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Comment by pawptart | Hacker News Reader

> Carbon bike frames are designed only for the loads in “normal riding” but not really for all the random things you might do by accident while owning a bike, like bumping it into walls, over-tightening the seat post, etc

I wish this myth would stop being perpetuated. Yes, aluminum frames typically crack or dent rather than splinter and become immediately hazardous, but if you hit something hard enough to break a carbon frame, you'll have done even more damage to an aluminum one. If you hit the deck that hard, you're best off getting the frame professionally inspected and replaced if necessary.

I think this fear stems from a gut feeling that metal is more ductile and durable than "plastic", but what you're dealing with is far, far stronger than plastic.

I always like to show these videos to illustrate:

https://www.youtube.com/watch?v=w5eMMf11uhM

https://www.youtube.com/watch?v=VfjjiHGuHoc

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The author was comparing carbon to steel, and that's very different.

Steel yields gently so most people have learned to tighten things until they feel the bolt start to yield, then stop. With aluminium that's harder, and with carbon it basically doesn't happen. You need to be careful, ideally use a torque wrench, and always check for cracks in your carbon bike.

A friend was "helped" by someone a while ago and their carbon MTB now has a crack where the seatpost bolt does up. It seems to be ok and they know to watch that area. But I'm ok with it because when it does fail... the seatpost drops and couple of cm into the frame and gets loose. That's pretty survivable.

Said friend-of-friend also helped change the oil in their car. Overtightening an oil filter is much less survivable (for the car). Filter split, oil everywhere, had to get towed to service centre and replaced. Oooops.

The author is a retrogrouch parroting the usual bike myths you hear over and over.

Aluminum, steel, carbon, etc are materials that can be engineered out to wazoo to have different qualities, many of them overlapping. Aluminum bikes can be wet noodles and steel bikes can be bone-jarring stiff. A big-box-store aluminum bike is nothing like a race bike made from high-end reynolds aluminum. Etc.

I badly cracked the head tube on my run-of-the-mill steel frame (Reynolds 501 CrMo) and a guy recommended by a bike shop welded it up for me for £25. Six years later it's still going strong.

I bought a carbon fiber framed Trek Remedy enduro/all mountain bike and repaired a fist size hole/delamination in the downtube at home for about $50 in materials [1]. Five years later it's also fine.

And I've taken the bike off 6ft drops and ridden it decently hard most of it's restored life.

CF repair can be done and plenty strong, it's just more risky because the properties aren't isotropic and simple like steel/alu so you can't just weld in a piece of metal of roughly the same thickness. I would not want to repair a bike I don't ride myself.

[1] https://medium.com/@fixingthings/fixing-a-trek-remedy-29-9-8... (yes, sorry it's medium)

I’d be careful welding aluminum. At the factory, post weld, the frames are then heat treated, and pulled back into shape (the heat treatment will distort them).

Welding after heat treatment will reduce the weld affected zone back to its basic untreated properties.

That's a very impressive carbon fibre repair.

The quality gradient between "cheap carbon" and "real bike" is way bigger than most aluminum and steel frames, not to mention all the myraid ways people inadvertantly ruin carbon frames... many of which don't even apply to metal frames. I've seen frames crack because of overtightened QR axles. Overtighten something on a metal frame and you damage paint.

Speaking of paint, if the paint on a carbon bike is damaged, you let in all kinds of new problems from UV brittling to galvanic corrosion (where the carbon meets a metal part). There have been recalls from pretty major manufacturers over glavanic corrosion (which mostly shows up in bottom brackets and the headset-crown interface on forks.

Long story short, carbon fiber is a great material for bike frames and parts! But caring for these bikes is a completely different beast compared to the horrific abuse metal bikes can take. Neighborhood kids will leave their metal bikes in the mud and snow outdoors all winter, then ride the thing to school all year. I wouldn't want to try that experimnent with carbon. It's my preferred frame material, but it's not a low-maintenance one.

Theproblem with carbon is that when it fails, it fails catastrophically, and will lead to a crash.

The most dangerous parts are rims, fork, steerer tube, handlebar. If one of those breaks at speed, you will crash spectacularly.

And what people are afraid of are manufacturing defects, testing a good frame is pointless if you worry about manufacturing defects.

I met a guy at a bike shop who was buying aluminum handlebars and he said that his carbon bars had failed on him twice! Both with bloody consequences! I've probably ridden 20,000 miles of my lifetime and had zero failures with aluminum bars!

> but if you hit something hard enough to break a carbon frame, you'll have done even more damage to an aluminum one.

The problem with carbon isn't what happens when you ride it, but what happens by accident. Tipping a carbon bike over can cause stresses in a direction where the fibers are weak, thus causing significant damage with minimal force. Some carbon bikes are even so thin in certain places that the frame can bend if you press it with your finger.

This type of problem is not present in aluminum frames.

> I always like to show these videos to illustrate:

The videos are fun, but of a very limited scope. I would like to see the corresponding videos made by an independent person, or by people who make steel, aluminium and titan frames. People who sell carbon frames at a higher price point have every incentive to bias the results of these experiments in their favor (for example, by intentionally producing less robust aluminium frames for their lower end products).

That seems unlikely as there's plenty of competition amongst bike manufacturers, so deliberately weakening a cheaper model would just lead to a drop in reputation. There's a reason that carbon frames are popular - it can be made extremely strong and still be light.

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