Busted Carbon

[Bigtwin]

Once carbon thing you certainly don't want...

http://www.velonews.com/article/93054/a-shattering-experience---a-post-recall-r-sys-wheel-failure

 

[Bigtwin]

And another (not that you can as they've been recalled)

http://cozybeehive.blogspot.com/2008/08/wolf-sl-fork-failure-2.html

And this, I suspect, probably gets how how a lot of the failures happen

Replacing the stem

Before doing anything you should remove any spacers under the stem - don't remove the fork or any headset top-cap, just the spacers. Clean and inspect the steerer tube for any damage, especially if it is made of carbon! You should also visually inspect the inside of the steerer tube to make sure that the compression plug or 'star fangled nut' is properly positioned.

The image on the left shows the stem removed leaving just one spacer in place - this is where the fork will drop out unless it's supported! The image on the right shows the compression plug inside the steerer tube, you can just see the top of it's holder in the image on the left. Tightening up the bolt through the top cap expands the plug and the pressure grips the inside of the steerer tube. Alloy steerer tubes use a 'star fangled nut' but they would wreck a carbon steerer. Replace the spacers on the steerer tube and, with the pinch bolts loosened off, slide the new or replacement stem into position so that it is hard up against the spacers which, in turn, are hard up against the headset. This is why having the front wheel resting on the ground is a help - it means that the fork cannot slip out of the headset.
Now re-fit any spacers or shims that may have been above the stem.
Place the top cap onto the steerer tube, then fit the compression bolt and tighten to 'finger tight' to keep everything in place. Now re-fit the handlebars, being careful not to damage any of the cables when lifting the bars in to place by re-fitting the face plate and bolts.
At this stage none of the fixings should be tight, follow this tightening sequence.

1. Rotate the handlebars into their original position (see note above) and tighten the face plate fixings, tighten the bolts in a diagonal pattern. Make sure that you tighten each fixing equally.
2. Tighten the 5mm compression bolt which sits on top of the stem checking for play and smooth operation. You can check for play by simply holding the front brake on whilst rocking the bike back and forth, if you feel no play that’s great!
3. Centre the stem to the front wheel; either by eye or use a straight edge to check from each side of the wheel.
4. Tighten the pinch bolts on the side of the stem.

The compression bolt is what holds the fork into the frame by pressing down on the stem, the spacers and the headset. The pinch bolts provide the grip that secures the stem onto the steerer tube so allowing the handlebars to turn the fork and so control the front wheel. You will often find markings on the stem that indicate the correct torque (usually expressed as a value like 6Nm which stands for Newton metres) to apply to the bolts. As with all carbon components, there is a danger that over-tightening can lead to excessive stress and potentially failure so don't just heave on the Allen key until it bends!!!

Wonder how many people actually use a torque wrench?

 

[yenrod]

I'm definately NOT getting carbon now !

My forks are CF

There's a really easy way to test for any delamination with carbon fibre parts. Delamination is when the different layers of fibre start to seperate, and will make the whole structure fail. All you need to do is take a 50p piece, and tap it along the part concerned. If the noise is a clean noise (ie no fuzziness) then it's fine. If there is any fuzziness or vibration in the noise, it has started to delaminate and is ****ed. You should do this regularly with carbon parts.

This is from a mate who makes F1 car parts out of carbon fibre, so knows a thing or two about it.

I was in a shop notsolongago and the touchy female proprietor was looking at me funny as I was tapping the CF tubes (of a bike I was checking out - may buy) with my nail..."why are you banging the tubes" (slightover reactionj) she said 'i'm checking them for thickness' replied myself...

Stupid or what (her) !

 

[yenrod]

Once carbon thing you certainly don't want...

http://www.velonews.com/article/93054/a-shattering-experience---a-post-recall-r-sys-wheel-failure

Mavic definately are not liking this !!!!!!!!!

 

[Bigtwin]

Mavic definately are not liking this !!!!!!!!!

Now you see that really does strike me a pointless pissing about with something that's tried, tested and proven to work reliably and safely - the conventional spoked wheel with all it's fairly minor variations - for bugger all purpose.

 

[Van Nick]

Indeed, but more important is most of the failures on that stupid site are of components involved in massive accidents such as being side swiped by a 42 ton truck.

