Originally Posted by
gauvins
In your opinion, how stable would a rack pack be, over a rack but without rear panniers?
We prefer to travel with front panniers only, but will have to deal with cold nights on our next trip, which will considerably increase luggage volume. The idea for now would be to ride with 2 front panniers and a rack pack.
I am not Doug but I will offer my thoughts.
In the attached photo you can see that the 31 liter Ortlieb duffel or they call it their rack-pack sits directly on my two rear panniers. It is only attached to the two panniers, not the rack and not the bike. (In the photo there was one strap around the middle to the rack, but I was on a very rough road and wanted to hold it in place better that day.)
Sometimes when it was only partly full, it might shift in position by a few inches, but the straps from the bag to the panniers held it in place quite well. The pannier straps on the Backroller directly clip into the upper buckles on the ends of the bag, refer to the photo in my previous post at this link.
https://www.bikeforums.net/20167316-post13.html
I would think that trying to balance anything that long on the rear rack without panniers would work really badly.
I think you need to find a roll top dry bag that has a diameter that is no more than twice the width of your rear rack platform width. And about the length from about the rear of your saddle to a few inches overhanging behind your rear rack. Roll top so that if it is not full, you can roll the top down to make it full by reducing capacity to make it less floppy. My Surly rack is 6 inches wide, thus I think a maximum of 12 inch diameter drybag would be stable lengthwise on that rack if you strap it down well enough. But my Tubus Logo is only 4 inches wide, on that I would not want to exceed an 8 inch dry bag. (Sorry for using inches instead of your country's units.)
The wider the bag on the rack, the better you need to strap it down.