Mix
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 posted on 29/4/04 at 10:51 AM |
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Wishbone tubing
Following on from the wishbone bending thread.
It would appear that RHS is stronger size for size than round tube, if this is the case why are wishbones generally made out of round or oval tube?
I can see there may be an issue with front wishbones not meeting SVA radii requirements but manufacturers of IRS kits seem to use similar materials
for the rears. Is this just for convenience?
I'm considering making my rear bones out of RHS as it seems to me construction would be easier. Do you think this is a viable option?
Cheers Mick
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stephen_gusterson
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| posted on 29/4/04 at 10:59 AM |
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My front wishbones are made of RHS - they will be fully covered by bodywork.
I was told once that nascars use rhs.
Mine is 25mm with 2mm wall.
atb
steve
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JoelP
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| posted on 29/4/04 at 12:11 PM |
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i've made mine (2nd time round) from 19mm 16gauge square tube. i also expect them to bend sometime...
still, the shocker mounting is almost inline with the balljoint, so there is less bending load than on book bones.
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undecided
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| posted on 29/4/04 at 12:59 PM |
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Round tube is stronger than square..try bending square, it bends easy round doesn't.
As for making wishbones in square tubing they would look awful
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MikeRJ
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| posted on 29/4/04 at 01:39 PM |
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quote:
Originally posted by undecided
Round tube is stronger than square..try bending square, it bends easy round doesn't.
As for making wishbones in square tubing they would look awful
I think that is a generalisation. Round section has a unform stiffness in all directions, square obviously does not. However, I'm fairly sure
a square section tube of the same wall thickness with a section equivalent to a round tubes diameter would be significantly stiffer in bending as long
as the bending force is applied to a flat side rather than a corner.
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flak monkey
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| posted on 29/4/04 at 02:22 PM |
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I asked about this after another discussion about whether round or square is better for spaceframes....
Square is stronger than round in bending. As it has a higher second moment of area (I). In the formula for working out deflection I appears in the
denominator, therefore a larger value of I means lower deflection.
BUT when square fails it will do so more dramatically.
Square is also stronger in compression (has a higher cross sectional area) and in torsion. But again when it fails it is more dramatic and sudden. In
torsion square gives less angular deflection than round for a given applied torque. But where as round will continue taking more power resonably
smoothly before it fails, square will not. Due to the high stresses in the corners of the section the square will suddenly fail....
This is all size for size, if you went weight for weight then it may be different (i cant be bothered to work that out as well!)
Cheers
David 
Sera
http://www.motosera.com
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stephen_gusterson
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| posted on 30/4/04 at 10:09 AM |
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nope they dont. If they are covered in bodywork, they dont look anything.
also- round tube spreads into an oval and crushes at a stress point - thats not much different to a square tube sides bending.
quote:
Originally posted by undecided
Round tube is stronger than square..try bending square, it bends easy round doesn't.
As for making wishbones in square tubing they would look awful
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Alfalfameister
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| posted on 30/4/04 at 01:01 PM |
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With all the "triangulation" going on, I wonder why no one made a triangle based tubing...
Just a thought
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flak monkey
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| posted on 30/4/04 at 01:13 PM |
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Think about the cross section (and therefore second moment of area) of a 25mm base triangle. The area will be half (i think  ) of a piece of 25mm
square tube. Making it weker in compression than even round tube. And therfore also weaker in bending than either round or square.
Structurally triangluar tube is weak. But triangulation in structures is a different thing all together....
Cheers
David
Sera
http://www.motosera.com
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kestrel1596
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| posted on 1/5/04 at 06:02 AM |
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I also went out and asked a similar question. The simple answer is: for the same diameter and wall thickness, square is stronger, but heavier.
One interesting thing that came up is this: I thought that it was the "sides" that gave the most resistance to bending in a square
section.
It seemed simple: take a flat piece of metal and bend it up or down through its narrow section and it bends easily, put it on edge and it is much
stiffer. Obvious right? Wrong.
In RHS it is the top and bottom that give you most of your resistance to bending when bending up or down. Why? Basically (if I have this right) the
top and bottom are not just being bent: they are in tension and compression respectively because of the shape of the metal.
That is why I-beams are used for load bearing, you can save the weight of one entire side member by "combining" the two sides into one
central web and still keep most of the resistance to bending ( I've forgotten the name of the characteristic: moment of something... ).
If I can find the thread that had all the best replies I got I'll post it Monday. Now if I can only remember where I put that...
Kevin B.
[Edited on 1-5-2004 by kestrel1596]
[Edited on 1-5-2004 by kestrel1596]
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britishtrident
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| posted on 1/5/04 at 06:38 AM |
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Square section is stiffer & stronger in bending but in torsion it is actually weaker because the corners stress raising effects.
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flak monkey
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| posted on 1/5/04 at 11:07 AM |
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There is no doubt more than one way to work it out, but the formulae i have (the general beam equation and derivations of it) all use second moment of
area (I), but then again second moment of mass (moment of inertia) is also given the symbol I in many equations...hence leading to much confusion.
Britishtrident is right in saying square is overall weaker in torsion. It fails at a lower stress than round (due to the high corner stresses). But
before it fails it deforms less in torsion than round for the same applied torque. Its stiffer in torsion, but not stronger.
Cheers
David
Sera
http://www.motosera.com
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