Thread: Lets get down to the nuts and bolts

With softer metal (low grade bolts) if you get a stress concentration, you get plastic flow of the metal so as to reduce the stress concentration. This may loosen the bolt, allowing movement, which will eventually result in failure - but you have a chance of seeing it before it fails - could take months or years. With harder (stronger) steel, this does not happen and if the stress concentration is enough, or cyclic, a crack eventually starts - and failure may be only a few hundred more cycles away. (one cycle = one corrugation!).

Of course, if the strength of the bolt is not enough, it will fail anyway. But fatigue is the enemy on towbars, and if properly designed there should be no call for anything higher than 8.8.