Well the Bug arrived and I fitted it on the rack, it's a slightly better fit than the Ortlieb Vario as the lower securing hook is attached to a more flexible bungee rather than the stiff rubber strap used by the Vario (which just didn't work to secure to lower part of the bag). However, with the Bug mounted as far back as possible on the rack, I've found that the bag does move forward about 1cm which is enough for heel-strike to occur. I'll try wrapping some insulating tape around part of the rack to prevent the movement but the Arkel's cam-lock fixing system doesn't strike me as being as durable or as secure as the Ortlieb systems. I did try to pull the Bug off the rack but it stayed put, but after years of use I wouldn't be surprised if the spring-loaded cam-lock mechanism became weaker, but only time would tell I guess.

For me though, the Bug is a bit disappointing given the price - it has no inner pocket or organiser which is essential for a commuter bag; granted you can buy a separate laptop bag with mesh pockets but this adds an extra £26 to an already expensive bag. The addition of a mesh organiser pocket inside the outer zippered compartment would be a great improvement to the Bug.

But it does seem that convertible pannier/backpacks all seem to suffer from the same problem, that the lower securing system is always a hook on an elasticated strap and unless that hook can be fixed directly below on the rack then the bag will move as it's pulled forward by the elastic strap. Unless your rack has horizontal dog-legs long emough to allow the elasticated hook to be attached vertically with the bag mounted at the furthest rear position on the rack, then you're more likely to encounter problems - I think a rack like the Tortec Expedition would work well in this case so maybe I should have bought one of those instead of the RackTime AddIt, but I didn't want a full-on touring rack for my commuting bike.

For bikes with short chainstays (435mm on my bike) and for riders with large feet, the elastic strap is more likely to be an issue, as I've found, so I'm starting to think that I'm better off just getting giving up on the convertible backpack/pannier and just plump for the Ortlieb panniers with their rigid QuickLock adjustable lower fixing system which would (I think) alleviate the issue with the pannier moving forward.

If The Ortlieb Vario and Arkel Bug dispensed with the elastic lower securing strap/hook and used a moveable plastic clip to secure the lower part of the bag, there'd be none of the issues I've encountered.