Cycling UK Forum

having a 36h hub generator, and some 32h rims that are due to go onto a bike with a 32h IGH in the back, a 32h rim on a 36h hub suddenly had an abnormally great appeal to me.

A quick search of the interweb showed that others had wanted to do this, but mostly hadn't. So I set to and I think I have succeeded, in such a way that I can present clear instructions that will make repeating this only a little bit more awkward than building a conventional wheel, despite the fact that it obviously requires a weird spoke pattern and several lengths of spokes.

The secret is to

a) have a clear plan in mind and
b) to work out the spoke lengths accurately (using fractional crossing values) and
c) to keep careful track of what you are doing as you build the wheel.

It is not very difficult to do this but I would not recommend it as a first wheel build or anything. The hub dynamo (disc compatible) I used had large flanges with the effect of both exaggerating the differences in spoke lengths and necessitating that eight different spoke lengths are notionally required.

In the workings I have used accurate measurements all the way; the time for rounding the values is at the end when you are choosing the spokes and/or cutting them to length. I use letters to identify spoke types/lengths, hence this is 'alphabetti-spoketti'...

This photograph tells most of the story;



It shows a quarter of the spoke pattern (eight spokes), which is (obviously) repeated thrice more around the wheel. There are two unused spoke positions per flange. On the opposite flange, the unused holes are almost opposite, skewed around by 1/36th of a turn. (NB If you have the unused holes set at some other random angle, the spoke lengths won't work out simply.).

There are four different spoke lengths amongst those eight spokes. I have labelled them A,B,C,D. I first worked out the angle error vs how those holes would appear on a 32h flange, and then reworked that into a new 'fractional crossing' value, so that the 32x3 pattern could be suitable adapted with four different spoke lengths per flange.

The hub I used had flanges of 74mm, centreline offsets of 19.5mm and 28.5mm and I used a 32h rim with an ERD of 595mm. A standard 32x3 pattern gave spoke lengths of 284.8mm (left) and 285.6mm (right).

The fractional crossing values and spoke lengths were calculated using a calculator that supports fractional crossing values ( used EDD) and came out as follows

Spoke Crossing Left length Right length

A--------x2.611------279.6mm-----280.3mm
B--------x3.278------288.8mm-----289.5mm
C--------x2.833------282.6mm-----283.3mm
D--------x3.056------285.6mm-----286.4mm

I cut and threaded spokes to the eight exact lengths as best I could (mainly to check the accuracy of my calculations), but in reality most folk would settle for 280mm, 289mm, 283mm, and 286mm. I stashed the different spoke lengths in holes in a piece of labelled card, so that I could keep track of them.

I recommend that you mark the flange holes that are to be unused, and first install all the A and D spokes on both flanges. The valve can go between two A spokes at the rim, i.e. opposite the D locations in the hub flange; in other words you can make one of your A spokes 'the lead spoke'. Then install the inside C and B spokes, and finally the outside C and B spokes. This will mean that you are unlikely to lose track of the build, whilst making for a relatively small amount of time consuming lacing of spokes.

In theory there is a small error that is introduced by the fact that the holes on one flange are not offset by 1/32 of a turn but are instead offset by 1/36 of a turn. In reality this error appears to be lost in the noise; it is probably about the same size error as you would normally get between the effective lengths of inside and outside spokes. Perhaps with flanges about twice the size it would be more significant.

If the spokes face opposite ways from the holes in the flanges (eg leading rather than trailing, and vice versa) then there is a different spoking pattern that requires different fractional crossing values in order to give accurate results, so be sure to copy the pattern carefully.

If building onto a 36h hub with smaller flanges, compute using the same fractional crossing values, and expect to find that the corrections to the normal lengths are very much smaller. You may find that you need even fewer distinct spoke lengths than in the example above.

When I had laced the wheel the way the spokes poked out of the nipples wasn't much less consistent than normal; the wheel trued up in a very normal fashion. With practice I don't think this method would be very much slower than any other wheel build, although there is obviously more time spent preparing the spokes beforehand.

Have fun!

cheers

Well done there.
I tried this some years ago with a 28 hole rim and a 36 hole hub - thought it would be easy, just leave out 4 spokes on each side. I never got beyond the pringle shaped wheel before i admitted defeat.

can you fit a 36H rim onto a 32H hub (and make a strong wheel)

rjb wrote:Well done there.
I tried this some years ago with a 28 hole rim and a 36 hole hub - thought it would be easy, just leave out 4 spokes on each side. I never got beyond the pringle shaped wheel before i admitted defeat.

I've done that one, and it isn't easy because 18 is not divisible by four, thus the unused holes are not uniformly spaced. IIRC I used a 'semi-Spanish' spoking, i.e. with four clusters of three on each flange, plus two odd radial spokes and four unused holes.

fastpedaller wrote:can you fit a 36H rim onto a 32H hub (and make a strong wheel)

yes, if you either

a) drill some extra holes and/or
b) make up some 'endless spokes'

Endless spokes are twice as long as would normally be required, and have nipple threading at both ends. They loop through a hub drilling so that one hub drilling effectively supports two spokes. You need to be careful not to overload the flange. It is easiest to use this approach on small wheels (you can more easily get spokes that are long enough to modify) and hubs with steel flanges. I have spoked a 28h rim onto a 20h hub this way.

cheers

BTW re using a36h small flange hub with a 32h rim. As predicted the spoke lengths vary by considerably less.

As an example a shimano front hub with 38mm dia flanges spaced 72.6mm apart is laced x3 onto a 595mm ERD rim.

