for “aircraft cable tension gauge” and marvel at the number of candidates.
In the late ’70’s, Hozan introduced the first commercial spoke gauges for factories.
In Palo Alto, Jobst Brandt took issue with its design and created an ingenious tool whose fundamentals survive today in the Wheel Fanatyk Tensiometer. The drawback to both was cost, Jobst and Hozan employed a precision dial indicator. In those days, these could only be sourced from high end measurement brands. Jobst used a Starrett gauge in his and it alone would set you back $100. Not much chance for commercialization.
A group of us at Wheelsmith decided to make a less expensive tool so tension awareness could flourish. Then as now, concerns include uniformity (consistency) and absolute (too low or too high) tension. Key in this adventure was Norm Ogle, a renaissance bicycle guy and gifted free thinker. With brother Jon and myself, we started by examining aircraft tools like this simple one by Aircraft Spruce.
Soon we were off to prototypes. One of the first shows the symmetrical shape of the final design, also shared with Jobst’s idea.
Next one held the spring needed to deflect a spoke so as to detect tension level.
By this third model, things were getting clearer. No stainless steel sheet but aluminum plates that would scissor with a spring captured between.
After a few simplifications and clean-up, we have a prototype from which our patent was written.
By the late ’80’s we were in full swing, eventually selling over 15,000 units.
Critical to accuracy is calibration and a Stanford engineering grad student, frame builder, and racer. Eric Topp, created the mathematical system that enables full calibration from a limited number of readings. I use it today just as originally conceived by Eric. The original test fixture employed a Dillon force gauge.
Except for a digital force gauge, today’s fixture is much the same.
Today, I’m announcing a recalibration service for any Wheelsmith tensiometer. Most are still in use, or should be. Charts may be mangled, springs bent or missing, scales peeled off, or just some confidence missing. Send them to us and, for $35 plus return shipping, all can be returned to brand-new accuracy. Here is one (from Bobby Kaufman in Chicago) with its original box and paperwork.
Now it has a new, laminated chart and is ready for decades more of usefulness.
So dig those tools out of your boxes, scoop them up in pawn shops, bid for them on eBay. Then send them for a little blessing and genuine recalibration. The service is described in our web store. The team that created these little marvels of accuracy and simplicity was intensely proud of the achievement. Nothing would please us more than to see them continue to contribute to wheel building. BTW, I can’t think of a more useful, durable, or practical tension tool (well, bias showing…) even after all these years.
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