5

I was riding my aluminium bike and I met some people who had been trapped in a locked park, so I helped them escape by holding the bike against the fence and letting them stand on the top tube to climb out.

If I had had a carbon fibre bike instead how much risk would there have been of serious damage to the bike? I understand that carbon components are engineered to be strong in certain directions and relatively weak in others.

Improve this question
17
  • 3
    I would say that there's a danger on any lightweight bike. The center of the tube is its weakest point.
    – Daniel R Hicks
    Jan 29, 2015 at 12:49
  • 2
    On an alloy bike you'd probably be OK standing on the top tube under the saddle, but it would still be better to stand on the saddle itself. Carbon is too fragile for anything out of the ordinary especially given how sensitive it is to cumulative damage.
    – Chris H
    Jan 29, 2015 at 13:22
  • 2
    Striking the top tube in a fall is not something you want to do but that is a stress you would expect a bike to take. I have and not hurt my bikes (cf, aluminum, or steel) in falls like that. Consider the falls you see on jump, downhill, and street. I stand on my CF in a park where I CX train to stash my coat in a tree all the time.
    – paparazzo
    Jan 29, 2015 at 13:38
  • 1
    I'd be as concerned as much about the stress on the carbon joints (I think they are bonded by epoxy resin) as the tubes themselves.
    – PeteH
    Jan 29, 2015 at 14:33
  • 1
    @Blam a Trek Madone frame is made of 7 pieces, which are moulded individually then glued together. However it is far too simplistic to think of these pieces as "tubes" in the steel sense. So it depends what you mean by "good" I suppose - I suspect there are several different manufacturing techniques going on, there's some really interesting stuff on the web about this, the Op should probably google this if he's sufficiently interested to ask questions on here.
    – PeteH
    Jan 29, 2015 at 17:29

4 Answers 4

Reset to default
7

At the risk of getting beat up on carbon will give it a try. CF seems to be a sensitive subject.

I don't agree with your assertion CF are engineered to be strong in certain directions and relatively weak in others. They are designed to have different flex in certain directions - not weak.

If I had 4 bikes I need to use as step stools I would go with this order (and I really have all but aluminum):
1) steel
2) titanium
3) carbon fiber (CF)
4) aluminum

There is a bit of discussion and comments that bearing weight on the top tube is not a stress a bicycle is designed for (and weak). That is a stress a bike must handle. If I take a tube and hold it at both ends and try and bend it that is the same type of stress as if I took the tube and put either ends on rocks and pushed on the middle. The primary force a top tube takes is bending from the ends. If you watch the video you can see top tube bend and when it fails the two pieces are bending displacement.
Santa Cruz Bicycles - Test Lab
And you can see the CF held up pretty nicely.

CF takes criticism as it fails brittlely which is true but it takes a lot to get it to fail.

I would not want to ride any bike that would fail with my weight on the top tube as that is a scenario (crash or drop off) I expect and need it to not fail.

With that said there is not one CF. A lot of variance in the construction / layout. If you are going to ride a CF then buy a good one.

I don't mean to advertise a brand but Niner has 5 year warranty including racing. I have crashed my Niner Air 9 Carbon hard and not even thought twice about the bike. I use it for CX training at a local park and stand on the top tube to stash my jacket in a tree.

There is stuff CF is not good at. Don't put a rack on a CF. Don't take a hammer or bat or hard object to CF. Don't take a drill to CF. Don't over or under torque bolts.

I am just amazed at the slow acceptance of CF frames. When CF forks first came out there was a lot a questioning. But now CF is the dominant rigid fork. A fork takes a lot of stress from a lot of directions. People think CF is too light to be strong.

