spinconn

I am not seeking arguments or controversy here. I have one bike and it is CF and I think it is as close to perfect for me and my riding as any bike can be.

However, I am not technically inclined and my research into CF has left me with a question I have not seen answered. Perhaps it has been answered and my technical ignorance prevented me from understanding the answer.

I read that I should not allow impacts to CF, should carefully inspect it for signs of damage, should not use certain chemicals on it, should not place it in a bike stand without some aluminum part such as a seat post to bear the weight, should not let it sit in the hot sun, etc., etc. Fine, every material will have an Achilles heel or two.

But I also read of the great strength of CF. What I do not understand is in what specific manner is it strong? Obviously it is light, and therefore must be strong to be functional despite light weight. So, I assume it is able to bear a greater weight on a pound for pound basis than some other materials. I also assume it is less likely to decline due to exposure to air and water.

But, is that it? Is there some other specific benefit? If not, that is fine, I love my CF bike. But I am curious whether there is more to it than that.

tagaproject6

Too much handwringing. Ride. Maintain. Ride. Repeat as necessary.

terrymorse 

Off the top of my head:

1. Impervious to fatigue failure.
2. Enables ability to directionally adjust stiffness/flexibility/strength.
3. Formable into any shape imaginable.

ljsense

It's more strong like a piece of floss is strong, and less strong like a rock is strong. It is not strong like breath can be strong after eating an onion.

It's a cloth made up of very thin fibers. Fibers are good at being strong when they are pulled. They have strength under tension. The glue that holds carbon cloth in the shape of a bike frame is not super strong. It's weaker than metal. Some things don't test the strength of carbon fiber as much as they test the glue that holds it together. Like clamping the top tube, or hitting it with a ice axe, or leaving a bike in a 500 degree oven.

Companies keep experimenting with better glues (epoxy resins, really). But that's the main weakness. The resins get soft at high temperatures. They don't have the same properties as an metal alloy.

indyfabz

Subscribed.

General Geoff

Carbon fiber is very light, strong and stiff, but it is brittle and more susceptible to chemical breakdown from sunlight over decades.

High end bicycles are mostly designed to be as light as possible while maintaining adequate strength for average rider weight and expected terrain. Typically, the lighter the bike, the weaker it is. Carbon fiber frames and components can be designed with adequate strength while shaving a few pounds off total weight, that's why they are prevalent, especially in competition.

86az135i

I just treat my bikes as a bike no matter the material. Somehow I manage.

Notso_fastLane

That's a good summary.

I'm an aircraft structural engineer, and I work fairly extensively with composites (not just CF). I'm not the local expert, though, so most of my knowledge is pretty practical with only some limited theoretical background.

I'm not actually sure what form of CF is used to manufacture bicycle frames. Ideally, it would be a combination of linear (often called tape) which is very strong (in tension) in a single direction and can be wound in places of high stress where extra strength is needed, weave or fabric, which has more uniform (in 2 dimensions) strength properties but isn't quite as strong ultimately as a tape. The resins can be made of chopped fiber resin, which has, as the name implies chopped (usually less than 1/4" average length, but often much less) CF in the resin mixture to improve it's tensile properties.

The fiber, as noted, is very good in tension, but it's like a really strong rope...you can't push on rope. The compressive strength all comes from the resin or epoxy that holds it all together. A lot of the resins are susceptible to UV damage, and as such are treated with some kind of protective coating. They are also fairly susceptible to temperature and can be affected at a much lower temp than metals. An example: Early composite aircraft, (from the 1990s) that had composite wings would have noticeable wing sag if they were kept outside without at least shade, or preferably in an actual hangar.

I hope that answers your questions. If you have any more specifics, feel free to ask and I will try to answer them.

spelger

you make it sound as if CF is nothing but heels.

berner

The video is of a Boeing wing test to destruction. After doing all the math you can think of, testing to destruction provides assurance that the math has told you what you think it has. The narration mentions that the wing is deflected 24 feet at failure and 154 % of the maximum conceivable load. So carbon composite can be tough but it all depends on good design and fabrication procedures. Bikes fabricated by known manufacturers will likely be build according to those criteria.

spinconn

That is remarkable.