The latest issue of Bicycle Retailer & Industry News (BRAIN) carried a blurb about the FSA tension gauge (sold here). Generous credit was given to its designer, Jobst Brandt. How exciting that this unique tool and its interesting history receives some press! Thanks to BRAIN’s sage and inquisitive technical editor, Matt Wiebe.
World's Lightest Tension Gauge
First, on Jobst’s behalf, while he is indeed the inventor of this device, and authorized the FSA version, he didn’t sign off on every detail. It’s a Full Speed Ahead product. Some minor specs were changed, from materials to a different model of dial indicator. The result is a beautiful tool with outstanding function. Hats off to Jobst for the original concept and bravo to FSA for giving it a go.
Let’s cover some of the tool’s details for all spoke tension nerds out there. It was created by Jobst about 1975 while he was still an engineer with HP. Jobst is the legendary author of The Bicycle Wheel, a pioneering work that unraveled the mystery of tensioned bicycle wheels for the first time. Translated into five languages and reprinted numerous times, it remains the seminal work on the physics of bicycle wheels.
This tool has some distinct advantages over others and I say this with some experience as the co-inventor of the Wheelsmith Tensiometer. The Wheelsmith tool is a practical, rugged little unit, but the only reason we made it was because the Jobst design was much more expensive and appreciation for spoke tension was too undeveloped in the 1970’s to support the price.
Both Jobst and I share a commitment to popularize and demystify the role of spoke tension. My approach was to introduce as inexpensive a tool as possible. Jobst’s approach was to make his design available without license to anyone. From day one, he gladly provided drawings and assembly instructions for free. At Wheelsmith, we kept copies of these materials and handed them to anyone who asked. Avocet, DT, and numerous do-it-yourselfers produced it. Ultimately, the cheaper Wheelsmith tool proved more effective at popularizing tension measurement and the Jobst design remained uncommercialized, sinking into obscurity.
FSA has showed the initiative and manufactured this beauty for lovers of bicycle wheels and the pursuit of accuracy and quality.
(1) Spoke thickness does not affect the reading. You put the tool onto the spoke, zero the dial indicator with your hand, gently release the spring "hammer" to bend the spoke, and then read the indicator. No matter how thick the spoke, the indicator ONLY shows deflection. All other tension gauges (Park, DT, Hozan, Wheelsmith) require long charts because the dial readings = spoke deflection + spoke thickness. Each column subtracts the spoke thickness. Many columns = slow to use.
One handed use
(2) The deflection spring is weak so spoke bending is very small (at 100 kgf, about 0.3 mm deflection), less than any other tension gauge. When spoke deflection is very small, you are closer to reading only tension -- the influence of spoke material stiffness and mass is diminished. The stronger the spring (larger deflection) the greater the influence of spoke material. For this reason, the FSA gauge has accuracy on many materials (steel, aluminum, Ti, composite). You can measure tension on Lightweight, Topolino, or Mavic Ksyrium wheels. The new Mavic R-Sys spokes are simply too stiff, being tubular, to be measured with deflection.
Hurray. Here’s a tool that, for all practical purposes, needs only one deflection-to-tension conversion column (not 18). Plus, it can effectively read the widest range of spokes. With today’s variety of spoke brands, models, and materials this becomes an big advantage.
Absolute accuracy is listed at +/-10%, because it’s unrealistic to pretend higher accuracy is possible. Here's why:
a) Although great care is taken in creating spoke wire, the diameter is not very consistent. For instance, butted spokes are made by by swaging. The center section can vary by 5% as a consequence of the shaping process. Without exact dimensions, tension measurements will vary.
b) A spoke’s path, from hub to rim, is not perfectly straight. Due to the lacing pattern, spokes often have a compound bend. This bend affects measurement. Moving a tension gauge over a spoke shows how the reading changes.
c) Where the tension gauge is placed affects the reading. A spoke is stiffer at its ends. The gauge should be centered between the rim and the spoke’s first cross (if any). Stay on a constant section of a butted or oval spoke.
So, even if a tension gauge were theoretically accurate to within 1%, that precision would be lost in due to the above factors. The solution is to optimize consistency, accuracy from spoke to spoke. The FSA tool is extremely consistent due to careful tolerances and a very high quality dial indicator (with 5 Å repeatability).
(1) Full jeweled dial movement; shock, dust, and water proof.
(2) ISO and JIS certified indicator.
(3) Precision ball bearing spoke rollers.
(4) Handsome, durable, and low friction carbon fiber main plate. Easy to handle and use.
(5) Each tool is calibrated and serially numbered.
(6) Accuracy is guaranteed for 10 years.
Here is the calibration fixture I keep at my workshop to be certain every tool meets specs. Various spokes can be substituted and tension varied, so each tool can be fully evaluated.
Less friction in the tool is better. Try putting a drop of light machine oil or dry graphite lubricant on the handle plunger and the hammer guide slots. The tool should exhibit no stickiness.
For easy, one handed use, it should be possible to zero the dial gauge with just your thumb. If your tool has too much friction for that, try a drop or two of oil just below the dial cap’s external knurling. Rotate the dial back and forth, notice it turns more freely.
What, if any, are this tool’s limitations? No tool is perfect, each is a compromise owing to the priorities of the designer. In this tool’s case, freedom from multi-column charts and ability to measure many spoke dimensions and materials are uppermost. As explained above, this led to a weak spring, small deflections, and a super-accurate dial. However, for higher and higher tensions, the deflection (already small) becomes tiny. This means accuracy at extreme tensions is sacrificed.
I believe that spoke tension approaching 150 kgf are excessive, but some brands claim them as features. To measure tensions that high with accuracy, a strong spring must be employed so the deflections are large; diminishing the tool’s ability to see past materials and gauge. So, you have to make your choice as a designer or purchaser of a tension gauge.
If you value beauty and the ability to measure many spoke types, than the FSA tension gauge is for you. Got ideas? Please send them along.