Hello, this year I am planning on trying to ride 2 centuries per month, but sometimes there is so much stuff to bring along it makes the back of a jersey sag. I have a very small underseat pack with a tube and c02 and multi tool, but do not have much room for more. The bag I am considering seems like it would be perfect for my centuries and other really long rides. What do you think about this bag? Do you think I can mount it on a carbon seatpost? https://www.topeak.com/products/Bags/DynaPack THANKS!

What about something in between, like a larger-sized seatpack?

Or, if you want something bigger but don't want to mess with your seatpost, perhaps a saddlebag? Minnehaha makes pretty nice, affordable ones
https://www.minnehahabags.com/canvas-saddle-bag-small/

I find that if I just need tools, a large-ish conventional seat pack (as opposed to the larger saddlebag) will fit what I need: Topeak Alien multitool, spare tube, glued and glueless patches, and maybe a small crescent wrench. A pump bolts onto the bike elsewhere.

So what are you wanting to carry on a century ride?

To do a ride of this distance then the bike should be well maintained. I know things can happen that require repairs but in theory- it should not. All I carry for rides up to 100 miles is the same as I do on any other ride. Tube- levers-patch kit and a multi tool fitted into a wedge under the saddle. I would fit two water bottles and a pump on the bike and in the pocket of the jersey are wallet- phone- and a bit of food. Clothing is the type that can be packed down and carried in the pockets or into its own pouch and carried round the waist.

Or you can go over the top and carry every tool and every bit of clothing to cater for problems- but that will require Panniers fitted to carry it all in.

alpacalypse has the sensible solution and a larger seat pack to carry the few extra's- as after you have done several century rides you will find what you do not have to carry on any ride.

I live in Wisconsin, the weather can vary a lot, so in the morning it may be 50, in the afternoon it may be 80. I already have a saddlebag on there, with the tools and tubes etc. The bigger bag in question would mostly be used to carry extra clothes, a camera, maybe a lunch in some of my rides go into the boonies. The size of that bag seems just about perfect, however the real question is whether it is safe to mount on carbon. Thanks for your input guys!

Just a suggestion from a long distance offroader: camelbak. Plenty of water, rain shell, tools, tubes, snack (sometimes a beer, too).

Or consider a handlebar bag.

I use a cheap ($10?) Bell version that's easy on/off. Great place to stuff jacket, shoes covers, wind pants for temp changes. I've even carried an extra quart of Gatorade when the need arose.

Downside is that it can interfere with hand position. Oh, and it looks totally Fred.

Some C.F.Seat post's are very light with thin walls and I would not mount anything on those. The thicker ones that are overbuilt may be OK and some are even C.F. over an alloy tube.

Providing you can keep the clamp tight enough so the box does not move- and providing you do not overtighten to Stress the post- it should be OK.

I would not chance it and an alloy seat post would be the best choice. They are not that expensive either.

I use this one, but it may not go with your fancy carbon fiber.

https://www.velo-orange.com/saddlebag.html