Originally Posted by
Schwinnsta
About half the weight of the rack, perhaps less, depending on how it is loaded, does go through frame. The rest goes directly though dropouts.
Most rear racks are centered over the rear axle, so nearly all the payload weight goes there, the rack stays just hold it from tipping over forward or back, very little load, if there was, most flat stays would buckle, especially since most people curve the stays (I don't).
On a Bike Friday New Word Tourist, most of the rack payload is behind the rear axle, it must be for panniers to have heel clearance when pedaling. My bike is not a BF but extremely similar configuration when unfolded, and if you look at the pic I posted of mine (post #3), you'll see that. This required extra-super-long rack stays that did not come with the rack but were purchased separately, and those are clearly evident as well; Much compression load on those stays and they would buckle and fold up instantly, they are so thin and long. With the panniers well behind the rear axle and supported at the dropouts, if anything, the rack stays would be in *tension*, which would actually slightly counteract the longitudinal bending moment on the frame due to rider weight. I carefully mathed out my rack configuration before building up the bike.