I get engineers are misunderstood (and certainly under appreciated) but a simple one foot drop landing on the front wheel is going to put more stress on a top tube than your weight. The frame did not even flex in the picture. I would not ride a frame I could not jump on.
enter image description here

Improve this answer
12
  • 1
    Steel for the win!
    – Rider_X
    Jan 30, 2015 at 20:42
  • That's worth a thousand words. I could probably stand to take that same picture on my carbon Stumpjumper, but I still wouldn't try it with my Tarmac.
    – Deleted User
    Jan 30, 2015 at 21:56
  • @ChrisinAK youtube.com/watch?v=HhabgvIIXik
    – paparazzo
    Jan 30, 2015 at 22:14
  • 5
    @Blam No, it's not at all. A drop places force upwards against the axles. The fork and the seat stays transmit that force to each end of the top tube. In a "drop" the top tube is being compressed from each end. It's much different than placing all the force against the middle of the tube. It's very similar to how a person standing on my shoulders would be tolerable, but a person standing on the center of my spine would not.
    – Deleted User
    Jan 30, 2015 at 23:14
  • 1
    @ChrisinAK: I agree and I’d also like to point out that tubes in lightweight bicycles tend to be made with great diameters and thin walls. So simply standing on the tube could put a dent in it.
    – Michael
    Oct 28, 2015 at 22:09
5

You could theoretically damage any frame in that manner. Tubes are meant to support a load from either end, not in the middle. No bike is designed to support a great deal of weight on the top tube. Theoretically carbon, steel and titanium could all flex/bend and resume their normal shape, where aluminum couldn't, but nonetheless it still isn't a good idea with any of them.

There is so much variation in material you could easily make a thick walled carbon frame that would stand up to such abuse or a paper thin steel frame that would bend and fail instantly. You are correct that carbon can be designed to be stronger in some directions that others, but it's still being shaped into a tube (or some variation of) that design and has it's own inherent strengths and weaknesses.

Improve this answer
10
  • Yes, a BSO from Wally World will probably support more weight on its top tube than any sort of fancy bike. In your traditional diamond frame there is very little "bending moment" on the tubes -- it's all tension/compression -- and the tubes do not need to be made to greatly resist bending. In the name of lightness the tubes of better quality bikes are made as thin as practical, and, eg, steel bikes generally have "double butted" tubes that have thinner walls in the middle than on the ends.
    – Daniel R Hicks
    Jan 30, 2015 at 0:59
  • @Blam I can compress the top ovalized tube of my Specialized Tarmac with my hand (noticable flex from top to bottom). I have trusted that bike to some pretty hairy descents and cobblestone roads. I would never stand on the top tube. I would certainly never let a stranger stand on it. I am certain most people's weight would generate more force than I can generate with my hand.
    – Deleted User
    Jan 30, 2015 at 21:09
  • @ChrisinAK Cool. Flex and fail are not the same. I would not ride a bike that could not take like 4x my static weight. +1 don't mean any disrespect. What you don't get is that stress from either end is stress in the middle.
    – paparazzo
    Jan 30, 2015 at 21:43
  • @Blam - It's true that in an crash an substantial amount of force might be applied laterally to the top tube. And it's not terribly unusual for bike frames (especially lightweight bikes) to be damaged in crashes. In ordinary operation, though, there is nothing present that would apply such lateral force.
    – Daniel R Hicks
    Jan 30, 2015 at 21:50
  • 1
    @Blam None felt. It's highly likely the Tarmac could withstand those forces, I still wouldn't try it. The direction of the stresses does matter greatly, however. Squeezing an egg end to end and then sideways is the easy eggsample.
    – Deleted User
    Jan 30, 2015 at 22:11
2

Bicycle frames are lightweight structures that have great strength when supporting the loads that the road imparts. Normally the top tube would experience compression or tension due to the weight of the rider. It also needs torsional stiffness to keep the bike stable. It is not designed to take the bending moment of a persons weight in the center. Aluminum bikes tend to have large diameter tubes for torsional stiffness and this gives them bending stiffness as well. Localized buckling of a thin wall is also likely.

Carbon fiber frames are not light because the material has magic properties. Carbon fiber has great axial strength it is true but much of the lightness of the frame is due to the finite element models that help designers put material exactly where it will do the most good. Before I would let anyone use my $3000 frame for a stepladder, I would think seriously about that.

Improve this answer
0
1

I would not be afraid to let anybody stand on my carbon fiber, aluminum, or steel bike. If standing on the top tube breaks the bike, it would be liable to break after any minor mishap, and you're safer getting it repaired or replaced. Bikes are sturdy -- even CF road bikes.

Improve this answer

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.