REI Co-op  Junction Seat Bag

I purchased this bag a couple of weeks ago and it has become my go to whenever I just need to bring the basics and an extra layer. It fits my mini U-lock, a tool kit, an extra tube, a couple of cylinders and an inflator head, an efficient outer or mid layer, and still has enough space for a foldable hat or some snacks. It is quick to attach or remove, so it works well for city trips or sketchier parking. For me, it’s been stable as a table. There are a lot of possible variables. My bike is a steel frame with no rear brake, no clearance issues, and plenty of seat post real estate for attachment. This angles it downward quite a bit and I have not once felt any swing or sway, even with a heavy U-Lock on the bottom. It fits enough to keep a bag off of my back and still barely sticks out longer than my saddle. Definitely fair weather fabric, but that fits my use case.

Big enough to fit tubes, tools, an extra layer, and a minipump. It's hard to find a seat bag that isn't too roadie-small or too bikepacking-large. Seems to be nicely made. Thanks, REI!

Bag is great, but seat post velcro is too short for the larger diameter posts.

Returning this after attaching to bike for examination. Upside: it's low cost. It has two small zip compartments that would be good so money, phone, keys do not get lost among gloves, hat, vest. Downside: The design details are a failure. 1. It wants to swing side to side even with the straps tightened. 2. The side buckles necessary for both attaching and tightening are ridiculous in size and quite a pain to tighten, because you must pull the straps up and toward the underside of the seat, where you cannot get your hand. 3. Why two zipper closures on the back door? It only needs one, two rattle and are just dumb. 4. Why a flat padded bottom? It does nothing useful. Why is it padded overall, and why does it have a liner? It is not waterproof, so these features do nothing but add weight/bulk. 5. It obscures your seat post of course, so your crucial rear light clips onto a strip of material on the back door, where it hangs completely loosely ready to fly off over a hard bump, and where it cannot be positioned stability to stay directly aimed at the traffic behind you. Ok, it's going to be a perfectly sufficient economical and even super useful bag for casual bike rides for people who do not focus on design, are bothered by things flopping, etc. But for riders who are all about great design it just annoys. Particularly annoying are the tightening/attachment buckles, the light hanger, & double zipper.

I am agreeing with CK in NH; the bag - or sack - swings around like a bull's sack. There is no way to reduced the swinging, any attempt to tighten the straps is not effective.