Do carbon seat posts really snap?
 

Or is it a bit of an urban myth?

Or is it like, only 1 in a bazillion?

A very good question.
Carbon fiber seatposts and handle bars and whatever tubular items we so choose, are all made up with the same general construction method. The fibers of carbon are woven into a fabric, which is strong in tension, then glued together with an expoxy resin, this gives it the compression strength. In order to make the part durable, they determine where they will have the most stress and build up the thicknesses accordingly. The flip side of that is the lesser stressed areas are thinner.
In the original days of carbon handlebars, they made them quite thin indeed. Just as with aluminum, carbon's big weakness is stress 'risers' which form from scratches and dents. These risers are typically caused by the clamps from brakes, shifters,seat tubes, etc. Before the general public was informed on this, there were more than a few failures, which led to the mass resistance to it that still persists.
The new carbon components have been more thoroughly engineered, combined with advances in the expoxy resins, effectively eliminating the incidence of failure. Easton Components(a leading carbon component maker) even has a policy to accept and test their components if you feel it has been compromised.

So, to answer in summary:
No. Carbon components do not fail, unless they are compromised(usually a pretty deep scratch or crack).
Be observant and the peices will take care of you.

Indeed so, you get a reputable company like easton or thompson and your carbon fiber product is going nowhere, the process of making carbon components is lengthy so that explains the price, but if a company is selling one at very cheap they are skimping somewhere. A cheap carbon component yea it may snap so keep that in mind

Do they show signs of stress before failure? IE small stress fractures as seen in metal frames? It's hard to tell if seatpost marks, etc. are structurally significant. I find it hard to believe that unless you're doing huge drops or jumps, or crashing, these components would fail with zero warning.

Well, ive never had a carbon component fail on me. Yes i hear they fail without warning, but really though, if you have a huge deep scratch in there, or a big chip, it's wise to replace it. I have carbon fiber handlebars (EC70) awnd they have minor scratches from shifters/brake levers but these are just surface scratches so that wont make any difference, just dont clamp it tighter than need be.

They teach us in aviation there is no such thing as a sudden failure. There will always be a minescule crack or something of the sort if there was impact damage. As far as scoring goes, a general rule of thumb would be(dont burn me at the stake for this) 5 percent of the wall thickness. They designed a certain ammount of redundency into the process, so if it does fail it generally was a progression of an earlier problem.
As you stated, the components if designed propperly and used within thier designed parameters don't fail. That is one of Carbon fiber's big selling points. Aluminum items have a fatigue life, after which they will crack or be suspect. Carbon has no fatigue life to speak of. Outside forces will damage it easier however.

If you're gonna be using carbon components USE A TORQUE WRENCH!

Also, if it's a seatpost don't grease it, carbon sucks up water like a sponge. Well, maybe that was an exaggeration, but it does soak up water.

There was one way of testing carbon components for structurally compromising scratches/dents, it had something to do with running an electric current through the part in question.

BTW I take care of over 15 bikes for me and my family and most of the adult bikes have some sort of carbon component on the bike somewhere. Seatposts, handlebars, cranks, seats, shifters, spokes, stems, forks and seatstays.

To date none of the carbon parts have failed or seem in danger of failing.

I can only say this... If my Bontrager carbon post hasn't snapped holding up my 275 lb. ass, there's no way in hell that anyone on here has anything to worry about. ;)

The testing method you are referring to is Eddy-Current. That is for conductive materials. The method you really want is Ultrasonic, such as the doctors use. Niether are accessible to the normal mechanic, I have both in my hangar but the machines cost mucho deniero.

May we refer to this as the Vincenzosi Test? :)
Tom

I saw a broken Specialized carbon seatpost this summer, and the owner was not very big at all. It broke just below the seat clamp, far from the clamp bolt, so it wasn't an obvious over-clamping issue. It was fairly new and broke about 2/3 of the way around, so the seat was a bit floppy if yo usat on it.

This is just one data point, but it made an impression on me... Though I still have a carbon Chorus post on one bike.

[

my bike mechanic, has applied a very thin film of grease on the seat post, it is still okay, he's reason is that it will stick to the seat tube in due time, He knows what his doing, as for the grease being absorb by the carbon material, that is not true, I have taken my seat post after a several months with the grease in it, there's no way that the grease has been absorb by the seat post

Thanks for the replies.