The standard x3 length is 291.7mm, using the fractional crossings as before yields

spoke crossing spoke length
A-----x2.611-----289.1mm
B-----x3.278-----293.7mm
C-----x2,833-----290.6mm
D-----x3.056-----292.7mm

which you could perhaps build using just two spoke lengths, eg 290mm and 293mm

cheers

Can one use a 36 hole rim with a 32 hole hub, is it then necessary to block the unused holes in the rim to keep water out or to stop the inner tube protruding?

it is, generally speaking, a much less good idea to use a rim without a full complement of spokes in it, rather than a hub. This is because the spokes are usually evenly spaced in the rim, and that this is done for a reason.

When spokes are not evenly spaced in the rim, they are arranged in clusters which

a) don't violate the angled drilling/offset drilling in the rim and
b) allows each cluster to be 'balanced'

By 'balanced' I mean that the cluster should not put a large net twist into the rim at that point, and that the spoke tensions should each represent the average pretty well. So for example it is OK to have clusters of three (in some rear rims) each consisting of two DS spokes and one NDS spoke, provided that there are net twice as many DS spokes as NDS spokes. It would be a much less good idea to have clusters of three in a wheel that has equal numbers of DS and NDS spokes; each cluster would not be balanced.

Not all rims are anywhere near stiff enough to take clustered spoking anyway.

Rims are consumables, moreso than hubs; with just a few exceptions it makes most sense to use a rim as it was intended to be used.

cheers

Why not just get a 36h rim or am I not the only one being dense?

Ivor Tingting wrote:Why not just get a 36h rim or am I not the only one being dense?

As I explained in my post, the rear wheel is a 32h one anyway in this set, and I have a nicely matched set of rims (which I am unlikely to use elsewhere) and a hub already. This is likely to be an ongoing issue in that some hubs are only readily available in 36 drilling and some new rims (the new Open Pro being a case in point) are only available up to 32 drilling.

In most cases I positively encourage the use of a 32h front wheel with a 36h rear wheel, but not the other way round.
Using a 36h front wheel with a 32h rear in this case would have been possible by buying new bits (and a matching rim would have cost more than another hub generator BTW) , but it just seems perverse....

BTW there is a trivial solution to the problem of building a 32h rim onto a 36h hub which is to use radial spoking. But it is no good for hub brakes or disc brakes.

cheers

The problem I see with this is the chances of spoke tension working very differently when the wheel is in use.
But given all the time it takes for calculating, cutting the spokes, building, tensioning; probably the builder will have enough time for looking after the wheel, it's something that could be done only for personal use.

I've built very many wheels with odd spoking patterns over the years, and none have given the slightest trouble. All that is required is that

a) the net torque from the spoking is balanced on each flange, I.e. so the sum of all the crossing values (taking leading spokes as positive and trailing as negative) is zero, and

b) that the balance between the radial and axial components of the tension in each spoke on each flange does not vary very much, and

c) that the radial component of each spoke tension on each flange is the same.

The latter point means that tangential spokes have more absolute tension in them than spokes with fewer crossings on the same flange, but this is a small effect; typically (unless the flanges are very large) a x3 spoke has a few% more tension than a x2 spoke, and the axial component is not greatly affected.

The easiest way of fulfilling all three requirements with several different lengths in each flange is to make sure that the leading spokes are exactly matched by the trailing spokes, i.e. if you have two spokes + x2.5 and two + x3.2, that you also have two spokes - x2.5 and - x 3.2. This is how the pattern I have used works.

In the pattern I have used there is a small difference only in spoke length and angle between the spokes used and therefore only a small (trivial) variation in spoke tension, and spoke stiffness. [NB If this were truly a problem, Spanish spoking simply would not work, ever... ].

The service (external wheel) loads are not perfectly shared amongst the spokes in such wheels, but then they are not perfectly shared anyway. IME there are considerably larger variations in spoke tension caused by rims that are not perfectly straight and/or sloppy building practice than are caused by the use of this sort of spoking pattern.

Fears that this kind of spoking pattern will in some way result in an inherently unsatisfactory wheel are quite without foundation.

cheers

I've not said that the wheel will be unsatisfactory, but that it could be a wild card in use.
Assuming the wheel is correctly built with an even spoke tension, a hub with fairly narrow flange offset as the one you've had here is not going to give much trouble, as the spokes don't lean much on the hub flange. This problem would obviously be exacerbated on a smaller rim (say 26") and a large flange hub with average flange offset (34-36mm, 100mm OLD front hub), but how many times this combination comes up?

to allay your fears; if you build a 540mm ERD 32h rim onto a 100mm dia +/-35mm hub with a mixture of x2 and x3 spokes there will be less than 7% variation in spoke tension and less than 7% variation in spoke stiffness arising because of the unusual spoking pattern.

To put this into perspective, x2 vs x3 is a much bigger difference than would be used if you build a 32rim onto a 36 hub as I have suggested, and anyway the 7% difference is a smaller effect than if, say, one flange was offset 35mm and the other was offset 32mm. In other words it is a very small tension variation indeed, not worth worrying about and not at all likely to cause any trouble. Hardly a wild card....

cheers

Did you take into account only the ideal angle (along the tangent only) or the relative one (including the offset of the flange)? The 7% could double if made in relation with other spokes working, to me is not an "absolute" value.
Still, it's an interesting concept and let us know how it goes in the long term.

the 7% difference arises because the bracing angles of x2 and x3 spokes are slightly different, because the spoke heads end up in slightly different places (just as they do, trivially, in either side of dished wheels).

In the 32/36 wheel the difference in the bracing angle because of the non-standard pattern is smaller than the difference between x2 and x3 spoking, so the tension variations arising will be less too.

Similar considerations apply to torsional service loads, with likely effects of similar magnitude.

cheers