If you remove the wheel and place a bolt through from the inside you gain a little clearance, and can put a nut on the outside. This is how I mounted the rack and fenders to my Azuki frame, where the eyelets weren't threaded. Use a nylock nut and you shouldn't have any problems with it coming loose. I regularly carry loads of between 50 and 90 lbs of weight on my rack(though I don't ride very far with 90 lbs!)
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"There is no greater wonder than the way the face and character of a woman fit so perfectly in a man's mind, and stay there, and he could never tell you why. It just seems it was the thing he most wanted." Robert Louis Stevenson
