Originally Posted by
hammond9705
What do you mean by "a beater"? If you want a bike that you can ride around town, use as a commuter and lock up outside your favorite bar or library, then alum might not be the best choice. Alum frames tend to have very thin walls, and dent pretty easily. They are great for road riding, but you have to be a bit careful that you don't drop them, or that someone doesn't drop something onto them.
If you still want an alum frame I wouldn't worry too much about the type of alum and instead consider the manufacturer. Cannondale is a favorite around here for alum with the CAAD series, but most manufacturers make alum frames. There are also a lot of cheap alum frames (Bikesdirect or performance for example) that are very ridable and could be easily replaced if crashed or dented.
The last half of your post is just about right but the first part is dead wrong. Aluminum
is soft and can be dented but it's no more prone to dents than thin steel or any other much thinner metal. If anything, aluminum has a thicker wall than just about any other material to make it more dent resistant. There are literally millions of mountain bike frames out there that are made of aluminum that get dropped, crashed and abused daily without issue.
ph4nt0mf1ng3rs: 6000 series aluminum and 7000 series aluminum are both used in building bikes. 6000 series aluminum is used on less expensive bikes because it's an inexpensive alloy that is mostly aluminum. 7000 series aluminum has lots of zinc, magnesium and copper in it so it's stronger, i.e higher yield and tensile strength. Stronger means that the frame can be made just as strong as the cheaper material but with less metal so the frame is lighter. It's also more expensive. That's the thinner tube that hammond9705 is talking about.
Low level bikes are generally 6000 series aluminum while more expensive bikes are 7000 series. A 6000 series road bike...the Trek 1.1 for example...is around $700. A 7000 series bike...the Trek 2.1 for example... is around $1400. There are component differences which make the price comparison dodgy but part of the cost difference is the frame material. Generally speaking, you don't hang really good components on 6000 series frames and you don't hang cheap components on a 7000 series frame.
If you are looking for a general purpose, don't worry about it bike, I'd go for the 6000 series bike. Save some money for a special uber bike later on.