Seatpacks or saddlebags attach to a bike's saddle rails, and often the seatpost too. While some are huge most are small, being designed to carry only essential spares and tools. Of course, different cyclists will have different definitions of 'essential'. The roadie who doesn't want to weigh down his carbon racer might be happy with a tiny bag that will take just a spare 700x23 innertube, a tyre lever or two, and a minimalist multitool. But you might also want to carry a puncture kit, phone, wallet, mini-pump, gas cannister, a second innertube, an energy bar, a compact wind or waterproof jacket, or lots of other things. Your best bet is to buy a seatpack from your local bike shop. That way you can take along everything you plan to put in it to see what size you need.
Most models have several sizes. Small is generally smaller than a pocket in a pair of jeans, while Medium might hold half a litre or more, and Large a litre-plus. Some seatpacks zip open to expand by as much as half their original volume, while those that strap to the saddle rails can be compressed to stop contents rattling around. A spare Buff, neckgaiter, or rag can be used to pad out other seatpacks, or to wrap a multitool to stop it poking a hole in your spare innertube.
The simplest attachment is by velcro straps: one around the bag and the saddle rails; the other around the seatpost. This works well. If you'll be taking the seatpack on and off the bike a lot, due to where you're parking it, a quick-release bracket that fits to the saddle rails is more convenient.
Most seatpacks will shrug off light rain, in part because they're shielded by the saddle and your body. Most cannot cope with the sustained spray of water from a mudguardless rear wheel, and you'll end up with a rusting multitool and a soggy wallet. Get a waterproof seatpack or fit mudguards.
Many seatpacks have reflective details, and a loop at the back for a small LED light. Bigger seatpacks might have dividers or pockets, so you can keep your phone separate from your spares, or get at your multitool without emptying the seatpack.
Here are three good quality seatpacks:
Vaude Racelight M
Also available in sizes S, L and XL, the Medium measures 7.5x7.5x15cm so is big enough a spare tube and a few other essentials. There are two small inner pockets to organise the contents. The rear of the pack has reflective stripes and a strap for an LED light. It attaches by two velcro straps. It's simple, but good value.
Altura Speed Seatpack - Large
This comes in sizes S and M as well. The Large is big enough to hold a smaller mini-pump as well as the usual odds and ends. It's nevertheless narrow enough not to rub your legs. Reflectivity is good, and the LED light strap is rubberised to stop the light jumping off. It attaches with two velcro straps.
Brooks Challenge Tool Bag
If you fancy a more upmarket seatpack for your classic roadster or cool fixie, how about this Brooks leather tool bag? First made in 1896, the Challenge is available in nine different colours in 2015. It measures 175x78x45mm, and there's a strap inside to stop tools rattling about. It's designed to attach to 'bag loops' on the back of traditional saddles. It's expensive but, being real leather, should wear well.
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