The part of a bike that everything attaches to. Bike frames are usually made of steel on bikes up to ~$600, aluminum on bikes from ~$600 up or carbon fibre or titanium on bikes from ~$2000 up. These are approximate figures, and there are still many good bikes made from chromoly steel, for example the Jamis Dragon, a very nice steel racing bike (~$3500 in Canadian dollars).
There are two general types of frames, hardtail or unsuspended frames and full suspension frames with a shock absorber in the back. I won't bother to go over full suspension frames, as the designs change from year to year and are fairly complicated.
Hardtail frames are usually made of 8 tubes. 2 chainstays which go from the cranks (what the pedals attach to) to the back wheel along the chainline. 2 seatstays, which go from the seat to the back wheel. The seat-tube, which the seatpost is inserted into, which goes from the seat to the cranks. The top-tube which goes from the seat to the front of the bike and the downtube which goes from the cranks to the front of the bike. This arrangement is known as the double-diamond and is very strong. This might seem a little overwhelming, but if you look at picture of a bike it should soon make sense.
When designing frames, bike companies have to worry about four factors. These are the material to build with, the weight and stiffness and the geometry of the frame.
There are a four commonly materials used in bike manufacture. These are steel,aluminum alloys,titanium and carbon fibre. Steel is about as strong as aluminum but about 3 times the density, rusts fairly easily, and doesn't fatigue very quickly. Aluminum is about as strong as steel but about one third the density, rusts although it isn't a concern, but fatigues fairly quickly. Titanium is stronger than steel or aluminum, about as dense as aluminum, and fatigues and rusts very slowly. Carbon fibre is up to 30 times stronger than steel, not very dense, doesn't rust, but fatigue is a problem as there aren't any visible signs of it.
Weight and Stiffness
Stiffness is a very important quality to consider when choosing and designing a frame. If a frame isn't stiff enough it will be inefficient, transmitting pedalling force to the frame instead of the wheels, while a frame that is too stiff will be hard on the rider, transmitting every little bump to the seat. It is also very important that the part of the frame where the brakes are mounted is stiff, because otherwise the bike will have weak braking. The reason for this is that when the brake pads on modern linear pull brakes touch the wheels, they start to push the brake mounts outwards. When this happens braking force is transmitted to the frame rather than the wheels.
The stiffness of a frame is determined by two things, the material used and the tube diameter. Larger tubes don't flex as easily, but won't be as strong because the tube wall will be thinner. This is where the density of the material is very important. Aluminum isn't very dense, so it gives designers more material to work with. It can therefore make wider tubes with the same wall thickness as smaller steel tubes. Generally aluminum is stiffer than titanium which is stiffer than steel. Carbon fibre is a special case, as it can be made to have varying densities and can even be designed to flex easily in one direction but not another.
Frame weight is generally determined by how much material is used, with aluminum frames being only slightly lighter than steel frames. Titanium and carbon frames are usually very lightweight.
Geometry is a very complicated subject. With a small change in tube lengths and angles a frame's handling can be greatly changed. The basics of geometry are that steeper angled seat and head tubes make for better cornering but at the expense of stability at speed and comfort. Usually racing bikes have the steepest headtube and seat tube angles, about 73-75 degrees, while downhill and freeride bikes have slacker angles. Slacker seat tube angles are also good for steep descents, keeping the rider's center of gravity off the front wheel and reducing the chance of going over the handlebars. The length of the top-tube is another important factor. A longer top-tube forces the rider into a more aerodynamic position, but a shorter top-tube is better for ascending and descending, while it is also preferable for women, who usually have shorter torsos.
This is another important subtopic. The main features that you should look for on a new bike frame are - predrilled holes for water bottle cages, etc. on the seat and down tubes; removeable derailleur hangers which are a must on all aluminum frames; mounts for linear pull brakes on the seat-stays. Another recent feature is the integrated headset, which increases strength and reduces weight. These introduce in 1998 as the ICBM (Integrated Cup and Bearing Module) on some Schwinn frames. The main disadvantages of integrated headsets is that they're harder to service, and it's difficult to get a replacement if you want to upgrade.
To learn more about bikes and bike frames, visit your local bike shop.