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The crowning insult in my ignominious departure from the Grand Old Duke's event in Aberfoyle yesterday was a snapped spoke in my rear wheel.
This is especially frustrating as the rear wheel has been an absolute bugger to set up tubeless and I'm noe going to have to unwind about four layers of rim tape to fish out the loose nipple and replace the spoke.
I've always reckoned I was a decent enough wheel builder but clearly a day spent winching up steep climbs in my lowest gear (and my current 'racing weight' of 92kg 🙄 ) was too much for the build. I snapped a spoke under similar circumstances last year climbing the Burma Road.
So what am I doing wrong? I typically tension the wheel based on the max tension of the rim (130Nm? I forget the units) on the driveside, and whatever the resultant tension is on NDS.
Tension is even all the way round, to within perhaps +/-5%, and I've pre-stressed the spokes.
Spokes are CX-ray sprint, so standard 2mm at the elbow.
I'm guessing I've just not been keeping an eye on the tension of the spokes and they've drifted out of whack. Only thing I might do differently is thread lock or similar on the nipples?
In future I'll probably go 32 spoke on the rear but would rather stick with my 28 hole hubs for now...
Are the spokes the correct length - i.e. full thread engagement (and support) in the nipples? The only time I've had similar happen was when the spoke lengths were marginal but ended up too short when I re-dished for a boost conversion.
How old are the wheels? IME at some point spokes just start snapping and it's time to retire the wheel. Or at least new spokes.
Are the spokes the correct length - i.e. full thread engagement (and support) in the nipples? The only time I've had similar happen was when the spoke lengths were marginal but ended up too short when I re-dished for a boost conversion.
The argument for longer spokes usually relates to nipples that fail at the 'head' because the spoke is pulling the nipple apart not compressing it.
DT actually recommend shorter spokes in their calculator if you specify longer nipples, so seems like even the manufacturers don't believe it and I've been using that trick when swapping rims for years. I build most of my wheels with 14mm nipples, then if they need a rebuild I can use 12 or 16mm to give me a wider range of replacement rim options, never had a nipple fail as a result.
Only thing I might do differently is thread lock or similar on the nipples?
I've started using the capillary action stuff. Annoyingly only available in medium so will probably need warming with a lighter to true in future but I wasn't paying the silly money being asked for proprietary bike stuff! As a bit of a fatty I think I overload my wheels a bit (especially the road ones), especially the softer rims will unwind at an alarming rate despite starting off with near perfect tension.
Are you measuring tension with the tyres mounted? IIRC tubeless tyres can compress rims.
Where did the spoke snap - head or elbow? Are you using a dial-type spoke tension meter, or a caliper type like Park? The latter are not very accurate, so could be simply a case of over-tightening? What spoke pattern? Lots of reasons for why spokes snap, but generally over-stressing the wheel is the reason.
DT actually recommend shorter spokes in their calculator if you specify longer nipples, so seems like even the manufacturers don't believe it and I've been using that trick when swapping rims for years.
Would this not be because DT Swiss nipples have different thread lengths depending on the nipple size?
This is especially frustrating as the rear wheel has been an absolute bugger to set up tubeless and I'm noe going to have to unwind about four layers of rim tape to fish out the loose nipple and replace the spoke.
This implies the nipple broke, correct? If so, I'd guess they're alu nips and the spokes are a bit too short so the head has broken off. Replace the nips with brass and you'll be fine.
Would this not be because DT Swiss nipples have different thread lengths depending on the nipple size?
Probably, but the point is the spoke won't protrude into the slot at the top, which is what the internet tells you will cause the nipple to fail.
I was just doing a bit of research to back it up but couldn't find a clear answer. In simple terms if you have a soft threaded material in a stiffer socket then the load is taken deeper into the socket as the spoke stretches and therefore shrinks in it's crosection slightly at the surface (analogous to a bird trying to pull a worm out of the ground), the opposite is a stiff material threaded into a softer one, which means the force is transferred in the first few threads, which is analogous to one of those Chinese finger traps. The trouble is stainless can have a modulus anywhere between 50 and 200GPa, and Brass is around 100GPa so it could be either way 😂. I was hoping Brass had the lower modulus to back up my point that all the load is in the first few threads anyway and depth is irrelevant.
Why not 36 spokes at the rear? Stronger wheel for trivial weight increase.
Probably still rideable if a spoke snaps
The only wheels I've broken spokes on in a similar manner to this, is also my only set built with CX rays.
Perhaps they're just not as super duper as the blurb tells us they are.
Are you using a bladed spoke holder to prevent twist when building? And just a thought but 28 spokes on the rear might be a tad light for 92kgs
This implies the nipple broke, correct? If so, I'd guess they're alu nips and the spokes are a bit too short so the head has broken off. Replace the nips with brass and you'll be fine
No, I do run aluminium nipples but have never had one fail. This is the third Sapim stainless spoke I've had go in that time though (different rims/buildd right enough).
