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Locutus
08-10-2008, 12:09 PM
When evaluating an old bike frame as a possible candidate for hacking, how can you tell the difference between steel, aluminum or cromoly? I've been told that aluminum is easy to spot because the weld beads are wider. But I think it might be tough to sort out whether a frame is cromoly or mild steel. It's an important distinction since welding cromoly requires before and after weld heat treating to prevent eventual stress related weld failures.

I suppose that in some cases you could contact the manufacturer and give them the model and serial number and they could then tell you what it is. But I suspect this is an impractical way to go about it. It would be better if one could tell by an eyeball evaluation.

As for parting out new bikes, nearly everything I see in the stores now is aluminum for the low to midrange bikes. Even the <$100 wally bikes. So they're okay for parts, but not for frame hacking. What's a bike hacker to do? :confused:

trikeman
08-10-2008, 01:12 PM
I carry a small magnet when I go bike "shopping." Many of the cheap walmart bikes have aluminum frames on the front, but still have steel rear triangles. I piked up an old bmx bike the other day with beautiful big welds. It was painted sliver in places and I could have sworn it was aluminum, until I stuck my magnet on it.

AtomicZombie
08-10-2008, 02:23 PM
The magnet trick is perfect for alum. vs steel. As for chromo, the tubing will often have a sticker at the top of the seat tube.

Stickers that read "high tensile steel" or "oversized tubing" usually = cheap depatment store frames... perfect for us!


Brad

bambuko
10-15-2009, 01:00 PM
...Stickers that read "high tensile steel" or "oversized tubing" usually = cheap depatment store frames... perfect for us!...

Just scored a donor bike - on the frame it says all the right things i.e. "Oversized High-Tensile Carbon Steel", but it also has a "CROMOLY" sticker :confused:
Am I correct in thinking that any frame bits are no good for me (MIG + mild steel) and only wheels etc are re-usable?
Can any of you master hackers, please confirm my diagnosis?
Or is there any way of checking whether it is mild steel or CrMn?
thanks, Chris
(my search for a donors continues ... )

Radical Brad
10-15-2009, 01:18 PM
What parts do you plan on using? Bottom bracket and head tube will certainly be fine to re-use.

Brad

bambuko
10-15-2009, 01:27 PM
Thanks!, that's better than I thought (I suppose droputs will also be OK?)
It least something.
I was hoping to use seat and chain stays (by welding it to new mild steel frame, but it sounds like I am better off not)
Chris
(better luck next time)

fultondp
10-15-2009, 01:42 PM
Don't be too scared about welding cromoly, or mixing mild steel and cromoly, it's not as sensitive as you've been lead to believe, at least for thinwall tubing. I've done it many times with no issues. I've never had any cromoly material over 14 guage thick, so I don't know how thicker material behaves. One thing I definitely don't do is fast cooling or quenching. That will crack your weld for sure. Just weld it up and leave it to cool in it's own sweet time.

If you are still concerned about it, there is always brazing!

Darren

bambuko
10-15-2009, 02:04 PM
thanks Darren,
at least I have Cromoly frame to experiment with and see whether it is worth the risk :)
Chris

Odd Man Out
10-15-2009, 04:39 PM
welding cromoly requires before and after weld heat treating to prevent eventual stress related weld failures. :confused:

True statement for anything over 1/4 inch thick. Not true and not needed for bike building thicknesses. What FultonDP said -- make sure the weld cools naturally without any breezes hiting it and you will be fine. And only TIG weld. there is little to no risk if you are careful.

Locutus
10-15-2009, 04:56 PM
True statement for anything over 1/4 inch thick. Not true and not needed for bike building thicknesses. What FultonDP said -- make sure the weld cools naturally without any breezes hiting it and you will be fine. And only TIG weld. there is little to no risk if you are careful.

When I build a planned all-chromoly trike frame, I'll be using my trusty arc welder and 7014 1/16" electrodes. I guess I'll just wait and see how that pans out.

trikeman
10-15-2009, 06:15 PM
Make sure you stay in a concrete bunker for the rest of your life, because there are thousands of homebuilt and old antique aircraft flying overhead which were built from Chromoly and welded with oxy-acetylene torches, which is what chormoly was originally designed to be welded with (before TIG even existed).

trikeman
10-15-2009, 06:37 PM
I do agree that for metals as thin as a bicycle frame, you do not need to heat treat chromoly unless you do something dumb like quenching it in water to cool it off. Chromoly doesn't like thermal shock, which is why it likes oxy-acetylene with its relatively slow heating and cooling of the metal compared to most electrical welding methods. Thin pieces seem to handle the shock better than thick pieces do. When in doubt, some builders heat the metal with a torch and let it slowly cool (just as it would if they welded it with the torch in the first place).

Odd Man Out
10-16-2009, 12:29 AM
Make sure you stay in a concrete bunker for the rest of your life, because there are thousands of homebuilt and old antique aircraft flying overhead which were built from Chromoly and welded with oxy-acetylene torches, which is what chormoly was originally designed to be welded with (before TIG even existed).

trite sacasm tman -- you are better than that.

trikeman
10-16-2009, 02:44 AM
Someone has to defend the world from the Tiggers who keep trying to spread the myth that Chromo must be tigged.

Odd Man Out
10-16-2009, 08:43 AM
Someone has to defend the world from the Tiggers who keep trying to spread the myth that Chromo must be tigged.

Not "must" -- should.
We need to emerge from caves sometimes...:jester::):jester::):jester:

sandyl
09-13-2012, 10:51 PM
Can anyone suggest a supplier for square tube chromoly.