Thank goodness for my local bike shop

[Pale Rider]

Today's ride ended at my local bike shop due to a couple of mechanicals.

The plan was to brave the showers and do one of my relatively easy 10-mile circuits on my Cannondale MTB.

Then swing by the local bike shop for some gear indexing, which I correctly judged the mechanic in the shop would fit in to his busy schedule, despite me not being booked in.

While indexing he spotted a broken spoke.

No worries, he was prepared to replace that as well while I waited.

Next problem was another spoke broke while the wheel was being re-trued.

The mechanic was happy to replace that as well, but his diagnosis was a full rebuild was the better solution.

Not wishing to waste any more of his time, I said I would leave the bike for the rebuild to be done.

The bike is well over a year old, but the mechanic said he would put in a warranty claim for the cost of the rebuild.

Not only that, the owner of the shop offered me a loan bike to get me home and use until the Cannondale can be fixed some time next week.

That's what I call good service.

Mind, I'm slightly underwhelmed by the Cannondale.

One of the reasons I was prepared to pay £900 for it was to get reasonable quality components.

I don't jump the bike or even bump off kerbs.

But it seems to me spokes can break for no apparent reason, so it's not worth fretting too much about it.

 

[ianrauk]

I had the same with an Ultegra wheel. Spoke kept on loosening. I bought the wheelset from Wiggle. I couldn't be arsed to send it back for refund/exchange so took it to my LBS who tried to repair it. Replaced the spoke but it went again. In the end he got an exchange for a new wheel on the shops warranty.
That's what a good LBS is all about.

 

[potsy]

What wheels are on there Pale Rider?

Even on a £900 bike I wouldn't imagine they'd be that great, they usually keep the costs down by scrimping somewhere and wheels are a common place to start.

 

[Pale Rider]

What wheels are on there Pale Rider?

Even on a £900 bike I wouldn't imagine they'd be that great, they usually keep the costs down by scrimping somewhere and wheels are a common place to start.

As per this pic, not sure of the brand, and of course, the bike isn't here to check.

I agree with what you say, although they do have Shimano hubs.

 

[Yellow Saddle]

Today's ride ended at my local bike shop due to a couple of mechanicals.

The plan was to brave the showers and do one of my relatively easy 10-mile circuits on my Cannondale MTB.

Then swing by the local bike shop for some gear indexing, which I correctly judged the mechanic in the shop would fit in to his busy schedule, despite me not being booked in.

While indexing he spotted a broken spoke.

No worries, he was prepared to replace that as well while I waited.

Next problem was another spoke broke while the wheel was being re-trued.

The mechanic was happy to replace that as well, but his diagnosis was a full rebuild was the better solution.

Not wishing to waste any more of his time, I said I would leave the bike for the rebuild to be done.

The bike is well over a year old, but the mechanic said he would put in a warranty claim for the cost of the rebuild.

Not only that, the owner of the shop offered me a loan bike to get me home and use until the Cannondale can be fixed some time next week.

That's what I call good service.

Mind, I'm slightly underwhelmed by the Cannondale.

One of the reasons I was prepared to pay £900 for it was to get reasonable quality components.

I don't jump the bike or even bump off kerbs.

But it seems to me spokes can break for no apparent reason, so it's not worth fretting too much about it.

As you say, that's a very poor show from Cannondale. Spokes break from fatigue, not strain, therefore jumping off kerbs and even house roofs has nothing to do with breaking spokes. It is due to poor choice of spokes (straight-gauge) and lack of wheelbuilding skills.

The replacement ones, if they are the same type, will have the same problem.

 

[RoubaixCube]

In my opinion (YMMV) but when you buy a cannondale, 99% its about the frame more than the hardware itself. I know their frames are absolutely top notch. but I see £900 running Shimano Sora/Tiagra or Apex & very rarely Rival groupsets whereas when it comes to their competitors £900 gets you a lot more for your money.


Just my $2

 

[Tim Hall]

As you say, that's a very poor show from Cannondale. Spokes break from fatigue, not strain, therefore jumping off kerbs and even house roofs has nothing to do with breaking spokes. It is due to poor choice of spokes (straight-gauge) and lack of wheelbuilding skills.

The replacement ones, if they are the same type, will have the same problem.

Are low spoke tensions, said to be common in machine built wheels, a factor in premature spoke breakage?

 

[Yellow Saddle]

Are low spoke tensions, said to be common in machine built wheels, a factor in premature spoke breakage?

Are low spoke tensions, said to be common in machine-built wheels, a factor in premature spoke breakage?

No. Low spoke tension causes other problems but not premature fatigue. Low spoke tension causes nipples to unscrew because there is not enough friction in between spoke and nipple but doesn't cause premature failure.

To explain the mechanism at work when a spoke breaks you have to use a bit of metallurgical license. Metal is crystalline and when you bend it, the crystals slide against each other. Imagine a piece of wire of some thickness, say 2mm, being bent. The outside crystals in the bend have to slide further over each other than the ones on the inside. In fact, the ones on the inside may even be sliding in the opposite direction to that of the outside ones. This demonstrates that there will be tension on the outside bend and compression on the inside bend. This sliding however, is not perfect. Some crystals will not slide properly and will sit in limbo between finding a new stable position and being stretched between stable and unstable. We can demonstrate this for ourselves by bending a piece of wire over a table edge and holding it exactly at 90 degrees. When you let go, it will shoot back to say 70 degrees. The only way to get it to settle at 90 is to bend it further than 90 - i.e. overshoot the bend a bit. This demonstrates that some crystals slide perfectly but a sizeable number of them don't. They resist the slide because they have not gone far enough beyond it.

This state of affairs in a spoke is true for three places - the threaded area, the bend in the elbow and the stamped-on mushroom head. In spokes, cyclical stresses happen as the wheel turns. At the bottom of the wheel, the spokes are relaxed by the weight of the bike and then immediately upon exiting the bottom position they return to a stressed position again. This happens 500 times per kilometer and the amplitude of this stress/relax/stress/relax situation is directly related to the weight of the rider and bike. More weight, more variance between the two positions.

However, if you can somehow get the metal to relax and have all the crystals settle in the fully slided position, it can tolerate exponentially more of these cyclical cycles. This is the basis of engineering and manufacturing. The trick is to stress relieve the workpiece. Be it a steam boiler, a bolt or, a spoke. There are several ways to do it. On welded pieces the metal can be heated after the welding job and then slowly cooled down. During high heat the metal automatically settles at crystalline level and as it cools down, the ideal crystalline configuration is frozen in. You could also stretch (strain) the workpiece beyond yield so that it takes on the new shape permanently. This is analogous to our piece of wire bent over the table end. If you bend it further than 90 degrees, it will settle on 90 degrees and be happy to work in that position.

With spokes we do the same. You overload the bends and threads so that the crystals settle in their new position and thus give you the benefit of a stress-relieved workpiece. It is so effective that someone who properly stress relieves a wheel can expect near infinite spoke life if the wheel is fit for purpose (has enough spokes and is not overloaded for its number of spokes).

Back to your question. The spoke doesn't care what the baseline tension in it is. As long as it is nowhere near yield, it will give near infinite life. In other words, a high-tension or low-tension wheel that is stress relieved will last forever. The difference is that the low-tension one will have spokes coming loose all the time.

Getting high tension in wheels is hard work and stress relieving is rocket science to most wheelbuilders, including most commercial builders, hence the rubbish we have seen described here.

Note that spoke life has NOTHING to do with spoke strength. It has everything to do with durability of the build.

Unfortunately metal doesn't like this limbo state. If you now apply cyclical stresses to a piece of metal that's in limbo, it starts to crack.