I don't know about hte bags never used one, they are pretty small though only about 15 L I think. Ok for some cloths and a bit of food and tools, would struggle to fit a sleeping bag and bivi bag in one though if you intended to camp. Check this guy out for tips http://www2.arnes.si/~ikovse/weight.htm he just uses a stuff sack or backpack strapped to the top very light. You'll cope with what ever you do though and adjust it if you don't like something it's not a big deal really.
I get you, I thought you meant seat post rack.
I don't know about hte bags never used one, they are pretty small though only about 15 L I think. Ok for some cloths and a bit of food and tools, would struggle to fit a sleeping bag and bivi bag in one though if you intended to camp. Check this guy out for tips http://www2.arnes.si/~ikovse/weight.htm he just uses a stuff sack or backpack strapped to the top very light. You'll cope with what ever you do though and adjust it if you don't like something it's not a big deal really.