Author Topic: Building a 40h hub into a 36h rim  (Read 665 times)

Building a 40h hub into a 36h rim
« on: November 24, 2019, 01:27:18 pm »
Sheldon Brown seems to say that it's possible so I am thinking of trying. The more usual situation would be a freewheel hub, which would allow the missing spokes to be all on the nds where the tension is lower (and presumably the stress less). I am thinking of doing it with a steel SA AW hub which will be virtually dishless so the same conditions don't apply. I think that I would go for one spoke in each orientation (one in, one out on each flange). The spoke lengths should vary in a maximum range of about 2mm as far as a rough play with a calculator would appear to indicate.

Has anyone on here done this already? Any ideas on how best to orientate the missing spokes (face to face, 180°, 90°)? Any advice at all? (including don't bother - but, even though I know SJS have a suitable 40h rim available, while I have the hub, the rim and probably the spokes I fancy having a go just for the intellectual challenge  8) )

Even if I succeed I don't exclude the possibility of undoing it all to build with the correct rim later!

LittleWheelsandBig

  • Whimsy Rider
Re: Building a 40h hub into a 36h rim
« Reply #1 on: November 24, 2019, 03:00:10 pm »
The few times I've done the same thing, I built the wheels with empty spoke holes at 180 degrees around the flange and each flange aligned as much as practicable. Mostly for aesthetics but it didn't seem to weaken the wheels. They were SA hubs, so not much offset to worry about.

Whenever I've built up 40 hole derailleur hubs, I used 40 hole rims, occasionally home drilled (harder to do well than it looks).
Wheel meet again, don't know where, don't know when...

Re: Building a 40h hub into a 36h rim
« Reply #2 on: November 24, 2019, 03:55:05 pm »
36h rim onto 40h hub is one of the worst ones you can do if you go for full spoke crossings on both flanges. I think you may need nine different lengths of spoke on each flange (which may boil down to about five different lengths if you are prepared to put up with an error of ~1mm or so).

It is much, much easier to build a 32h rim onto a 40h hub; you only need two different spoke lengths to do it.

I have documented my approach to building odd combinations here

https://forum.cyclinguk.org/viewtopic.php?f=5&t=118718

It really is not difficult to work out the spoke lengths provided

a) you have a spoke calculator that supports fractional crossing values and
b) you have a good plan and
c) you know what fractional crossing values are needed.

 although I've built several, I have not listed the fractional spoke crossing values for 36rim/40hub in the link above.

If I get a few spare minutes I'll do that.

cheers

Re: Building a 40h hub into a 36h rim
« Reply #3 on: November 24, 2019, 03:56:04 pm »
Re-shell the hub!  It's easy and cheap.
Never tell me the odds.

Re: Building a 40h hub into a 36h rim
« Reply #4 on: November 24, 2019, 04:55:37 pm »
Re-shell the hub!  It's easy and cheap.

If that was the option I would just go for the new 40h rim or stick with the 28h wheel that I have already - but aesthetics indicate a 28h front which I don't have, it doesn't look very vintage and my 40h hub is an old one (1957 vintage I think - or 1959, can't remember which). Could do 32h front and back with GEL 280 sprint rims (which would be different) but I feel like trying the hard route. Added to which I live in a country bereft of SA bits and pieces!  :'(
36h rim onto 40h hub is one of the worst ones you can do if you go for full spoke crossings on both flanges. I think you may need nine different lengths of spoke on each flange (which may boil down to about five different lengths if you are prepared to put up with an error of ~1mm or so).

It is much, much easier to build a 32h rim onto a 40h hub; you only need two different spoke lengths to do it.

I have documented my approach to building odd combinations here

https://forum.cyclinguk.org/viewtopic.php?f=5&t=118718

It really is not difficult to work out the spoke lengths provided

a) you have a spoke calculator that supports fractional crossing values and
b) you have a good plan and
c) you know what fractional crossing values are needed.

 although I've built several, I have not listed the fractional spoke crossing values for 36rim/40hub in the link above.

If I get a few spare minutes I'll do that.

cheers

9 lengths of spoke and only 9 holes in use = a different length for every spoke. Given that spokes are sold in 2mm increments I think that means pulling it up round and grinding the sticking out bits in my case. Oh well I am using a single wall rim so the grinding stage is not too difficult. I will read your article (which I think I have already seen). I think the macro for spocalc handles fractional values so if you feel like working them out I would be very grateful.
No I don't have a spoke thread roller (sometimes I wish I did!)

