Spoke tension isn’t something I’ve spent much time worrying about. Mostly, I ride on wheels that I have built, and mostly those wheels have stayed true and problem free. I haven’t ever had a broken spoke on any wheels I’ve ridden. The vintage wheels I use are usually well built but sometimes need some truing and dishing. So, I’ve never had to re-tension a wheel…until now.
The wheel in question is the Swytch e-bike front motor wheel which I installed about 7 months ago on my Bridgestone (see previous post). A few months ago I began hearing some strange noises from the wheel when braking hard or when going over a bump. It was a noise I recognized as a spoke noise. Spoke noises should not be ignored, as they could indicate a wheel on the verge of failure.
On the roadside after a particularly concerning noise, I flipped the bike over and spun the wheel to see if it was true – it was. Then I began looking at the spokes and found one spoke VERY loose – almost just hanging there. So, I did a quick tightening job with the adjustable wrench I carry and got it up to the same tension as the other spokes, and then rode home. Unfortunately, that fix did not cure the spoke noises. So, once home I again examined the wheel by plucking the spokes and became concerned when noting that the right side spokes were way less tense than the left. It look me a moment to realize that this front wheel is dished. The above photo shows that the flanges of the hub are not centered, to allow for the wheel magnets on the left side motor to have room to spin.
I didn’t have my wheel building tools at home, only a very basic spoke wrench of the wrong size (the spokes are 2.3 mm diameter – meaning a 13 gauge nipple). After squeezing pairs of spokes on each side of the wheel and observing a great deal of movement in the spokes, I realized I needed to up the tension to a minimally acceptable level in order to safely ride the bike over to my shop. Using my adjustable wrench I turned each spoke a quarter turn. I played a “G” note on my phone to aim for a reasonably correct initial tension, based on past wheels I have built. To achieve this I needed another quarter turn on all the spoke nipples. The wheel stayed fairly true, even after another round of “stress relieving”.
(C) Avocet, Jobst Brandt, the Bicycle Wheel, p. 57
What is stress relieving? According to bicycle engineer Jobst Brandt (RIP) in his famed classic, the Bicycle Wheel, “stress relieving is one of the most important operations in wheel building”. He goes on to argue that failure to relieve spoke stress is the most common cause of spoke failure.
To perform this procedure, you place your hands opposite each other on the wheel and grab pairs of parallel spokes, and then squeeze hard, continuing around the wheel until all the spokes have been stretched. If the wheel goes out of true in “two smooth waves” after this it means that spoke tension was too high. Brandt recommends getting spoke tension as high as possible, just short of causing rim failure. This will create the strongest wheel.
While I have always performed the stress relieving procedure on the wheels I have built, I haven’t considered that I should up the spoke tension to the absolute maximum. Instead, I have aimed for a musical note – somewhere between F and G – and also use a Park tension meter and chart to check my work.
When you buy a Park Tension Meter, you are also supplied with their handy tension guide which is organized by type of spoke (round, butted, bladed, etc.) and spoke diameter. So, once I was in the shop I located a correct spoke wrench, then determined what meter reading I was aiming for – the middle range for a 2.3mm spoke – which is a reading of about 27 or 28 on the Park Tension Meter. My initial reading was about 21, which did not even register on Park’s chart, being below the minimum recommended kgf of 54. This was after tightening the spokes 1/2 turn while at home! After continuing 1/4 turns around the rim, I was able to bring the tension up to the mid range of the recommended kgf of about 92, equating to a reading of 27 on the tension meter. After this, I performed the stress relieving operation, then did some minor truing. I checked the dish which remained correct, and then began riding the wheel again.
The wheel seemed fine for awhile but now I am hearing spoke noises again, meaning that the wheel is continuing to de-tension itself. I am going to guess that possible causes include bad spoke threads and low quality nipples. I’ve notified Swytch and they are sending me a new wheel. But will this solve the problem?
Jobst Brandt had a number of strong opinions about machine built wheels, arguing that the wheel building process for machine built wheels is essentially incomplete. An archive of his on-line writings can be found here. I’m going to research this issue further and may end up rebuilding the old wheel with my own choice of spokes and nipples. At a minimum I will perform the stress relieving and re-tensioning procedure on the new wheel to insure that I start out at the correct tension. And, I may consider upping the tension to the higher range on Park’s tension chart, just to be on the safe side.
