I have a
Brompton B bag.
It's not a back pack, but it does have quite a comfy shoulder strap, and the package is convenient to carry onto buses, through airports and so on. It's got little wheels, so on smooth surfaces you can trundle it like a wheely suitcase. It doesn't look like a bike, and I've flown with it packed in this bag several times. (Bag + bike weigh exactly 15kg - the limit for Ryanair).
When the bike's not in it, it's a little bulky, but it has straps to help you roll it up, and I just strap it on the front of the bike over the front bag with a bungee. It's no problem carrying it like that.
I would have thought a backpack was a little pointless, in that:
- If you need to carry the bike any distance, you're better off riding it.
- A bike strapped sideways across your back is bound to jab other people in tender places. If there's no-one about to jab, why not ride it?
- It's probably better carried pointing fore-and-aft, like a briefcase or suitcase, because of its bulk. You're better able to see and control what you bump with it under your arm than across your back.
