headasunder

finally managed to kill the rear rim on our tandem, running tubeless with too high a pressure than specd over a couple of years caused the internals to crack. A new rim has been ordered with a wider internal channel to allow a tyre width that I prefer. I didn't build the existing wheel but I did re tension it up too spec as is was down on tension causing the odd spoke breakage at the nipple non drive side(that seemed to fix that issue). The rim/hub is 32 hole (minimal for a tandem but 32 hole rims offer a wide choice so it is what it is), now the existing wheel was laced 2 cross and I have to say once tensioned to spec I had no issues with it, it rode well and took plenty of hits and even now with a decent crack in the spoke nipple access hole bed it was still true just bulging a bit in that area with the tyre inflated. New spokes need to be ordered to accommodate a different erd so do I just stick with 2 cross is there any benefit to going 3 cross will it result in a more resilient wheel? if so how/why,
hub is dt swiss 350 hybrid
rim DT Swiss GR 531 DB Gravel Rim 700c 24mm ID 32h

CliffordK

A little will depend on your hub. But, for a fairly ordinary low to mid flange hub, I prefer 2x for 32h, and 3x for 36h. I think it keeps the angle at the nipple fairly straight and exits the hub at a good angle.

mpetry912 

32 is a low spoke count for a tandem but for sure gives you access to lots of rims.

The DT rim series are exceptionally strong, damn near indestructible. the DT 440 is a great rim.

I would not run 2 cross on a tandem rear, at least not on the drive side. I'd recommend 3 cross for this application

/markp

headasunder

Please explain why you would run the d/side as 3cross how is this more durable I'm happy to lace it up to what ever pattern as long as it's backed by logic I'm not a prolific wheel builder so inexperienced but I'm fussy when it comes to build process

FBinNY 

Don't overthink this, but try ordering your priorities.

More crosses will increase the torsional rigidity, but is that really an issue when flanges are already large?

OTOH consider the nipple angle at the rim. Generally, I find that having the "parallel" spokes meet the flange more than 4" (or so) apart causes nipples to seat poorly. Rims vary, so you might thread a spoke and nipple into the rim and flex sideways to determine the wiggle room.

Once you know whether or not you have enough wiggle room, you'll have the basis for a judgement call about that extra cross.

If you're still on the fence, consider the marginally improved lateral rigidity of fewer crosses as the tie breaker.

Andrew R Stewart 

I question the rim cracking was caused by, or even helped on by, tire pressure or tubeless sealant. I'd love to see the rim cracks and if they are much the same as the typical nipple hole cracking that many rims have had. I suspect they will look the same. Did the rim have eyelets at the nipple holes? Andy

grumpus

I agree that spoke angle is an important factor - with only 32 of them you need to keep them happy. The other consideration is hub wind-up - for the same reason you wouldn't build a back wheel radial (at least on the drive side) 3X might be better than 2X.

grumpus

Imagine a radial spoked rear wheel; when you apply torque at the hub it has to rotate relative to the rim before it can transfer torque because the spokes are hinged at the hub and flexible - they can't push sideways. Received wisdom is that a spoke tangential to the hub transfers torque best because the applied force is purely tension along its length. I suspect that as long as the wheel is adequately tensioned to resist hub wind up the angles aren't important as the force vectors cancel out and the hub doesn't twist in the wheel - no work is done in a non-useful direction. But I've not put much thought into it - maybe I should the next time I build a wheel, rather than watching TV.

mpetry912 

the 2 cross pattern is not suitable for the torque loads the hub will see on a tandem.

has to do with the spoke departure angle from the hub flange

suggest you use 3 cross. Or build another one with 2 cross and see what happens !

/markp

mpetry912 

When I took Ric Hjertberg's wheel building class, we did this experiment with his spoke tensiometer (which I have)

Spoke tension is reduced by up to 10% on a clincher rim when the tire is inflated ! seems counter intuitive but it is true

/markp

headasunder

FBinNY 

Sorry, but no.

Has nothing to do with the angle. The main consideration is the lever arm length from spoke head to axle ( on centers).

This is the reason it's perfectly OK to reduce crosses with larger flanges.

Andrew R Stewart 

headasunder- Wow! I have to admit I don't think I have ever seen a rim with such a series of cracks. Thanks, it's a good day when you learn something. Andy

Lombard

More importantly, did the rim that cracked say "Mavic" on it?

