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I think Mr. Fusion seems perfect, except they don't recommend it for carbon seatposts

https://porcelainrocket.com/collections/store/products/mr-fusion

Q : Why is Mr. Fusion not recommended for carbon seatposts?

A : Some folks tend to get a bit ham-fisted when tightening bolts on their bicycles, and Porcelain Rocket cannot be held responsible for damage to expensive carbon seatposts, due to over-torquing of the clamp bolt. The clamp bolt is meant to be tightened to NO MORE than 4Nm. This is about the same torque that you would apply to MTB brake levers; tight enough to hold the stabilizing rack in place reliably, but not so tight that the rack cannot rotate on the seatpost, in the event of a crash. If you do decide to install Mr. Fusion on a carbon seatpost, this is done at your own risk.

Does it say long as you don't tighten the clamp bolt too tight, you should be fine? (English is not my first language and can't say for sure what the above lines means)

Has anyone used Mr Fusion on carbon seatposts ?

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    I edited the title so that it is more general and hopefully clearer. Feel free to propose a different edit.
    – Weiwen Ng
    Nov 29 '20 at 15:04
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    If you want to be on the safe side, replace your carbon post with an alloy post just for that trip.
    – Carel
    Nov 29 '20 at 18:41
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The item in question appears to be a bikepacking seat bag which has a set of struts below the bag. Those struts clamp onto your seatpost.

Without having used this clamp myself, my reading of the language is that Porcelain Rocket, the manufacturer, think you are likely to be fine clamping this to a carbon post if and only if you don’t over-torque the clamp. This requires a torque wrench, and 4 Nm is not high torque (the clamp on my carbon gravel bike says 5-8 Nm, and I think I use about 6-7 Nm to prevent slipping). The manufacturer is clearly saying not willing to assume any responsibility for damage to your carbon posts, which seems prudent and reasonable. They clarify in the last sentence that if you clamp this to a carbon post, this is at your own risk. Had they meant to prohibit this saddlebag on carbon posts entirely, they would have said something like, “Do not mount the Mr. Fusion to any carbon seatpost.”

This is nitpicky of me, but ultralight carbon posts may not be safe to be clamped in that fashion. Their wall thickness might not be sufficient for that purpose. I don’t know if any posts are made with variable wall thickness, but it seems possible that someone could lay up a seatpost with less fiber in the area where it will not be clamped. Of course, this is a thought exercise only, since a you probably aren’t mounting a bikepacking saddle bag on a road racing bike with that light a seatpost.

One thing you could consider is to inspect the clamp for any burrs or sharp edges when you get it. It does seem possible to me that a sharp edge on the bottom side of the clamp could eventually start cutting through fibers in the seatpost, which would naturally be bad. You could just sandpaper the edges of the clamp until they are smooth. This statement is admittedly not based on any practical experience, just on my general (and limited) knowledge of how carbon fiber can fail. I would not expect there to be any burrs or sharp edges, as this should have been caught in quality control, but I suppose it's not impossible.

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If your seat post can be clamped in that area (i.e. there are no markings that it mustn’t be inserted below a certain point) then it should be fine with the forces&torques exerted by the small clamp of this saddlebag. Of course you should only tighten to the specified 4Nm.

I’d regularly check that it hasn’t come loose because rubbing from a loose clamp could damage the seat post over time.

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