Snapped spoke tackling 900ft climb

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johnnyb47

Guru
Location
Wales
Hi again.
Tonight I was feeling a little energetic and tackled one of the many hills i have around where i live. Straight from my front door i went out on one the the harder hills which lasted for 2.5 miles and climbed 900ft.
Just before i got to the top i heard something snap and noticed the rear wheel was not running true. A quick look revealed a spoke had snapped right were it fits on the hub. Needless to say i gingerly rode it gently for the next ten miles of my run after removing what was left of my spoke.
I was just wondering what actually cause a spoke to snap on a road bike. I can understand how a spoke would snap on a MTB with the punishment that's inflicted on them on rough terrain, but on a road bike slogging at a low speed climb on a smooth road , i don't understand.
Anyway i will nip over to my local bike shop tomorrow and sort it out. Would it be wise to let the shop sort it out, or do you think it's just one of those things that goes from time to time and i should fit in a new spoke and forget about it.
Many thanks for reading and any replies given.
Johnny.
 

Skuhravy

Veteran
Location
Darlo
Drive side spoke?
 

lpretro1

Guest
It happens - road riding can be rough on wheels due to uneven surfaces and many potholes - sometimes it can be down to a poorly built wheel. Get a repair but if you find that any further spokes go shortly afterwards then it may be worthwhile getting the wheel re-built (assuming it is a quality wheel - if only cheap then a new wheel is a better option
 
Location
Loch side.
I was just wondering what actually cause a spoke to snap on a road bike. I can understand how a spoke would snap on a MTB with the punishment that's inflicted on them on rough terrain, but on a road bike slogging at a low speed climb on a smooth road , i don't understand.
.
Johnny.

A spoke doesn't snap in the strict sense of the word. It cracks and breaks. Indulge me with some semantics, I'll explain. Snapping implies overload and sudden give-away. A spoke doesn't break in this way. It breaks because of metal fatigue. I'm sure as a naughty child you used to bend coat hanger wire until it gets hot and then burnt your buddy with it. If you haven't, you haven't practiced science yet. Anyway, if you had, you'll know that eventually, with a few more bends, the wire breaks. It doesn't make a snapping noise, it just kinda gives up and separates into two pieces.
That's exactly how spokes break - through metal fatigue.
To understand how metal fatigue breaks a spoke you have to look at the elbow (in this case). That's the J-bend where the spoke goes into the hub. The spoke make a 90 degree bend where it enters the hub (actually exit, but let's pretend). Now, if you were to tension this spoke you can imagine this 90 degree bend becoming something like a 95 degree bend because of the pull on the spoke. With each wheel revolution, as that spoke in observation gets to the bottom of the wheel just above the contact patch with the road, it relaxes and bends back to 90 degrees. And so on and so on. 10 000 times per 20 kilometers. Millions of times per year.

Eventually the spoke's metal crystals separate and a crack develops at the outside of the bend radius and grows inwards. This is slow. If you look at the spoke you'll see that area is darker, even with some dirt ingression. Eventually the crack has grown to a point where the tension in the spoke is bigger than the last remaining bit can stand that that bit then snaps in tension. It is a small bit, about one 20th of the total area of the cross section.

Potholes have a very, very very very small effect on this. A pothole serves to relax (not tension) the spoke more than just a normal wheel revolution. However a pothole only occurs every zillionth of a revolution and is thus no big deal in the big scheme of things. What matters most is just riding along over time. Your weight and the distance does all the work for you.

Right side spokes (on road bike rear wheels with rim brakes) are under more tension than the left side ones. Further, right hand spokes transmit torque to the road, not left hand ones. They therefore undergo bigger cyclical strain than the left hand ones and, lo and behold, break quicker.

It wasn't the climb that broke the spoke, but the fact that you reached the maximum number of revolutions that the spoke could tolerate, that broke the spoke. It could have happened anywhere and would have happened in anyway, had the climb been a flat section.

Spokes don't snap, they break.
 
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OP
OP
johnnyb47

johnnyb47

Guru
Location
Wales
Drive side spoke?
Hi buddy.
It was on the opposite side to the drive side ( if that makes any sense). I've just had a good look at the wheel and the spoke directly next to the damaged one is very loose. I've now tensioned it up to what i feel is about right.
 
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OP
johnnyb47

johnnyb47

Guru
Location
Wales
A big thankyou to you Yellow Saddle for taking the time to explain that in your post buddy..That really makes a lot of sense now as to why they can fail at any time. I will get it all sorted tomorrow as I'm looking forward to a good weekend of bike riding.
Cheers buddy :-)
 
OP
OP
johnnyb47

johnnyb47

Guru
Location
Wales
It happens - road riding can be rough on wheels due to uneven surfaces and many potholes - sometimes it can be down to a poorly built wheel. Get a repair but if you find that any further spokes go shortly afterwards then it may be worthwhile getting the wheel re-built (assuming it is a quality wheel - if only cheap then a new wheel is a better option
I will take your good advice on this buddy. The wheels are a set of Mavics so if a spoke does decide to fail again within the next few weeks i will get it rebuilt.
 

