Best way to strengthen a wheel?

Question

I'm a relatively heavy chap - 95kg or so. (15 stone, 210 pounds) I've got a commuter bike which uses a NuVinci N360 hub. The hub itself is quite heavy at 2.5kg and I suspect contributes to what is already a heavily loaded wheel due to my weight. The wheel itself is a 700c 28 with 32 spokes. All in with bike, padlock and commuter luggage rack and bag I get to about 115kg.

I built a wheel myself for the first time about 4 months ago using 14G double butted Halo spokes. The large size of the hub doesn't allow for anything more than 2 cross which is I what I did. I did not use a spoke tension gauge as it seemed expensive at the time and I was hoping not to build wheels too many times. The wheel was pretty true at the build and I thought I did an OK to good job. but who really knows without a tension gauge.

The wheel was fine for about 4 months, doing 12 road miles a day, but has now started popping spokes - about 1 or 2 a week and clearly needs a rebuild. All the spokes have gone at the elbow - right where it attaches to the hub. No sign of nipples pulling through the rim, which isn't eyeletted. There are obviously quite a few parameters in the above which are not optimised for weight (28 tyre; 32 holes; 14G spokes; amateur wheelbuilder etc). I quite like the belt driven continuously variable transmission for commuting (scratch that, I love it) and so unfortunately the hub and the 32h is something I have to live with. But of the below parameters, how would you prioritise them in order to arrive at something that doesn't pop spokes long term? Really I guess I'm asking if my problem is more due to system spec or system build quality.

1. Use 12G or 13G spokes. Or maybe triple butted 14G is better?
2. Put a bigger tyre on to reduce peak loading
3. Buy a spoke gauge when doing the next build
4. Get a professional wheelbuilder to do it
5. Get an eyeletted rim which allows more tension
6. Anything else you might think of?

Ideally I'd also like to get my son on the back which would add another 15-20kg by the time you pay for the seat. Is it realistic for me to do that without going on a severe diet?

Thanks advance for your thoughts.

EDIT: Added a few more details

Answer 1

When the spokes break, do they snap at the J bend? Or elsewhere?

If the little mushroomed cap is gone at the hub end and there's a spikey "claw" then your spoke tension is low which is stressing then tensionining the spokes on every wheel revolution.

I'd start by lightly tapping the wheel spokes with a screwdriver, key, or something light. And then compare that TINK sound with other bike wheels. Or if you're a hands-on person then gently squeeze two spokes somewhere outboard of where they cross. Compare that hand-feel with other people's bikes of similar wheel size and construction.

I think you could probably tighten every spoke nipple by a half turn or even a full turn all round. Be aware that a weak spoke might choose this time to let-go.

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If you don't feel up to all the messing around, there's no shame in taking the wheel to a LBS for a professional build. A good wheel is the ultimate outcome here.

Do you know what pattern its laced with? Radial spoking is rarely if ever done on the back wheel, so you're likely to have some cross pattern. 2-cross or 3-cross are most likely. If you have a 1-cross then the spokes don't leave the hub at a great angle, and are not far off radial, so this stresses the spoke's J bend more than 2/3/4 cross.

No idea how large the nuvinci hub is, but I've put an alfine11 in a 26" wheel and had to go 2 cross. I guess you'd be best with 3-cross on a 700c (29") wheel.

Thicker spokes is always an option too, but if you do this then the whole wheel needs relacing. I wouldn't mix gauges on the same side, though I might accept having a different gauge on the left vs the right.

I'd probably go with straight spokes. The cost of butted, double and triple aren't worth it, they're more for weight saving than adding strength. Butted spokes have a thinner piece in the middle which can stretch a little more under impact, leading to a "better" ride.

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Aside, 95 kilograms is not ridiculously heavy - I was there only 6 months ago.
Sure we're more than a GCN rider in their 60s and 70s, but more mass means more muscle, right ?

You might not be good on a svelt road bike but I'm guessing you're on a rigid commuter/hybrid bike. Your weight is not the root problem here, though it will make potholes much harder on your bike, and brakes will wear a little faster. On the upside, more weight means more exercise per metre travelled.

The hub is probably not really helping, given its a lot of weight compared to a 100 gram hub and a 400 gram cassette etc. The hub's position means it has the least "suspension" effect and can make the wheel harsher. If your bike has rear suspension, the hub is still unsuspended weight and that contributes to the impacts, stressing those spokes.

Basically don't ride off kerbs/curbs and don't bulldoze your way through potholes or speedbumps. Learn to unweight your saddle, then "ballerina" your bike as it passes over the obstacle, effectively halving the weight for an instant. You're not trying to get air or off the ground, just unweight for a brief instant. Timing is everything and cleats can help a lot.

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The eyeletted rim is only relevant if you're causing damage to the rim. If the nipples are pulling through the rim holes then you would benefit from eyelets. I doubt this is required in your case.

A bigger tyre would be great if you can fit it in the frame. I rode 23mm for a while, then went 28mm and then 32 for a short while Each step was an improvement in comfort, but other changes meant I had to return to a 25mm on the current road bike.

The pinch points are your chainstays, seat stays, and rim brake bridge or calipers if you have them. Sometimes mudguards can be limiting too, but they're easier to adjust than the frame.

Answer 2

In addition to @criggie's answer, it sounds like you didn't stress relieve your spokes:

... spokes in a newly built wheel have locations where stress is near yield, some more so than others. Because fatigue endurance of a metal at or near the yield stress is short, cyclic loads in such spokes will cause failures at high- stress points. In normal use, a wheel only unloads spokes, but with spokes near yield, even these stress cycles readily cause fatigue failures. Only the lightest riders on smooth roads might be spared failures with a wheel whose spokes have not been stress-relieved.

And one of the locations on the spokes that will be at nor near yield stress is the spoke elbow, because of residual stress from the bending process used to produce the elbow.

How to do it? Here's one way (same link as above):

Stress relieving to relax these high stress points is accomplished by over-stressing them in order to erase their memory. It is not done to bed the spokes into the hub, as is often stated. Bedding-in occurs sufficiently from tension. However, stretching spoke pairs with a strong grasp at mid-span, can momentarily increased tension by 50% to 100%. Because spokes are usually tensioned no higher than 1/3 their yield stress, this operation has no effect on the spoke as a whole, affecting only the small high stress zones where spokes are near yield. By stretching them, these zones relax below yield by as much as the overload.

That's how I do it: after truing up the wheel, grab a spoke cross point for both the drive-side and non-drive-side spokes, and squeeze hard with both hands. Work all the way around the wheel, grabbing and squeezing each spoke crossing.

There are other methods, but the "grab spokes and squeeze" is IMO the easiest and safest - it doesn't put sideways stress on the axle bearings and won't cause your wheel to taco if you overdo it a bit, nor do you need any tools.

You might have to retrue the wheel afterwards.