Technical > Wheels, Tyres and Brakes
Cracked nipples
trailplanner:
I think I have found the root cause :)
Using a spoke length calculator - https://www.prowheelbuilder.com/spokelengthcalculator I can see the spoke lengths are different for these asymmetric rims and the wheel has been laced with the same length spokes (290mm).
The non-drive side (rear) should be 291.4mm. The non-disc side (front) also should be longer (291.4mm). Note the asymmetric rims are laced to compensate for the centre line offset, which is different rear and front, so they are laced differently (cassette takes up the dish on the rear, and the disc takes the dish on the front). The idea of asymmetric rims is to even up the spoke tension to build stronger wheels.
All the nipple failures are non-drive side rear and front non-disc side. So I conclude the spoke lengths are too short (1.4mm) which is not enough to fully thread into the nipple and hence the fractures. If I look at each nipple they are clearly not long enough in each alternate hole, front and rear.
So I need 32 new spokes length 292mm (plus replace the fatigued nipples).
The spoke tensioner is great. My daughter's new wheels had a spot on even tension, but greater difference drive side vs non-drive side. I tensioned my wheels using the Park Tools WTA online app. Then I fine-tuned afterwards to correct any lateral movement, and that worked well. The spoke tension difference was less using the asymmetric rims.
I'll ring SJS, as they built the original wheels, and see if they agree. It is, as Dan suggests, a useful skill to have in the field. It would be nice to know I can take a good shot at rebuilding a wheel.
I tried the spoke frequency, but a spoke tensioner is much faster.
JohnR:
As I had noted here http://thorncyclesforum.co.uk/index.php?topic=14150.msg106789#msg106789 , I've recently had a nipple failure on the front wheel of my Mercury which is just over one year since purchase and has now clocked up 6,000 miles. As the local distortion of the rim was dowards the disc side then the broken nipple would have been on a non-disc side spoke which has me wondering if it suffers the problem of all the spokes on that side being slightly shorter than optimum. This leaves me with a dilemma: Should I pull off the nice tubeless rim tape (two layers - the broken nipple was conveniently near the end of the tape so I peeled back the first layer and poked a hole in the 2nd layer to get to that nipple, no trace of the head to be found) in order to check if the spokes come to the top of the nipples or just be aware of the risk of nipple failure? Changing to slightly longer spokes where needed is effectively a complete wheel rebuild (what is the warranty on Thorn bikes although this problem might be classified as a latent defect)?. I'm not completely sure but I reckon that the failure occurred when I encountered a particularly nasty pothole while checking the route on my Garmin.
Danneaux:
John,
I suggest you contact Thorn/SJS Cycles so they are aware of the problem and see what develops from there. They may have some ideas or suggestions to offer.
Best,
Dan.
JohnR:
I've been in contact with Robin Thorn who noted (i) brass nipple failure is not common (unlike alloy nipples) but anything CAN fail; (ii)
sometimes the steel eyelet will cut into the plating and this causes a stress riser; and (iii) They normally build with boiled linseed which acts as a lubricant and stops the nipples loosening.
He asked for some photos of the heads of other nipples so I removed the rim tape and took the attached photos, labelled according to whether they are disc side or non-disc side spokes. Effectively the ends of the disc side spokes are level with the top of the head while the ends of the non-disc side spokes are level with the groove in the head. I would have thought that this is enough penetration as the threaded part of the spoke above the bottom of the groove won't provide any significant increase in overall strength.
I also found the head of the broken nipple which has a fairly rough and dirty surface. The latter may be the result of sitting in the rim for a few weeks.
I therefore see this as an isolated failure, perhaps caused by the pothole impact. Any thoughts?
trailplanner:
I suspect an isolated incident. My wheel builds definitely had spokes too short on one side and the spoke head was 1-1.5mm below the nipple crown and these consistently failed. I rebuilt the wheel myself investing in the tension gauge (park Tool) and borrowing a friends truing stand. Robin was very helpful and got me back on the road. I have since bought some HUNT wheels and I am very pleased with them and reuse my 700C wheels across a few bikes I own to change them from fast road bikes to touring capable, with stronger wheels. Discs allow you to do that easily, after adjusting the calliper alignment, which only takes a moment.
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