Carbon seatpost
 

Apologies, if this subject has been beaten to death, but the search function on this forum is not very helpful.

Looking to replace the alloy seat post on my touring bike. Of course it's 30.8 mm, not the most common size, but do-able. I see the biggest selection for that size is carbon. Some cost $20, others cost $200. Presently looking at what's called 3K carbon.

Any education and opinions would be greatly appreciated, regarding reliability, warnings and any special things to keep in mind.

Thanks

I've never had any problems with carbon seat posts before, but I've never gone cheap on those due to my weight. The only special thing I've done with them is use a torque wrench and carbon assembly gel.

FWIW carbon posts feel no different than alloy ones, IME. I've run both on my steel road bike and cant tell the difference when riding.

Your Touring load will make your bike ride smoother because of the mass...

I have a couple bikes with a seat post sizing shim ,, the Inside is 27.2 , the outside in your case would be 30.8. 1.8mm radial difference .

Id go* for a Cane Creek Thudbuster seatpost rather than expect any great comfort gains from your metal to carbon change
of seatposts ..

* In fact I have 2, one of the long travel, and one of the short travel, both 27.2 shim sleeved out to fit in a larger frame ID..


...

What is your frame made of? If Al, I would stay with an Al seatpost. If CF, go with a CF seatpot. If you mix, make sure to pull it out regularly. Google "galvanic welding".

The "3K" is just the cosmetic outer layer. The inner layers will be made of whatever they make inner layers out of.

Just a thought. Carbon is strongest in tension. Seatposts bear their loads in compression. Don't go light.

I ruined a carbon seatpost once by over torquing the clamp. The seatpost crushed. Nowadays I use a torque wrench and have had no problems.

How bad do you want carbon?

Post -- $$$
Torque wrench -- $$
Assembly snot -- $
Total = $$$$

Get a decent alloy post and let a little air out of your tires. Of course, your rolling weight will be a little heavier with all the money left in your wallet.

-Kedosto

I agree with Mr Fietsbob, a Thudbuster ST (short throw) seat post works very well for me on paved roads. The LT (long throw) is for gravel and trail riding, especially if you are on a rigid frame (or if you're riding a hardtail and your butt complains). A carbon post helps somewhat but nothing like the Thudbuster. And both the LT and ST maintain the seat-to-pedal distance during the compression.

My apologies, if I gave the impression that I was going carbon for the comfort aspect. It's more of the fact that more carbon posts are available in my size. My current seat post was slightly damaged in a high jump and my heavy weight stripped one of the position grooves. Nothing more. I do not want to spend much. If I find a decent alloy for a reasonable price, I will get it. Just wanted to get some experienced opinions

I had the same need. I wanted a post with two bolts at the saddle. In the end the best option for me was to use a cane creek shim to get down to 27.2. After that you have a zillion good options. I went with a oval on eBay but you should find more cheap options than you can count.

My question would be why? There is virtually no weight difference. The cheaper seat posts are very likely to be seconds from other makers. And the correct sizes of aluminum seat posts are usually available for not staggering costs as you often run into with carbon seatposts.

Another thing - many of today's carbon posts have that damned one bolt adjustment that is forever slipping. You can get two bolt adjustment aluminum posts that are extremely easy to adjust to the proper tilt.

What a load of rubbish. Oh, its on the internet, so it must be true!

Sure, you should use common sense and get your bike checked out after a crash. As you should with any bicycle. However, to just scare people away from carbon fiber is just stupid.

Tens of thousands of people buy and ride carbon fiber bicycles each year, and carbon fiber bikes have been around for at least two decades now. I have yet to see a large number of people getting injured or killed from riding carbon fiber bicycles or parts.

