All steels have the same mass and/or densities. Chromoly steel has been made stronger than carbon (hi-tensile) steel, by the alloying of iron. This requires the addition of the elements molybdenum and chromium. Since these steels (which are alloys of iron) have been made stronger by these additions, they can be butted, or made thicker at the ends, where they are joined. This is true, because that's where most of the stress will be experienced by each bicycle. Butting is usually achieved by making the middle portion of a tube, thinner than at the ends. This is done to make the overall bicycle lighter in mass. It therefore, is performed mostly upon road racing bikes, and not so much with hybrids, cruisers, and mountain bikes.

There is a distinct difference between hi tensile steel and chromoly steel. Hi tensile steel is not as strong as chromoly steel. It is also more susceptible to rust or oxidation. It is however, almost as durable, and can be made to be just as strong as chromoly steel, by making the tubular walls thicker, or by increasing the diameter of the tubes.

Chromoly 853 is the same as 4130 chromoly steel, except that it has to go through an air-hardening process, to acquire its strength. 631 chromoly is almost as strong as 853 chromoly, but not quite. The stronger the steel, the more it can be butted, and the less it will weigh when racing. Therefore, 853 chromoly bicycle tubes can be made thinner than 631 chromoly tubes. Both types of chromoly bicycle tubes, can be made to be thinner than 4130 chromoly tubes. All chromoly tubes can be made thinner than hi-tensile steel bicycle tubes.

Whenever looking for a steel bicycle frame, always buy chromoly!