How sturdy are rigid carbon forks?

i snapped some aluminium handlebars, trust me, it was a very sudden failure and my face hit the ground quite hard.

Thankfully it was in the woods and the ground was soft earth.

Why is when a carbon part breaks, people blame the material??

Carbon fork breaks – its because its carbon fibre
Steel fork breaks – its because its badly made/designed

Interesting links, Macavity, although it does look like someone's been copy & pasting instead of doing their own original research. 😉

I feel a bit more reassured now about the reliability of my carbon forks now. The idea of sudden catastrophic failure is still a bit of a worry though.
Both frames developed a mysterious creak shortly before they broke. I would imagine that if I had searched more thoroughly I would have found the cracks before they spread.
A crack on a steel or aluminium fork may spread slower, but it may still be too quick for me to stop riding before it fails completely.

I'm still curious as to why there is a rule of thumb 80 hour limit for carbon handlebars and no similar limit for frames or forks.

TJ….please please please dont scare people with your theories..

stick to what you do for a day job, and dont come on here as a mechanical/electrical/aero engineer?

please?

scotia – I did say I accepted my fear might be irrational – and then please tell me what is wrong with what I said?

At the end of the day it's down to design and manufacture, rather than material. A correctly designed carbon part will not fail catastrophically as it will be designed not to fail in use. Likewise a steel or alu part. If a steel fork or an alu fork cracks it's because it wasn't designed properly and the material has been taken outside it's safe envelope, just as with a carbon part. The instantaneous failure that seems to scare everyone also scares me. I've not had bars fail, but I've had forks fail. 2 of them. Both steel, but fortunately both instances they bend plastically and were still loosely ridable. Not sure that really matters though as both times I'd crashed and come off the bike in the process. Had they been carbon forks they'd probably have shattered, but I'd still be off the bike.

Heres a link to a picture of a broken Pace RC31:

http://jasperscyclingdiaries.blogspot.com/2010/05/epilogue.html

Heres a link to a picture of a broken Pace RC31:

And a picture of a broken steel chain, does this mean i need to go to a belt drive? as my chain could fail catastrophically throwing me over my bars. 😆

Some important points from that link;
Only one leg broke.
It didn't break completely.
The rider was able to stop without crashing.

I also have irrational fears, including having one fork leg snap clean off, which then overloads the other one causing it to also snap, leading to a nosedive.
It looks like that may not necessarily be the most likely scenario.

Carbon eh ? What a can of worms. As I said earlier I run RC31s both very old and newish ones.I have thus far had no problems but now it has been mentioned "the bad juju man" might just spook mine into to failure. Fit them,ride them, check them when you wash you bike after every ride (yer as if) and stop fretting.

if you look at those broken rc31s they've failed exactly how they should have. Instead of snapping the crown assembly has bent. Unfortunately other companies dont do the same thing :S

Ive ridden some stuff on my exotic carbon forks that a lot of people wouldnt do on XC suspended bikes and not had problems. Get the odd creak and stuff from the front end but ive not stopped riding them yet.

Just dont smash them on anything and make sure they dont crack or anything and you will be fine.

and then please tell me what is wrong with what I said?

Well there's this bit.

Put an large overload force ( ie in a crash) thru a metal component it will bend and fail gradually. Put the same overload thru a CF product it will fail catastrophically – turning into dust.

It you put the force required to break a CF product (e.g. handlebars) through a similarly designed metal product it will fail just the same. It won't "fail gradually", it will just fail. The minutae of the failure mechanism might be different however you, as the person experiencing the failure, will not notice the difference during the failure episode. You'll crash just as hard and you'll hurt yourself just as much. It will make no practical difference whatsoever.

Are you sure of that gonefishing – there won't be bending rather than fracture?

I have seen bent metal components
Edit – like this?

Would a CF bike be in two pieces if in the same crash?

"Instead of snapping the crown assembly has bent."