You should be more concerned about getting hit by a car, that is how cyclists really get injured and killed, no matter the material their bike is made of.

https://www.irishtimes.com/sport/oth...ries-1.1879653

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In terms of technology three-time Tour de France winner Greg LeMond was a pioneer. When steel dominated, LeMond rode bikes made of carbon-fibre composites, then an exotic material mostly used by the military.
At this year’s Tour, carbon fibre is the only material used for bikes and it has also replaced aluminium in wheel rims. Strength, weight and design flexibility has ensured the material’s dominance. And its most extreme form, the time-trial bicycle, was on display in Saturday’s 20th stage.
But there’s a catch: unlike steel or aluminium, carbon fibre does not bend in crashes. The bikes and wheels frequently shatter, often hurling riders to the road and, many fear, increasing severity of injuries.
“Anyone in a team who’s being honest with you will tell you how frequently their bikes are breaking; everybody knows,” said Mark Greve, a physician and assistant professor of sports medicine at Brown University who studied injuries to 3,500 competitive cyclists. “Few people in the public appreciate how many bikes a pro team will go through in a season, because they break
.”
Having conquered professionals, who ride frames that retail in the United States for $5,000 (€3,700) to $6,000 with forks, carbon fibre is making its way to increasingly affordable models available to the more casual riders.
A code of silence exists among riders, even retired ones, and mechanics and team officials over the carbon bike and wheel durability. The teams and riders exist, in part, to act as powerful marketing tools for bicycle makers. But when they spoke on the condition they not be identified, their stories emerged. Riders described landing on the top, horizontal tube of the bikes during crashes and ending up on the road after frames collapsed. Even small spills often require a bike change. Mechanics say they sometimes return the shattered remains of frames to manufacturers in bags intended to hold a single bicycle wheel."

When lawyers are lining up you can bet that there's something under those circling vultures:

https://www.atlantainjurylawyer.com/...-injury-o.html

In 2015, the U.S. Department of Transportation published a report that noted pedal-cyclist fatalities had increased by 12 percent year-on-year. Now there could be many reasons for this but bicycle recalls are routinely announced by the Consumer Product Safety Commission (CPSC). The safety agency has recalled millions of bicycles due to cracking frames, breaking forks, faulty chains, failing brakes.

https://theracery.com/blogs/news/is-this-carbon-safe

We are continually told that aluminum and steel are no more safe than carbon fiber. And yet we didn't have people crying about sudden catastrophic failures of steel or aluminum.

It must also be said that you obviously do not appreciate the changes in carbon fiber bikes over the last five years. They have gone from "we don't trust this material so we will overbuild it" to "who cares - racers want the lightest possible". The end result is several people bragging about having bikes with all up weights of 12 lbs.

You can absolutely rely on people who have paid $6,000 for a bike to defend it to the death. And let's hope it doesn't come to that.

Well check this out then


or this


Look, it is not as simple as one is "better" or "stronger" or "more durable" than the other. Fact of the matter is, carbon fiber does not fail more easily than aluminium full stop. First of all, there is the difference between impact and fatigue resistance.

Carbon fiber has better fatigue resistance than aluminium or steel (also depending on design, but generally). So it will last longer under normal conditions in fact, because "fatigue" is a particular material property of metals. Carbon fibers and resin are not metals.

Impact resistance: It depends on the design. Carbon fiber is stronger (more resistant to impact) for the same weight. But if you design it to a lower load or impact limit because of additional weight savings, there is where it can become weaker. You can also design one aluminium frame to be less strong (less resistant to impact) then another for example.

You are also talking about two different things: crashes versus everyday use. In everyday use, carbon fiber bicycles are totally safe in terms of both impact and fatigue resistance. You will not exceed what the material can handle by going over potholes, etc.

In terms of crashes: you are talking about professional riders in the above article. Multiple crashes occur during a typical race, at 50 kmph. Of course the carbon fiber frames are (potentially) destroyed after a crash like that. Is that really a surprise? Aluminium frames could similarly be bent after an impact (look at the first video of impact testing of aluminium). As a kid, I bent a steel fork on my bicycle crashing it into some stairs for example. Problem is: with steel or aluminum the damage will be more visible apparent, while carbon might have hairline fractures that are difficult to spot with the naked eye. You should get your frame inspected (X-rayed) after a crash. If you are talking about recreational riders: you will crash seldomly (hopefully), and at lower speed if you do crash. But yes, after a crash, it is possible your bike has sustained catastrophic damage.


