Straight pull hub failure – advice/opinions please

Your thoughts/advice please.

A real JRA here: the interface/flange on non drive side of rear hub has broken off. Just a wierd clang at low speed on flat smooth tarmac. Rider is 70kg at most.

Wheel has one by owner only,  1500 miles on it, was straight and had never been near a spoke key before the hub failed. Tyre was fine and still at 75psi when I removed it to inspect and return the wheel, it was not quite wonky enough to rub the chain stay, so was gently ridden 3 miles home and still not been touched with a spoke key.

The wheel is a nice-ish (£400 a pair) thru axle road disc, straight pull, 8 nds and 16 ds spokes, and now with one of the eight nds spokes flapping in the breeze, it’s well out of true and the hub is unrepairable.

I will not name the shop or manufacturer as according to the shop, the U.K. distributor is not warrantying it, claiming that impact/rim damage caused this. I think the distributor’s assessment does not add up, and I have disputed this. Their very first point is how untrue the wheel is, that this is suggestive of trauma of some kind. Well yes, one of only eight nds spokes is gone, of course it’s not true any more 🤦

my question I suppose is how does a straight pull hub fail at the flange/interface rather the spoke or rim hole? I have repaired/rebuilt from broken spokes, nipples, cracked rims but never once binned a hub from a failed flange/spoke bed.  A big enough pothole at speed maybe? but that is the distributor’s word against mine. However the dent in the rim they report (I dispute this too and have requested a photo) is apparently adjacent to the spoke whose flange has broken.

I have only built 25 or so wheels over the years, and rebuilt or just re-trued maybe 30 more, but my understanding is that under load of the rider, the hub hangs off the uppermost part of the rim in effect, so even on j-bend but especially on straight pull, a big impact actually stresses the spokes immediately opposite the point of impact on the rim not next to it.

I also understand how overzealous tensioning can bring about unexpected wheel failures even without impact, but this was a one-owner wheel with low mileage and never needed to go near a spoke key.

sorry for long post but trying to answer the obvious questions in the description of how it broke, what it looks like now and what the warranty person advises.

cheers all

an impact on a rim spreads the load over a whole load of spokes on the opposite side of the rim. It is feasable that if you only had one done up reasonably tightly (all the others loose) then smacked it at just the right point it’d go, but I think that'[d be unlikely.

that said, with only 8 spokes (!) theres more of a gap. A smart shock to one side of the rim in just the wrong place could maybe over-tension it? if the rim has pulled to the nds where the damaged spoke is, that would be indicitive of that kinda impact (if just out of true due to the missing spoke it should be pulled over to the drive side)

only 8 spokes (!)

Yeah, that seems odd for a disc hub. 16/8 isn’t that uncommon for a rim brake road wheel, but I’d have thought the additional load for disc brakes would warrant a more even spoke count.

Their very first point is how untrue the wheel is, that this is suggestive of trauma of some kind. Well yes, one of only eight nds spokes is gone, of course it’s not true any more 🤦

I think if it was catastrophically untrue they’d have a point, but given as it was rideable without rubbing on the stays, that’s pretty much what I’d expect.

but never once binned a hub from a failed flange/spoke bed.

Some of the Hope lightweight hubs (regular J-bend) used to fail that way. Hope were always good at replacing them, no questions asked, even for decade old hubs!

I can understand not naming the shop, but if the distributor is not playing ball, why wouldn’t you mention the brand?

Some of the Hope lightweight hubs (regular J-bend) used to fail that way.

indeed, I had a hope pro-2 fail that way. Mine was second hand so I just bought another second hand and rebuilt it. Back then hope were trying to build brand reputation and they achieved it. Doesn’t sound like OPs distributor are much bothered about their reputation. So go ahead and name them OP.

Yep. Please name and shame !

As for the cause, do you have any photos ? Particularly the fracture surfaces as they can tell a story (if the photos are sufficient resolution).

A 16/8 spoke pattern suggests either Campanology or Fulcrum:

https://www.fulcrumwheels.com/en/technologies/mtb-wheels-technologies/two-to-one-disc-brake

I found those too, but the Fulcrums look to be 7(!) disc side spokes.

Back then hope were trying to build brand reputation and they achieved it.

Maybe I’m too naive, but it never felt like it was a calculated choice, just that they stood by their products.

my question I suppose is how does a straight pull hub fail at the flange/interface rather the spoke or rim hole?

I had this on a Mavic wheel years ago. My assumption was a material defect gradually propagating a crack.

You say 1500 miles, but how old is it? Outside the normal warranty period?

Yeah, I’ve seen both Mavic and Hope fail like that. We were able to work with both companies to get warranty support. Based on our in-store assessment, neither company suggested that it was user error/crash damage.

my question I suppose is how does a straight pull hub fail at the flange/interface rather the spoke or rim hole?

Flaw in the forging, nick when it’s been machined, over/unevenly tightened spoke from day 1.
It’s one of the weaknesses of some straight pull hub designs, pulling on a stub of metal, rather than a ring.

I’ve always put flange failure down to spoke tension. There is only so many time you can true a wheel before the build up of pressure on the spokes causes something the break.

On my rare retro wheels, I was informed by the wheel builder that it was best to completely detension and rebuilt from scratch, to keep the pressure from building up in one area. Which seems to make sense.

There is only so many time you can true a wheel before the build up of pressure on the spokes causes something the break.

That doesn’t really make sense. You don’t just keep adding tension to spokes.

You don’t just keep adding tension to spokes.

Some people do. Don’t realise that truing is about balancing the tensions, rather than just adding more.

Should be a warranty imo*, what could the rider have done to prevent this?

*If in time