Wheel tension

Discussion in 'Bicycle Mechanics and Repairs' started by Yellow Saddle, 11 Jan 2019.

  1. Yellow Saddle

    Yellow Saddle Veteran

    Loch side.
    In this thread (warning, it may hurt sensitive people), @Milkfloat posted a link to a DT Swiss web page talking about spoke tension.

    This graph grabbed my attention.

    Wheen tension DT SWiss.jpg

    DT Swiss apparently takes a "fingerprint" of each wheel as it comes off the production line. The fingerprint shows the tension, of that specific wheel, in each spoke. Also, in a very apt graph, which helps to understand what's going on.

    This graph is of a 28-spoke wheel and represents the tension in either side. The red shows the tension on one side, the black on the other. As you can see, maximum tension is about 1200N. This could be rear or front wheel, provided, if it is a front wheel, it is a disc wheel, hence the difference between left and right.

    The graph fits in nicely with my postulation that a wheel that's perfectly true cannot have equal spoke tension and conversely, a wheel with equal spoke tension cannot be equally true.

    I used to teach that to students in my wheelbuilding classes. They usually have a question such as, "why don't we just make all spokes equal tension and then the wheel will be true?"

    The answer lies in the rim. It is an extruded rail, bent into a hoop and welded at the one end. Then it is drilled for spokes and drilled for a valve. Extrusions are not homogeneous. Some places have more material, others less. Further, the valve hole makes it weaker in one place and the weld thickens it at the opposite end. This all means that different tensions all around the wheel are required to make it run true.

    If you look at the graph you can infer where the valve is - at position (spoke) 6. The weld, which is always placed opposite the valve hole is at position 13. Strangely enough it is not exactly at the opposite end. That's because the welding doesn't heat the joint perfectly evenly and, there's a lug inside that may not be placed exactly centre inside the joint.

    The other variations are due to extrusion irregularities.

    I love it when I see a great graph that does what it is supposed to do.

    Thanks Milkfloat.
    Drago, JhnBssll, Ajax Bay and 9 others like this.
  2. meta lon

    meta lon Guru

    I true'd a buckled wheel using zip ties on the frame cut to where i wanted the rim.
    Adjusting the spokes while rotating 360° to align it so i had a straight wheel.
    It was perfect.

    So yes i agree all spokes will have different tension.
    JhnBssll likes this.
  3. Ajax Bay

    Ajax Bay Veteran

    East Devon
    Inserting the link for ease of access.
    Thank you for your commentary on the DT Swiss graph. Admit I'm surprised (and informed) by the difference in tensions between one side an the other - and I think this differential suggests that this graph shows a heavily dished rear wheel designed for rim braking (rather than a front wheel with a bit of dishing to accommodate a disc).
    Have they won your trust with this, or do they need to sort the double butted issue out?
  4. OP
    Yellow Saddle

    Yellow Saddle Veteran

    Loch side.
    There is a huge difference in left/right tension on a rear wheel. BTW, all Shimano rear wheels are dished exactly the same and all Campag the same. Campag is dished more. Typically, on a 32-spoke wheel we use tension of 1400 right, to 1000 left. If the spoke count reduces, the problem starts. Often you have to have so much tension on the right side that the nipples gall, yet not enough on the left to prevent nipples coming loose.

    Does one set of graphs give me confidence in DT? Absolutely not. They've just graphed a bunch of measurements without interpreting it. But that's not where I distrust them, it is in the description of the reason they make 2.3mm spokes. Have a look at it for yourself.
  5. rogerzilla

    rogerzilla Guru

    Thinner DT Revolution spokes (2.0/1.5/2.0) on the LH side of a dished wheel help with the "coming loose" problem, although Revs are a pain to build with because they twist so readily. I don't believe in plain gauge spokes on the RH side though, like some (e.g. Hewitt) - I'd just use DT Competition. In fact, I don't like plain gauge spokes anywhere unless no other option is available in that length.

    I suppose more than half the rear wheels I build are actually dishless, because I have a lot of fixie/SS/hub geared bikes. OK, Sturmey-Archer rear wheels aren't totally dishless, but they're very close to being so.
  6. Ajax Bay

    Ajax Bay Veteran

    East Devon
    Why? The spoke tension required on the LH side is going to be the same whatever (DB or not) gauge of spoke, no? More threads engaged since the thinner spokes will elongate more?
  7. rogerzilla

    rogerzilla Guru

    The spokes are indeed strained (elongated) more than the RH spokes, so the spoke is less likely to completely lose tension when you hit a big bump. It's like the difference between a bungee cord and a piece of rope.
  8. Ajax Bay

    Ajax Bay Veteran

    East Devon
    Yesbut; this is how I see it (apologies for my poor analysis if I err):
    On a rear dished wheel the LH spokes are at a lower tension than the RH spokes. Even with thinner spokes on the LH side (and the spokes will be slightly longer) the strain they exhibit will be dependent on the stress (ie the tension) in the spoke. This may or may not result in a strain (elongation) more than in the RH spokes. Hitting a "big bump" will result in an equal drop in tension in the lowest spokes of both sides. The LH spoke is just as "likely to completely lose tension when you hit a big bump" whatever its gauge so will not mitigate any nipple "coming loose" problem.
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