I'm not sure what you mean there.
It's not very clear from that photo, but it looks to me like the carbon leg has snapped immediately below the aluminium crown.
Or is there a spigot on the crown extending down inside the leg and it's the spigot that has bent ?

Would a CF bike be in two pieces if in the same crash?

I'd have thought that a similarly designed CF bike would have withstood the crash just fine rather than bending. CF will withstand an impact that bends a metal component without failure. The reverse is not true.

As I said the minutiae of the failure mechanism may be different but a failure is still a failure.

if you look at those broken rc31s they've failed exactly how they should have. Instead of snapping the crown assembly has bent. Unfortunately other companies dont do the same thing :S

No, it's snapped at the crown from what I can see?

At the end of the day if you have a lightweight designed anything it will fail in a fairly rapid way anyway. Light alu bars will fold and fall off pretty much instantly, just as carbon. Forks are possibly the safest place to have it as you have redundancy of sorts, the second leg will share the load should one begin to lose strength unbeknownst to you, so as above you should see the failure before it goes big-time. But at the end of the day if you hit something hard enough with any fork it will snap clean off, or if it doesnt it'll bend and become so weak it might as well just fall off.

No-one else taken a road bike on a BMX course as a kid and bent the forks, leading to forks that bend on any further mild impact and eventually shear right off due to fatigue?

Gonfishing – who knows?? Of course a failure is still a failure but one that leaves you with a bent component and one that leaves you with a broken one is different.

I fully accept my fear may be irrational as I said – but I personally could never be happy riding CF forks for this reason.

I fully accept my fear may be irrational as I said – but I personally could never be happy riding CF forks for this reason.

TJ, that's fine and I don't think anyone has a problem with it, however statements like this

Put an large overload force ( ie in a crash) thru a metal component it will bend and fail gradually. Put the same overload thru a CF product it will fail catastrophically – turning into dust.

are a long way from that position.

Or is there a spigot on the crown extending down inside the leg and it's the spigot that has bent ?

Production technique on that style of fork is to press a chromoly or even titanium tube into the cnc'd crown, then bond the mandrel wound carbon fork leg over the top.

I have some sympathy with TJ – structural components that break in the way carbon does just give me the heebie-jeebies.

Steel/alloy can often be bent back and get you home. I accept this is all irrational and carbon is probbers way stronger but I doubt I'll evet own much carbon.

Gonefishin – but is that not the case? a tube put under a bending load and the load increased until it fails – the metal one will buckle and bend whereas the CF one will break? Metal will undergo plastic deformation before failure CF won't

Thats my understanding. Is that wrong?

edit I also accept that CF is stronger for the same weight.

Thats my understanding. Is that wrong?

That statement isn't wrong, but the conclusions you are deriving from it are. As I said the exact mechanism of failure between a CF and metal component may be different however the effect will be the same. Just to be clear any force sufficiently strong to break a CF component will also break a metal one and whilst the precise failure mechanism of the metal will be different to that of a CF one it will still fail, and you'll hurt yourself just as badly. The reverse however is not necessarily true, i.e. the force required to break a metal component is not necessarily enough to break an equivalent CF one.

To put it another way, a little knowledge is a dangerous thing.

Sorry gonfishing = I may be being dense here. If overloaded does the steel tube bend and the CF one break? Yes or no?

Edit – relative strengths surely depend how how they are made

Double edit

So ride your bike into a wall at speed. Teh CF one breaks the steel one bends? Both hurt

The important factor with any material is will it fail in such a way that you can dismount safely? (eg disintegration versus bend)

@gonfishing why are you assuming that the force required to break a carbon component will break that of a metal component? it relates down to specific stiffness, material UTS figures and geometry, and without quoting those that argument is fairly redundant.
Anyway when a steel fails it fails progressively and plasticly, this act dissipates energy, mitigating (arguably minutely) any failure. carbon fibre failure modes are poorly understood and catastrophic with zero plastic flow, do with that what you will…
oh and putting a small ding in steel/alu isn't the end of the world, whereas with carbon you get all sorts of nasty effects such as debonding, crack initiation and delamination.

