How strong actually is carbon fibre?
 

I often see stuff regarding how strong carbon fibre is, but I also see stuff about how risky carbon is as its literally fibre networks, I'm assuming its much stronger than aluminium, however whats the chances something happens, around the same as aluminium bending? Is it really something to worry about if I get paint scratches and stuff? What should I do about them, and are they safe against water etc? Thanks.

It's strong enough, and has been proven for long enough, that we don't worry about it when it comes from reputable manufacturers. If it gets damaged, beyond a simple scratch, one should have it looked at by a qualified professional - as one would with aluminum.

This is my 20+ year old carbon MTB. It's beat to hell. It still rides just like it did when it was new, and I have zero concerns about it exploding. I still ride it multiple times per week.
https://i.imgur.com/WgI78DQ.jpg

This might address come of your concerns. CF exceeded aluminum in every test.

Depends on the design and construction of whatever is being made, same as with any material.

If designed and constructed properly, CF is much stronger than any metal used for bike construction. Let's say a Surly steel touring bike frame weights 6 lbs - if some company made a properly designed 6 lb carbon fibre touring frame, it would be much much stronger than the steel frame.

The 'IFs", of course, are the unknowns - is the frame properly designed and constructed? You can ask the same thing about steel or aluminum or titanium frames. All we have to go on are the reputations of the manufacturer.

This is kind of what I was thinking about, I've seen a lot about how scratches in carbon can be detrimental, but any cosmetic scratches or scrapes should be absolutely fine? And the level of severity that is dangerous is similar to aluminium, like cracks or dents, however carbon is just more prone to such damage, is this correct?

Cosmetic scratches in CF are a non-issue. Impacts hard enough to actually damage a CF structure will also likely be hard enough to damage an aluminum structure. CF is NOT more fragile than aluminum.

I crashed an aluminum-frame bike, with a carbon fork, into the back of a car at probably 15+ mph (the car had a stop sign and entered the intersection; I didn't have a stop sign and was headed downhill). The collision crumpled the aluminum down tube. The carbon fork had no visible damage (although I'd be hesitant to use it again). A very small sample size but demonstrated to me that carbon can take a lot of impact.

it it not yes/no stronger or not kinda answer a material can be strong or weak, but the end product strength and durability depends on the design and build and quality control.

as an example a quality carbon mountain bike will be engineered with lots of strength to take the stress of jumping and with enough material to deal with crashs etc, where a high end road racing bike will be engineered for strength where needed and less strength where not needed to keep it light. (often in the top tube)

There are reasons some manufacturers state don't use the top tube for clamping or transport

so in some cases the bike can fall over and you can damage it like in this thread https://www.bikeforums.net/bicycle-m...bon-fiber.html
and in other cases you can crash a mountain bike with no problems

from what I have seen the chief issue with carbon is that when it fails, it fails fast without warning and it's overall structural integrity is as whole composite, so a deep cut into the composite can cause loss of integrity, so in general a lighter carbon bike should have more TLC and should be checked for potential deep cuts

Quality made carbon fiber products are fantastically light and strong. Modern carbon technology has come a long way.

More than 10 years ago, I wound up t-boning another cyclist in a freak accident while riding an old lugged late 70's steel Italian bike (A Bertoni). The other bike was a fairly new carbon road bike of some flavor. He rode off nothing more than a little irritated, no damage to the bike. My bike, however, had the frame and fork bend so far as to push the wheel into the downtube.

Slightly off-topic, but I wonder if older carbon bikes are stronger than newer ones. Were early carbon frames and forks overbuilt to avoid failure, since the technology was not as well tested at the time? The bike I crashed had a much bulkier carbon fork (2006 Scott CR1 fork, part of Speedster S20) than my newer bikes.

