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!

My favorite bag. The large size, simple form, light padding that gives it shape, large opening, and a couple pockets make loading and accessing everything easier than any saddle bag I've used in my 50 years of riding. I can fit a light windbreaker, light wool cap and neck gaitor, knee warmers, as well as two tubes, tool kit, a small lock, and my EDC pouch without a struggle. can also squeeze in a mesh string backpack for carrying overflow items. I don't have any problems at all with the things other reviewers have nitpicked about other than the tail light mount being too low causing my tail light to dangle (which I solved by sewing a small nylon strap higher on the panel) and the obnoxious REI logo (which my safety triangle completely covers). I've never felt the bag sway even though I don't keep the mounting straps too tight. I also have the smaller version of this bike that works great for smaller loads.

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

This REI 3 Liter under saddle bike bag (“Junction Seat Bag”) was a fortunate find for me. The typical 1.5 L bike bags are too small and large is often 8L+. It has two inside netted pouches, which is useful. It installs very easily and sits close under the seat. It swings a little from side to side, but I don’t notice it at all on my 50+ mile rides. The external loop on the zippered closure is well positioned to hold my taillight, so I’m easily visible to traffic. The bag is lightweight and appears strong. I haven’t tested it in the rain yet. I would have liked a bag that allows some external storage, like a zippered side pouch or a bungee setup, but I didn’t find one in this size. I’ve used it over about 200 miles so far and am glad I got it. It’s big enough for a mini-pump, multi-tool, several snack bars, a wallet, and still has room for small items.

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.