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Half-lugs?

Old 03-27-22, 03:02 PM
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cormacf
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Half-lugs?

Question for those of you who know things (I am not among them): I've seen a few bikes (like the example below from VO, but I think I've also seen some similar frames from Crust in the past) with a kind of sem-lug added to a TIG weld near the headtube.

Is this an aesthetic decision, or is there a performance / durability reason? TIG welds seem pretty capable on their own without the extra piece of metal, but maybe it's just there to make it even MORE durable?

Thanks!

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Old 03-27-22, 03:47 PM
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They're gussets. Supposed to strengthen those joints which sometimes fail although I'm doubtful about the justification for them: if properly welded there's no reason why those joints should fail.
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Old 03-27-22, 06:12 PM
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And if you are to mimic these gussets don't weld down the end that is furthest from the head tube. Others with more engineering skills can say why better that I can. Andy
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Old 03-28-22, 01:26 AM
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Originally Posted by Andrew R Stewart
And if you are to mimic these gussets don't weld down the end that is furthest from the head tube. Others with more engineering skills can say why better that I can. Andy
Interesting. Is that because that would basically pretension the gusset along its length (welds always shrink a bit as they cool) thus adding to the fatigue problem at the joint that adding them is supposed to solve in the first place?

The reason I don't like them is that a good weld is stronger than the tube. It will fail where the butt ends, and these just transfer stress to that point as well as putting more heat into the tubes.

For a really gnarly off-road bike a better solution is to use a Columbus triple butted tube (or a Reynolds "dual zone butted" one which is the same thing). These get even thicker walled right by the joint. But the transition is perfectly smooth, cold-worked, etc.

But realistically the friends/myself that I make bikes for aren't sending the huge steazy bangers and hucks to flat etc that you really need that extra strength for!
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Old 03-28-22, 07:32 AM
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They are there for bikes that will see big loads from jumping, but have gotten more common because the European testing requirements will often fail a properly done TIG weld. The European standards apparently assume you are going to jump your touring bike. There was a guy that runs a small volume bike company that used to post his problems with joints failing the EU tests in here until he gave up on us. My thought is the VO bike in the OP only has them to pass testing.

I think successful gussets will allow a certain amount of compliance or they'll just tear along the outside. I am not sure the practice of not welding them down in the center of the tube really does much, but I'm also not looking at a stress distribution.
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Old 03-28-22, 07:43 AM
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I have read of tube cracks starting at the welded shut end of a gusset. I believe it has to do with the change of stiffness, a welded on plate will be a very stiff section compared to the tube. So the stresses will focus at the boarder/weld. By leaving the end of that gusset open it allows some flexibility and spreads the stress focus to a wider area.

As to Eric's comments about passing the EU testing- When I first learned of the actual testing procedure I felt that it was way overkill for the vast majority of riders and how they use their bikes. But I do understand strength standards are often more about politics within the legal and manufacturing communities than in the field product performance. Andy
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Old 03-28-22, 08:03 AM
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The stresses will be concentrated along the sides too. But if you don't weld the ends it will be a little more compliant, because the gusset can bend along its width (oil canning) at its own rate. So it probably does avoid a little bit of cracking. One of the more vocal anti-gusset builders* was last seen looking for information on how to add gussets because his personal frame broke pretty easily. But he was testing it on 8 foot drops to flat. So if you do jumps like that, a gusset probably will help. Failure in rupture is worse than fatigue failures. Don't know if he ever added gussets, I get turned off by his "only I know how to do things right" attitude.

*he always said that gussets just show the frame where to crack, which is probably true.
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Old 03-28-22, 09:20 AM
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Originally Posted by unterhausen
They are there for bikes that will see big loads from jumping, but have gotten more common because the European testing requirements will often fail a properly done TIG weld. The European standards apparently assume you are going to jump your touring bike. There was a guy that runs a small volume bike company that used to post his problems with joints failing the EU tests in here until he gave up on us. My thought is the VO bike in the OP only has them to pass testing.
Does anyone know how high-volume you have to be in order to be subject to the EU requirements? Presumably they don't affect custom builders? Or maybe they do. They're probably the reason most off-the-shelf steel frames are horrendously overbuilt.
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Old 03-28-22, 09:35 AM
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I don't know, CPSC in the U.S. doesn't require small builders to be compliant. Not that it affects me.
I think the Asian factories mostly want to be compliant with the EU standards no matter where the bike is ending up. Which doesn't affect those of us in this forum.
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Old 03-28-22, 11:08 AM
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Originally Posted by unterhausen
The stresses will be concentrated along the sides too. But if you don't weld the ends it will be a little more compliant, because the gusset can bend along its width (oil canning) at its own rate. So it probably does avoid a little bit of cracking. One of the more vocal anti-gusset builders* was last seen looking for information on how to add gussets because his personal frame broke pretty easily. But he was testing it on 8 foot drops to flat. So if you do jumps like that, a gusset probably will help. Failure in rupture is worse than fatigue failures. Don't know if he ever added gussets, I get turned off by his "only I know how to do things right" attitude.

*he always said that gussets just show the frame where to crack, which is probably true.
This has a PVD vibe to it. Those super slack head tube angles do improve the attack angle for the front wheel in forward motion, but it greatly increases the leverage on the HT/DT joint when landing flat. I don't think this type of gusset is going to help him.
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