Darth Lefty 

How not-brittle and impact-tolerant do you think the carbon fiber is? Strain at break is about 1.5%. It would be a pretty terrible paint that couldn't follow that.

Koyote

Paint and gelcoat are two different things.

Branko D

There is paint and clearcoat to protect it from UV. Material fatigue is just unavoidable with use, though.

Bike going soft over time is a better failure mode than ride -> crack.It would be cool if there was a commercially viable (read: cheap) method of assessing bike frame damage.

Darth Lefty 

Hey thanks for that. Lots of things are different than other things. Some of them are pretty equivalent, though, like a pigmented coating that can go on top of a surface and protect it from sunlight and other elements. Gel coat survives the strain on something actually designed to a margin of safety like a sailplane wing. Bike frames are pretty overbuilt by comparison, being designed to a stiffness and not a strength

Mojo31

Your post doesn't make much sense.

Raw carbon fiber is fairly brittle, but when laid up with epoxy to form the tubes of bikes and other things, it is not so brittle. The layups are designed to accommodate structural movement in places where it needs to flex and rigidity in other places. The layup is not the same in every part of the bike. For example, the head tube and bottom bracket will be made with a layup that is much thicker and stiffer than the top tube. In the areas where flex is desired, it will be layed up in a manner to accommodate that without it cracking when it flexes.

Carbon fiber is subject to impact damage, much more so when the impact is to the side of the tube, and much less so when the impact is to the end of the tube. But, it is not, when properly designed, brittle in the sense that it will crack under load.

Gelcoat is not a good finish for carbon fiber that is made with an epoxy resin as there can be compatibility issues. Normally, when CF is going to be coated with gelcoat, it is layed up with a polyester or vinylester resin. Polyester and vinylester resins are not the best for bikes because of load bearing issues, which is one of the reasons why CF bikes are made with an epoxy resin.

Gelcoat is typically applied to polyester or vinylester resin layups such as fiberglass boats because it is well suited for that use in recreational uses. Gelcoat is used to finish those products because it is longer lasting than paint and more subject to wear such as the friction from shoes. Part of that is due to the thicknesses at which gelcoat is applied. You can see the brittle nature of gelcoats by looking at almost any boat that is not new. You will see spider cracks at surface curves and other areas of point loading. Those cracks occur due to the constant flexing of boat hulls and decks as they go across the water. They are generally cosmetic and do not usually affect the underlying fiberglass layup.

Carbon fiber is being used for boat hulls and decks, especially those built for racing purposes. Those boats are more often than not painted due to the compatibility issues between epoxies and gelcoats. In addition, for reasons I'll explain below, the builders would not want to use a gelcoat on them Masts, booms and tillers are also made from CF for sailboats, but they typically need UV protection is they remain uncovered long term. CF masts and hulls, for example, are generally painted with a high UV rejecting paint such as Awlgrip.

For other things like bikes and cars, gelcoat is not a very desirable finish. One of the big issues is weight since gelcoat is applied at a 18-25 mil thickness versus 2 mil for automotive and bike finishes. The long, flat surfaces on RVs lend themselves more to a gelcoat finish.

So, if you want a CF bike that is coated with gelcoat, you will have to give up a lot of weight and will have to be happy with finish cracks in the areas where the tubes are joined or where they flex/stress.

For metal bikes, gelcoat does not adhere to metal. And, I don't think anyone wants a fiberglass bike.

genejockey 

Give it to Mark Cavendish?

ctak

Well said. As damage tolerances improve, gradual softening of the carbon matrix is much more likely than a sudden snap or crippling crack (conventional wisdom) for the vast majority of users.

qwaalodge

My experience with raw carbon fiber is different. I actually found the raw fibers flexible It feels quite like wood fiber to the touch and I could bend it 90 degrees without breaking, just like wood fiber.

The CF-epoxy product on the other hand would break long before bending 90 degrees. It's the epoxy that is brittle.

Mojo31

In the composites industry, raw CF is considered a brittle material. Its flexibility depends on its construction. You may have had a low modulus fiber in your hands.

Carbon Fibres: Production, Properties and Potential Use | Material Science Research India (materialsciencejournal.org)

Carbon Fiber Grades: It's all About Tensile Modulus (rockwestcomposites.com)

Once layed up, the break characteristics depends on much more than the use of epoxy. Twill weave, regular weave, high-low-intermediate modulus fibers, type of epoxy, layup thickness, tube or flat, type of impact etc. all play into it.

