The Ortlieb Back-Roller Classic is the most well-known model in the Ortlieb series. Both bike bags in this set are identical. They're made of double-sided coated polyester and have the classic rolling closure. This makes the bag 100% waterproof, so you can cycle through the rain without worrying. Firmly attach the bags to your bike rear rack with sliding hooks. Do they fit around every bike rear rack tube? Yes they do, because with the included rollers, you'll attach it in a totally customized manner. That way, your bag won't move up and down at every speed bump. The bag consists of 1 large main compartment, but the bag also contains a handy mesh compartment with a zipper, to store things you want to be able to access quickly.