Should I avoid sitting on top tube on my carbon bike?

Question

In another bicycle discussion forum I encountered a thread about possibility to damage carbon frame by sitting on top tube while stopped.

I see some sense in it, as generally tube walls are a lot thinner in the middle to save weight and are not designed for this kind of load. The problem is, I have not seen any warnings from manufacturers about this habit, nor there are stickers on the frame suggesting to not do it.

So the question is - Is this a real problem, should I change my habits?

UPDATED: There is one local person who does carbon repairs, he claims that he has repaired multiple frames which had been damaged this way.

Answer 1

I have never had or heard of a problem.

Over at www.velominati.com, one of the rules is

Rule #80 // Always be Casually Deliberate.

Waiting for others pre-ride or at the start line pre-race, you must be tranquilo, resting on your top tube thusly. This may be extended to any time one is aboard the bike, but not riding it, such as at stop lights.

While some of what they say is tongue in cheek, this one is not. Most people I know do it, and have done for years.

Answer 2

You need to seat at right spot of your but. There is a big vein on back side of the upper leg can be squeezed. It can cause a problem with blood circulation before race or workout.

Answer 3

You correctly stated that the carbon layup (i.e. the number and orientation of plies of carbon used) is likely to be lighter in areas where the frame isn’t typically as heavily loaded. You also correctly identified that CF is anisotropic, meaning that its material properties can be made to vary by direction, unlike metal tubes.

I heard on one of the podcasts I frequent that at least one carbon frame model was made so light that the manufacturer explicitly warned against sitting on the top tube. I don’t recall which model, nor even which podcast or which episode (for reference, it would have been Marginal Gains by Josh Poertner, or the Cyclingtips podcast, or the Nerd Alert podcast which is a separate podcast under Cyclingtips’ umbrella). I also believe it isn’t a general practice to warn against sitting on the TT. Consider that we aren’t putting our full weight on the tube, we are typically sitting on it while stopped with one leg on the ground. That said, Lennard Zinn of Velonews recently asked three manufacturers and two repairers about this issue. All three manufacturers said that this shouldn't be a problem. One repairer said not to do it, and the other repairer said it could be a problem, particularly if the top tube were ovalized.

Without discounting what the carbon repairers in that letter and in the OP said, be aware that they experience what statisticians would call survivorship bias. People come to a repair shop because their bikes are broken. The repairers don't observe all the CF bikes where the owner did sit on the top tube, but the bike didn’t explode.

Thus, if there's no warning sticker, I would think that sitting on your top tube is fine. Exercise more caution if the bike is light, or if you're heavy, or if the top tube is ovalized as described above.

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This is a digression for interested parties. Related to the paragraph above, here’s the Wikipedia page discussing survivorship bias. In World War II, the US Air Force was examining bullet holes on returning B-17 bombers. They initially added more armor to the areas that got hit the most. However, Abraham Wald and colleagues told them to add armor to the parts where surviving planes did not have bullet holes: the engines and cockpit. In hindsight, it seems obvious that hits there would disable the bomber entirely and prevent it from returning home at all. This is one example of how survivorship bias can distort our conclusions.

Answer 4

According to a VeloNews advice column, opinions on this are very much mixed. Basically, the answer is that it depends on whether you have an ultra-lightweight bike.

Here are some quotes from the article (emphasis added):

• From Brady Kappius, founder of Broken Carbon (carbon repair business):

  This is something we highly recommend against doing, especially on road bikes. The sitting forces exceed the magnitude and direction of forces that the top tube sees during normal riding conditions and can cause damage. We see it quite often. Some manufacturers even have decals on new frames that say do not sit. The lighter road frame top tubes are easily flexed by just the squeeze of a hand! Their wall thickness is close to only 1mm.

• From Craig Calfee, founder and CEO of Calfee Design, which does both frame and component manufacturing as well as carbon repair:

  Sitting on the top tube is normal behaviour for cyclists. But we have had a few frames come in for repair from different manufacturers where the top tube was cracked after the rider sat on the top tube. And these were not covered by the manufacturer’s warranty. I recall these had oval top tubes as well, which would not do well with a compression load on the flat side of the oval.

• From Ryan Cannizzaro, founder of Alchemy Bikes:

  Although the top tube is not built to be a seat, it is plenty strong to sit on. Although it is beyond the normal use of the top tube.

• From Mark Schroeder, Engineering Director at Specialized (who also directs Specialized’s testing lab):

  Specialized has not found any problem with riders sitting on their frame’s Top Tube.

  However, we completely agree that under no circumstance should any frame tube be clamped in a workstand!

Answer 5

Carbon fiber is strong. Really strong. The kinds of loads that your frame sees when you hit a pothole is far bigger than what you would get sitting on the bike.