Quite true. I am a certified Golf Equipment Professional and have worked with CF for over 30 years. The material is strong enough to withstand impacts of over 100 mph when used in a golf shaft, That does not imply that CF can withstand a 100 mph collision with a golf ball. That impact will break a golf shaft in two. I have seen countless occurrences of this. But if a tube structure is properly made with high quality pre-peg it can withstand all the twisting and torsional forces placed on it in a bile frame.
CF is used in a lot of other applications other than bike frames. Some sections of airplane tail fins are CF, as are the entire bodies of a few $180,000 automobiles. CF is susceptible to extreme heat but the damaging temperatures are over 350* F, and that is unlikely to be applied to any bike frame. The most damage CF sustains is through high speed impact or wear from abrasion. Things that are unlikely for a bicycle frame. HTH, Smiles, MH

A carbon fiber bike frame or component is generally strong enough if its replacement cost is simultaneously high enough to give one pause and low enough for one to be able to afford to actually replace it if it is damaged.

Common logic would certainly lead you to believe the older, thicker stuff is more robust, and it's probably true that early carbon fiber was overbuilt out of an abundance of caution. However, the reality is, modern, lighter carbon is as strong if not stronger than its older, heavier counterpart. At this point, there's been a few decades worth of production, experiments, testing, analyzing, etc. This allows manufacturers of carbon fiber, in any industry, to know how to best lay the sheets, where to add more layers, where fewer layers won't compromise structural integrity, better bonding agents for the carbon and so on.

While steel is not exactly the same, the concept is and can be seen when comparing a car from the 60's to a car of today. Back then, pretty much the same kind of steel alloy was used on the entire car. Where the frames needed to be tougher, simply more of it was used. Made for a heavy and sturdy feeling car. A modern car of similar dimensions will be far lighter while being more rigid and dramatically safer for the occupant. This is thanks to leaps and bounds in metallurgy and our knowledge of it. Different alloys are used in different places depending on what that part is needed to do. Some of just the sheet metal on newer cars is so hard that a normal drill bit will hardly scratch it. Same can be said for the frames.

I know that was a bit long winded of a reply and I could have probably said the same with half the words, but you get the idea regardless.

That being said, I'd love to get my hands on a mid-90's Trek OCLV or Kestrel. There's such a Trek for sale locally, in my size AND my favorite color and I am so sorely tempted to drop the dough on it.

you need to define “strong”. CF is stiff and light, AFAIK has great fatigue resistance, but has poor impact resistance, won’t hold a thread like metal, and will shatter if bent. Scratches etc aren’t a problem as long as they don’t reach the fibers

I have a 1993 Trek 5900 OCLV, their top of the line road bike at the time. It's still fine. Fork is heavy by contemporary standards, partly because it used a steel steerer tube. The forks appear to be all carbon fiber, but very tightly compacted to the dimensions of typical steel forks. With a newer style hollow fork and carbon fiber steerer, that bike could still be built up to around 17 lbs or lighter. The steel steerer is the single heaviest thing on the bike. Origin8 and a couple others still sell those style forks -- carbon fiber forks made to the dimensions of older steel forks, with steel or aluminum steerer tubes.

My nearest LBS is a longtime Trek dealer, since the 1970s or early 1980s, with the original owner still in charge. He was so surprised to see a decent 5900 OCLV after so many years we chattered for about an hour. Usually he'd try to sell me a new bike, but he seemed so impressed by this one we just talked shop and looked at old catalogs. (I bought some stuff anyway, as I usually do when I visit that shop -- tubes, whatever.)

Very different from the larger dimension but hollow carbon fiber forks with carbon fiber steerer tube on my 2014 Diamondback Podium. When I got that bike second or third hand I disassembled the fork to examine the steerer tube, headset, frame headtube, etc., to be sure everything was okay. The fork has a nick on the paint on one side, apparently due to scraping it against something while leaning, not due to crashing. No problems.

Coming from an old school steel bike background, I was a bit concerned about both bikes at first but after plenty of miles on some rough roads, no worries now. I've cracked some rims on 1980s super light low profile aluminum rims intended for mountain stages (Wolber Super Champion Alpines, Araya CTL 370), but haven't managed to harm these carbon fiber frames or forks.

https://dragonplate.com/what-is-carb...igher%20values.

