Seat packs, as the name suggests, are small aero shaped packs of varying size which sit under the saddle and are a very convenient way to carry spare tubes or tools. They are usually attached with Velcro straps, or a dedicated locking attachment depending on model and compatibility, on to the saddle rails and seatpost which holds it securely even on the roughest of surfaces. Depending on size they can carry anything from a single tube right up to spare rain capes as well as tools and spares. Being aero shaped they are unobtrusive and therefore don’t rub on shorts or tights which can irritate skin as well as ruin your clothing. Speaking of clothing, they can also prevent the rear of your jersey from sagging by taking spares out of your pockets.
For bigger loads, larger seat packs can be used which need a specific carrier rack which extends from the seatpost and is ideal for those who need more space but do not want to or cannot use traditional racks and pannier bags. Please check compatibility with your current bike set up before purchase as certain seatposts may not be suitable for such seat pack racks, Merlin Cycles staff will be happy to help with any queries on this