1

Shimano service instructions for hubs show that out of the four sides, three should be "forward-facing spokes are heads-in", and one, the drive side on the rear wheel, the other way around:

Shimano instructions for spoke direction

What is the reason to specify one way out of the two?

2

With disc or hub brakes the braking forces are the reverse of pedaling forces, so it makes sense to have the spokes reverse from drive side too. Any braking or pedaling torque transmitted through the hub would move the rim to side if the , so it makes sense to build the front wheel wheel which isn't driven with mirror symmetry.

Now, why this orientation? When torque is transmitted from hub to rim, one half of spoken are tensioned and other half slackened. The spokes are arranged so that the slack spokes are on the inside and the tension pushes spokes inwards, away from derailleur or brake caliper.

  • On the last picture, the drive side (marked with "Z") pedaling force would tighten the "backward-facing" spokes. And they are "heads-in", i.e. when straightening under increased tension, they move the third intersection point (not visible on the picture) outward? – Konstantin Shemyak Jan 16 at 19:52
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    @ojs There shouldn't be any 'slacked spokes'. In a well-spoked wheel all the spokes are pre-tensioned to such a high value, e.g. >900N, so that all pedalling or braking forces, or the force of gravity of the rider, only reduce the pre-tension to about half, but never close to or below zero. So there should never be an inward push! The reason the relaxed spokes are mounted heads-in is what is called 'maximal dish': the triangular structure of the hub, spokes and rim that gives both vertical and lateral stability. For a picture, see en.wikipedia.org/wiki/Bicycle_wheel#Dish – mathieu van rijswick Jan 17 at 20:21
  • @mathieuvanrijswick You are absolutely right about the tensions involved. What you missed is that spokes are flexible, and when the spoke on one side of the crossing has three times the tension of the other side, the crossing will move to the side with less tension. – ojs Jan 17 at 21:01
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When you accelerate, the backward going spokes of your rear wheel are put under additional stress; and when you use a disk brake, the forward going spokes are stressed. The lacing guide you cite tries hard to put all the spokes that get these stress peaks on the outside with their heads facing inwards.

Why is this important?
The flange of your hub typically makes a right angle with the axis of the wheel, but the spokes do not. Instead, they angle away towards the center plane of the wheel. This angling of the spokes relative to the hub's flange puts an additional bending force on the spoke's neck. For inner spokes that have their heads going outwards, this bending force adds to the bending force that is caused by pure spoke tension. For the outer spokes, the two bending forces are in opposite direction, partially cancelling each other out. Thus, inner spokes are naturally under higher danger of breaking at the neck than outer spokes.

Putting the spokes which are stressed by accelerating and braking on the outside (head facing inwards) avoids them getting the structural stress that the inner spokes are subject to.


I learned this the hard way, because I used to lace my wheels without considering this effect. At some point, I realized that it was always the inner backwards facing spokes on my rear wheel (stressed in acceleration) that broken when I accelerated hard. Learning from this, I laced my rear wheel with the backward going spokes on the outside, which significantly reduced the frequency of broken spokes.

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