Lifespan of carbon forks?

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Sweep
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Re: Lifespan of carbon forks?

Postby Sweep » 21 Nov 2020, 7:35pm

tim-b wrote:"Carbon fiber is five-times stronger than steel and twice as stiff." https://www.innovativecomposite.com/what-is-carbon-fiber/

tim-b

Forgive me if I'm being thick (quite possible) but am somewhat puzzled.
As I thought some cycling folks on rides had told me that carbon was good for soaking up road buzz/some sort of suspension.

How does that square with being stiffer?

Be easy on me - I may have just revealed again my total lack of knowledge of materials technology.
Sweep

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foxyrider
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Re: Lifespan of carbon forks?

Postby foxyrider » 21 Nov 2020, 9:26pm

Sweep wrote:
tim-b wrote:"Carbon fiber is five-times stronger than steel and twice as stiff." https://www.innovativecomposite.com/what-is-carbon-fiber/

tim-b

Forgive me if I'm being thick (quite possible) but am somewhat puzzled.
As I thought some cycling folks on rides had told me that carbon was good for soaking up road buzz/some sort of suspension.

How does that square with being stiffer?

Be easy on me - I may have just revealed again my total lack of knowledge of materials technology.


its all to do with resonance! To say its stiffer isn't correct, you can make metal frames equally as stiff but they will be far less comfortable to ride as the wall thickness of the tubes needs to increase which in turn allows vibration to travel more easily along its length. CF on the other hand can be made as thick as you like without this downside, the vibration cannot easily pass through the layered construction causing a damping effect. So it is possible to build a stiffer CF frame and it still be more comfortable as the material does indeed 'soak up' most of the road buzz.

There's a lot of techno jargon of course but thats the basics as presented to me by a Trek technician a few years back. That session involved jumping up and down on one of their CF frames, smashing it against walls and finding not even a paint defect afterwards!
Convention? what's that then?
Airnimal Chameleon touring, Orbit Pro hack, Orbit Photon audax, Focus Mares AX tour, Peugeot Carbon sportive, Owen Blower vintage race - all running Tulio's finest!

Bonzo Banana
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Re: Lifespan of carbon forks?

Postby Bonzo Banana » 21 Nov 2020, 10:58pm

tim-b wrote:
slowster wrote:
tim-b wrote:testing costs. Unless you have something that you want to preserve then new is cheaper

snip
In the OP's case another option might be to get a pair of custom made steel forks. They would cost more than an off the shelf pair of carbon forks, but that would be my preference for a frame requiring a fork with a 1" steerer and especially if it was a bike which meant a lot to me.

"Carbon fiber is five-times stronger than steel and twice as stiff." https://www.innovativecomposite.com/what-is-carbon-fiber/
CF would do the job quite easily. I do understand that you may have other reasons for preferring steel, including that skinny, classic elegance that CF designers don't seem to use
Regards
tim-b


Safety would be the primary reason to go steel. Steel normally gives a warning before complete failure as the metal becomes more pliable at the point of where it is going to fail, it is easy to inspect and takes much abuse even hard impacts in contrast to carbon fibre which can fail at any moment and gives no warning, it is not abuse-able and cannot be safety inspected easily. Yes major damage would be obvious but internal structural issues require scanning.

As ever though its about how much personal risk you are willing to take, yes you can improve your safety with steel forks but that doesn't mean carbon fibre forks are deadly they just have a much higher risk of failure. A performance road bike is like a performance car it will cost more, be less reliable requiring more expensive maintenance and be more dangerous but you get more street cred and you benefit from a slightly faster bike and dare I say enjoyable bike. Some people have their lightweight carbon fibre road bike as their weekend bike and use a more low end bike for commuting etc others may use such a bike as their main bike. I'm personally more of the weekend bike view but that doesn't mean I think its in anyway wrong to use such a bike everyday.

tim-b
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Re: Lifespan of carbon forks?

Postby tim-b » 22 Nov 2020, 7:10am

Hi
Be easy on me - I may have just revealed again my total lack of knowledge of materials technology

I won't pretend to have a huge knowledge and foxyrider has better access to explanations than I do, but it's all in the layers
Carbon fibres can be made as a woven fabric or as a uni-directional (UD) fibre and can be laid up in laminations in any direction and tapered wall thickness at the whim of the designer (who's really an engineer), which is difficult in thin-walled steel
You can build characteristics such as light, stiff, comfortable, etc. into a tube through laying different carbon fibre laminations in different directions and (or a combination of) tube cross-sections, and is where foxyrider's explanation fits
You'll see marketing trumpeting "UD CF" because it's stiffer, but the reality is more nuanced (link)
Regards
tim-b
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Brucey
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Re: Lifespan of carbon forks?

