so that the lower tab stops the bag from sliding forward under braking lets say, and the upper attachment point stops the pannier from sliding backwards. (or the other way around, whatever works best)
This is a valid point.
Something to consider is heel strike as you are pedaling. This is a bigger consideration with the larger "rear" panniers of the Ortleib sets as they are "wider" (side to side, which means "longer" fore/aft with respect to the bike). You have less room with that extra width.
To combat that you can adjust the top latch hooks to set them for places on your racks where they work best - to eliminate the sliding that you are referring to.
This past late winter/early spring I took my folding bike on a day trip and used the large rear panniers. I had never used them on that bike before, but with the gear (clothes, cook set, coffee, etc, etc) I decided to see if I could get the big ones on there. I had the latches in the furthest forward positions I could get (so the panniers were pushed furthest backwards) and was able to get them to work just fine. I did have to force the rear latches on both sides a little bit because the spot where they fit was also where the rack started to bend. I suppose I could have gone up a size with the inserts, but it wasn't that tough of a fit and works fine.
At the 50 second mark in the video linked below they show the adjustability of the top latch hooks.
Originally Posted by
debade