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Old 06-08-08, 08:16 PM   #1
Russ
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Is there a way to determine what type of steel a frame is made from?

I've got an old mountain bike frame, probably from the mid-80's (based on the fact that it has a chain stay mounted U brake). The thing is, the frame was previously stripped and repainted, so there are no labels, stickers, or any sort of identifying marks. The geometry resembles a road bike frame (horizontal top tube), but with 26" tires. Mechanic told me the dropout was forged, vice stamped (which I assume is a good thing).

Is there any way to tell whether this is a quality chromoly frame, rather than a cheap pipe frame? It does have a lugged seat tube/top tube connection, not sure if that's any indication. The other tube connections are welded.

Trying to figure out if it's worth fixing up, or if I should start with a new frame. I've ridden it, and it fits me well.
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Old 06-08-08, 09:06 PM   #2
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Unfortunately, there's really no way to tell other than tubing wall thickness which translates to weight. The forged dropouts are a good sign.

If the tubing is hi-ten non-butted 18 gauge carbon steel it's going to be heavy. If it's butted or double butted 4130 chromoly or a manganese-molebdenum alloy, it'll be lighter. Typically, if the seat post is 27.2 mm or larger, the tubing is better than straight gauge carbon steel.

There are instruments like the Olympus Series 35 ultrasonic thickness gauges that can accurately measure tubing wall thickness, thereby providing an indication of the tubing quality.
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Old 06-08-08, 09:20 PM   #3
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I was at a bike shop in the 80's and the old guy tapped the top tube with a key and said "hi tensile" based on the ping. It was really a check of wall thickness I would guess, but impressive.
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Old 06-08-08, 09:26 PM   #4
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Geeze I just gotta ask, if ya like the bike what the heck do you care other than bragging rights?
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Old 06-08-08, 09:34 PM   #5
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Beg to differ on seat post diameter, but during the mid '80s 26.8 was common for mountain bikes, almost to the point of standard. 27.2 was standard for road bikes.
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Old 06-08-08, 10:19 PM   #6
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you can dig through old catalogs and try to find the frame
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Old 06-08-08, 10:44 PM   #7
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Geeze I just gotta ask, if ya like the bike what the heck do you care other than bragging rights?
alf
Good point - it's the sort of question one asks on a lazy Sunday evening, with no other pressing issues.
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Old 06-09-08, 11:41 AM   #8
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Sounds similar to an 89 Bianchi Grizzly I used to have. That was made out of "Tange 900 for MTB", which is double-butted 4130, I believe.

I've also seen an older Stumpjumper and a Trek 930 made out of Tange MTB, which leads me to believe it was a fairly common material for mountain bikes of that era. I found it to be nice and flexy on trails, if a little on the heavy side.

You can find out a lot by going to the Classic+Vintage section of MTBR.com
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Old 06-09-08, 12:34 PM   #9
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No uniquely-identifying features like serial number under BB ?
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Old 06-09-08, 05:49 PM   #10
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Sonic testings good for finding if it is butted or not.You can tell the difference between normal steel and high carbon steel with a spark test.You need to touch the frame somewhere(hidden) with a grinder and watch the shape of the sparks.A knowing eye can tell you in a second what it is.If you have a good eye,you can find spark patterns online.

If you have a big scrapyard near your house,alot of them have a gun they point at the metal and it will tell you EXACTLY what it is.
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Old 06-09-08, 05:53 PM   #11
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Good point - it's the sort of question one asks on a lazy Sunday evening, with no other pressing issues.
I thought your originally stated reason was good:

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Trying to figure out if it's worth fixing up, or if I should start with a new frame. I've ridden it, and it fits me well.
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Old 06-09-08, 09:14 PM   #12
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Have you weighed it?
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