BMX riders want a light and stiff frame, making flatland and aerial tricks easier to perform. The differences between aluminum and chromoly for frame materials do affect the way a BMX bike handles, but the performance characteristics of a frame depend heavily on the way the frame is constructed. Knowing the strengths and weaknesses of these two materials can help you decide which is ideal for your style of BMX riding.
Bike Frame Characteristics
The differences between aluminum and steel for bicycle frames can be separated into three primary characteristics: modulus, yield point and specific gravity. Modulus refers to the effort required to bend the material; yield point is the amount of stress the metal can withstand before breaking; while specific gravity refers to the weight. Aluminum has one-third the modulus and yield point of steel, meaning it breaks more quickly and is more easily bent under stress. However, aluminum also has one-third the specific gravity of chromoly steel, allowing for thicker tubing without sacrificing low weight.
A steel BMX frame is generally made from chromoly steel. Chromoly is forged steel that contains the alloying elements chromium and molybdenum, which give the resulting steel tubing exceptional durability, stiffness and workability. Steel, in comparison with aluminum, lasts a very long time, and can be repaired if bent or broken. Since BMX bikes have no suspension to absorb the shock from jumps and impact tricks, the long-term durability of steel makes it an ideal frame material.
BMX riders, especially racers, want stiffness to minimize flex, especially side flex at the bottom bracket during pedaling. Since aluminum is one-third the weight of chromoly, aluminum frames can be made with much thicker tubing, which improves the stiffness of the frame when compared to most chromoly frames. The thick tubing of an aluminum frame prevents flex that would sap efficiency from the rider. This stiffness is also ideal for flatland tricks, where the rider is often standing on the frame itself. Unlike chromoly, an aluminum frame usually cannot be repaired if it does end up with a major crack.
The Best Frame Material
The debate between aluminum and steel is ongoing, and there are clear benefits to each material. Steel is a great value as a frame material due to a better long-term life span, and a good steel frame can match the stiffness of aluminum through its construction and design. Aluminum frames are ideal for racers or flatlanders looking for exceptional stiffness and light weight. Generally, high-end alloys of either material outperform lower-quality frames, and new techniques in frame building and metallurgy are constantly improving the performance of BMX bikes made from either material.
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