Funny you should bring this up at this time 'cause I just got through making my floorboards work better. You can only tighten that bolt as far as the shoulder. But the boards are still loose and floppy, yes. A huge part of the problem is the holes in the boards are too large (the bracket holes are not as bad) for the bolt, allowing the bolt to move around in the hole. This is problem because the boards won't sit flat... they end up slanting down a bit which in turn throws the knees away from the bike and the rider has to tense leg muscles to keep the legs close to the bike. Over a long ride, this is a real drag.
So I made up some 1/8 steel spacers with tight bolt holes and added them on the nut side of the bolt. Now the bolt can't sit crooked in the holes which once again, would provide enough slack for the boards to rest at a downward angle.
The picture shows that the spacer I added fits tight between the underneath of the board and the bracket (when the board is down of course). So my boards now sit flat (90 degrees to the bike) and don't rattle.
