Originally Posted by SquidPuppet

... Also, and this is critical, do something to completely immobilize the strap. That small amount of up and down motion repeats itself every time you transition from accelerating to braking and vice versa. Every time. That's what caused that big nut to work itself free. Make a shim out of soda can or whatever it takes so there is no slop between the strap and the chain stay. Or you can measure the diameter of your chain stay and buy a snazzy new Sturmy Archer strap in the right size for just a few bucks. And get your self a bigger replacement bolt for the strap if there is slop between the bolt and the strap/arm hole. You want that bolt to be snug in the hole.