Specialized Tricross disc snapping spokes.

[Globalti]

I've just sold my little-used 2011 Tricross to a colleague who loves it and has started commuting on it. He's quite a large gentleman and this morning he tells me he broke a spoke on the way to work, so I'm going to go with him to the LBS at lunch time to fix it.

Curiously this also happened soon after I bought the bike when I lent it to my cycling buddy for about three miles and got it back with a broken front spoke. So now I'm worried and a little embarrassed. Does anybody have any experience of this? Anybody got a Tricross with a spoke snapping habit? I wonder why it's been the front wheel both times not the rear? Would it be worth getting the wheel rebuilt with stronger spokes? I'd gladly pay for that to keep my colleague happy.

 

[lpretro1]

If it has only snapped two spokes in its' life then that isn't a cause for concern - esp given the state of our roads. Plus you already said your pal was a large chap. If it was breaking spokes every few rides then yes rebuild it otherwise I'd be advising my large friend to take weight off when riding over bad surfaces by lifting his bum off the saddle a tad just to help with the bumping/bashing

 

[Globalti]

I've dropped it at the wonderful LBS (Cooksons in Whitefield) and Chris the mechanic is going to replace the spoke for me by tea time. That's the value of a good LBS.

Looking at the spokes carefully I see that they all depart at an angle from their nipples; the nipples are perpendicular to the rim not pointing towards the hub end of the spoke. Ths must stress them at the point of bending where they enter the nipple, especially as there is thread visible forming a great stress riser. You wouldn't expect this from Specialized. I've told my colleague I'll go halvers with him on a proper rebuild with SS spokes if another one breaks.

 

[Yellow Saddle]

Spokes break from cumulative metal fatigue. The fact that a heavy person rode them for a short while before the spoke broke is not his fault. He was just the last straw that broke the camel's back.
Your observation about the spoke departure angle at the rim is valid. Whilst standard external nipples can swivel a little bit, if the spoke's arrival angle exceed the two or three degrees of swivel, the spoke itself takes up the rest of the bend and that at it's weakest point - the first thread. Have a good look at the angle and decide whether the nipple takes it all up or whether the spoke actually bends. If the spoke bends, the wheelbuilder chose the wrong number of crossings for the job at hand.

Edit: Stronger spokes are not an option. This is because spokes don't break from tensile weakness but metal fatigue. A stronger i.e. thicker spoke will just last a little bit longer. By little I mean a few thousand revolutions longer, which is just a few kilometers. It is futile to increase thickness to prevent metal fatigue.

 

[Globalti]

Thanks. He later told me he was braking hard and hit a pothole simultaneously. I had also pumped the tyres to their maximum pressure before selling it to him so I've dropped them to 75 and swllowed the £14 bill for the spoke repair. I wonder how long my warranty will last for though?

 

[Yellow Saddle]

Thanks. He later told me he was braking hard and hit a pothole simultaneously. I had also pumped the tyres to their maximum pressure before selling it to him so I've dropped them to 75 and swllowed the £14 bill for the spoke repair. I wonder how long my warranty will last for though?

People always tend to look for a single big reason why a spoke break but "the event" is never the reason. A spoke breaks because it has developed a crack and as the crack propagates through the diameter of the spoke, it reaches a point where the tension in the spoke is bigger than the remaining strength of the last little bit of metal that's not yet cracked. Anything can trigger the final break but it is hardly ever a pothole or pedaling hard.

It doesn't really matter what the exact nature of that last straw is that breaks the camel's back but understanding is sometimes useful or just interesting/boring, depending on which side of the science/don't care border your personality falls.

We know it is tension that causes the last break and that it is NOT tension that causes the crack to start. We also know that as a spoke is loaded by the rider's weight, only the bottom spoke is affected and it is affected counter-intuitively. It relaxes rather than tensions up. Therefore, loading doesn't break it because technically the loading reduces stress in the spoke. However, when the wheel rolls and the spoke goes back to tension, then it could break. So, it is coming out of the pothole rather than hitting it that's the last straw.

In your case you say he said he was braking hard and hitting a pothole. By braking on a rim brake wheel, the entire front half of the wheel's spokes, from 12 O'clock to 6 O' clock relaxes (a bit) and the entire rear half increases in tension. You'll never know where the errant spoke was when it broke. You'll also never know whether the pothole was the culprit or not.

What you do know is that all spokes fatigue equally and it is only minor differences that makes the one break before the others.

To cut a very long story short, your 14 pound spoke will outlast the others, which are all right now considering cashing in their pension. It is just a matter of when.

 

[Globalti]

The bike has disc brakes, which introduces a new set of stresses into the equation, I would expect.

 

[Yellow Saddle]

The bike has disc brakes, which introduces a new set of stresses into the equation, I would expect.

Yes, with these brakes every second spoke loses tension and every other spoke gains tension during braking. Net tension in the wheel thus remains the same and differences are no more than 5% for even extreme hard braking. Thus, braking does nothing remarkable to the spokes. It is just normal load carrying that creates the fatigue since spokes lose up to 50% of tension during a standard roll-over.