Originally Posted by
HillRider
If you can, mount the rack's leg to the inside (against the dropout face) of the fender strut. That puts less bending moment on the bolt from the rack's load.
1. It's not moment of inertia that matters here; it's shear strength.
2. Actually, shear strength doesn't matter very much either. The bolt supplies the normal force. The friction between the rack and frame is what holds most of the weight.
3. It's usually not possible to mount a rack on the inside of the dropout. The smallest cog on the cassette is in the way.
Put the fender strut on the outside of the rack, use washers, and tighten the bolt well.