Unless you ride a tourer equipped motorcycle with panniers and a top-box, you can't carry very much cargo on your motorcycle. The tiny space under your seat and the pockets of your jacket will just about hold your mobile phone, wallet and a candy-bar but not much else. You need a bag to lug on your back.
There are so many backpacks on the market that it's impossible to list only a few without missing out some good things of others. As a rule of thumb, it's well worth investing in a proper motorcycle backpack rather than a cheapo type from the local discount market. So what to look for when buying a motorcycle backpack?
Unless you're a woman, your backpack really shouldn't be regarded a fashion accessory. But we can't ignore the rise of the man-bag and the unerring desire of some motorcyclists to make sure every bit of their kit makes a statement about their 'individuality'. In the groups of motorcyclists I know, this trend is led by Kaeng, who tries to convince us that an absurdly expensive leather satchel is the mark of an urbane dandy. We're all unconvinced, but there's a lesson to be learnt. It might be okay to pick a bag that looks smart and complements your riding kit, but falling for form over function will only make you look stupid.
Don't make the mistake of buying the first bag that catches your eye. The best question is: How much do you need to carry? Bags range in capacity from a little 15 liter to a good 40 liter. If you're planning on using it for your supermarket visits, for instance, then biggest is best because there's always that unexpected discount buying to keep in mind. If you're just carrying your fried-rice with spicy chicken lunch to work or a day's refreshments to a bike-meeting, it makes sense to go for a compact backpack.
Smaller ergonomic and don't flap around as much, particularly when riding along at speed on a sportsbike. If you're going long-distance touring, say going to visit the in-laws for Songkran, it's best to use motorcycle-mounted luggage rather than trying to stuff everything in a backpack. Too much bulky weight on your back will become uncomfortable, restrict your movement and the straps will dig into your shoulders, not to mention the potential for injury should you fall off and land on your back.
Some larger backpacks have chest and/or waist straps, which are useful for keeping the payload balanced and steady.
All backpacks affect aerodynamics and the bulkier ones have the worst impact. If you intend to ride fast, it's best to choose a compact backpack that tapers from a wide base to narrow top, sits flush to your back and thereby causes minimal disruption of airflow.