I already have a tiny surface indentation on my Cervelo post, so I'm not gunna use it.

Even if there's the slightest risk, why bother?

Grease? Water? Since when is grease made of water?

I've had one experience with carbon. A seat post. It broke. My fault for treating it like metal. Showed me there is no margin for error. I value my @rse too much to use it again.

I'm with Raiyn. Aluminium doesn't require a torque wrench (like, who's going to carry a torque wrench on the road when you need to make a 2mm adjustment on the post?). My alloy seat posts have lasted... like... forever so far. And for the CF post I had, with the clamp and all, it was the SAME weight as the alloy post/clamp it replaced.

I agree; I reckon they're not worth it. They aren't all that light (I wouldn't use a feather weight one anyway), they seem to get damaged easily, and, like most carbon stuff, they're usually way too expensive. I only had one because it came with the frame.

I'm also a dubious about this "comfort" factor they're supposed to provide, especially on
non-sloping bikes with only 5" of post showing.

Well CF does absorb some of the road shock indeed, dont make the mistake of thinking its going to be like a suspension but it does absorb a small amount, road shocks really.

If you are the kind of person that breaks a lot of stuff carbon parts can be tricky. They do tend to be very durable but they don't let you know of impending failure by bending or cracking like a piece of steel or alluminum will. That is they will flex or spring until the reach the failure point and then they stay bent or fatigue out and start to crack. Carbon flexes until it reaches the failure point then it breaks. Carbon usually does not fatigue(Well it does, but it takes hundreds of thousands of cycles)
I think the Specialized posts were breaking where two parts were bonded together.

Unfortunately, I'm the kind of person who's having hamstring problems, so I've been adjusting my seat height almost every ride.
All that clamping and unclamping can't be good for a carbon post.

I also have a bad habit of slamming back onto the seat during hard efforts......YEEOUCH!

for real...my girl had the same post and it craked...It was a big a$$ crack too...and the bike was 2 weeks old.

I am telling the truth... Anyhow we turned around and got another one b/c it seemed to be a defected post...my girl has not had any problems with the new one...

There have been recalls on some products and there is a way to look them up...i can't remember.

Carbon fiber constrction is pretty much like fiberglass except the fabric in the resin is a lot tougher. It is much more difficult to do right as it is a precise process.

My problem with carbon fiber is that the end user must trust the person or machine that made the part did it right. There is no reasonable way to inspect or test the construction of these things. I agree with the earlier comment about cheap carbon fiber stuff. The manufacturer usually knows what his product is worth.

It is a tough material, but it is prone to bonding issues and to flaws in construction. A great deal of the destruction in the Fastnet sailing race several years ago was caused by improper construction of carbon fiber rudders on a whole class of sailboats. These rudders were built by the Lotus formula one tub builders who were the pioneers of the process.

That being said, I've seen a race car tub that was repairable after a 135 G hit. No tubular frame or monococque aluminum tub could have stood that impact.

I personally would not use a carbon fiber seat post as there are more than adequate alternatives.

That is only my opinion and what do I know, I'm old ;o)

Doc

how about you get an aluminum one and not worry about it ;)

EDIT: thanks hobbes, now that i read what i typed, it makes no sense at all :p

If they don't fatigue then it seems to me that smart companies would provide pressure testing (and perhaps that ultrasonic checking as well) of their product before selling it. Yes that would increase the price but its already an expensive product. I think any customer who would clunk down CF level of cash in the first place would appreciate such reassurance that it was done right.

@dirtbike Perhaps you meant to say "get a aluminum one" and not "don't get a aluminum one"? Otherwise that sentence doesn't make much sense.

Easton Components does test. Because of the massive liability involved with something like a handlebar or post cracking they can't afford to have a careless mistake cause a lawsuit. I do beleive I had a magazine article (Mountain Bike Action?) from a few years back that showed all thier torture tests. Nonetheless like he said, you have to ask yourself is the reward within reason for your use. its not for freeriders or most douwnhillers, but for those who take care to inspect and maintain carefully, it can reward you with much longer service lives and lighter, corrosion free performance. Personally, if I had the budget I would have a carbon frame. But, I don't have the money so I am stuck with metal, which for lack of a better term is cheap, and fixable with normal means.

I know of a carbon post that snapped during the Hawaii Ironman and the result really sucked. It may be rare, but it does happen. SuperGo didn't recall all of those bars last month because of poor sales!