I've just bought and installed some brake calipers on my rear wheel, only to find that I can't get the brake blocks to line up with the rim, the arms seem too short to do this, so they're rubbing on the tyre. These are Shimano Tiagra calipers-identical to the previous brakes. Now I'm wondering: are the front and rear different, as there is no indication on the pack to tell me? I just assumed they were the same. Where am I going wrong? Can anyone help?