Spinney

Bimbleur extraordinaire
Location
Back up north
I will take your good advice on this buddy. The wheels are a set of Mavics so if a spoke does decide to fail again within the next few weeks i will get it rebuilt.
If one spoke has had enough cycles to cause a fatigue fracture, then other spokes may be close to that stage as well.
 

raleighnut

Legendary Member
A spoke doesn't snap in the strict sense of the word. It cracks and breaks. Indulge me with some semantics, I'll explain. Snapping implies overload and sudden give-away. A spoke doesn't break in this way. It breaks because of metal fatigue. I'm sure as a naughty child you used to bend coat hanger wire until it gets hot and then burnt your buddy with it. If you haven't, you haven't practiced science yet. Anyway, if you had, you'll know that eventually, with a few more bends, the wire breaks. It doesn't make a snapping noise, it just kinda gives up and separates into two pieces.
That's exactly how spokes break - through metal fatigue.
To understand how metal fatigue breaks a spoke you have to look at the elbow (in this case). That's the J-bend where the spoke goes into the hub. The spoke make a 90 degree bend where it enters the hub (actually exit, but let's pretend). Now, if you were to tension this spoke you can imagine this 90 degree bend becoming something like a 95 degree bend because of the pull on the spoke. With each wheel revolution, as that spoke in observation gets to the bottom of the wheel just above the contact patch with the road, it relaxes and bends back to 90 degrees. And so on and so on. 10 000 times per 20 kilometers. Millions of times per year.

Eventually the spoke's metal crystals separate and a crack develops at the outside of the bend radius and grows inwards. This is slow. If you look at the spoke you'll see that area is darker, even with some dirt ingression. Eventually the crack has grown to a point where the tension in the spoke is bigger than the last remaining bit can stand that that bit then snaps in tension. It is a small bit, about one 20th of the total area of the cross section.

Potholes have a very, very very very small effect on this. A pothole serves to relax (not tension) the spoke more than just a normal wheel revolution. However a pothole only occurs every zillionth of a revolution and is thus no big deal in the big scheme of things. What matters most is just riding along over time. Your weight and the distance does all the work for you.

Right hand spokes (on road bike rear wheels with rim brakes) are under more tension than the left hand ones. Further, right hand spokes transmit torque to the road, not left hand ones. They therefore undergo bigger cyclical strain than the left hand ones and, lo and behold, break quicker.

It wasn't the climb that broke the spoke, but the fact that you reached the maximum number of revolutions that the spoke could tolerate, that broke the spoke. It could have happened anywhere and would have happened in anyway, had the climb been a flat section.

Spokes don't snap, they break.

Whilst I agree with you on most of this I'd raise 1 point.

You say drive side spokes transmit torque/drive to the rim, last time I looked at my rear wheel there was a solid aluminium hub connecting both sides together and I hope that doesn't 'twist' under load.
 
Location
Loch side.
Whilst I agree with you on most of this I'd raise 1 point.

You say drive side spokes transmit torque/drive to the rim, last time I looked at my rear wheel there was a solid aluminium hub connecting both sides together and I hope that doesn't 'twist' under load.

How much your hub winds up between left and right flanges is a function of the hub diameter in the centre. How much torque gets transmitted from right to left is entirely up to how large tube connects the two flanges. In most hubs such as Shimano, the torque tube is too small to transmit any significant torque and most of it is taken up by the right spokes via the right flange.

For a wheel to transmit even equal torque on right and left, the hub "shaft" has to be huge. The best example of this can be seen on some Mavic wheels which have radial spokes on the RIGHT. Radial spokes cannot transmit torque. So, all torque on those wheels have to be transmitted by the right spokes through the torque tube to the left spokes. You'll notice that those hubs are fat. You will not find a working example of a wheel with radial right spokes and a thin aluminium torque tube.

Solid is not solid. Your hubs are tubes and narrow diameter tubes are poor torque converters. A simple search for the formula for torque transmission via tubes will reveal the secrets of this obscure fact.

Besides, why do you think the right side spokes break before the left?
 
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raleighnut

Legendary Member
How much your hub winds up between left and right flanges is a function of the hub diameter in the centre. How much torque gets transmitted from right to left is entirely up to how large tube connects the two flanges. In most hubs such as Shimano, the torque tube is too small to transmit any significant torque and most of it is taken up by the right spokes via the right flange.

For a wheel to transmit even equal torque on right and left, the hub "shaft" has to be huge. The best example of this can be seen on some Mavic wheels which have radial spokes on the RIGHT. Radial spokes cannot transmit torque. So, all torque on those wheels have to be transmitted by the right spokes through the torque tube to the left spokes. You'll notice that those hubs are fat. You will not find a working example of a wheel with radial right spokes and a thin aluminium torque tube.

Solid is not solid. Your hubs are tubes and narrow diameter tubes are poor torque converters. A simple search for the formula for torque transmission via tubes will reveal the secrets of this obscure fact.

Besides, why do you think the right side spokes break before the left?
A solid shaft of the same weight will twist more under torque than a tubular one, tubes are excellent at resisting 'twist'.
 

raleighnut

Legendary Member
Yes, but larger tubes are much better than smaller ones.

Like I asked, why do you think it is that right side spokes break more often than left side spokes?
They're under more stress especially when a rider is 'honking' up a hill and throwing the bike from side to side beneath them using their entire body weight as a lever.
 
Location
Loch side.
They're under more stress especially when a rider is 'honking' up a hill and throwing the bike from side to side beneath them using their entire body weight as a lever.

No, in your model (I'm making assumptions about what you say), if torque is evenly transmitted, then stress on both sides will be equal. Remember, stress is just force in the spoke. Tension in this case. That's the corollary of what you say above.

Stress can only be more on the right if less than half the torque is transmitted to the left. You can't have it both ways.
 
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