How long should a crank last?

[mickle]

It was more than two years old so out of formal warranty.

I'd give it a go anyway. A crank should *never* fail like that. Shi**no should be taking a very close interest in your crank's failure. If it can happen to yours it could happen to any one of the hundreds of thousands of other identical cranks out there.

 

[classic33]

Here's a face on photo of the broken crank

View attachment 101093

The diagonal "line" on the bottom piece of the crank, is different in formation to the metal either side of it.

 

[Yellow Saddle]

Here's a face on photo of the broken crank

View attachment 101093

The picture reveals it all. Thanks for posting it.

Look at the section on the left. The dark area at the bottom, left of the hole, is where the crack started. In other words, it started where the pedal spindle flange ate into the crank and caused a stress riser. If you turn your head and arrange the crank in your mind, you'll see how this fretting action worked with each pedal stroke. It is exactly where you would expect it for a downstroke. The dark area took some time, possibly months, to develop. It grew slowly from the stress riser, travelling inwards in an arc shape. Then, the crack jumped to the other side of the hole. That's the reddish stain you can see there. That phase lasted a short while too, with at least one wet ride whilst it developed. Then suddenly, boom, the silver section is where it cracked in one go - possibly within a few kilometers from start to end.

I suspect your pedal was not tightened enough and there was some play in there that made it all worse. The black rouge in the thread itself reveals that there was movement between pedal and crank, which should not be there.

It is not a faulty casting (cranks are forged after casting so they are made with the best and most expensive techniques possible) nor is it a warrantee repair. It is faulty assembly of pedal to crank.

 

[Danny]

The picture reveals it all. Thanks for posting it.

Look at the section on the left. The dark area at the bottom, left of the hole, is where the crack started. In other words, it started where the pedal spindle flange ate into the crank and caused a stress riser. If you turn your head and arrange the crank in your mind, you'll see how this fretting action worked with each pedal stroke. It is exactly where you would expect it for a downstroke. The dark area took some time, possibly months, to develop. It grew slowly from the stress riser, travelling inwards in an arc shape. Then, the crack jumped to the other side of the hole. That's the reddish stain you can see there. That phase lasted a short while too, with at least one wet ride whilst it developed. Then suddenly, boom, the silver section is where it cracked in one go - possibly within a few kilometers from start to end.

I suspect your pedal was not tightened enough and there was some play in there that made it all worse. The black rouge in the thread itself reveals that there was movement between pedal and crank, which should not be there.

It is not a faulty casting (cranks are forged after casting so they are made with the best and most expensive techniques possible) nor is it a warrantee repair. It is faulty assembly of pedal to crank.

Thanks for the detailed response - very informative

Why do you think "The black rouge in the thread itself reveals that there was movement between pedal and crank, which should not be there". Couldn't it just be an accumulation of muck which has attached itself to the grease I put on threads before I screw the pedals in?

 

[Yellow Saddle]

Thanks for the detailed response - very informative

Why do you think "The black rouge in the thread itself reveals that there was movement between pedal and crank, which should not be there". Couldn't it just be an accumulation of muck which has attached itself to the grease I put on threads before I screw the pedals in?

Aluminium reveals a fretting (rubbing) movement very quickly. For instance, think of the black muck that comes off your rim's brake tracks when you use your brakes in the wet. That black is not the rubber, but the aluminium.
Further, the grease you put in there wasn't black and road muck is almost always brown due to the high soil content in road slurry.
Whenever engineers work with aluminium they will look for telltale rouge which indicates the type of fretting we're talking about here. As I mentioned earlier, the thread on a pedal spindle doesn't fix the spindle properly to the crank and there is always movement due to the use of a flange instead of a taper like on a wheelnut. If pedals are torqued properly, they don't show this black rouge. Mine (ahem....torqued properly) still shows the bright copper colour from the copper compound I use on them instead of grease.

Another place where aluminium rouge tells a powerful but subtle story is inside older, silver non-anodized tubby rims. The black rouge tells us that tubbies glued to the rim still move against the rim. This is a large source of rolling resistance on tubby wheels that's not understood by the bicycling industry. For a tubbie to display lower rolling resistance than an equal clincher tyre, it has to be glued with hard glue such as shellac.

 

[Andrew_P]

Aluminium reveals a fretting (rubbing) movement very quickly. For instance, think of the black muck that comes off your rim's brake tracks when you use your brakes in the wet. That black is not the rubber, but the aluminium.

Is that a fact? Where does the rubber go as the brake block wears down?!

 

[Dogtrousers]

I remember my mate's crank shearing just outside Grantham on tour in the late 70s. I don't remember which end broke. He rode one-legged to the nearest bike shop, which fortunately wasn't far.

Do I get a prize for most useless post on this thread?

 

[fossyant]

Ah well. Opportunity for an upgrade. Never miss an upgrade opportunity

 

[Yellow Saddle]

Is that a fact? Where does the rubber go as the brake block wears down?!

Yes it is a fact. The rubber doesn't grind down into a fine powder like aluminium oxide. It grinds down coarsely and in the dry, disperses in the wind. In the wet, it washes away without staining the water. My brake blocks are red and the rims certainly don't turn red.

 

[gbb]

I remember my mate's crank shearing just outside Grantham on tour in the late 70s. I don't remember which end broke. He rode one-legged to the nearest bike shop, which fortunately wasn't far.

Do I get a prize for most useless post on this thread?

No, but you can have a 'like' for it...