Square or round tube?

[ed]

bill, you should clarify actually - was the original failure 'bent' or 'compressed'

and my words in the 3rd post

*taps table, waits for bill*

[clubagreenie]

Note Williams lak of contribution sonce first post. He's captain sensible and gone to bed. Just 'cause you're away doesn't mean you don't have a bed time

[thechuckster]

pics are in this thread on PF - registration is probably required - http://www.performanceforums.com/for...7&postcount=36

[Billzilla]

Morning guys.
Looking at the other two threads I posted on two other forums on this, it looks like round tube a size larger will do the job.
(And maybe slipping the clutch a little off the line next time!)

Thanks!

[kingmick]

We have to replace the trailing arms on the rear suspension on the racing car, as one of them bent very badly under compression last weekend,
My question is that for a set size (say, 30mm) what would be stronger in terms of resisting being bent, a 30mm diameter round tube or a 30mm to a side square tube?
And how much more would the square tube weigh?
Same wall thickness for both tubes.

Go round get the weight as low as possible.

[merc-blue]

chomoly with a welded swage (cant think of the exact term right now)

we did some control arms for a project supercharged 308 Boss Cobra and thats how we went, put them in the press and got some pretty good pressure before they deformed (as oppsed to flexed)

[ed]

failure in compression

wouldve been nice to have those pics in the first place bill !

larger round is the choice

[roadsailing]

there is more to it thtan just using bigger round tube, there are a few issues that need ot be addressed, seeing the parts im sure bill is aware of this, but here are my thoughts.

have a look at the links of the pictures already posted, mine have red all over them.



what rear end does this car have? if it is live axle i can imagine it moving to one side, allowing this link to touch the body, if not i dunno.



a strap on the top balljoint mount will really help things out.

[Billzilla]

No the trailing arms can't touch the sides, the Watts Link stop any lateral movement. It's got a somewhat modified Datsun 1000 Ute diff.
And yes, in the rebuild it'll get double shear in all those joints.

[Lambolica]

In terms of strength Square over Circular. Circluar will always be stronger for any given Cross-sectional area when the loads are applied in tension or compression.

To use Ed's Keyboard sketching...

-->============================<-- Compression

<--============================--> Tension

For a visualisation of this... Get a piece of paper roll it and tape it together to make a tube. Place it on it's end on a flat table.

Do the same thing with another piece of paper but fold it to make a Square Hollow Section. In both cases make sure the tape is running the entire length of the join.
With both sitting on their ends on a flat table, lightly place a book on the top of both (make sure the book is of appoximate same weight for each member) keep loading the members with addition books until failure.

Note as you place load on them that the Square section will start to flex in the flat side walls (paper will exaggerate the buckling) this will with additional load cause a fault in the side wall and the member will Buckle under torsional failure and collapse. the circular member will fail in a similar mannor but with more load.

Ed is correct in saying that for a similar Over all sized Member (like a 30mm Circular hollow Section and a 30 x 30 Square hollow section that have the same wall thickness) that there will be between 25-29% more material (depending on alot of factors) However this will not equal 25-29% more strength between the 2 shapes. that being said the Square would still be stronger.

Cheers
Simon

[Jonny Rochester]

If the tube bent under compression, increasing the diameter will be more effective than increasing the wall thickness.

[The Last Streetfighter]

"Bending (buckling) under compression" is to do with Slenderness Ratio. So a larger diameter should help that a bit.

[Lambolica]

^^^ Yes... Most definatly correct, increasing the wall thickness with have results as well but larger diameter will give a net better result.

Cheers
Simon