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raceinc.2
post Nov 25 2008, 11:50 PM
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is mild steel heavier than chromoly (IMG:style_emoticons/default/huh.gif) ?
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mr coasterbrake
post Nov 26 2008, 12:00 AM
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simplified answer is no, not in equal parts.

CRMO is stronger, thus allowing you to use less of it to get the same strength.

i'm sure the "metals" guys can add to this...
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raceinc.2
post Nov 26 2008, 12:26 AM
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in essence, wouldn't a mild steel frame be heavier than the same version in chromoly?
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mr coasterbrake
post Nov 26 2008, 12:36 PM
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depends on how the metal was used.

typically, yes, mild steel would be a heavier frame because they had to use more material to achieve "X" level of strength.

but, if you built a mild steel frame and a CRMO frame using all the exact same dimensions (tubing diameter, wall thickness etc) the weight would be nearly identical. in that case, the CRMO frame would be stronger, but not lighter.
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mxer746
post Nov 26 2008, 02:25 PM
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QUOTE (raceinc.2 @ Nov 26 2008, 12:50 AM) *
is mild steel heavier than chromoly (IMG:style_emoticons/default/huh.gif) ?

I know of an A pro years ago who built a mild steel frame in shop class. It was a nice frame, but complete bike was over 30 lbs.
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raceinc.2
post Nov 26 2008, 03:27 PM
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let's say this. in the case of the mild steel looptail quad vs the very early cromo looptail quad, is it inferred that the mild steel frame would weigh more than the cromo? we have been told from the "powers that be" that the wide loop quad is mild steel and that all the tight loops, no matter what, are cromo. I have a problem with this theory b/c it seems to me that the wide loop quads would use more material than the tight loop quads. this alone would make the wide loop quads heavier than the tight loop quad. I have asked several looptail quad owners to weigh what they have and have come up with some interesting information. The wide loop quads, serial #'s that begin with "00", weigh about 1 pound LESS than the very early tight loop quads- serial #'s that begin with "1". How is it possible that a wide loop quad using more heavier material would be lighter than the less material tight loop quad?
Also, I was told that at one point, SE was selling both material frames at the exact same time and that Tim Cook has a copy of the ad print. How 'bout this theory: the "powers that be" are wrong in their assumptions. It could be that the serial #'s designated what metal was being used: the "00" frames with wide loops are in fact cromo and the early quads with serial # "1" are the mild steel frames. It would be much harder in the beginning of these frames to bend the cromoly tubing in a tight loop than it would the mild steel frames.....what do you guys think? This whole idea would set the looptail quad theorists on their butts....
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Mark 'Gonzo...
post Nov 26 2008, 04:38 PM
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Some material property discussions that may or may not be of interest.

Mild Steel (major alloying element carbon)
example: AISI 1018

density: 0.284 lb/in³
elastic modulus: 30,000,000 psi
tensile strength: varies with heat treatment
yield strength: varies with heat treatment

Chromoly Steel (major alloying elements chromium and molybdenum)
example: AISI 4130

density: 0.284 lb/in³
elastic modulus: 30,000,000 psi
tensile strength: varies with heat treatment
yield strength: varies with heat treatment

Both steel alloys are more than 98% iron, so the density is virtually the same. A similar equality applies to something called the elastic modulus of these materials, which is the ratio of stress to strain.

Visualize a piece of sample (mild steel or chromoly steel) material 1.00" wide, 1.00" tall, and 100" long. If you pull on this piece longitudinally with 3,000 pounds, it will elongate (stretch). For these materials, it would elongate 0.010" so the new length (under load) would be 100.010".

Stress is force divided by area. Strain is the change in length divided by the original length. Therefore, in our example, the stress is 3,000 pounds divided by 1.0 square inch or 3,000 psi, and the strain is .010" divided by 100.000" or .0001. Since the elastic modulus is stress divided by strain, the elastic modulus is 3,000 psi/.0001, which is 30,000,000 psi.

Since these materials are linearly elastic, we can perform the experiment with 300 pounds or 30,000 pounds, but each time we would compute the same elastic modulus of 30,000,000 psi, assuming we stay below the yield strength. More on that next.

OK, now about strength. Two parameters are usually used to describe the strength of a material: tensile strength and yield strength. Tensile strength is the maximum stress the material can handle, and leads to failure. Yield strength is the maximum stress the material can handle and still return to it's initial shape when the load is removed. If you stress a material beyond it's yield strength, it will bend, and stay bent.

Values of tensile strength and yield strength for these two materials varies wildly, depending on the heat treatment used. Also, things like welding and forming can change these values. Some make the material weaker, some actually make it stronger.

I would be interested to hear what heat treatment is used on bicycle frame materials, both mild steel and chromoly steel.
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