l0rd
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| posted on 24/9/10 at 05:38 PM |
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To bend or to weld? That is the question
Since i haven't seen anyone else asking this and although i am not building anything at the moment here goes The silliest question of the
century.
Without considering cost implications
What is better if you use round tube for chassis?
straight tubes welded or a long tube mandrel bended?
was thinking if getting a 7m long tube mandrel bended in the same points as the square tube.
[Edited on 24/9/10 by l0rd]
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big_wasa
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| posted on 24/9/10 at 05:43 PM |
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Worth checking but a lot of the thin wall stuff is made in 6m lengths 
I wouldnt even atempt to bend it that long and get any accuarcy.
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mark chandler
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| posted on 24/9/10 at 05:48 PM |
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It will be weaker, not a good idea
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l0rd
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| posted on 24/9/10 at 07:02 PM |
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quote:
Originally posted by big_wasa
Worth checking but a lot of the thin wall stuff is made in 6m lengths
I wouldnt even atempt to bend it that long and get any accuarcy.
I believe that if it is done properly, it can be accurate enough.
If looking at the haynes roadster chassis, the bottom rails are approximately 2.4m
so 2X2.4=4.8m and then you have about 1.2m to do the bend and the front nosecone section which is approximately .4m which gives you a grand total of
5.2m
quote:
Originally posted by mark chandler
It will be weaker, not a good idea
Why will this be?
Welded areas are supposed to suffer from heat when they are welded.
[Edited on 24/9/10 by l0rd]
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scudderfish
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| posted on 24/9/10 at 07:10 PM |
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Didn't Robin Hood used to do this? I seem to remember they advertised not having any joints at any of the corners.
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mark chandler
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| posted on 24/9/10 at 07:14 PM |
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Putting bends in at the outset cause a weak point as you have already bent the structure, okay it may be at the end not the middle of a length but it
will still collapse here as the load is not pointing at the joint.
[Edited on 24/9/10 by mark chandler]
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l0rd
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| posted on 24/9/10 at 07:23 PM |
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quote:
Originally posted by mark chandler
Putting bends in at the outset cause a weak point as you have already bent the structure, okay it may be at the end not the middle of a length but it
will still collapse here as the load is not pointing at the joint.
[Edited on 24/9/10 by mark chandler]
Thank you for that.
I knew there must be a justified reason apart from the cost.
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hillbillyracer
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| posted on 24/9/10 at 07:23 PM |
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I see no reason for it to be weaker, it will be weaker than before it was bent as it's suffered compression & tension deformation but a
welded joint is even more compromised.
The real problem will be getting the bends in the right place & angle relative to each other, it'd take a bit of experimentaion &
practice before you start for real.
A thing to consider first though is are you solving a problem by making it that way? How much of a problem do the welded joints cause, are you solving
a problem that only exists in theory?
There'd still be a lot of welded joints anyway, you'd only be able to do the top & bottom rails really, the rest is pretty much
straight lengths
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l0rd
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| posted on 24/9/10 at 07:56 PM |
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quote:
Originally posted by hillbillyracer
I see no reason for it to be weaker, it will be weaker than before it was bent as it's suffered compression & tension deformation but a
welded joint is even more compromised.
The real problem will be getting the bends in the right place & angle relative to each other, it'd take a bit of experimentaion &
practice before you start for real.
A thing to consider first though is are you solving a problem by making it that way? How much of a problem do the welded joints cause, are you solving
a problem that only exists in theory?
There'd still be a lot of welded joints anyway, you'd only be able to do the top & bottom rails really, the rest is pretty much
straight lengths
I was looking at a video where the person doing it managed to get the correct bends so easily that it got me wondering.
Yes, you can only do the top and bottom rails but as it would only be one piece of metal, it would be stronger than welding loads of smaller ones.
Don't know, for some reason, i always hated welds and thought that they would just crack in case of an accident.
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hillbillyracer
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| posted on 24/9/10 at 08:07 PM |
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I was dead set on doing a round tube chassis because it'd be better & worth the hassle of fish-mouthing ends of tubes etc. The I went to a
hill-climb & folk were driving their sevens much harder & with more power than I probably ever would & all of them had square tube chassis
so I decided that it'd do for me too & save much time & hassle
Then I went & bought an unfinished project as it'd get me on the road so much quicker & I cant find the time to do much with it so I
doubt I'd ever have got far with the round tube!
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iank
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| posted on 25/9/10 at 10:32 AM |
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quote:
Originally posted by mark chandler
Putting bends in at the outset cause a weak point as you have already bent the structure, okay it may be at the end not the middle of a length but it
will still collapse here as the load is not pointing at the joint.
[Edited on 24/9/10 by mark chandler]
But is that amy weaker than cutting and welding two straight lengths at the same point? I don't think it will be any weaker overall when
everything is taken into account.
--
Never argue with an idiot. They drag you down to their level, then beat you with experience.
