The Fizik Saddle Bag with Strap comes in two sizes, small and medium, depending on how much you are looking to carry. The pack has enough room to carry a tube, CO2, levers and a basic multi-tool. The Fizik Saddle Bag with Strap will work on road, mountain, cyclocross bikes and triathlon bikes thanks to a simple strap design that wraps around your saddle rails. The pack features a small loop to mount a rear taillight to help you be more visible in traffic or while riding in low light conditions. If you are riding a 29″ mountain bike you may consider ordering the larger pack due to the volume of the tube.