Re: Building a 40h hub into a 36h rim
« Reply #5 on: November 24, 2019, 07:08:19 pm »
well I've done it, and it made my brain hurt. No wonder I hadn't bothered to write it down before....

FWIW I use EDD (Lenni) and it handles fractional spoke crossings just fine.  If you are prepared to tolerate spokes being +/-1mm then you can get away with using just five different lengths when building an SA 3s hub.

cheers

Re: Building a 40h hub into a 36h rim
« Reply #6 on: November 24, 2019, 07:24:51 pm »
Sheldon Brown seems to say that it's possible so I am thinking of trying. The more usual situation would be a freewheel hub, which would allow the missing spokes to be all on the nds where the tension is lower (and presumably the stress less)…..


Using a standard rim, it is very difficult to build a good wheel with a different number of spokes in the DS and NDS. The reason is that the rim is usually drilled with a stagger to the spoke holes, and of course you can't avoid 'using the wrong stagger' in many of the holes.  This leads to bad nipple kinks and short spoke life.

A few rims are drilled with no stagger at all and you can do what you like with these, which means you are just left with a (worse than normal) job of calculating the DS spoke lengths.

FWIW with an unstaggered rim you can build a dished  36 spoke 2:1 wheel (should be good on a touring bike) by usinga 48h hub, or by drilling an extra 12 holes in the DS flange of a 24 spoke hub, or by drilling an extra 6 holes in a 36h hub. 

All sorts of fun and games to be had... but it is probably easier (and better) to find a way of building the wheel with less dish in the first place TBH.

cheers

Re: Building a 40h hub into a 36h rim
« Reply #7 on: November 24, 2019, 08:39:00 pm »
The more I look at this the more I think that using the correct 40h rim is where it will end. I was thinking in terms of X2 (or even X1, which I have used on a tandem wheel, but with a very tall hub). I have been looking around again and building on 32h rims might be more sensible than on 36 (especially since I have a pair, but that also means building a new front wheel). I will do some drawing and calculating and see what comes out. BTW Brucey could you post your 40/36h calculations please (or PM). I might very well need them!

Re: Building a 40h hub into a 36h rim
« Reply #8 on: November 24, 2019, 09:43:19 pm »
I've added them to the link above.

cheers

guidon

  • formerly known as cyclone
Re: Building a 40h hub into a 36h rim
« Reply #9 on: November 24, 2019, 09:45:01 pm »
Spend the money Jo! Much less grief in the long run  :smug:

Re: Building a 40h hub into a 36h rim
« Reply #10 on: November 25, 2019, 11:54:20 am »
Spend the money Jo! Much less grief in the long run  :smug:

But it's been a while since I have had a decent chance to extend the learning curve. Anyway no progress without grief; il faut spéculer pour accumuler as they say at the PMU  ;D

S2L

Re: Building a 40h hub into a 36h rim
« Reply #11 on: November 25, 2019, 12:19:37 pm »
There is no good way of doing this. Get a 40 holes rim or a 36 holes hub, whichever is more convenient.

Re: Building a 40h hub into a 36h rim
« Reply #12 on: November 25, 2019, 12:31:13 pm »
All you need is a 36h hub shell.  A modern SRF3 alloy one would be an improvement for spoke elbow support, although ISTR that it's not significantly lighter than an AW steel shell (the old 1950s SA alloy shells are very light, but a little fragile).

All A-series hubs*, and the SRF3, have the same shell dimensions.  The LH ratchet can vary in the number of teeth, but is compatible.

*except the weird-ass ASC, which has a different LH ball cup
Never tell me the odds.

Re: Building a 40h hub into a 36h rim
« Reply #13 on: November 25, 2019, 01:00:27 pm »
There is no good way of doing this...

there is. See the link above.

Getting a replacement hubshell is an easy way of making the job straightforward.  A modern SRF3 hubshell will work but (like all since about 1987) will have a 20T ratchet ring in the LH end of the hubshell.  Older hubshells have a 10T ring and have more take-up lash in low gear, but are also less likely to engage just one pawl and cause problems that way.