I don't think there was any impact involved with this poor chap with the pedal and carbon crank arm hanging from his foot

 

[dellzeqq]

I don't think there was any impact involved with this poor chap with the pedal and carbon crank arm hanging from his foot

fine and dandy, but I've had an alloy crank arm fail, and an alloy seat post (a magic moment..) All these things are engineered to almost fail.

 

[Bigtwin]

I've had an alloy crank arm fail, and an alloy seat post. All these things are engineered to almost fail.

Yes of course they are.

 

[Chonker]

If they were thicker/heavier they wouldn't break, but you wouldn't buy them!

Aluminium alloys work fatigue badly, it's part of the reason why the aerospace industry uses as much composite material as it does.

I agree with the torque wrench comment, I bet alot of these steerer problems are down to overtightening.

 

[Bigtwin]

Aluminium alloys work fatigue badly,

It works certain types of fatigue badly - flex forces in particular of course.

I must have been riding Al bits from pretty much when they came in - racing, touring, road and MTB. I'm not at the light end of the scale, having been a sprinter, but I have never had any Al post or crank fail - or component fail come to that. Yes, if you bugger up threads and over torque bolts etc as plenty of ham-fisted home mechanics do, you can get it to happen, as with most things.

If someone is breaking Al cranks in normal, non-abusive within the design envelope use, they were either cheap-shite components in the first place (why do people buy things on eBay from China for 10% of the UK price and think they will be equal?), or faulty from the start for some reason. The forces required to snap a properly made and fitted alloy crank are simply huge. Likewise, the weight penalty to move from "engineered to almost fail", whatever that concept is, to pretty much bomb-proof, is marginal.

In today's litigation culture one thing is for sure, manufacturers do not engineer things to "almost fail".

 

[dellzeqq]

my failed crank was Campag.

That was another magic moment. Going up hill in West Norwood (a dull car-strewn suburb of South London, with apols to Norwoodidlians here) and all of a sudden a sort of loose feeling, followed by a sort of stopping feeling, followed by a sort of whoops! I'm falling over feeling....

 

[Bigtwin]

And what was the nature of the failure?

Slightly interesting site featuring crank failures amongst other things;

http://materials.open.ac.uk/mem/mem_ccf.htm

 

[dellzeqq]

Sheared between the spider (this was the chain-ring side) and the arm. And I am not Mr. Big and Beefy Hillclimber. Far from it.

I've also seen a (carbon) Campag seatpost fail under a rider who could not be more than 10 stone - and at 67 years old he was hardly throwing what little weight he had around. So, all in all, I don't think it's about the material. As somebody said, if there was extra weight we wouldn't buy it. My frame comes in at about 1.4kg including forks. That's both wonderful and completely barking.

 

[Bigtwin]

Sheared between the spider (this was the chain-ring side) and the arm. And I am not Mr. Big and Beefy Hillclimber. Far from it.

I've also seen a (carbon) Campag seatpost fail under a rider who could not be more than 10 stone - and at 67 years old he was hardly throwing what little weight he had around. So, all in all, I don't think it's about the material. As somebody said, if there was extra weight we wouldn't buy it. My frame comes in at about 1.4kg including forks. That's both wonderful and completely barking.

But people do buy it - all the time. Downhill bikes, touring bikes, commuter bikes to name but 3, are examples where people don't buy the lightest stuff, they buy durable, reliable stuff. Steel forks are another classic example. Yes there is a tribe of weight weenies that buy the lightest, then drill it, shave it, whittle it for all their worth and stick bits on digital scales - RBR is full of them, but most people don't.

And the world is full of people who buy stuff and fit it paying no attention to instructions and warnings - everything from bolt torque, max stack height, max post exposure, min post in frame, weight limits, you name it. And who buy "bargain" stuff from feabay etc knoeing nothing of its history, whether it's been fitted properly (c.f. those stems I posted yesterday). And how many people ever actually inspect components regularly - or at all? Especially when they drop them, bang them, ram them into car boots and all the rest of things people do? Pretty low proportion I'll wager.

Campag did seem to have a period of some dodgy design on spiders a while back.

This is an interesting read, not least on account of the section about pedal eye failure avoidance:

http://books.google.co.uk/books?id=...X&oi=book_result&ct=result&resnum=8#PPA262,M1

 

[alecstilleyedye]

i'm not sure that seat post failures (of alloy or carbon) aren't the result of the current fashion for sloping top tubes and large exposed areas of seat post.

an old-fashioned(ish) straight top tube requires less exposed seat post and hence less stress being applied to the post at the point it joins the seat tube.