Originally Posted by
alan s
It's the diameter of the tube. My Topeak rack is 8mm, and Ortliebs come with little inserts that more or less permanently fit into the hooks for the smaller diameter tube.
As an aside, Backrollers are huge, so unless you really need all that capacity, you may want to get Front Rollers and use them on the back.
Thanks Alan.
Yes there is a gap between the rail and the inside of the hooks. However, with the third point of attatchment to the lower part of my rack, the bags seem pretty secure. Would you still think it necessary to use the inserts? I am hesistant, because if I decide to use another rack with wider rails someday, the inserts might be tough to remove.