Snapped carbon crank

[Beekeeper]

Carbon v Aluminium v Steel

who wins?

 

[Mikerb]

Carbon v Aluminium v Steel

who wins?

Too basic...each has different properties. Carbon composites can be made super stiff for a specified direction of stress whilst being very light and easy to form into different shapes. It is rarely impact resistant unless the composite includes something like kevlar, not uv resistant unless an inhibitor is added to the resin, quality of mat varies greatly and integrity of the finished product is determined largely by the knowledge and skill used in the mainly non automated manufacture process. Invariably complex shapes have to comprise several sections which are then bonded together...again by hand.
There are applications where the extra costs involved are justified. I windsurf and virtually every component of a complete board/rig set up is carbon composite.....and it is justified on the basis of light weight, anti corrosion and the ability to create stiffness (eg board and boom) or fast reflex flex ( eg mast and fin).
For emtb I see no justification at all.

 

[KeithR]

That's not an impact resistance test, and that's where the problem with carbon fibre lies. Compression resistance isn't impact resistance, and for the most part, bike components outside of the suspension aren't under significant compression anyway.

But it does prove my earlier point - the aluminium deformed rather than fragmenting - pretty much into dust - like the carbon fibre.

I know which quality I want in my bike's structural parts, given that critical damage to carbon fibre can be impossible to detect visually before the part fails.

A bent aluminium crank arm will still get you home, and is far less likely to put you in hospital...

I know most people aren't nerdy enough to read academic research papers, but I am. This one is particularly revealing, as it specifically features carbon fibre in bike production:

https://www.tandfonline.com/doi/full/10.1080/21642583.2017.1311240

For me, the TL;DR take-away is in section 2.1 (with my emphasis):

There are many reasons why damage occurs and it can be certain that once there is damage that this will perpetuate further

That's a characteristic that has no place in a material used to build mountain bikes, IMHO.

And given that your local bike shop is unlikely to have the equipment necessary to detect non-visual damage:

Analysing existing failure detection methods of composite materials, it can be seen that the majority of methods require the composite structure be either taken to a test house or that relatively complex and large equipment be taken to the structure site. In each case the equipment is large, requires a high level of competence and is typically expensive.

It's not for me.

I'm still not trying to talk anyone off their carbon fibre bikes, but facts are facts, and there are good reasons not to be a fan of the stuff as a structural bike-building material.

 

[Mikerb]

That's not an impact resistance test, and that's where the problem with carbon fibre lies. Compression resistance isn't impact resistance, and for the most part, bike components outside of the suspension aren't under significant compression anyway.

But it does prove my earlier point - the aluminium deformed rather than fragmenting - pretty much into dust - like the carbon fibre.

I know which quality I want in my bike's structural parts, given that critical damage to carbon fibre can be impossible to detect visually before the part fails.

A bent aluminium crank arm will still get you home, and is far less likely to put you in hospital...

I know most people aren't nerdy enough to read academic research papers, but I am. This one is particularly revealing, as it specifically features carbon fibre in bike production:

https://www.tandfonline.com/doi/full/10.1080/21642583.2017.1311240

For me, the TL;DR take-away is in section 2.1 (with my emphasis):

That's a characteristic that has no place in a material used to build mountain bikes, IMHO.

And given that your local bike shop is unlikely to have the equipment necessary to detect non-visual damage:

It's not for me.

I'm still not trying to talk anyone off their carbon fibre bikes, but facts are facts, and there are good reasons not to be a fan of the stuff as a structural bike-building material.

Actually the front triangle is under compression load and a composite laid with UD fibres along the tubes will effectively resist those forces. The same could be said for a rear triangle. Both however are subject to torsional forces so the layup has also to cater for that....easy enough if it is effectively a square tube. There are also forces which act at 90° to the tube especially the top tube. UD fibre layup is weak in that direction. So 45° weave is probably used...which is a compromise. Regardless neither can withstand an impact with a hard object without causing potential fibre damage. In windsurf kit a special protective layer is added on areas likely to suffer abrasion and that also protects against minor knocks. In the end you only know if a boom or mast is damaged when it snaps under load. At the very least on a carbon frame the entire front face of the downtube should be protected.

 

[KeithR]

Actually the front triangle is under compression load and a composite laid with UD fibres along the tubes will effectively resist those forces.

It's not significant compression load though, Mike - I was careful about how I couched my comment. The action of the suspension - and the stress-dispersal aspects of the frame design - will markedly mitigate the overall loading on the front triangle.

(For complete clarity, I mean significant enough to be capable of causing frame failure).

In terms of likelihood of failure, impact is a far more likely cause than normal, in-spec compression forces; followed by manufacturing faults.

The same could be said for a rear triangle.

Which, coincidentally, is aluminium, more often than not...

(Aye, I know that it's cheaper, too.)

Both however are subject to torsional forces so the layup has also to cater for that....easy enough if it is effectively a square tube. There are also forces which act at 90° to the tube especially the top tube. UD fibre layup is weak in that direction. So 45° weave is probably used...which is a compromise. Regardless neither can withstand an impact with a hard object without causing potential fibre damage. In windsurf kit a special protective layer is added on areas likely to suffer abrasion and that also protects against minor knocks. In the end you only know if a boom or mast is damaged when it snaps under load. At the very least on a carbon frame the entire front face of the downtube should be protected.

Yep, with you on all of that.

 

[Beekeeper]

For what it’s worth, I still have my steel shafted golf clubs.

all my graphite ones are long gone. Split, shattered or exploded.

 

[Mikerb]

For what it’s worth, I still have my steel shafted golf clubs.

all my graphite ones are long gone. Split, shattered or exploded.

what do you expect with pencil lead

 

[Mondfuchs]

what do you expect with pencil lead

I do hope you're not flying at all - in some cases you would be wondering what keeps you 10'000 meters above the ground...