If you can, you should call the shop at which your GF will be buying the bike rack. Talk to a salesperson about your bike and your needs and have him or her suggest various racks that might work for you. (If you can, you should use the Internet while you're on the phone to the bike shop to look up and look at the models the salesperson suggests). Then pick out a rack. That way, your GF only has to go into the shop and buy the rack you've already picked out.