OnyxTiger

Just saw this on YouTube and thought this to be very interesting. Of course, the testing involved torsional stress vs. isolated tangential forces or other stresses perhaps specific to bikes, but there's got to be some eye-opening stuff here for others, as there was definitely for me. Certainly gives me more confidence in my rig.

https://youtu.be/hjErH4_1fks

Hard to claim any other currently used material as superior to CF in my book now. Any thoughts?

corrado33

*Sigh*

CF, VERY strong in the direction the fibers are laid, in this case they're laid helically around the shaft. However, it's WEAK in the opposite direction. CF also wears heavily (doesn't last as long as a steel/aluminum thing.) Also, one crash/ding/scratch and a CF structure is significantly weakened. This is fine in racing because generally if you crash you're done anyway, but that's not the point. CF fails catastrophically, steel more so than aluminum but both more so than CF fail SLOWLY. Failing slowly on a bike is a GOOD thing.

Steel and aluminum are great wearing materials that are good "all arounders" in terms of stress resistance. CF is not a good all arounder. It's a thoroughbred. Meant to be used a few times (or a season) then retired. (Bit of exaggeration there of course.)

So yes, take an extremely expensive, purpose built THING and test it against something that is opposite, in all respects, and the purpose built thing will win every time. I highly doubt they used an expensive steel driveshaft.

Look at it this way. There are trucks out there with much greater than 1000 ft lb of torque. That's ~ 1300 Nm, yet they're not breaking their steel driveshafts anytime soon... I mean heck, the 2016 Ram has 900 ft lb of torque... stock... do you really think they're using a driveshaft that fails at ~1000 ft lb? You'd be stupid if you answered yes.

PepeM

Assplosion.

Carbonfiberboy 

CF driveshafts are obviously the way to go if weight is important. And it's going to keep becoming more important. Their issue is the end attachments. We installed a CF driveshaft in a custom boat we built. One end came off after about an hour's use. It was specced for our build by the driveshaft manufacturer. Getting CF to bond to metals is always tricky and has to be done perfectly.

hooCycles

These threads never end. CFRP usually gives a better strength-to-weight ratio and stiffness-to-weight ratio than aluminum or steel or titanium or magnesium. As coraddo33 alluded to, and this is important to me, steel has far better fracture toughness than cfrp. Steel tubing is also usually thicker walled which gives a higher plastic section modulus. Both of these mean that in general steel will fail in a less catastrophic manner than cfrp. Given that I don't inspect my bike for cracks and don't value weight as much as some people here, steel is my choice.

hooCycles

Also, keep in mind that in pure torsion the stress is axisymmetrically constant across the cross-section (and nearly constant in general for a thin-walled tube). In bending, as is the case for many bike tubes, the stress is not constant across the section. Therefore in bending a steel would probably exceed yield strength at the top or bottom of the section, causing that area to locally yield, work harden, and not fail until much more of the section yielded. CFRP likely does not behave so nicely when the yield strength is exceeded and so once the area of highest stress in the cross-section reached yield limit, the tube would fail (crack formation would probably occur where the fibers delaminate from the polymer matrix and further concentrate stress).

In other words, pure torsion is a 'nicer' (more uniform) loading scenario for CFRP than bending. And not many of your bike components are subjected to purely torsional loads, as a driveshaft is.

OnyxTiger

From other sources I've read that CF has an infinite fatigue life. There's an article where different material bike frames were put under something equivalent to ~260lbs of pressure on each crank for hundreds of thousands of cycles. Aluminum tended to fail first, then steel, then CF. I'm curious what you mean when you say than CF wears heavily.

Regarding not using an expensive steel driveshaft... that's not really anything anyone can confirm. We do know that the steel driveshaft was substantially heavier as well as substantially weaker. It'd be pretty accurate to assume that - weight wise - if we compared an "expensive steel driveshaft" with a CF driveshaft of equivalent weight, the CF would still end up stronger.

Ratzinger

I'm still confused about what kind of driveshaft I should be using on my bike.

PepeM

No mentions of the resonance of steel yet? Someone needs to add some SCIENCE to this discussion.

StanSeven

Bike? You got in the wrong place. This is auto forums. Need help finding you way out?

corrado33

You're going to argue that their "driveshaft" that barely withstood 1300 Nm was a good representative sample of steel driveshafts, even when STOCK trucks produce nearly that much torque? I'd love to see a specifically designed steel driveshaft go up against the CF. Sure, the CF would win, but at probably double to triple the price, so is it really a good comparison?

Let's take a quote from Bicycling.com.

So tell me this, have you NEVER hit your bike on another bike, or accidently dropped it? Have you ever had your bike in the bed of a truck with a half dozen other bikes? Have you ever seen what happens when you shove that many bikes onto the tailgate of a truck?

