Hi-tensile and Chromoly are the two main types of steel used the cycling industry. Chromoly is a blend of chromium and molybdenum alloys, this is what quality steel mountain bike frames are fabricated with.
Cro-moly steel has an excellent strength-to-weight ratio and it absorbs shock like no other. The advantage of Cro-moly steel is that it's cheap, reliable, and simple to fabricate frames with. However, steel is “density challenged” – frames are usually heavier than their aluminum or titanium equivalents, the extra weight of a steel frame can hamper mobility.
You can choose the steel bike if you likely value tube design, professional welds, and frame longevity. Cro-moly steel is the classic material used in frame fabrication. In fact, many MTBs nowadays still sport this classic material in their build.
Today, aluminum frames are designed for an array of mountain biking’s demands. Aluminum alloy as the MTB frame material, it is durable and reliable. Aluminum is the second lightest of the materials per cubic inch behind Carbon so providing the rider with a lightweight and nimble experience.
Nonetheless, aluminum lacks in elongation, it is a more brittle and weaker material than steel so it needs to use more material and larger diameter tubes to make the aluminum frames durable. As such frames tend to be slightly overbuilt to make frames stronger and give a larger safety margin.
If you are looking for a mountain bike with a light frame, aluminum frame such as the one for you.
Titanium is only used by a few niche mountain bike frame manufacturers today. The biggest advantage of Titanium is as strong as steel, but a lot lighter. Titanium is half the weight of chromoly and has the most amount of elongation, gives the rider smooth and vibration damped riding experience.
Titanium is incredibly hard-wearing and corrosion resistant, is a desirable material of mountain bike but it takes a lot of effort to refine and process into the tubing used for frames. This means that it costs more to make a titanium frame, and its price is much higher than that of other materials bike frames. Besides, its dense less than steel but heavier than aluminum, it still does not offer the lightness of carbon fiber in a weight-conscious cycling industry.
Carbon is a composite rather than metal, CFRP has incredible strength and stiffness for its weight, allowing very lightweight frames to be built. Carbon is also quite versatile in its structure, different characteristics of carbon fiber will be used in different areas of the frame, it makes the entire construction a lot more resilient.
Carbon fiber is that it is an inherently brittle material, the main flaw of carbon fiber is its lack of elongation. When carbon breaks, it will fail suddenly, breaks all at once. That said, a crash that destroys a carbon frame is likely to destroy most other frames as well. You also need to keep in mind that carbon is that its production has a larger environmental impact than many other materials, as it isn’t recyclable.