I just picked up my new Cannondale CAAD12 bike the other day. During my first ride I occasionally was sitting on the top tube (waiting for traffic lights etc). After the ride was over I actually realised this might be bad for the frame and I started to worry, especially because the CAAD12 frames are so light considering they are made from aluminium. Any ideas if sitting on the top tube of a lightweight alu bike like the CAAD12 is a no-no? I inspected the frame and it seems ok though, but I am not an expert. My weight is around 170 lbs.
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1@Criggie Presumably sitting like this. A matter of comfort, rather than necessity. The pros do this all the time. They're rather lighter than the asker but I still wouldn't guess it'd do any harm.– David RicherbyCommented Jun 13, 2017 at 9:12
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1@Criggie The first one doesn't seem implausible if the lights have a long cycle (no pun intended).– David RicherbyCommented Jun 13, 2017 at 12:20
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1It doesn't seem any slower to get going from sitting on the toptube (as at the beginning of the video @DavidRicherby linked) than from standing over it. If I'm expecting a long wait (just seen the lights change) I have to do that or risk getting cramp in my calf, especially at the end of a long ride -- or lean the bike further than I'd like to in the presence of cars– Chris HCommented Jun 14, 2017 at 15:04
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1Related bicycles.stackexchange.com/questions/28758/…– paparazzoCommented Jun 14, 2017 at 15:48
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1I'll note that a bike designed for routine use should be able to take, without damage, a "hit" to the top tube when the cyclist falls.– Daniel R HicksCommented Jun 14, 2017 at 16:13
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2 Answers
If sitting on your top tube causes damage to your bike, the bike wasn't very safe to begin with. I imagine while sitting you have at least one foot on the ground, so your entire mass is not resting on the tube.
The amount of force that a frame must withstand is much greater than the weight of the rider. If simply placing 170 lbs as a static load was enough to damage your frame, there would be no way to safely ride the bike.
Update: While not the most scientific test, I decided to try a worst case top tube sitting scenario. I went for a 5 mile ride last night on my alloy road bike. While not as high speed, low drag as the OP's rig, it is all I have. I, a rather large 6'2" 210lbs rider, sat on my top tube while riding for as much of my ride as possible. I do not recommend this, as it turned out to be very painful, and I got some looks form other riders for some reason. Most of the ride was on roads that are not in the best condition (holes, cracks, gravel). Again, I do not recommend this. After completing my 5 miles on the top tube, I brought my frame to work for some x-rays. I x-rayed the entire main triangle, as well as the welds to the rear triangle. I can report that there was no signs of any damage (cracking or deformation of the alloy). There was a bit of damage to me however, as apparently it is a bad idea to sit on the top tube while moving.
Update 2: the mechanical engineers I work with and I conducted an experiment. We got some aluminum alloy tubes (approx 1.25mm wall thickness). It as standard 6061, which is not exactly what is used in the OP's bike. We welded up a main triangle as close to the geometry of the OP's bike as we could. We have learned a few things from this:
- None of us should quit our day jobs to build bike frames. Man is that hard.
- The strength of the main triangle is insane, assuming the forces are perpendicular to any side. We were about to load a static load of over 350lbs on the "top tube" before it buckled.
- Bike frames are not designed to handle rotational forces. The frame will twist up with relatively low force (a torque of about 200lbft.) This force was exerted at what would be the head tube.
- Science is great.
In conclusion, go ahead and sit on your top tube at lights. Since I read this post I have started doing it and it can be quite comfortable.
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1Sitting on top tube is also a load the bike is necessarily designed for. Think about a soda can: strong enough to stand on, but still easily crushed from the sides.– ojsCommented Jun 13, 2017 at 17:48
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But the yield strength of the side it's still significantly higher than the maxim force that soda will exert. I spoke to some of the mechanical engineers I work with about this (also cyclists). They don't see am issue with this, but we will be conducting some experiments on this in the next few days.– CRobertsCommented Jun 13, 2017 at 19:45
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I'll be waiting for updates. I wouldn't expect the tube to fail either, but I'm not trying it on my own bike.– ojsCommented Jun 13, 2017 at 20:11
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1So we have a crunched frame that we will be taking measurements from. We will then build a simulated frame and play around. We will be using 6061 alloy of a know thickness and will then try to take our findings and relate then to this exact model. We could do this through math, but things are slow right now and this should be very fun.– CRobertsCommented Jun 13, 2017 at 20:58
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That's quite an experiment. Nice. I hope you recover quickly.– Chris HCommented Jun 14, 2017 at 15:01
It is a force the bike is designed for
Consider this 
below.
Force 1 and 3 are the axles and they carry your weight.
Bending the ends of the top tube mimic force 2.
Is that force 2 equal to your weight - no.
But it is a force the top tube is designed to handle.
This is a force from the axle in a different direction that resulted in a bend failure of the top tube. The top tube must be designed to take some very strong bending forces.
Think about if you want to bend the top tube. You could apply bending on both ends or you could stack it on two blocks and stand in the middle. At the molecular level it is resolved shear stress. I have stood on all my bikes - steel, aluminum, titanium, and carbon fiber.
Jump bikes but you see you see riders come down hard on the top tube.
Racers sitting on the top tube
Seat on top tube
answered Jun 14, 2017 at 18:33
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3I'm quite sure force 2 in first picture is not how physics works. Also, the question is not about bending the top tube but denting it.– ojsCommented Jun 14, 2017 at 21:16
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1Your second photo looks like a total bodge job and intended for a child to sit on. I don't think it demonstrates anything, other than that somebody has put a small weight on a top-tube and it hasn't broken yet. (Sure, I agree that top tubes should be strong enough to cope with people sitting on them; I'm just saying that this one photo is very poor eveidence that does nothing to strengthen your case.) Commented Jun 14, 2017 at 23:16
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@ojs Be sure. If you think the question about denting then feel free to present evidence a butt will dent a top tube. Commented Jun 15, 2017 at 2:50
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I'll note that, in better-quality steel frames (I don't know what the practice is for Al or fiber), the tubes are "butted", meaning the walls are thicker at the ends than in the middle. So your torque on the top tube is being applied to a thick portion of the tube -- probably 5-10 times stronger than the tube in the middle. Commented Jun 16, 2017 at 0:01