MarkMcM wrote:Max. Tension = Kt * W / N
It appears that you are looking at compressive stress on the rim, but most rims these days are failing at the nipple holes. Some are much more prone to this than others, and I think the alloy used has a lot to do with it. The shape and thickness and contour have an effect too, of course.
Maybe I'm just lucky (knocks on wood and strokes rabbit's foot), but the number of my builds that have suffered from this is zero out of more than a thousand... though I hear stories all the time on the forums. Someone even managed to crack several Kinlin XR270s... though this seems like one of the least likely rims for this to happen to.
So... I think the build details matter also. Things like chamfering the nipple holes, forming the spokes above the nipple, using a marine antiseize, and thorough stress-relieving.
The 340s are very light and still pretty new, so this should give anyone cause to wonder about their durability. Stan's rims are made using 6061 aluminum, which should be a good choice... my favorite for hubs also. It isn't a very strong or exotic alloy, but it more than makes up for this in these applications by having an excellent resistance to stress corrosion cracking.
The people at Stan's seem confused about tensions and spokes and wheelbuilding in general. Even though the tubeless tires can reduce spoke tension by up to 30kg they still claim that the you cannot exceed 95kg on the DS without a tire. Well... if I don't use a tubeless tire, then I'm running 90kg DS and 45kg NDS... and if I do use a tubeless tire, it's 65kg and 32kg. Since no one rides without a tire installed, what sense does that make?
Also CX-Rays are a fine spoke to use... and they are just like any other light spoke. They do not "require or work best with high tension". Compared to 2mm straight gauge spoke, they will be stretched ~80% farther at a particular tension level, which means that much greater deflection can be tolerated before they go slack. Of course they are less stiff as well, but you can deal with that by optimizing your bracing angles or adding more spokes.
Another thing about "max tensions" for rims. The static load isn't the only consideration, because you have some control over the cyclic loading (and unloading) also. With radial forces, the spoke tension decreases *below* static levels. Lateral loads can of course result in a tension increase, but these are not high very often and also vary considerably depending on riding style. Torque seems like the most severe service, especially if you are heavy and/or ride on steep grades often. The pulling spokes can easily see a 30kg increase with every stroke (and the pushing spokes, a corresponding decrease) on typical rear wheels with 12 crossed spokes. You can get a lot of stress cycles if you climb a lot.
Which makes me wonder about alternatives like 1x DS and 2x (or 3x) NDS for rims that might be prone to cracking. There are some issues with using the NDS as the primary side for torque transfer... mostly because the spokes are lightly tensioned and can go slack fairly easily. But they will also see lower max tensions due to torque. A large NDS flange helps... and not too big of an offset. Most hubs (at least the S drivetrain models) can be laced 1x heads-in on the DS and still have derailleur clearance. The Rotax hub looks like it might be a good candidate for this lacing, but I'd have to see the offset dimensions.