I haven't tried Deuter on bike, just hiking and XC skiing. Can't you just keep the hip belt open? Tuck the loose ends somehow, so they don't get tangled with anything? Not sure if you can do anything about the top bumping your helmet though.
Deuter makes biking specific backpacks, but I'm not sure they're large enough... see the Cross Air for example.
http://deuter.com/en/DE/product/1-76...sAir20EXP.html
--J