Originally Posted by
garethzbarker
I know this a few weeks old but I have an 09 roubaix and I am about to mount the srq on it for my carradice pendle bag. I don't know why everyone is so scared of a carbon seatpost. That seatpost is strong as hell. I'm 99kg and it holds me fine (it held me at 110), if I was worried about a 7 to 10lb bag attached to the strongest part of the tube breaking I'd never ride the bike. A 5lb load on a 400gram seatpost rack is not going to break your bike. But I would choose one with a nice wide clamping area and that wont scratch the post all up.
A lot of distance riders ride carbon bikes and use saddlebags/handlebar bags and I can only guess that some of those guys have to install little seatpost racks to hold the bags.
Carbon fiber holds weight as well as any other material used for bikes, but it's a little more delicate when it comes to something being clamped onto it. You could theoretically crush any seatpost - steel, aluminum, titanium, etc if you clamped down a seatpost to much. In practice, this wasn't a problem as you either needed power tools or a enormous lever to do so by hand.
But you sometimes can crush carbon fiber with a strong arm and hand tools. If you look at a bike with a carbon seatpost, it will often specify how many foot-pounds (torque) should be applied to tighten it correctly, often requiring a torque wrench to get it precisely right.
There's also something about carbon fiber where it's only strong in the direction it's designed to hold weight, but I don't know that much about that topic.
I think if anything is clampable, it's the seatpost - you gotta clamp it down to secure it anyways. Just don't wrench on it as hard as you possible can and it should be fine. I'm not so sure about some of the other parts of the bike - those rear seat stays are usually unbelievably thin, and they're not designed to be clamped down onto.
The Axiom rack mentioned above, and most other racks, simply attach at the bike in locations designed to bear weight. Usually it's the rear wheel skewer (which is designed to hold much of your body weight) and right where the brakes are attached (which needs to withstand the forces of bringing your 200 pound body weight to a screeching halt from 30mph or more).
I totally agree in general with what you're saying - what's another 5 pounds on a bike designed to hold a 300 pound rider? It's just a matter of not crushing something that wasn't designed to be squeezed, and putting the weight on a bike in a location that's designed to handle it.
I wouldn't hesitate to put a rack on a Roubaix (the right kind of rack). With it's more stable handling, it would be fine. Putting a rack on some of the twitchier bikes brings up issues of handling with the added weight (depending on how much weight, etc, be seriously - some of them can be really squirrelly).