If there's one area of your bike you don't want to cut corners on, it's the frame. The reason is simple -- you can easily upgrade gearsets, brakes and other features, but you can't really upgrade the frame without basically buying a new bike.
The two aspects of mountain bike frames to consider are material and geometry. Here are the most common frame materials, along with their pros and cons.
- High-Tensile Steel. This material is inexpensive and strong, but very heavy. This is used on low-end bikes, and serious mountain bikers won't touch it.
- Chromoly Steel. A different alloy (using chromium and molybdenum) than high-tensile steel, it's lighter and more rigid, but costs a bit more.
- Aluminum. An aluminum frame is lighter than chromoly, but not quite as strong. A well-made chromoly frame is better than a cheap aluminum one. Aluminum is a good compromise between weight and cost for intermediate bikers.
- Titanium. This material is very light and strong. However, welding titanium is tricky, so some titanium frames have a reputation for broken joints. The cost of titanium frames has come down considerably, but is still mostly found in high-end, expensive bikes.
- Carbon fiber. While extremely light and rigid, carbon fiber is very expensive and prone to impact damage. Carbon fiber frames are for serious competitive mountain bikers.
The higher-quality frames undergo a process called butting, in which the walls of the frame tubes are thinned in the middle, with thicker tube walls at the ends. This saves weight and makes welded joints stronger. You may see frames described as double or triple butted. Non-butted frames are known as straight-gauge.
The frame geometry you choose will depend on whether you want a rear suspension and how advanced a rider you are. Beginners and many intermediate mountain bikers do well with traditional diamond frames. To accommodate a rear suspension, you may need more exotic type of frame geometry, and competitive riders may prefer exotic geometries to save weight or increase performance.
It is also recommended that you get a frame with mounting spaces for optional equipment. A basic mountain bike can be upgraded easily as long as the hubs are able to mount disc brakes and the front suspension can be changed. If you want to take your bike for longer rides at some point, mounting spaces for mudflaps and cargo baskets will come in handy.
One last note: Look for a sloping top tube (the tube that runs from the seat to the front of the bike). Almost all mountain bikes have them, since they provide extra clearance for the rider in the event of a crash. Don't take it for granted, though. One painful encounter with a top tube will instantly remind you why sloped is better.
How do you keep a mountain bike under control during steep descents or while negotiating trail obstacles? We'll talk mountain bike suspensions in the next section.