Originally Posted by
tatfiend
The Topeak bags have a plastic outer bottom piece which is intended to slide into the plate that runs the length of the center of the rack. The plate has inward angled edges on the sides which mate with the bag bottom. The whole bag slides on the rack from the rear and locks into place via a quick release locking mechanism so the Topeak bags are only intended for ue with a Topeak rack unfortunately.
I'm not so sure about the new ones but my 2 year old Topeak MTX expandable trunk does fit on other racks. It has these handy little velcro straps built into the corners for in-case you need to use it on a non Topeak rack. Of course this means it won't just slide on and click into place, you gotta fidgit with it for a bit, but it works. I suppose they would also be good for stabilizing the bag to the Topeak rack for when I'm taking the mtn bike trail home in the summertime. But I never remember to do that and in 2 years of commuting with this bag the slider and clip hasn't failed me yet. Good stuff.