Brompton rear trailing & leading spokes lengths differ ?

[hoogerbooger]

They seem to ! in which case if you don't have a tension meter like me presumably they'll resonate at a different pitch ?

or am I being a berk?

I have just had a first stab at building a Sturmey Archer SRF5(w) into a wheel for my Brompton. The flange positioning is a little different on the non-drive side to my BWR wheel (& spoke length longer on that side) - BUT one thing that surprised me is that as far as I can see the trailing spoke length ( outside of flange) seems to be longer than the leading spoke length (inside of flange) on the same side of the wheel.

I copied the BWR spoke pattern of 2 cross non-interlaced. Normally to start tensioning from, I'd set all the spoke positions the same by just removing all the showing thread - But it was not possible as the starting point as it immediately became obvious that with the same amount of thread showing the tensions were very different between trailing & leading. Looking at the spoke angles and thinking trigonometry it seems reasonable for a little difference - but having tried to find a starting point by equalising 'wiggle-ability' of all spokes, there seems several millimetres of difference. (If it wasn't a double walled rim I'd get 14 punctures at a time !)

Which gets me back to my current uncertainty. How do I best check if the tensions are suitably equal on each side. Normally (once at tension) I'd check by equalising the tone they made when I tap them with a spare spoke. But I'm guessing that a different length will give a different tone.

I compared with my old BWR wheel (which is on a single walled rim with no difference in length obvious at the nipples !) and the leading and training spokes do indeed largely have a different tone between trailing and leading spokes. But then again I've been told Brompton wheels are noted for often being badly built.


So what does your BWR wheel sound like ! what is the best way without a meter to check tension - or can you spot and explain that I'm being a berk ? ( please be gentle)

( or of course I could just ride it and see if it stays true)

[Brucey]

there is usually a small difference in the lengths of spokes from the inside positions and the outside positions on any one flange. That the spokes are braced partly compensates for this, and also if the wheel is built symmetric (i.e. all the leading spokes are 'inside' spokes and all the outside spokes are trailing spokes) then you may not see that the hub is wound up one or two degrees vs the rim when all the nipples show an equal amount of thread.

However if the wheel is built 'skew' (i.e. inside spokes are leading on one flange and trailing on the other) and the crossings are unbraced (as is usually the case on Brompton wheels) then you will definitely see that the nipples are not showing a uniform amount of thread; all the inside nipples will be slightly further on the spokes than the outside ones.

As you have noted this also allows all the inside spokes to have a different tension to all the outside spokes (whereas by contrast this is impossible with the symmetric build pattern) and because the lengths are fractionally different and the outside spokes may be touching the hub flange (they should be) they won't sound quite the same either.

I prefer to build wheels (especially rear wheels) symmetric rather than skew, but

a) this is not the way recommended by shimano for disc-braked rear wheels (rightly or wrongly) and
b) this is not the way that is regularly advised for beginner wheelbuilders to build wheels.

The skew pattern at least allows you to see that the spoke lengths are about correct when less than half the spokes are in, whereas the symmetric pattern does not, [unless you add some outside spokes to one side of the wheel before you have finished all the inside spokes, which makes lacing the remaining inside spokes on the other flange unnecessarily awkward]. Symmetric builds also require that you are able to wind the hub up against all the inside spokes before you can start with the outside spokes; it often seems like the spokes are the wrong length when you are doing this and novices often lose heart at this stage.

So regarding the spoke lengths; don't worry, it will be fine, and regarding the tensions, if you are really worried about it, have someone check the tensions with a tension meter.

BTW the reason so many Brompton rear wheels fall apart is, I think, that when they are built they are not stress-relieved/bedded enough. And I understand why; stress-relieving wheels with short 13G spokes in is very difficult; I use a hammer to do it and my wheels don't fall apart. Often after stress-relief every spoke goes up another turn on the nipple, which means that if the spokes had settled similarly in service, the spokes would definitely be loose already and the wheel would be merrily falling apart.

cheers

[Gattonero]

...
BTW the reason so many Brompton rear wheels fall apart ...

How do you get this, by seeing it or assumption?
Over thousands of Bromptons, I can recall only a couple with loose spokes, none actually "falling apart".
The new rims with angled drilling seem to have sorted spoke breakage too, haven't seen a broken spoke on a Brompton in a long time now.