Where did the spoke snap - head or elbow? Are you using a dial-type spoke tension meter, or a caliper type like Park?
The previous breakage (different rim and different spokes, but same scenario) was at the head, this one was at the elbow.
I'm using a knock-off Lifeline caliper type. My reasoning was that it would be good enough for checking consistent tension, if not giving me a perfectly accurate 'absolute' tension. Perhaps though I'm just stressing the spokes enough that I really need to try for the highest absolute tension I can.
I'd always thought manufacturers recommended tensions were without tyres, but it's worth a shot 👍
I'd always thought manufacturers recommended tensions were without tyres, but it's worth a shot 👍
They are supposed to be, but needs must sometimes to get useable wheels.
It's more of an issue on road rims, a 25mm wide tyre, on a 25" rim (the approx. actual diameter of a 700c rim) at 100psi then the compression on the rim is 1 x 25 x 100 = 2500lb or about 1.25 tones, more than enough to result in a measurable* reduction in diameter!
*albeit you'd probably need a dial gauge to measure it
this one was at the elbow.
Did you seat them properly in the flange? Work round them with a rubber mallet after the initial tensioning to make sure the the heads are seated completely into the flanges.
Are you using a bladed spoke holder to prevent twist when building? And just a thought but 28 spokes on the rear might be a tad light for 92kgs
Just reading that has made me double check my VEL 50 RL specs, I'd correctly remembered 110Kg max system weight (which I'd literally just be the right side of with two 750ml bidons on my road bike, with me being ~96Kg), but surprisingly the are only 24 spokes front and rear! 😮
Poor road bike is being neglected this outdoor season since getting the egravel bike, but I have pondered using the front VEL for my usually road ebike rides, to help get above the 15.5mph motor cutoff quicker (small 250Wh battery) and get more speed for my own pathetic efforts above the cutoff these days.
Are you using a bladed spoke holder to prevent twist when building? And just a thought but 28 spokes on the rear might be a tad light for 92kgs
Yeah, I have the spoke holder, no twist.
But yeah, in future I'll be building 32 rear minimum, I still think of myself as my 85kg 25yr old self but 'system weight' of me plus full hydration pack and bottles and food is easily 100kg.
I have a replacement spoke and replacement Sapim SIL nipples in the post, maybe the threadlock thing will help.
My MTB wheels are 28 spoke also, will compare tensions. They seem to have survived a good few 1000km and were presumably machine built, it was only a £1200 bike.
Work round them with a rubber mallet after the initial tensioning to make sure the the heads are seated completely into the flanges
Oh, that's a good idea. Wrecks the heel of my palms and thumbs doing it manually.
Breaking at the elbow generally means too little tension and it's cyclic fatiguing rather than impact driven.
The M.O of snapping under climbing fits.
Id be checking a the validity of any figure spat out by your tension meter and I have always done my tensions with tire mounted it makes a fair difference I had a number of similar issues to you with factory built Stan's in the early days when people were shit scared to tension them properly and tension meters were rare..... Moving to having correct tension on under the compression of the tire pretty much solved them over night with no degeneration to rim life.
More confused than ever now, just getting round to replacing the broken spoke and rebuilding with locking nipples.
The existing tension was high, easily 160kgf non-drive and seemingly 'off the chart' on the non-drive. Clearly something amiss with the tension meter or the supplied conversion chart, although doesn't surprise me as I built to a high tension then added tension later to try and account for the rim hitting the brake blocks at times (yes yes, I know extra spoke tension doesn't increase lateral stiffness, but I did it anyway 🙄 ). By the same chart though my MTB wheels are *only* 100kgf on the drive side.
I can't actually fathom what the maximum rated tension of CX Sprint spokes is, but I know the rims are tested to 300kgf.
Whilst researching I found a comment from DCR (wheel builders and rim suppliers) saying CX Sprint were actually less durable than CX Ray, despite being thicker. I guess due to lack of 'give.
However maybe the smoking gun was this beauty, immediately adjacent to the one that snapped. Clearly a rock strike or something. Wondering what effect a spoke with a visible kink whilst under tension (photo is with tension wound off a bit) would have on neighbouring spokes...
With rear wheel builds it’s often a case of getting the drive side to true and evenly tensioned and the non-drive is just what it takes to keep it true - trying to get all three together is often just chasing your tail. Add super-stretchy spokes into the mix and it gets even more fun, with a light and flexible rim it’s fun, fun, fun. 🥳
easily 160kgf non-drive and seemingly 'off the chart' on the non-drive.
Clearly I meant 'off the chart' on the DRIVE side, d'oh.