Without knowing the load, it's pointless speculating on the failure mode.
Yes, when it fails, carbon fibre will fail completely. However, given a high enough load (for the design and material spec.) steel will also 'appear' to fail completely. The progessive, plastic, region is only so long.

Best reason I've heard is that is easier to extract metal shards from you person than carbon.

I have carbon forks on the commuter (Bonty Switchblades) and really it's only the weight – the compliance over P2s was nothing you could distinguish after you've added even a 1.5 tyre at 60 psi. I also have carbon bars on the full suss and I'd say the same about them.. but there the weight saving isn't even so apparent.

"Production technique on that style of fork is to press a chromoly or even titanium tube into the cnc'd crown, then bond the mandrel wound carbon fork leg over the top."
That makes sense. So in that picture, it looks like it is the pressed in metal spigot tube that has failed, by going way past its elastic limit and permanently deforming, while still remaining in one piece. The CF fork leg itself doesn't appear to be damaged at all.
So if we take the Cf out of the equation and imagine that as a full chromoly or titanium fork leg pressed in to the aluminium crown, it would have failed in exactly the same way.

"Just to be clear any force sufficiently strong to break a CF component will also break a metal one…"
There's a lot of variables and assumptions in that statement.
I watched a bit of the Olympic track cycling on the telly.
There was a crash where a rider hit another downed rider and endoed over the top of her. The bike could clearly be seen flying through the air in two halves.
Presumably it was a CF frame and a single minor crash had exceeded its design limits.
I wouldn't take that as evidence that my CF forks will break the first time I crash with them, more that track bikes are built down to a weight with a minimal safety margin.

Ok – can I sum up – lets see if I get it right???

CF forks will be ( assuming properly designed and made) strong enough for most xc riding and at least as strong as metal ones.

In the event of exceeding the load capability of the fork the steel one might give you a bit less catestrophic failure but this might make no odds at all.

Some of us are paranoid about Cf components?

Im no expert but i ride with someone who knows far too much about cf stuff and loves his RC31s. He told me they have a spigot or something from teh crown assembly down into the carbon fibre tube. IIRC he said this was the place that gets the largest amount of stress placed on it.

When they do break you hope the spigoty thing bends instead of the CF snapping.

And i do believe that has broken as i said, the picture isnt big enough but the carbon has what looks like a very straight cut instead of a big fracture.

Apparently whoever bought out Paces fork department (DT swiss??) have shown a RC31 prototype with the titanium/alloy spigot made of carbon fibre. Talk about missing the point.

Good summary TJ – that pretty much does it all.

Ask Mark Webber how strong CF is.

Dont know much about the science but back in 2004 did have one leg of my months old set of Rc31s shear completely through about one third of the way down – 9 stone rider doing moderate XC.

how about that video from niner bikes. hang on

bloke lampin carbon forks with hammer

Still stuck on the nuke proof forks being made in same factory as others, or being rebranded. I keep reading about this — how do we know nuke proof don't make their forks?

I'm right on the weight limit of the RC31's and mine have cracked … at the drop out.

The carbon legs held up fine for a number of years, but the magnesium brake side dropout has cracked … forks unusable. I'd say that if i were to get another set, then I would be going for dropouts which are less liekly to suffer the white powder corrosion like the Pace ones did.

As for overall strength, I would say that mine held up well. I avoided drop-offs and like and a wheels on the ground rider, but other than that, they were great.

TandemJeremy
I fully accept my fear may be irrational as I said – but I personally could never be happy riding CF forks for this reason.

Mountain biker makes component choice based on irrational preconceptions rather than sound, peer reviewed, scientific evidence shock.

nuke proof, wide WHITE industries, on-one, someone else and exotic are all made in the same factory i believe. If you want proof just compare the products in detail. The dropouts/crown are the obvious places.

Can you stop this now as i have some rc31 for sale 😯

rather than sound, peer reviewed, scientific evidence shock.

Is there scientific data available on any mountain bike components?