Anonymous
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Bluemoon
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| posted on 25/9/10 at 11:47 AM |
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Not quite sure what you mean exaclty, but a space frame chassis is made from straight tubes and "point" intersection, tubes are only in
compression or tension this is why the space frame from is strong (with triagnulation).
An uncooked piece of spaghetti is very strong under compression/tension but as soon you but a bending load on it will snap.
A bend in a a tube between the fixed end point will weaken the tube, it will first fail under compression/tension at the bend..
Dan
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iank
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| posted on 25/9/10 at 03:46 PM |
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All true enough, but taking the following isolated part of a spaceframe which is stronger, the one with the bend, or the one with the cut and weld?
We aren't comparing a bent tube to a straight tube, but to two straight tubes at an angle with a weld.
 
Description
I think in isolation both are probably very close in strength (once taking into account weld quality, cutting accuracy etc.)
The problem with the bent one is it's much harder to triangulate into the bend to make a spaceframe, compared with the cut and joined version,
but there are plenty of places in the non-ideal spaceframes we use where such things could be used (for example the BR1, BR3, BR5 bottom rail on the
haynes roadster, which could be fabricated as a single tube with 2 bends if you had the equipment to bend it accurately enough)
The problem with the bent tube robin hood 2b chassis was they didn't bother putting in much triangulation.
--
Never argue with an idiot. They drag you down to their level, then beat you with experience.
Anonymous
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l0rd
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| posted on 25/9/10 at 04:31 PM |
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quote:
Originally posted by iank
(for example the BR1, BR3, BR5 bottom rail on the haynes roadster, which could be fabricated as a single tube with 2 bends if you had the equipment
to bend it accurately enough)
The question is, would you be able to do it for both sides as a single tube.
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TQ_uk
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| posted on 28/9/10 at 03:36 PM |
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Off the top of my head, without having the Haynes book in front of me for reference, I believe what you proposing has crossmembers welded at these
points?
I.e. there is a point of triangulation attached on the inside of the bend point, so its not as if the bend is 'unsupported'
Am I following your thinking? In which case I would imagine it pretty much equal in strength, but nicer looking
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Rod Ends
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| posted on 28/9/10 at 03:50 PM |
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The Blessed Colin (we are not worthy) called bent tubes 'pre-failed'   
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l0rd
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| posted on 28/9/10 at 05:04 PM |
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quote:
Originally posted by Rod Ends
The Blessed Colin (we are not worthy) called bent tubes 'pre-failed'
and the welded ones?
Postfailed after a hacksaw?
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coyoteboy
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| posted on 30/9/10 at 09:46 AM |
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Bent metal will have "necked" during the plastic deformation (Thank's Poisson) and therefore will be a thinner wall thickness on the
outer edge of the bend, possibly stayed the same on the inner (depending on bend method) and therefore have, overall, lost strength depending on load
direction. During a cut and weld, assuming the weld is done well, the filler material and the weld will actually be stronger than the original
material (thicker, possibly higher strength filler material used-depends on the weld), but you need to consider heat effects in the surrounding tube.
If this is kept in check, the weld would be stronger IMO. So in essence, I agree with the bends=pre-failed concept.
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smart51
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| posted on 30/9/10 at 10:59 AM |
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MNR do all their chassis in round tube now that they have a tube bender. They bend long lengths to shape. The tube is not weakened if the
triangulation supports it at the bend. It is then a series of straight tubes but with bent rather than cut and welded nodes.
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coyoteboy
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| posted on 30/9/10 at 11:11 AM |
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quote:
The tube is not weakened if the triangulation supports it at the bend. It is then a series of straight tubes but with bent rather than cut and
welded nodes.
Depends on the radius of your bends. You still have a thinned wall on that bend which is ultimately weaker than a good weld. It's just a case of
how much difference and in what load considitions you're talking.
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MikeR
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| posted on 30/9/10 at 12:11 PM |
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Suspect part of mnr's reason for bending is speed of fabrication.
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TQ_uk
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| posted on 30/9/10 at 12:16 PM |
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Suppose this is more relevant for thin walled tube, as roll cage for example have to be bent (for certain regulations), at key corners, & not
welded....
But then they also have to be a minimum thickness
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coyoteboy
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| posted on 30/9/10 at 12:56 PM |
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That's due to the unquantifiable quality of welds I think. They want something certified and of a fixed, known strength, rather than it being
the best way of doing it. If you can fix the material and construction you can know the strength. If you just fix the material and get a moron to weld
it you could have a useless crumpled mess in an accident. It doesn't mean the method is stronger.
[Edited on 30/9/10 by coyoteboy]
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TQ_uk
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| posted on 30/9/10 at 01:54 PM |
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Agreed...
I suppose the train of thought is almost, bend it if "one's"* not confident of "one's" welding proficiency -
though that raises the question whether you should be let loose on making your own chassis!
(* not directed at any one!)
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coyoteboy
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| posted on 30/9/10 at 02:15 PM |
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I probably shouldn't
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