How can you tell chromoly vs. common steel?

[pengyou]

I am shopping looking for a frame, either by buying a new frame only or a new or older bike that is intact and will strip the frame of anything I don't need. If someone tells me that a frame is Chromoly how do I know they are telling the truth, aside from grinding some of the metal off and doing some kind of spectro analaysis?

[Greg Fox]

Some type of chemical analysis is the only way to be sure and even then often the main triangle is the only chrome moly part. Some scrap yards use portable x-ray units to sort metal. I don't know that they are sensitive enough to see the small amount of chromium or molybdenum that is in something like 4130. If it is a relatively light frame it is more likely to be chrome moly than if it is heavier just because of the strength properties of the different alloys.

[Peterpan1]

Weight would be the main tip-off. It certainly isn't a proof of materials, but a bike frame made of CR will weigh about twice to three times what it should. Of course that won't help you tell if particular pieces of the frame are to the same standard. You can check also in vent holes. CM frames will be about .9MM or less. However, you have to know frame parts since even something that is CM like a chain stay can have much thicker parts.

[alanbikehouston]

Yup, weight is the best clue to the type of steel used. And, except for French made frames, the seat post size is often a good clue. The lightest steel frames had thin-walled seat tubes, and use the larger diameter seat posts. The thick walled seat tubes use small diameter seat posts.

But, weighing the frame tells you the whole story.

[Falanx]

But cost 25 grand