Not bad as far as a manufacturer's/vendor's "marketing spiel" for their product goes in terms of being obviously biased - but still IMO a marketing spiel designed to sell a product vice an unbiased evaluation.

The following link IMO provides a far better evaluation of CF's pros/cons as a frame material. Link is to the CF section of a much longer article written to give non-metalurgists insight into the trade-offs of one material vs another in bicycle frames.

https://bike.bikegremlin.com/11843/m...or-cyclists/#5

It's dated, but best I can tell it's still valid in terms of analysis and evaluation. It also appears pretty evenhanded.

It points out something engineers have known and been dealing with for literally centuries: perfect solutions do not exist; there are always trade-offs.

I thought it answered the actually question in the post title reasonably early in the speil.

"The modulus of carbon fiber is typically 33 msi (228 GPa) and its ultimate tensile strength is typically 500 ksi (3.5 Gpa). High stiffness and strength carbon fiber materials are also available through specialized heat treatment processes with much higher values."

It does, but only gives a partial and misleading picture of reality.

Tensile strength and modulus are only two important factors for a bike frame material. Both are very important, but others - ductility, elongation, "toughness" (defined by Nichols as "the ability to absorb energy by deforming plastically before fracturing"), compression strength, and the material's fatigue limit - are also very important when it comes to a bike's frame. The article you quoted doesn't address them. That's probably because except for fatigue limit (not sure about carbon in that area), carbon doesn't exactly shine in those areas. Those limitations also have to be considered and addressed.

Here's an example: as Nichols points out, monocrystalline silicon (the substrate used for many electronic components, like microprocessors) is vastly superior to aluminum in a number of areas. If you only focused on those good properties, you'd think it's a great choice for a bike frame.

Unfortunately, it's also brittle as hell (low elongation and not "tough" as Nichols defines the term). So it's pretty much useless for building frames.

Bottom line: you can make many things look "perfect" if you focus exclusively on the things they do well while not mentioning the areas in which they don't. But proper design demands that all essential factors be considered and any shortcomings addressed - otherwise you're deluding yourself.

Carbon is a good frame material, with limitations that must be addressed. The same is true of steel, aluminum, and titanium. Great frames and absolute dogs can be made from all four - and with "mix & match" combinations (carbon forks on new titanium frames seem to be effectively standard equipment these days).

And what's "great" and what's a "dog" depends on the intended use. A Tour de France racing frame will have very different design criteria than does one intended for consumer use over multiple decades - or it should, in any case. The TdF frame only has to last a month, maybe 3 or 4 if it's used for training prior; a consumer frame should last considerably longer than that.

Or, in other words: perfect solutions and materials don't exist; there are always trade-offs. (smile)

The Boeing 787 fuselage is composite and carbon fiber. As far as I can determine, there have been no issues with this design

My favorite non-cycling related use of carbon fiber to cite whenever these discussions come up is the telemetry antenna mast(s) on the Voyager spacecraft. As of last week these things were over 14 billion miles from Earth, travelling at speeds of over 35,000 mph through interstellar space. The antenna mast is still intact 44 years after being launched.

True. But I'm not sure that's a particularly good example to use.

An unpressurized item in free-fall in a vacuum (like a space probe) doesn't generally experience much in the way of forces and stresses other than those produced from thermal expansion/contraction and gravitational forces during close fly-by to another object. (Launch is a different story.) And as I recall, even those forces/stresses on the object's structural components are rather low and easily accommodated by proper design. Since only inertia needs to be overcome, forces needed to move components in zero gravity are likewise generally pretty small if internal friction is kept low by proper design.

In contrast, the Boeing 787 does experience significant forces due to aerodynamic drag, thrust, landing impact, etc . . . .

^^^Fair point. Plus I recognize that CF layup technology has advanced significantly since 1977. But I would think surviving the occasional impact with space dust and/or micro-meteors @ 35,000mph might still be reason enough to convince recreational cyclists that the chance of their frame asploding while JRA is misguided.

and the design and requirements for a fuselage and a bicycle frame are not any any way similar, this is simply not a valid comparison. it gets made all the time, all the time but is apples to avocadoes