Your LBS can give you little loops that attach to your seat stays for mounting your rack at three points. I personally use rack for road bike (alum frame) and backpack for mtb. Just stylistic differences really. A road bike with rack and rack trunk is a pretty hard combination to beat for commuting imho. The very best combo I found though was leaving a duffle bag full of clothes at the office, and carrying nothing but water and tools on the bike.