Originally Posted by
striknein
Seatpost racks clamp in (mostly) the same way threadless stems clamp. You can put a hell of a lot more force on a carbon steerer than you would loading a trunk bag with 20 lbs. of crap.
good stems (and seat post clamps) use an asymmetric clamp to prevent point loading the steerer. im not saying seapost racks dont have this but i have seen carbon posts fail when subjected to a clamp on rack in the same way carbon steerers can fail with the wrong stem.
bad clamp:
good clamp:
stems use similar designs.