matmannrobin

I bought a transit seatpost rack and it on the directions say I cannot attach to a carbon fiber seatpost. Why not?

kokomo61

Because the added weight on the seatpost (from the cantilevered rack) puts different stresses on the CF seat post than it was designed to handle. It's designed to handle the weight from the saddle, not additional stress from the seatpost rack. With enough weight in the rack, the CF seatpost can crack and fail, which wouldn't be fun at all.

If you really need a seatpost rack, switch to an aluminum seat post. Problem solved.

matmannrobin

Thanks for the info. Guess I'll have to use the backpack tomorrow.

ax0n

Dude, I talked to my bike shop about this.

He took a run of the mill carbon Bontrager seatpost and beat the hell out of his metal work bench with it, and tossed it to me. A scratch or two, perhaps. Dents? Nowhere to be found. He said the only thing he's seen damage a CF seatpost is when someone clamped the seat tube down way too hard on it. It splintered a bit. For carrying an extra pair of shoes, your lunch, work clothes or whatnot, it would be fine. You can't put panniers or anything heavy on it, of course, but you wouldn't want to do that with ANY seatpost, carbon or otherwise.

What were you planning on carrying on the rack? I'd say if it's under 10 pounds you'd probably be just fine.

xB_Nutt

It's not just about adding weight to the post. The clamp for the rack will indent the carbon post possibly causing it to snap at that point. To me it would not be worth the risk. Carbon Fiber is nasty when it cracks. Sharp, jagged, splinter laden edges that will more than likely find there way into your back side...

kokomo61

It's not JUST the weight - every time you hit a bump with a weighted seatpost rack, that movement gets transferred to the CF seatpost. If you clamp the rack too tight, nick the post, or if you accidentally ding the rack, it can cause a small crack in the post. Now, it could be that the manufacturers are just covering their butts against a potential lawsuit, but there's enough of a possibility that it could fracture that I'm not doing it.

Full disclosure: I bought a seatpost rack in case I decide to use it to carry stuff during a June century. If I use it, I'll be using an aluminum seatpost, not my CF one.