SRF3 shells have no lubricator fitted, so if you want to run the hub in oil then you need to fit one or oil it via the axle. They also vary in what size/shape upstand there is in the LH end of the hubshell, which makes a difference to how water (getting in)  and oil (getting out) are handled.

If you fit another hubshell it will look you are running a hub that type/age, which you may or may not want.

Building a 36h rim onto a 40h hubshell isn't that difficult and it will work fine but it is a bit of a challenge by comparison with building a standard wheel.

cheers

S2L

Re: Building a 40h hub into a 36h rim
« Reply #14 on: November 25, 2019, 01:12:45 pm »
I might even have a 36 h hubshell for threaded freewheels somewhere... unless it ended up in the metal recycling bin when we moved last summer, which is entirely possible

Re: Building a 40h hub into a 36h rim
« Reply #15 on: November 25, 2019, 08:43:21 pm »
The driver would be threaded...and it's for a track sprocket, not a freewheel.  A freewheel would be a bit pointless since the hub already has one.

Threaded drivers are quite rare and definitely worth keeping safe if the bearing surface is unpitted (which it usually will be; they're well sealed).  One of my AM-equipped bikes has an 18T Surly track sprocket.
Never tell me the odds.

Re: Building a 40h hub into a 36h rim
« Reply #16 on: November 25, 2019, 09:29:04 pm »
The end result of all this will undoubtedly be a conventional wheel built on a 40h rim, which is a much easier solution than swopping hubshells (and I want to keep my late 50s hubshell anyway and am in the middle of France so tracking down a vintage 36h hubshell is likely to be a right PITA). Before I get to that stage I will undoubtedly try building onto first a 32h rim and then a 36h for the intellectual challenge the exercice may present. I will probably order the 40h rim fairly soon while they are still available. It's one of these (shallower section than the Velocity that I have seen on other machines on the web) https://www.sjscycles.co.uk/rims-700c-29er-622/?holes=40

There is another small point. The hub would have to be in a wheel before I could get the hub internals out to put them in another hubshell (so even if a 36h hubshell was available I would still have to find a wheelbuilding solution for the 40h hub first).

Re: Building a 40h hub into a 36h rim
« Reply #17 on: November 25, 2019, 10:02:16 pm »
I have had good luck with releasing the centre from loose SA hubs; there is enough inertia in the hub that using a hammer and drift is usually effective if you go about it in the right way.

The method I use is to put the hub on something softish (like a piece of wood) and to stand with one foot on it, such that the hub will 'jam' into a wedge shape formed by your foot and the piece of wood when twisted. Then (using a hammer and drift) give the ball ring a couple of smart hits. Then turn the hub 180 degrees and do the same in the other ball-ring notch.  Repeat as necessary.

This works surprisingly well, better than I expected.  I think it works OK because

a) It is a twin-start thread so doesn't ever get as tight as a single-start thread would
b) there is always a radial clearance on the thread
c) the hubshell itself is slightly flexible

these things usually cause the ball ring to come free after a few goes.

I have always thought that it would be possible to make a jig to unscrew internals from loose hubs by arranging a corner to trap the hubshell in (eg by screwing/glueing chunks of wood together) and if necessary adding a few spokes in critical places so that the hub cannot turn.  However I have not yet had to do this.

I can report that I have had most trouble with removing internals from 'bad wheels' in which the spokes have gone very slack; using a hammer and drift the hub just deflects, and trying to use  spanner on the ball ring usually results in the wheel threatening to implode before the ball ring comes undone. In such cases it is probably best if the edge of the ball ring is supported (eg on a piece of wood) so that the hammer and drift can be used to better advantage.

cheers

Re: Building a 40h hub into a 36h rim
« Reply #18 on: November 26, 2019, 09:32:41 am »
I can remove them even if the hub isn't built into a wheel.  It does mark the cutouts in the ball ring a little.
Never tell me the odds.

Re: Building a 40h hub into a 36h rim
« Reply #19 on: November 29, 2019, 07:30:51 pm »
Thank you Brucey for those calculations. I hadn't seen that there were three pages to that post so I missed it first time round.

 Now after a free afternoon reading and calculating I even have a third option, 28rim on 36h hub with crow's foot spoking (I always fancied the idea of having a crow's foot wheel; a bit different in a world of low spoke count factory wheels). It all depends on what length spokes I have in the garage.