Postby Brucey » 22 Nov 2020, 8:32am

Sweep wrote:
tim-b wrote:"Carbon fiber is five-times stronger than steel and twice as stiff." https://www.innovativecomposite.com/what-is-carbon-fiber/

tim-b

Forgive me if I'm being thick (quite possible) but am somewhat puzzled.
As I thought some cycling folks on rides had told me that carbon was good for soaking up road buzz/some sort of suspension.

How does that square with being stiffer?

Be easy on me - I may have just revealed again my total lack of knowledge of materials technology.


like the supposedly 'infinite fatigue life' claim this is -in a nutshell- little more than irrelevant nonsense when it comes to real structures.

The things to bear in mind are that

a) strength (and stiffness) is normally measured in uniaxial loading and is expressed in terms of the loaded cross section area, or in terms of the unit weight.
b) fibres are, er, fibres so are capable of withstanding loads in one direction only.

So real structures may see primary loads in one or two directions but between more complex real-world loading conditions and the fact that (in real structures) there are always shear stresses generated within the structure, you need fibres going in many different directions in almost any real structure, if you want it to work and last. [ Imagine trying to build a house using string; not easy is it...? And you certainly wouldn't expect the house to be as stiff as any one piece of the string, would you...?] This means that the strength and the stiffness (per unit area or unit weight) in a real structure are nothing like the 'headline figures' might lead you to expect.

This fairly simple fact is nothing like as well understood as it should be, and it means that by the time you have a real structure with real loads on it, there ought to be more CF in the structure than you might expect, and it may be seeing loads in ways you wouldn't expect too.

The fibres also don't (in fact can't) be packed to 100% density, any more than a bundle of sticks won't have gaps in it. There has to be a percentage of resin in the structure and the resin is responsible for transferring the load between fibres and between layers; without the resin a CF object would just be a floppy mess. If the resin matrix starts to deteriorate in any way, this can initiate a failure; fibres are then exposed to stresses they wouldn't normally see which then causes them to start breaking. Often this process is near-silent, faster than you might expect and potentially deadly. Any time you see a woven layer in CF, this comes with a small penalty in terms of packing density.

It won't have escaped your notice that any laminated structure resists through-thickness stresses about as well as the glue (resin) between the fibres/layers/joints does, i.e. not very well. [Joints? Yes, most CF objects have joints in them, because this simplifies production. You can mould an entire frame in one go but this is rarely done because it is more expensive to do it this way.] In real structures the through-thickness stresses are never zero and if they are not allowed for in the right way then they can become the source of failures, pretty much in the same way as something made of wood will usually break in such a way as it will include some splitting along the grain.

So headlines such as 'infinite fatigue life' sound great but real structures contain stress concentrations and complex loading conditions which make this a nonsense. Talk of relative strengths of materials is also meaningless unless the tests represent the loading conditions seen in the real object, and they don't; simple as that.

CF is a wonderful material, but there are a lot of buts concerned with its use.

I have refrained from commenting in this thread thus far; the only type of reply which could possibly make the OP happy is one whereby it would be well known that

a) CF parts are always uniformly well designed and manufactured
b) it is easy to inspect CF structures and detect damage/ incipient failures well before they are likely to cause trouble
c) the failures were always easy to detect before the part actually fails catastrophically and
d) failures are in bike parts which have built in redundancy, where a failure can be tolerated at least for long enough for you to bring the bike to a halt.

Unfortunately none of the things on the list above is true, not for a CF fork, handlebar or stem, not even from a single manufacturer. The 'real worry quotient' is (or should be) something to do with risk x consequence, (where a), b) and c) are mostly to do with risk and d) is mostly to do with consequence) and for some uses the numbers never stack up to something tolerable. By the time you factor in age and the possibility of deterioration/damage the numbers (for parts like forks) rarely stack up to anything that will give you any comfort whatsoever.

"The light that shines twice as bright, burns for half as long"....?

Something like that....?

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

pq
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Re: Lifespan of carbon forks?

Postby pq » 22 Nov 2020, 2:51pm

[quoteI do understand that you may have other reasons for preferring steel, including that skinny, classic elegance that CF designers don't seem to use][/quote]

The existing carbon forks are skinny, and the ones I'm likely to replace them with, Columbus Minimal also have that look, the reason being the only people who buy 1" carbon forks use them on old steel (or in my case titanium) frames.
One link to your website is enough. G