Ya’ know how whenever you start to do something, you have to do something else first, and then you forget what it is you were going to do? Happened to me.
In this week’s post, I was going to use spoke tension, and specifically change in spoke tension, to prove that vertical flexibility of bicycle wheels is for all intents and purposes a non-issue. My plan involved measuring spoke tension on a loaded and unloaded wheel with my Park TM-1 Spoke Tension Meter.
So I figured it would be fun to calibrate my tension meter before I started, and that’s where I got sidetracked. I hung a spoke from the ceiling, with a bucket full of dumbbells below it. I knew the weight hanging from the spoke – the tension – was 118 lbs, because I weighed each dumbbell, and the bucket, with my Park DS-1 Digital Scale. My dumbbells, by the way, were all bang on their stated weight within an ounce.
Here is my tension measuring jig – a spoke hanging on a cord from the ceiling with a bucket full of dumbbells suspended a few inches off the floor. My spoke tension meter is the Park Blue thingie mid-picture.
I proceeded to measure the tension in my hanging spoke, and instead of 118 lbs, I measured 167 lbs! Wow, could my calibration be more than 40% off?
A distraction within a distraction here: I checked the diameter of my 2mm nominal round spoke. It was 2.01mm, a 1% difference in cross-sectional area.
Time to read the TM-1 instructions: “Return to Park Tool for recalibration.” Really? I have a spoke with a known tension. I can calibrate this thing. It’s a very simple device, right?
It turns out measuring tension in a spoke is not that simple, and it’s been driving me nuts. The TM-1 spoke tension meter measures tension by displacing the spoke laterally a small amount (a few millimeters) between two points 100 millimeters apart. A spring provides a (fairly) constant force, so the amount of displacement is inversely proportional to the tension in the spoke. But it’s not a linear relationship, and it depends a lot on how the tension grows with the lateral displacement.
In my test rig, the bucket of weights is lifted a small amount due to the geometry imposed by the lateral displacement. The angles involved are miniscule (less than 2 degrees). So it seems to me that the tension in my test spoke remains essentially constant throughout the measurement (unless friction at the TM-1 contact points isolates the mid-section of the spoke during the measurement – hhmmm?)
A spoke installed in a bicycle wheel is constrained within a complex system comprised of the rim, the hub, and all the other spokes. It’s not at all clear to me how spoke tension responds to lateral displacement during measurement. I can’t help but think it grows faster than it does in my test rig. This is what has been baffling me about the 40% error in my calibration. I would have expected the error to be in the other direction.
Be that as it may, tool calibration must take tension growth response into account. Below is a graph of the Park TM-1 Spoke Tension Meter response curve for a 2mm round spoke.
Tension in Kg on vertical axis.
TM-1 reading on horizontal axis. (Smaller numbers = larger displacement.)
The very competent people at Park, with a bigger research budget than I, have surely applied some experimentally derived calibration factors into their response curves. Did I mention that there are different response curves for spokes of different dimensions and materials.
Should I worry if my spoke tensions are off because of a mis-calibrated tension meter? If I were building wheels from scratch, definitely. If I am truing wheels and trying to maintain uniform relative spoke tension, not so much. In the latter case, resolution is more important than accuracy. If I measure five spokes that all really have the same tension, I better get near the same answer on every spoke. A spoke with 20% more tension had better read about 20% higher on the TM-1.
Now I’ve blown another Saturday afternoon and I still haven’t addressed vertical wheel flexibility. Maybe next week – unless I decide to send in my TM-1 for calibration.