Originally Posted by
EvilV
Unless you were carrying the bike in a back pack as a form of pentitential right, I'd suggest that it would need to have some kind of back protector built into it. This shouldn't be too hard to arrange though. You could even just use a sponge sleeping mat folded suitably and placed between the bike and your back if you didn't need to carry it too far.
I'd recommend hard plastic over something spongy. My experience is that when it comes to wearing a folder on your back hardness isn't the problem as much as pointy parts stabbing you and a hard barrier will better protect you from that than a soft one.