Carbon fiber bikes wear more heavily because they can't take the day to day abuse like that sort of stuff. A steel bike could take that forever. CF? No. A sharp hit in the right place (like if your bike fell onto another bike) could break it. Don't believe me?

https://www.youtube.com/watch?v=khJQ..._channel=M9USA

Comparing the tubing of course, not the specifically designed CF foam composite meant to deal with impacts, as it's an unfair comparison. If you put a block of steel under there half as thick as that CF composite it wouldn't even move...

So answer me this. What's the biggest enemy of a CF composite other than impacts? UV! HEAT! Yes, theoretically carbon fiber resin is "stable" indefinitely, but practically it's not true. CF resin breaks down when exposed to UV and heat. Ok, so paint the CF white! Great idea! However, even the best "UV resistant" paints succumb to UV eventually. (I know this because I used to do accelerated testing of the paints used for buildings...) Even more quickly if the paint is on a surface that could possibly get beat up like... oh I don't know... a bike? What happens if you leave a steel or aluminum frame out in the sun for years? Maybe a bit of surface rust?

So, not only do steel and aluminum deal with the daily life of being a bike better than CF, but they also deal with being a bike LONGER than CF.

Yes, a carbon fiber bike CAN last for as long as you want given tedious proper care, but a steel bike WILL last as long as you want, even if you abuse it. So again, carbon fiber is a thoroughbred, fast and light and good for exactly it was designed for, but requiring care and not great for normal people who can barely keep a phone from breaking every 2 years...

OnyxTiger

Well, considering we weren't talking about cost as much as we were talking about strength-to-weight, I think the comparison is within context here. CF bikes are almost always going to be more expensive than a steel framed bike, so there's no revelation here.

No. No. No. And no. I actually treat my bikes well, and haven't raced it.

If your definition of day to day abuse includes crashing a bike, dropping it, or putting it in a bed with half a dozen other bikes, I don't know what to tell ya. And, btw, the Carbon Matrix sample actually held up best in that video.

Bit of a stretch for your argument here. Who would put a solid block of steel inside CF tubing?

Though I'd like to find substance in your argument here, it's frankly pretty difficult. Do you have any sources that show the degradation of CF composites to UV? I'm also pretty sure the technology has matured more since the CF builds of the 90s.

Tedious proper care is what I would plan on doing to my bikes, both my Alu and CF, and eventually my Ti and Steel as I expand my collection.

Carbonfiberboy 

Stupid test. The tested items should have all been the same weight. I guarantee that wasn't the case.

I've dropped my carbon bike countless times. I've been down on it a few times. I've left it out in the hot sun. 50,000+ miles and frame and fork are still perfect. Yes, UV will cause surface degradation of the epoxy in the laminate. Totally simple to prevent: either paint or coat with a UV absorbing clear coat. My last steel bike was rusting out when I got rid of it, and I took better care of it than the carbon.

Mr IGH

Back in the early 80s, Chevy changed the rear leaf spring from steel to CF, the cycle life before failure went up ~10x. The traverse spring is inches off the road, lasts over 500k miles and decades whilst reducing unsprung weight.

gregf83 

My steel bike (Colnago) lasted about 10 years before breaking. Nothing lasts forever. I have a couple of carbon bikes and an aluminum one with carbon forks. They all seem equally sturdy and I don't baby any of them. I've crashed a few times with no problems.

lightspree

Others have also shared their experiences with carbon fiber, including a few of the gritty details of their specific serious injuries (which is the part that gets through to me), some permanent:

Carbon Bicycle Forks: Cautions, Facts and Misconceptions | IsolateCyclist

lightspree

I can't help taking these experiences into account. How important is it to a recreational cyclist to have a CF fork when there are perfectly good alternatives?

lightspree

Everybody has bad days, including the blokes in the Chinese factories who are laying up the carbon for .25 an hour. Manufacturing defects happen.

TGT1

for driveshaft applications the stiffness and low mass of CF solves critical frequency issues. (the speed where the shaft whips like a guitar string)


Last one I sold



It's twins, installed 30 years ago are FRP (fiberglass)

Tis all irrelevant to bicycles.

Doge

Strength and stiffness are just one measure for what makes a bike good to ride. CF is not better for gears/chain - or not currently. But, yes, I'd take the CF drive shaft and CF frame.

gregf83 

Did you read the article you linked? A fork of any material can fail. Catastrophic failures of any forks are extremely rare and not worth worrying about.

lightspree

To answer your question, Yes of course I read the article, along with a number of others and additional sources I've taken into account.

According to the first quoted excerpt above, from that article, among other sources, these failures are not "extremely rare."

How do you know how rare they are, and have you done as much research, or seen as much evidence, as all those who see it differently, including the writer mentioned above? I'm certain he's way ahead of you in these areas.

gregf83 

Perhaps he is but I saw no statistics in the article you linked. What I did read was:
Like I said not worth worrying about.

lightspree

You also read, but omitted to mention,

lightspree

To me, worry is not the real issue.

As far as statistics go, where are yours?

He comes a lot closer than you in that area.