Ben is taking on the very worthwhile task of trying to insure he doesn't have a rack failure that causes him to fly over the bars.
The cases where this CAN happen is on a rack with three points of attachment. If the crown attachment fails, and the rack pivots around the fork blade attachment points and hits the tire. If the tire is knobby, and "grabs" the rack, pulling it harder down onto the tire, then this locks the front wheel and flips you over the bars. This is what we want to avoid. It has happened a few times and it's really bad for the rider. Here's a questionnaire about the situation:
1. Does your rack have a welded-on metal tab, designed to allow you to attach a headlamp or reflector? If YES, that's worse (more likely to send you over the bars). If NO, that's better (less likely to throw you over the bars)
2. Does your bike have fenders? If YES, that's much better (if the fender makes it impossible for your rack to ever hit the tire, then it can't possibly lock up the front wheel). If NO, that's worse.
3. Do you run knobby tires? If YES, that's worse (knobs can grab the rack and lock up the front wheel). If NO, that's better.
What I see on Ben's bike is that if the diving board fails, the rack will pivot forward onto the fender. The fender will rub the tire, make noise, and slow Ben down. I don't think it could possibly lock up the front wheel and throw Ben over the bars.
I recommend that Ben unbolt the diving board from the rack and loosen the bolts at the mid-fork braze ons so the rack does pivot downwards. Convince yourself that the rack will land on the fender and the fender will press on the tire. That's a brake, but it's not a self-energizing launcher.
If Ben convinces himself that's the worst that could happen, I'd recommend that a failsafe could be to replace one of the two diving board to rack bolts with a very long one, run through a spacer and through the fender itself. Bolt the rack to the fender, and the fender will act as the failsafe for a failed diving board. No strap needed.
Bill Lindsay
El Cerrito, CA