Mojo31

The CF bristles on my record cleaning brush are very flexible. There are lots of types of CF.

SoSmellyAir

​​​

According to the F1 polymer scientist in the video, it is merely a measurable reduction in stiffness or elastic modulus, which he (a) questioned whether most cyclists can even feel, but (b) conceded may make a small difference in professional racing. Softening makes it sound like the epoxy resin is melting and turning into caramel or putty.

According to this BikeRadar article (Inside FFWD's Dutch factory, where 'handbuilt' is the mantra - BikeRadar), FFWD X-rays its carbon fiber rims to check for latent defects.

This forum has given me the impression that a fair number of road cyclists are also dentists, of whom some are named "Fred". Every dentist I have visited has an X-ray. Maybe I am onto something here?

Darth Lefty 

You've talked me out of gel coat. Let's paint!

ctak

Agreed that many cyclists aren't likely to notice this kind of subtle material degradation over time, irrespective of whether this scenario is more likely than an outright catastrophic failure.

I'm no domestic pro but when fresh can hold 400w on a steep 2-mile local climb that has been part of my weekly routine for years. This is a hard effort, above threshold, with plenty out-of-the-saddle moments. At this power output, my intuition tells me that one of my higher end carbon bikes feels noticeably "softer" (less punchy / responsive to inputs) in the bb area than it used to on this particular climb. OTOH, there are a million variables at play so I can fully appreciate quantifying such things outside of a lab is beyond my capability...

jaxgtr

<cough> iPhone </cough>

merlinextraligh

uhm, Cavendish is a little guy, who is also extremely aero. So he produces a lot of speed out of watts, that would not be exceptional for an amateur racer that weighs 100 kilos.

I’d wager my fat ass puts close to the same stress on a frame that Cavs does.

ctak

+1 on this, not to mention a pro cyclist's torque and power phase values are going to be considerably smoother during medium/high power outputs

genejockey 

Pedantry - it's what's for dinner!

Seattle Forrest

I tell people they need to replace their CF frame once a year, but only if it's a 58.

Koyote

And you offer a convenient frame disposal service.

merlinextraligh

Pedantry is a slavish attentions to formal rules or minute detail.

My comment went directly to the heart of your argument. A small pro cyclist does not necessarily place significantly more stress on a frame than a larger heavier amateur. Thus how long a frame might last for a pro cyclist is not a reasonable basis upon which to extrapolate its lifespan for others.

genejockey 

Pedantry is also mistaking a joke for an argument.

Psimet2001 

Assuming this is a legit question - Mad Fiber was a part of the Divine Cycle Group debacle. This was the era when everyone thought that these funding groups were somehow a good thing. They led to the death of Blue, Serotta, and Mad Fiber. I can't remember if the Guru mess was theirs or not. Guru machinery ended up with allied BTW.

There was an auction at Mad Fiber. I considered it after getting the head up from Rukus Founder Shawn Small. They wanted way too much for what it was and the money needed to buy it all and transport it was ridiculous. They also split the company into 2 lots - 1 with the IP and Patents that either Hjertberg or like SRAM bought up and the physical assets from production excluding the parts really needed. Some plywood molds and fixtures but not much else. No ovens, etc.

I still see a few sets every now and then for maintenance or repair. The riders swear by them but tbh - they never really did anything for me.

Psimet2001 

Didn't watch the link but yeah carbon bikes will eventually "break down" enough to change the ride quality. Not so much the matrix but rather allowing some of the fibers to move in the tiny gaps that the composite had as a result of layup. Should be less of a factor on more modern frames as techniques have gotten better. I know quite a few racers that compete at a high level who would talk about how their main bike would start to ride differently than their other frames towards the end of the season. Those riders are not "you". You are fine and yes you can ride your carbon bike for the foreseeable term of your life.

eduskator

Everything in the universe has a specific lifespan. That's the beauty of it!

I do believe that 99% of us, mere mortals, could not notice the difference from a fatigued CF frame versus a new one if the 2 bikes were ridden back to back.

ctak

Thanks everyone for weighing in on yet another debatable GCN topic. Certainly some strong and varied opinions floating around on the lifespan of this mysterious fiber and its binders.