Simon
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| posted on 22/2/09 at 07:21 PM |
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Stress analysis q
Chaps,
Going to make a bike rack that fits into the towbar (instead of towball). It's a Witter detatchable towball thingy
Main support will be made from (approx) 3mm wall 1.5" shs and will project approx .8m from towbar bracket and have side projections to pop bikes
tyre into (yes, it'll be subject to my interpretation of SVA rules re projections etc)
Now, my q is this. I have no doubt that main support will be more than capable of handling the loads placed upon it, but if I were to insert another
(round) tube, similar wall thickness, inside the square one and weld through every 6" or so (incl the ends), roughly how much stronger should it
be.
My reasons for asking are that with sq tube, I'm basically covering the vertical and horizontal loads, but not the twisty (tech term )
torsional ones, hence the insertion of round tube (see propshafts for proof of concept!)
Cheers very much
ATB
Simon
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britishtrident
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| posted on 22/2/09 at 07:40 PM |
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Using torsion theory --- Round tube isn't stiffer in torsion than square tube of the same dimension.
However using square section in torsion can cause problems where it is welded to the main structure.
3mm thick 38x38 should be more than strong enough in bending and torsion the key to the problem the detail design and quality of the welds.
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mark chandler
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| posted on 22/2/09 at 09:54 PM |
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I thought round tube was stronger in twist, second square rule of area and all that, its certainly spreads the load evenly.
That aside bike racks area really light and made from thin stuff, what you are suggesting would be much more durable than anything you would buy off
the shelf. I would make it, jump on it and if it does not bend call it good myself.
Cheers Mark
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Simon
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| posted on 22/2/09 at 10:08 PM |
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quote:
Originally posted by mark chandler
I would make it, jump on it and if it does not bend call it good myself.
Cheers Mark
Mark,
That's my idea of stress analysis
ATB
Simon
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BenB
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| posted on 22/2/09 at 10:55 PM |
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Yup. Pragmatic structural analysis. When I modified my engine cradle recently I lay under the car and did a chin up (okay, more of an inverted press
up as I don't have a lift). It took that okay, which was good enough for me.... Did some burn-outs at the weekend and nothing fractured so
suggests the chin-up test is now a proven engineering test procedure . Sadly a water feature eminating from my f$%#*ng leaky Samco hoses ended my
fun and games early. When I took the bonnet off (after the blat  )and reved the engine there was quite an impressive water fountain in evidence. I
really can't believe I had much coolant left at the end of an hours blat.... perhaps I invented the worlds first air cooled ST1100 engine... It
was certainly getting sufficient cooling during my blast up the M1... In fact, rather too much I suspect when I blatted past the hidden police car
I'm hoping the spray of steam and water coming out of the offside will have obscured the photo of my no plate!!! Otherwise I may be off the road
for longer than it takes just to do a quick HG swap....
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