Zinn technical FAQ: Sitting on carbon top tube, through-axle security, hill gradients

Published August 1, 2022 11:44PM

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We all know that you never clamp a carbon frame’s tubes in a bike stand clamp, but what is the bike manufacturers’ position with riders sitting on their carbon top tubes?

If you go on your local club ride, stop at the red light, or even watch some riders after a race as they chat away, you’ll find many are sitting their bums on their carbon bikes top tubes….

That is an excellent question. Indeed, my inquiring mind would like to know as well. So, I put the question out to some carbon frame makers as well as to a couple of companies who repair carbon frames, and I got a wide variety of answers. Turns out, there is not universal agreement that it is okay to sit on your carbon top tube. It’s great that you brought this up.

Here is what Chris Cocalis, founder and CEO of Pivot Cycles, has to say about it:

“Unless they have something sharp in their pocket that could damage a thin tube, there should be no issue with sitting on the top tube. Highly concentrated pressure is the issue. Even if you have buns of steel this shouldn’t be a problem – even on the lightest for road frames 😊.

If he’s still concerned, tell him to get a nicer chamois to give that top tube a little extra padding.

“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.

“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!

So, there you go. Far from universal agreement on this one. I think the takeaway is that, if you can avoid sitting on your carbon top tube, do so. And if it is a superlight carbon road frame with an oval top tube, it is even more important to avoid it. My guess is that carbon mountain bike frames, especially ones with a shock mount at the center of the top tube, have sufficiently beefed-up top tubes that there is less of a concern about sitting on their top tubes.

I saw Richard’s question about thru-axles and wanted to mention that I have definitely seen thru-axles come loose on mountain bikes at least. I do think this is a fairly common problem, as others I’ve spoken to have had similar issues to the point that checking the play in the wheels has become a standard pre-ride routine for us. Perhaps this will never happen if correctly torqued, but as you say, sometimes you need to put a wheel on and don’t have access to a torque wrench. As you mention, they need quite a bit of torque, and are typically aluminum, so I find myself feeling like I’m about to strip the bolt head every time I get to the required torque. I don’t even know if I’d be able to apply >12 nm using a multi tool on the road.

These are quite different from how quick-release axles work, because they don’t have a cam, which I think is what Richard was getting at. There are cam-style thru-axles, which I personally much prefer to the regular bolt-on style. The number of different axle styles makes it very difficult to figure out what will fit your bike, so it’s not easy to switch the axle unfortunately.

Dear Lennard,
Just read your response on through axles and had to reply. I have on going issues with front and rear (mostly rear) through axles vibrating loose on my specialized MTB. I check and tighten before every ride and still get the rear axle to loosen during a ride. I’ve had to use thread locker to keep them from working out. Never had any issues with the old axles and dislike through axles for both this issue and removing the wheel to transport the bike.
– Dave

This is a surprise for me. While I do check my through axles before almost every ride, I have never found them to have worked loose at all. I wonder what the difference is… Thanks for letting us all know.

Regarding the question from May 24 of perceived effort climbing two hills with average grades that appeared to be the same, I have a potential explanation. Using a GPS based cycle computer (Garmin) can only report % grade to the nearest integer %. This leaves the possibility that a 6.51% and a 7.49% slope would both be shown as 7% on the head unit, in spite of the difference being virtually 1%, which would be enough to feel as different efforts. If a tracking application such as Strava is used and the hill is a segment, the average gradient could be calculated by dividing the elevation gained by the distance traveled in feet x 100.

Another factor would be the wind direction and on short hills, the speed at the bottom of the hill as the climb starts.