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Old 04-16-19, 09:46 PM
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Join Date: Apr 2005
Location: Northern CA
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Bikes: Cannondale tandems: '92 Road, '97 Mtn. Mongoose 10.9 Ti, Kelly Deluxe, Tommaso Chorus, Cdale MT2000, Schwinn Deluxe Cruiser, Torker Unicycle, among others.

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Originally Posted by reburns View Post
I should reiterate that Spinergy exceeded my expectations by replacing my rim gratis a year after the warranty expired. They immediately sent me a new rim at my request as I felt like that would be simpler and quicker than sending the wheel to them for repair. Replacing the rim was painless. I zip tied the new rim next to the old one and moved the spokes over in the truing stand. It trued up perfectly with the two sides within 3% for tension, and all individual spokes within less than 10% of average. So the rim started out very round and true before mounting.

I tensioned the spokes to around 100 kgf. Spinergy recommends closer to 120, although the wheels were at more like 80 before I started messing with them and I donít think they changed from the factory as they always ran pretty true. Spinergy cautions against over tensioning because the spokes are more flexible than steel and will over-tension the rim if done by feel. Clearly, too much tension could cause cracks around the spoke holes, but I suppose that having spoke tension too low could allow the rim to flex too much, also resulting in fractures...?

Given our team weight plus bike plus cargo, and our weight distribution, the rear wheel static load could easily be 100 kg. We do frequently climb short grades upwards of 15%, either sitting in a really low gear (26/34) or sometimes out of the saddle in a gear as low as 39/34 for short bursts. Those are probably the times that the rear spokes see the greatest tension, other than hitting potholes at downhill speeds. We also get on unpaved surfaces occasionally, but rarely. Our hub drum brake puts less strain on the wheel than a disc.

You see a lot of these wheels at tandem gatherings. They are comfortable and light. I plan to keep an eye on the rim and monitor tension occasionally, and hopefully get more miles and years from the wheels.
You're a light team. And hub windup during sprints or climbs adds very little to overall and individual spoke tensions from what I've read on the issue. And that's pedaling forces. Disc braking forces on the front wheel will cause more windup tension spikes than pedaling, but I think it's also inconsequential. There's no way a wheel with spokes at 80kgf will come anywhere NEAR max tension while riding on a tandem, steep climbing with low gearing or under hard disc braking or not. It simply is not an issue. (Now, high torque loads blowing up rear hubs ARE a problem! But that's another issue altogether.)

I'm thinking the low spoke tension is the culprit. We know that spokes fatigue prematurely when they go through a tension / zero tension / tension cycle as the wheel rotates. The heavier the load, the easier this issue will rear its head. A tandem is the perfect example of much higher load on a single wheel, causing sub-optimal tension spokes to fall to zero tension while riding. Since the rim maker specifies a 120 kgf spoke tension maximum, I don't know why you wouldn't want to approach or reach that maximum. Especially on the rear wheel (assuming it's dished - but I think you mentioned it's dishless).

Well, if spokes fatigue prematurely going through this cycle, it seems to me that a rim bed should suffer the same consequence. After all, it's metal going through a fatigue lifecycle as well. But what supports this argument even more is that the rim is ALUMINUM. And we know that 6000 and 7000 series aluminum alloys have a much shorter fatigue lifespan than stainless steel. So I think the culprit is the exact OPPOSITE of overloading (due to a "heavy" team, climbing out of the saddle, or carrying loads), but "underloading" of the low tension spokes and associated spoke hole bed in the rim as the wheel rotates.

I've had several rims crack at the spoke holes over the years, but I just assumed I outlived the rim's useful life. But in thinking back, I seem to recall these failures occurring on the rear wheel NDS spoke holes - exactly where you'd expect zero-tension issues to occur.

I think that was the cause of your rim failure.

And to be clear, what spokes are you using? I realize the photo you show does indicate a non-standard nipple and a larger spoke diameter. I just assumed it was the close-up photo skewing the viewer's perspective. I ask because I'm curious how you're determining your actual spoke tension. Your spoke tension meter will provide spoke tensions for a certain spoke material, usually 1.5mm to 2.0mm stainless steel. If you change gauge or material, you have to determine what tension a measured deflection means. In other words, you have to calibrate your meter for each spoke you use. For example, a butted 2.0/1.8 stainless spoke will have wildly different deflections than a straight gauge aluminum spoke.

I use a decent Ice Toolz tension meter. However, it comes with a rudimentary deflection chart for only three spoke gauges. This quickly became an issue as I built and serviced wheels with many more spoke options. Rebuilding a Ksyrium meant I needed to solve this problem. I eventually purchased a strain gauge and made a jig so I could measure deflections at tensions of 80, 90, 100, 110, 120 and 130 kgf. (I copied a YouTube guy's design, but used wood.) This completely resolved any questions I had about what actual tensions I was achieving during wheelbuilding. I hope you've considered this when measuring and determining spoke tensions in your wheels.

Good luck!
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