So I have a 20" folding bicycle to which I have attached panniers on the front. I wanted them up there to improve the weight distribution. But now I have a problem: My commute distance is pretty long (~ 20 miles) so I've been taking the bus one way and riding back. And although this is a folder, our buses are *really* crowded, and with bike-carrier space usually available that's where it goes. The spring-loaded arm is supposed to fit over the front wheel, but now the pannier rack is in the way. If I turned the bike around, it seems like it would fit fine over the back wheel.

Hence my question: Does anyone know if those bike carriers are labeled "front wheel" and "back wheel" just by convention, to fit two or three bikes handlebars-to-seat-to-handlebars? Or is it really important that the steering end of the bike be the end clamped down?