Bottom line: carbon fiber bikes are totally safe in daily use, and actually have a greater fatigue (not impact) resistance than aluminium (see GCN video for fatigue testing). So if no crashing occurs, carbon is actually SAFER, and will last longer than aluminium.

Impact (crash) resistance is a different matter, and you should have the frame inspected by a professional after a crash. There is no "conspiracy theory" or "omerta" here about carbon fiber bicycles. Everyone with common sense knows that a bicycle can be damaged in a crash. The only "danger" is people potentially being stupid and continuing to ride a carbon frame after a crash without proper inspection. Carbon fiber CAN sustain damage not visible to the naked eye, unlike aluminium or steel (you can't even physically ride a bent frame or fork).

Sadly, people generalize all of these aspects and take them out of context. Steel, aluminum, titanium, carbon fiber, all have their uses in bicycle manufacturing.

:lol:

If you believe those tests to be an accurate depiction of what occurs with a carbon fiber failure then all I can say is that you are not an engineer and I am. I have several boards on the original three part International Space Station. I am presently on the list of approved engineers for Lockheed. I look at that test and I am so shocked that anyone would look at that and not understand that it isn't what happens that I simply cannot understand how ANYONE could use that as support for anything.

OF COURSE a new faultless CF frame fails in a different manner than an AL frame. What surprises you about that?

If the CF frame had a few bubbles in the layup and it exploded apart at 50% of the load of the AL frame what would that prove? Not a damned thing.

The failures of carbon fiber occur over time. The resins begin to harden and crack. When enough of those cracks interconnect with each other the frame fails dramatically. The AL frame is about the same strength after 10 years as new. My steel frames are from 1990 and have exactly the same strength as when manufactured.

I'm not sure how to get an attachment on this but I'll give it a try. This failure occurred on a Colnago C40 that had been babied its entire life. It occurred at below 5 mph and the owner is permanently disabled.

Wow that's nuts. Do you have a link to the story? I googled but couldn't find anything on a cracked C40, 5 mph, disabled rider, etc.

TIA

I guess I should have expected this to get into a tangent debate. The one thing I've noticed is the people who are claiming how unsafe carbon is and for the number of members there are on this forum, I have not seen any direct stories, only internet hyperbole.
Thus, what I have appreciated until now is; use a torque wrench.

Thank you for the input!

To me, it seems that way. Maybe I dozed off...


The C40 looks like faulty design. The Engineers probably didn't know better at the time. :D

Lol, I see an impact test and a fatigue test, and I see what impact and fatigue resistance the materials have. How does that not prove anything?

As a matter of fact, I do have an engineering degree in construction engineering, because you brought it up. I took a materials science class or two during my studies, including some lab experience with carbon fiber. I won't profess to being an expert on carbon fiber though. In my college lab, this is how impact resistance of materials was tested, exactly as shown in those lab tests in the videos. I don't see anything wrong with their methodology.

It doesn't. That was my point, they are different materials, they fail differently, have different properties. Both are good materials if designed with properly.

The polymer resin is supposed to "harden" during manufacturing lol, that is what the resin does. Do you mean it gets more brittle over time? I have never heard of this concurring. Do you have some studies to back up that bold claim? I believe resin is very brittle to begin with, but since CFRP is a composite material, you cannot look at the properties of any one component, but rather the whole composite. The fact that you refer to the resin as "hardening" versus "becoming brittle" seriously makes me doubt you have a materials science engineering degree.

In the case of the Colnago C40: They used carbon lugs bonded to carbon tubes on those models. The failure seems to have occurred where the lugs were bonded to the tubes. Perhaps in those early C40 models (manufactured from 1993 onwards) they got something wrong with the bonding of the tubes to the lugs? You cannot really compare this to a monocoque frame (made out of a single piece).

Again, I would love to see some actual scientific study or paper showing that the resin in the carbon fiber becomes more brittle over time. Have never heard of this, but I would be genuinely interested.

