Front and rear suspensions have enabled riders to subject their bikes (and themselves) to incredible abuse. There are high-speed races down mountains strewn with rocks, fallen trees and gullies.
The increased abuse and rear-suspension systems and have placed additional stress on the frames of mountain bikes. Frames constructed simply by welding together steel or aluminum tubing are quickly being replaced by more complicated structures.
The round tube is not the most efficient shape of tubing for use in bike frames. A round tube is equally strong in side-to-side and up-and-down bending, but the stresses in frames tend to be more in the up-and-down direction and less in the side-to-side. The rectangular beams and joists used in building your house can support lots of up-and-down weight without bending much, but if you turn them on their side they can barely support any weight at all.
If the frame is made using shapes that are more rectangular, taller than they are wide, you can gain strength in the up-and-down direction, sacrificing a little side-to-side strength that you didn't need anyway.
Making these more complicated shapes required some advances in material-forming technology. Some of these pieces are made by hydroforming the metal. This is a process whereby sheets or tubes of metal are placed inside strong dies (kind of like molds) and then pressurized with water. The pressure of the water forces the metal to conform to the shape of the die. This technique allows for the formation of complex metal shapes.
Some bike frames are constructed from carbon fiber. This is a material that is built up in sheets over foam forms. Sheets of carbon-fiber cloth are placed over the foam forms and epoxied in place. The result is a very light, strong structure that can have almost any shape.
These non-circular-section frame structures make certain rear-suspension designs possible. One example is the motorcycle-style suspensions that have no vertical tubes tying them into the frame.
A suspension like the one in the picture above makes it difficult to find a place to mount conventional brakes. That's one of the reasons why some bikes have been using disc brakes, as we'll see in the next section.