My mountain bike has rear suspension, and this make it difficult to mount a full cargo rack. This is a great rack, especially for the price. My seat post is large, and I didn't need the spacers, but it should fit all sizes. If you load it down, it may shift. You may consider drilling a small hole through the rack and seat tube to prevent that.

Pleasantly surprised. Sturdy and easy to fit. The photo I ordered from shows some crummy looking flat shims that adapt it to various sizes of seat posts. Mine came with three telescoping cylindrical shims that you use in various combinations depending upon the size of your seatpost. I used two and it made a PERFECT fit on my 27.2mm post (one of the most common sizes). The bungee cord seems to be good quality and the cap to the small compartment underneath has holes to mount a standard rear reflector. Seems to have been much improved over the product that most of the reviews refer to. i saw many minor complaints but bought it any way, I was expecting to have to do a good bit of re=engineering or maybe just have to return it. But no, everything was perfect, just opened it and installed it and used it, no problems at all. Rare excellent product, and great price; Walmart at its best!