Spokes for a tandem wheel

[Panoramix]

I need to build a new rear wheel for my tandem as it is now a spoke eating machine, I will also change the rim.

At the moment it is a sturdy sun CR 18 rim, 36 heavy gauge plain spokes (not double butted, not sure of the actual diameter), the hub is a 135 mm hub.

I don't really want to cold set the frame at 145mm so would like to stay with a 135mm MTB hub.

I was never happy with this wheel, the only thing that I can think to make it better is to use the biggest double butted spokes I can find  to spread the load better between spokes and tension it fairly well without going OTT.

Can anybody suggest a better approach?

Most spokes have failed on the non drive side at the hub, I need to check but I think it is laced with 3 or may be 4 cross. It was probably initially laced by SJSC (got the bike second hand but it is the initial spec), so presumably well built.

[Ian H]

Spokes breaking on the non-drive side suggests an under-tensioned wheel.  I'd go for DT triple-butted spokes. They're 13 gauge at the hub which makes them a tight fit in the spoke holes*, so better supported. Butted spokes, with a thinner main section, are easier to tension properly than plain gauge.

*time was when hubs were drilled according to the spoke gauge to be used, but no longer.

[Biggsy]

Also, after the build, make sure the spokes are stress relieved by the Brandt method: grabbing pairs of spokes and squeeze together HARD.

[The French Tandem]

I can only agree with Ian and Biggsy. Although it may seems strange, making the spokes thinner in their middle section actually makes a stronger wheels. I would go for DT Alpine III.

If you want to increase the spoke count to 40 or 48 without cold setting your frame there is another option:

1) buy a Shimano HF08 hub, sold as a 145 OLN hub,
2) unscrew the non-drive-side lock not,
3) take off a couple of washers, and make sure these washers add up to 10 mm,
4) re-tighten the locknut,

et voilà! That's what I did for our good old Follis and it worked perfectly well.

[Panoramix]

Thanks, so it will be DT triple butted, for the hub I will try to stay with the same for now.

Finally what's the best lacing, 3 cross?

[The French Tandem]

mtb hubs with small flanges & 36 holes: 3 cross
tandem hubs with large flanges & 48 holes: 4 cross

Never, ever, use radial lacing on the non-drive side, as some people do, unless your hub is specifically designed for that!

[Biggsy]

Unlike with front wheels, tension won't be high enough on the non-drive side for radial lacing to be bad for the hub.  The spokes will be prone to coming loose, though, if not threadlocked.  2 cross is an (arguably) reasonable compromise sometimes used for the NDS.

[The French Tandem]

The spokes will be prone to coming loose

Loose spokes are certainly not something you want  on a tandem wheel!

[Panoramix]

Do you reckon that 2 cross on the NDS will strengthen the wheel?

I think that the wheel died of bearing too much weight more than anything else, certainly not of an excessive power input. So a more radial lacing should improve this, am I right?

[The French Tandem]

I think that the wheel died of bearing too much weight more than anything else

Basic engineering tells us that what makes a spoke break is usually not tension but fatigue, and fatigue
is always a function of the ratio between the max and min load:

-Minimum load occurs when the spoke points toward the road
-Maximum load occurs when the spoke points upward.
-When the spokes are too loose, their minimum load is zero, and the max/min ratio is huge.
-When the spokes are properly tensioned, the min load is not much different from the max load, and the ratio is closer to 1.

Now, the question is: does a 2 cross lacing allows a higher tension than a radial lacing? I tend to say yes, but I'm not sure. The limit on radial lacing is the hub flange resistance. Based on this argument, a 2 or 3 cross lacing should allows for a higher spoke tension before hub failure becomes an issue.

I hope I have not introduced too much confusion!

[Panoramix]

I understand the thing about fatigue,

My thinking is that weight on the wheel creates radial forces whereas torque creates tangential forces so if i want to improve weight bearing characteristics, I should have a more radial lacing. But I am speculating and don't know for sure.

At the moment I am not sure between a 3 cross + 3 cross pattern, 2 cross on the NDS + 3 cross or radial + 3 cross (though not keen on spokes coming loose!). In my mind 2 + 3 is the safer option, but I would like to remove the guesswork out of the equation!

Also, presumably more radial lacing -> harsher ride ? My stoker will probably notice this!

[Biggsy]

Also, presumably more radial lacing -> harsher ride ?

Some people imagine so.

Another practice is to use thinner spokes on the NDS than the DS - but any benefit of this or 2x is marginal*.  You might as well go for the same spokes and 3x on both sides if in doubt.

* I've done both these (on a solo bike) just to save a few grams for fun, and in the hope that it enables higher tension to help stop the spokes undoing.  I'm not sure if it really makes any significant difference.

[Panoramix]

Another practice is to use thinner spokes on the NDS than the DS - but any benefit of this or 2x is marginal*.  You might as well go for the same spokes and 3x on both sides if in doubt.

* I've done both these (on a solo bike) just to save a few grams for fun, and in the hope that it enables higher tension to help stop the spokes undoing.  I'm not sure if it really makes any significant difference.

Weight isn't really an issue, I just want to be able to carry 4 panniers + tent without replacing spokes regularly. So 3X it will be.

[The French Tandem]

Also, presumably more radial lacing -> harsher ride ? My stoker will probably notice this!

It might be MARGINALLY harsher, due to slightly shorter spokes, but I'm not sure I could tell, just by riding it, whether a wheel is radially spoken or not.
Anyway, I can't see how crossing the spokes could reduce the radial bearing capability. It just redistribute the load among a larger number of spokes.
Radial lacing is allegedly a matter of aerodynamics, but I'm not fond of that theory. I suspect its more a matter of aesthetics, and certainly not a matter of load bearing.

[Ian H]

...so if i want to improve weight bearing characteristics, I should have a more radial lacing. But I am speculating and don't know for sure.

Probably only true for a stationary bike.

[mrcharly-YHT]

Radial spoking is a bad idea if you are wanting to build a strong wheel. Higher-spoke tensions help with wheels that need to take high loads.

Radial spoking puts the tension on the hub in the direction with the least metal opposing the tension.

[Panoramix]

Thank you all, so it will be 3 X, DT triple butted and very tight!