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Strength of bottom bracket housing with nothing inside?

Old 10-12-20, 10:03 AM
  #1  
Badzilla
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Strength of bottom bracket housing with nothing inside?

I realize it might be difficult to figure and it varies from bike to bike, but generally speaking...

What would you guess is the strength of a bottom bracket housing tube? I mean, just the round steel tube with nothing inside of it. The tube that the three (or four) parts are welded to. The part that a bottom bracket screws into. Less generally... I'm talking about a larger tube that requires an adapter to screw a modular bottom bracket into. I suspect the larger diameter bottom bracket housing would be less strong. But I'm only asking for guesses about anything.

If you use the bottom bracket area with nothing inside of it, does that weaken it to the point of breaking? I'm guessing the innards provide some strength, but maybe not so much that it makes a big difference.

I'm asking for guesses since the subject might never have even come up before.

Thanks.
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Old 10-12-20, 10:23 AM
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70sSanO
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Although I doubt there is any chance that the bb housing itself will fail, I would guess larger diameter tubes in general are stronger.

John

Last edited by 70sSanO; 10-12-20 at 10:29 AM.
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Old 10-12-20, 10:45 AM
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Strong enough to tolerate what?
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Old 10-12-20, 10:55 AM
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They're usually very strong although they're available in different sizes. They're all 34.8mm on the inside (English threads) and some are as much as 40mm on the outside. That's 2.6mm wall thickness. And they're usually cromoly as well (rather unnecessarily given how thick they are). The other tubes on the bike are usually about 0.5mm increasing to 0.8mm at the ends to give you an idea how thick that is.
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Old 10-12-20, 11:02 AM
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How could you test this? Without the spindle, bearings, cups and crankarms installed you can't ride the bike except to push it along with your feet on the ground. I expect that wouldn't put much strain on the bottom bracket
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Old 10-12-20, 11:07 AM
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Originally Posted by 70sSanO
Although I doubt there is any chance that the bb housing itself will fail, I would guess larger diameter tubes in general are stronger.

John
Assuming the same wall thickness, the larger tube will be weaker from forces hitting the tube perpendicularly. It will be stronger in other ways (twist, mainly), but nonetheless.

That said, I'd buy a cheap bottom bracket and install that if I was worried about transporting a crank-/BB-less frame.
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Old 10-12-20, 11:34 AM
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Originally Posted by Badzilla
If you use the bottom bracket area with nothing inside of it, does that weaken it to the point of breaking?
I think it would depend on how you use it. Uses for a bike frame w/o a crank seem pretty limited. Wall art, yard art, mail box support.... Give us a clue
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Old 10-12-20, 11:42 AM
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Strong enough.
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Old 10-12-20, 12:27 PM
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It's not too hard to find the mechanical properties of 4130 steel. ASM Material Data Sheet

The real question is what kind of strength are you looking for? resistance to crushing? shear strength?

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Old 10-12-20, 12:35 PM
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It is strong enough for its intended use and that includes holding the BB assembly. Its equivalent to asking the question of "how strong is an innertube."
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Old 10-12-20, 12:40 PM
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This sounds like someone wanting to use a bike frame for something with a motor in it.

Which would probably not be a great idea.
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Old 10-12-20, 04:51 PM
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Not sure if you are trolling or simply clueless but a BB with nothing inside of it should last forever, never wear out and never break, because after all it won't be used for anything.
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Old 10-12-20, 05:10 PM
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Originally Posted by dsaul

It's not too hard to find the mechanical properties of 4130 steel. ASM Material Data Sheet

The real question is what kind of strength are you looking for? resistance to crushing? shear strength?

Not very many frames are made from 4130, most are generally higher quality. I think the last one I had was a bmx frame in the late 70's. Cheap dept store bikes might be using it.
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Old 10-12-20, 05:26 PM
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Originally Posted by cxwrench
Not very many frames are made from 4130, most are generally higher quality. I think the last one I had was a bmx frame in the late 70's. Cheap dept store bikes might be using it.
You're thinking of 1030 steel or the equivalent. 4130 is chromium-molybdenum, e.g., Columbus SL tubing.
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Old 10-12-20, 05:26 PM
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Originally Posted by cxwrench
Not very many frames are made from 4130, most are generally higher quality. I think the last one I had was a bmx frame in the late 70's. Cheap dept store bikes might be using it.
Huh? Do you mean mild steel? Most tubes are 4130, with different butting and heat treatments.
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Old 10-12-20, 05:30 PM
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Originally Posted by cxwrench
Not very many frames are made from 4130, most are generally higher quality. I think the last one I had was a bmx frame in the late 70's. Cheap dept store bikes might be using it.
I build steel bike frames. I can assure you that almost all of the bottom brackets in quality welded steel bike frames are made from 4130 tube. Dept. store bikes are made from mild steel or Hi-tensile steel.
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Old 10-12-20, 06:26 PM
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Originally Posted by Trakhak
You're thinking of 1030 steel or the equivalent. 4130 is chromium-molybdenum, e.g., Columbus SL tubing.
Originally Posted by aggiegrads
Huh? Do you mean mild steel? Most tubes are 4130, with different butting and heat treatments.
Originally Posted by dsaul
I build steel bike frames. I can assure you that almost all of the bottom brackets in quality welded steel bike frames are made from 4130 tube. Dept. store bikes are made from mild steel or Hi-tensile steel.
Yep, you're all correct and I made the error...thanks for the clarification!
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Old 10-13-20, 06:21 AM
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I don't think any kind of spindle and bearings will add strength to a shell.
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Old 10-13-20, 07:25 AM
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Originally Posted by andrewclaus
I don't think any kind of spindle and bearings will add strength to a shell.
But cups would.

That said, I think we need to know the OP's ultimate plans in order to assess the matter properly.
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