Lombard

Not counterintuitive at all. The tire acts as a boa constrictor around the wheel and the inward pressure counteracts the tension on the spokes.

Lombard

Yikes! You're lucky that wheel didn't totally collapse on you while riding.

cyccommute 

I’ve had several rims that this has happened to. It’s probably a metallurgical problem and is only tangentially related to tire pressure. I haven’t had it happen in roughly 20 years or more.

The wheel isn’t likely to collapse. In my experience, the problem presented as a sound initiall…. a distinct “creaking” noise. Then the wheel started to go out of true. I could true it but it quickly went back out of true. Eventually the rim wouldn’t true at all. I didn’t remove the tape until the rim wouldn’t true at all. That was when I saw that the rim was cracked from spoke hole to spoke hole. On a double wall rim like this one and the ones I’ve experienced, the wheel will hold together as long as the spoke bed wall is intact. It’s not optimal but the wheel isn’t going to collapse.

FBinNY 

I'm pretty surprised to hear that you've never run into this yet.

This used to be pretty rare, but is becoming less so, and relates to the difference between rim maker design considerations, and changing rider trends.

Makers place a priority on weight, and design the center web around the minimum requirements based on expected outward tire forces. Those are a proportional towidth X pressure.

However, trends are to wider tires at higher pressures, taking spreading forces beyond expectations.

Combine the higher deadload, brittleness of the alloys used, and normal flexing, this becomes more likely.

The reality is that wheelbuilding is an arms race between builder choices, spoke, rim, and tire makers and periodically the balance shifts. Expect rim producers to rethink design parameters, but in the meantime, don't be surprised to see this failure.

IPassGas

Could we see a picture of the top surface where the nipples contact the rim? If no indication of cracks, then the cause is overly high tire pressure for the rim, rather that anything to do with spoke angle/tension, since the nipples make no contact with the outer wall. If the top section is also cracked, the cracks may be initiated in the top portion by nipples and propagated to the lower section.

cyccommute 

My experience is the exact opposite. It used to be fairly common and is now less so.

Again, that is not my experience nor the current trend. Tires are wider but people are using wider rims and much lower pressures than in the past. I, being a luddite, continue to run higher pressures on very narrow rims…one of my bikes has Velocity Deep Vs (13mm internal width) run at 80 to 95 psi and I’m not a little guy. Past rims…like 20 years ago…with similar widths and pressures have seen this kind of cracking but I haven’t had one crack like this since.

Metallurgy and quality control has gotten much better with time. If I recall correctly, the rims that have cracked like this in the past were cheaper rims from kind of off-brand makers. This has never happened to me with Mavic and Velocity. I would not be surprised if this kind of crack is related to the drilling of the rim and/or some flaw in the extruder both of which could give rise to stress risers. Once a crack starts tire pressure and load would quickly cause a cascade failure around the rest of the rim.

Lombard

Don't you mean lower pressures?

FBinNY 

No, I meant what I said.

Over the (many) years, tires have improved considerably, so we now have 2" wide tires with pressure ratings in ranges that used to be high for 1" tires.

When I started riding, tubulars were the ONLY tires one would pump above 75psi. Eventually, wired-on tires broke that barrier, but only for the narrowest road tires at first.

As I said, it's like an arms race, as rim makers have to up their game to keep up with improvements in tires. However, since wheels are durable, and tires are replaced often, rims are at the disadvantage here.

cyccommute 

I’m not sure when you started riding but tires throughout the last close to 50 years have had higher maximum pressure ratings then 75 psi for clinchers. Although I have never ridden tubulars, my understanding was that they were run at even higher pressures than wire-on tires of the day.

I haven’t done an exhaustive survey but of the current 2” tires (45mm)that I can find, they have maximum pressures of ≤65 psi. And almost no one runs them even close to that pressure.

equinoxranch

Time again and again I read of concerns where the problem is as a result of doing things wrong.........NOT building from strength - always trying to cut corners, save "weight", etc.. The results are always the same : Failure. It never ends...................

Andrew R Stewart 

For a brand to be successful it has to appeal to the market. I am ok with saying that the market favors the dreams promised, not keeping the old running. I'll also say that it's in the industry's interest to keep the market looking for that next dream. Andy