Image source: http://www.giant-bicycle.com
The frame of a bicycle is one of the most important components we will look at. It defines the comportment of the bicycle on the road. The frame making up a great amount of the overall material of the bicycle, it also affects its weight quite noticeably. Some lower-cost bicycles still use steel tubing. You should avoid this type of frame as much as possible. Nowadays, most frames are made of various alloys. The most commonly used is aluminum. When the first aluminum bicycles were put on market, some exhibited soldering problem and were subject to break under heavy usage. This is less of a concern today. You can safely buy an aluminum frame in all confidence. Aluminum has become the common frame material for mid to high-end bicycles. At the end of the spectrum, we are seeing carbon frames. Carbon frames have many advantages. They are very strong (resistant), and at the same time, more flexible than aluminum tubes. What does it implies? While they are more robust, the small flexion they allow creates a frame that is more comfortable to ride. Thus, some of the vibrations coming from the road imperfections are dissipated by this flexion of the frame, instead of being fully transmitted to the cyclist. What is good for the comfort of the cyclist is unfortunately bad for the energy efficiency of the bicycle. As the cyclist pushes on the pedals, the frame absorbs a percentage of the cyclist energy. That energy is not transmitted in the transmission system of the bicycle. Many high-end race bicycles use carbon forks to provide some sort of shocks absorption, to the detriment of a very small performance loss. This is a far lighter way to provide shock absorption than using real shock absorbers, which would be unpractical on race bicycles. Even if the frame rigidity may absorb a small amount of energy, you have to keep in mind that it is only one of many factors making a bicycle a high performance one or not. For instance, the weight reduction provided by a carbon frame surpasses, by far, the energy dissipated by the frame due to flexions. A frame that is a bit less rigid can actually save some of the cyclist energy as it is imposing less long-term fatigue to him as he rides.
No matter the type of bicycle you plan to buy, you should always try to get the lightest bicycle of its category. The bicycle is there to carry your own body weight. It is not there to interfere with your movements. The lightest bicycle will always provide the most enjoyable riding experience to its user. A good bicycle will feel transparent to you. If you are carrying your bicycle instead of being supported by it, it is not going to be a pleasant ride.
Shopping tip: You should always weight every bicycle you look at by hand. To get an idea of the weight of a bicycle, place your hand under the top frame tube, at equilibrium position. Then lift it a few inches from ground, with your fingers under the tube, not the palm. The minimal number of finger that let you safely hold up the bicycle gives you a numerical value u can remember for comparison with other bicycles you shop. Always remove the little finger first, going to the index gradually. Each finger does not support the same amount of weight!