I use an Arkel Tailrider that I bought specifically because it's smaller than most trunk bags. (Although it's far from cheap.)

My reasoning was that, left to my own devices, I tend to carry everything with me. The Tailrider holds everything I absolutely need, with no room leftover for crap I'd carry but never use.

It holds tube, patch kit, tire levers, multi-tool, spare cash, wallet, sunglasses, keys, comb, flash drive, camera, lunch (two sandwiches), snacks (two apples, two oranges, two granola bars, two trail-mix bars), and a work shirt. Once a week (in each direction) I roll up a pair of jeans and slip them under the outer straps on top. I can just barely squeeze in a hardcover library book if it's not too thick. Coming home (sans lunch and snacks) I can fit in two or three hardcovers.

Once a week I add my grocery panniers to shop on the way home. When I have more books to return than will fit in the Tailrider, I return them on grocery day.


The Tailrider on a jeans day.