8

I built a wheel for the first time recently, so I checked it after a few hundred km. The spokes seemed to all have lower tension (by pitch) than when I built the wheel. I tightened them all about 1/4 turn (except one that needed more) and had to slightly retrue one section of the wheel.

I assume this even loosening was due to the spokes stretching, but do they stretch significantly after the wheel is trued? Should I build it to a slightly higher tension/pitch than I expect to run long term?

4
  • Just out of curiosity, which hub was it? Of all the wheels I've built I've had this happen on two early 2000s Campagnolo front hubs and and nothing else.
    – ojs
    Oct 9, 2019 at 8:05
  • @ojs it was a rebuild on an SP-PD8 dynamo hub, using new spokes and a lightly used rim. My crash in June wrecked a rim (I hit a rock hard enough to put the rim through not just the tube but a Marathon Supreme) but I had a spare
    – Chris H
    Oct 9, 2019 at 11:07
  • 4
    When spokes are initially installed they need to "work in". This involves "seating" in the hub holes, developing a bend where they cross, and simply straightening, from the slight curve they often have on delivery. A skilled wheel builder will "stress" the wheel in various ways to accelerate this process. Alas, many factory-built wheels are not pre-stressed sufficiently and rather quickly develop problems. Oct 9, 2019 at 11:55
  • @DanielRHicks it sounds like this novice wheel-builder didn't stress the wheel enough - but I suspected that might be the case and checked it. I might not have helped myself by building the wheel when I was still unable to ride, so it sat built but not test-ridden for a few weeks when a quick test-ride and check might have freed things up nicely.
    – Chris H
    Oct 9, 2019 at 12:04

2 Answers 2

6

They don't. Spoke stretch is a very theoretical effect that mainly happens when tensioning loose spokes or over seriously long term usage.

What you have experienced here is most likely due to the spokes finding their final place and form, possibly combined with twisted spokes working their ways straight. These are common and expected things especially when using the J-bend spokes, but they can be migitated, although not by over-torquing the spokes as you suggest.

New J-bend tend to leave the hub at an angle parallel to the hub flange, but obviously they'll end up with a bit of an inwards angle to meet the rim. Before tensioning, when all the spokes are in place, you can gently push each of the outer spokes inwards where they leave the hub, thus deforming the spoke a bit and causing it to have the proper angle even before tension is applied. If you don't, some of the tension may be lost afterwards when the tension itself causes this deformation.

Additionally, spokes may twist when tensioning. This applies to all spokes, but is of course harder to spot with round ones. What you need to do is lubricate the spoke threads beforehand, try and hold the spokes with appropriate pliers when tensioning them, and most importantly, you should try to release this torsional tension during the wheelbuilding process so you can re-check and adjust accordingly.

Releasing this tension happens by grabbing two spokes where they cross and squeezing them together, or by grabbing the rim at opposite sides and pushing the hub/axle against something. You'll hear slight 'pings' when it happens. Reiterate for both sides of the wheel, on multiple different spots on the rim.

5
  • 1
    Also keep in mind that comparing spoke tensions by the pitch may work for spokes of equal length during the same wheelbuilding session, there's very little point and quite a lot of room for human error when comparing a current pitch to one you heard a couple hundred kilometers ago.
    – Wsal
    Oct 9, 2019 at 8:33
  • wrt your comment I'm not very good at pitch, so on both occasions I was comparing to the back wheel non-drive side. As built: slightly higher; a few hundred km later: slightly lower, though I did actually try a spectrum analyser on my phone.
    – Chris H
    Oct 9, 2019 at 11:03
  • It was new spokes on an old hub and rim (same lacing pattern), and I tried to relieve the twist both by squeezing and at the end by riding 100m and checking the wheel was still true/tight. So the major factor was probably the J-bends
    – Chris H
    Oct 9, 2019 at 11:05
  • Note that machine-built wheels use spokes with a gentler bend, to move more smoothly through the machines. This bend tends to deform over time, causing spoke stretch. Some custom wheel builders use special spokes, while others use small washers on the standard spokes, to avoid this problem. Oct 9, 2019 at 11:59
  • @DanielRHicks these were Sapim Race so a popular spoke for hand-built wheels. No washers and I haven't compared the bend to other spokes. The previous wheel on this hub was also hand-built, but by a pro
    – Chris H
    Oct 9, 2019 at 12:09
5

It could be caused by the tyre, assuming your first tension measurement was immediately after completing the wheel and before fitting the tyre.

An inflated tyre will compress the rim and relieve some of the tension in the spokes. Measuring the tension before and after fitting and inflating a tyre can give readings that differ by as much as 100 N. The effect is more pronounced (I believe) in tubeless setups because the beads of the tyre are so damn tight.

Of course, as soon as you remove the tyre, the tensions will return to what they were when you built the wheel.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.