Repeated broken rear spokes on Trek District 4 Equipped

Sounds like you'll just see how the new wheel works now.  I've not had issues but I've done much less distance.  Probably had somewhere north of 500 miles in the last build of this bike (theft, recovered it but needed a rear wheel rebuild) with no issues.  Couple of hundred this time round.

 RABBITHOLE ALERT!

I had a quick look online and the Alfine wheels of this diameter I could see all had the same spoke pattern.  The angles at rim do seem to be greater on this hub (because the wider hub flange?) though.

For deeper dives into wheelery-spokery there are of course SpokeCalc and FreeSpoke.  The latter gives you a nice visualisation with angles.  Plus a wealth of nerding from engineers, the late great Sheldon Brown etc.

Which is all well and good, and hopefully built well, but we still don't know the cause of the previous failure.

HoarseMann wrote:

Looking at the photos on the Trek website, the angle that some of the spokes are leaving the nipple is quite extreme. I think Trek have made a mistake using a 2-cross pattern on a 32-hole hub - especially an internal geared hub that has wide flanges. (edit: if it is a 2-cross pattern, in some photos it looks like it might be 3-cross!).

On lower spoke count wheels with a smaller flange hub, the spoke angle is less. See below a rough example, the green triangle is the same size, you can see how much greater the spoke angle is on the 32 spoke wheel of the District 4 compared to a 24 spoke Domane.

I'd ask them to rebuild the wheels (or at least the rear) in a 3-cross pattern.

Interesting - I wouldn't normally think that 2-cross vs 3-cross would make much difference to the spoke angles. Also, I would expect the nipple to deform slightly to accomodate that if they (more likely the nipple flanges) aren't sitting flush with the rim. I'd say that 3-cross would be stronger than 2-cross, but mainly for resisting sideways forces on the wheel (including the forces when braking/pedalling).

A rebuild to 3-cross is a good idea though as it'll force them to use a different set of spokes due to the longer length required.

I would guess that multiple recorded failures would point at design flaws and one or two instances of failure pointing at manufacturing or QC failure (spoke tension).

I might be wrong about it being 2-cross, but the spoke angles do look quite high, this one in particular:

I still can't work out if it's a 2-cross or 3-cross lacing, but it's something to check. The angles of some of those spokes do look like they are bending quite a bit at the nipple.

https://www.purecycles.com/blogs/bicycle-news/156387911-speaking-of-spok...

It's a regular enough pattern, but I don't like the way the spokes run in pairs to (?) adjoining holes on the rims.  3x (the only method I've used) is always better. I'd recommend Sapim Strong for for DS and Sapim Race for NDS if you go the rebuild route. 

I assume the breaks are happening at the J? 
 

ive never got to the bottom of spoke tension on rear wheels.  Even on each side, ping on the NDS and pong on the DS. 

I know you said "don't tell me the spokes aren't strong enough", but for whatever reason(s) you've got too much flex (e.g. weight, riding surface, less likely - riding style) and the spoking you've got cannae take it, Captain.

They're unlikely to be extra tight, you'd have a particularly hard ride and potentially many punctures, unless short spokes have been used and we go back to their QC dept. Again the wheel being true radially and axially doesn't really mean that much if the spoke tension isn't within tolerance. I'll assume that when spokes have been added that attention has also been paid to other spokes across the wheel.

First port of call for me would be checking spoke tension.

It'd do no harm to strip it down and rebuild it, if Trek can't sort it out under warranty, there's nothing to lose.

It's possible that they ran out of correct length spokes and had to overtighten a shorter spoke or resort to 2 cross and overtighten, but having seen production wheels being built, I can't see this sort of thing happening. Might be the frame material, might be a tightly laced wheel causing harshness. Easiest way is to eliminate the possibilities and get that tensionometer to do its thing.

Scared to get into an engineering debate, but:

I don't think spoke tension has much of an effect on wheel stiffness/ride feel, provided the spokes are tight enough that the bottom ones don't lose all their tension under load.

The stiffness of a spoke depends upon the Young's Modulus of the material and its cross section, neither of which change that much in "normal" operating/assembly conditions. Here's a link to some experiments.

There's a slim chance the wheel was built with the rim backwards, so the nipples come out at the wrong angle and the spokes have to bend a long way to compensate.

Overall, however, I'm betting on a root-cause of "crappy spokes"

Creakingcrank wrote:

Scared to get into an engineering debate, but:

I don't think spoke tension has much of an effect on wheel stiffness/ride feel, provided the spokes are tight enough that the bottom ones don't lose all their tension under load.

The stiffness of a spoke depends upon the Young's Modulus of the material and its cross section, neither of which change that much in "normal" operating/assembly conditions. Here's a link to some experiments.

There's a slim chance the wheel was built with the rim backwards, so the nipples come out at the wrong angle and the spokes have to bend a long way to compensate.

Overall, however, I'm betting on a root-cause of "crappy spokes"

I maybe misunderstood teacher.

don simon fbpe wrote:

They're unlikely to be extra tight, you'd have a particularly hard ride and potentially many punctures, unless short spokes have been used and we go back to their QC dept. Again the wheel being true radially and axially doesn't really mean that much if the spoke tension isn't within tolerance. I'll assume that when spokes have been added that attention has also been paid to other spokes across the wheel.

First port of call for me would be checking spoke tension.

It'd do no harm to strip it down and rebuild it, if Trek can't sort it out under warranty, there's nothing to lose.

I also share CreakingCrank's scepticism of how much spoke tension contributes to the ride feel. I'd expect tight spokes to put extra force on the rim which can lead to rim failure (it goes from a circle to a (mathematical) saddle shape) but does it really affect how much the spokes stretch?

Also, how does spoke tension affect the numbers of punctures - I haven't heard that theory.

Correct length spoke - ends up roughly flush with top of nipple. Spoke overtightened - sticks out of nipple on the inside - pierces rim tape - punctures tube. Less likely on a box section rim, but potentially a problem on a "basic" shaped one.

Creakingcrank wrote:

Correct length spoke - ends up roughly flush with top of nipple. Spoke overtightened - sticks out of nipple on the inside - pierces rim tape - punctures tube. Less likely on a box section rim, but potentially a problem on a "basic" shaped one.

Aha - that makes sense now.