By the way: if you are an engineer for Lockheed, you surely know that the new F-35 fighter jet, made by Lockheed Martin, is using a lot of CFRC in its body. I suggest you never test fly one if you are so concerned. https://www.compositesworld.com/arti...e-f-35-fighter
Or never get into a Airbus A350, which uses 52% CFRF in it's body: https://en.wikipedia.org/wiki/Airbus_A350_XWB

That guy is a retired Federal Investigator and an NCIS teamleader and he doesn't put any information on the Internet for obvious reasons.

I call troll at this point.

Sad you don't have anything better to do than spread misinformation on the internet to people who are actually looking for good advice from people who are knowledgeable.

As a construction engineer I would expect you to be more knowledgeable about resins than that. Resin never really cures completely and continues to harden over their lifetime. Progressively slower and slower of course but at the same time the resin grows more and more brittle and is degraded by UV light. This hardening in the older bikes didn't much matter because they were so overbuilt that even the resin hardened almost to glass hardness was stronger than any expected load on these bikes. But the new bikes are not built for strength, they are built for ultimate lightness. Several people have bragged that DuraAce equipped bikes with DuraAce wheels (not the lightest around) are under 12 lbs. My brothers 10 year old Giant TCR-0 weighs 16 lbs. Giant well knew how to build lighter bikes then but didn't. Now you ain't got a bike unless it's well under 16 lbs.

The test of aluminum frames and carbon fiber frames when the strength of the CF is at it's highest expected lifetime strength is hardly an accurate test don't you think? Colnago has some of the best CF engineers on the planet and they give a 3 year warranty on their C60 - what, a $20,000 bike with a complete group and reputable wheels for a bike that expensive?

Why would you expect a company that is supposed to be one of the best on the planet to offer a warranty that short? Because they think they might get sued? Doesn't work that way. They're an Italian company and there aren't laws that allow that sort of thing. What's more, if they were that worried about it they could simply close their US distributors and be proof against that sort of thing.

When these CF bikes are first completed they have their resins something like 92% cured under heat and they are at the point where they have the highest strength while having the greatest flexibility of the resins so that it is proof against brittleness.

And if you read my treatise above you would see why they can get away with using CF in the F-35. Would you expect a supersonic aircraft to be built for lightness or strength? What's more there is a "useful lifetime" for a fighter. And I would love to see what it is expected to be for the F-35. Aluminum fighters are lined up by the mile in the desert storage yards in the sun. B52D's that I worked and occasionally flew on in the Vietnam War are still sitting on the pad at March AFB and they could still call me up to work on them. That model first flew in 1954. I will guarantee you that any fighter with CF parts will replace them on a timely basis. The Buff-D would need fuel. And I could design a new bomb/nav system the size of a PC and a Laser radar 10 times more accurate instead of almost half of the total electronics on the aircraft and weighing perhaps a ton.

Anyway try looking at https://www.google.com/search?q=carb...7vwIxNcvhCn1M:

Many of these failures were from crashes. They were crashes that any aluminum frame would have lived through with little damage but I think it's still unfair to count crashes as a CF failure. But plenty of them were just plain failures for no reason whatsoever. And in the TdF I saw one of them occur near the back of the pack. On the flats is where the climbers congregate and they have special very light bikes built for them.

Remember that these bikes for the grand tours are built just shortly before the races and they are at their peak strength.

So those tests on Santa Cruz frames are unreliable because besides being on new frames that are at maximum strength the forces are applied slowly and smoothly. A crash is liable to give full loads in milliseconds when an aluminum frame would generally flex and bounce back. Putting a load like that over such a long period of time is not a good test of the strength of the frame material.

I didn't say I was an engineer at Lockheed. I said that I am on their list for projects in the SF bay area and I suppose they don't have any right now. Since I got that initial contact I figured it was a spam and didn't answer it. But I got a second "please respond" and tracked it back to Lockheed Grumman so I know it's the real thing. At one of the companies I worked for I designed and programmed boards used in the International Space Station which at that time was only three modules. Now I think that it's more like 15. Elon Musk says he's going to expand that.