l0rd
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| posted on 13/2/10 at 02:47 PM |
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Something i have been thinking of asking for some time
Now, daft question.
I have seen quite a few broken wisbones since i joined the forum due to fatigue of the metal near the joint where it was welded.
So am i right thinking that it would be a good time to redesign this part with a longer tube?
I know that new bushes would have to be made but isn't it a logical way of thinking?
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fesycresy
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| posted on 13/2/10 at 03:13 PM |
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If I were to make a set today.....
1. correctly size the crush tube
2. take time to dress the wishbone
3. TIG weld a suitably thick (wall diameter) end tube
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The sooner you fall behind, the more time you'll have to catch up.
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mistergrumpy
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| posted on 13/2/10 at 03:17 PM |
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Is/was the general consensus that the failure was happening due to the quality of the weld looked at on an individual basis as with some sort of non
destructive testing (x ray?) the correct penetration coudn't be assured. This would mean that regardless of the length of the weld it would
still be prone to failure when the forces are acting on it.
I do remember that some where caused by incorrect dimensions and fitting of the bush and crush tube which again would mean that a longer weld
wasn't needed, just the correct fittings.
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MikeRJ
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| posted on 13/2/10 at 05:57 PM |
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Also use stainless crush tubes with plenty of silicone grease. Mild steel one will rust and could seize within the bush causing much higher loads on
the welded joint.
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iank
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| posted on 13/2/10 at 10:15 PM |
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Caterham r500 bones don't seem to break while being made of much smaller diameter tube.
What they do have is a cross brace down at the bush end (making the bone a triangle rather than a V shape.
I'm of the opinion that this is to prevent fatigue at the wheel end due to compliance in the bushes causing the bone to flex, they certainly
don't weld it on to add weight 
Some pictures and discussion here
http://www.locostbuilders.co.uk/viewthread.php?tid=120605
--
Never argue with an idiot. They drag you down to their level, then beat you with experience.
Anonymous
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MikeRJ
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| posted on 15/2/10 at 01:04 PM |
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The Caterham wishbones are a superior design because 1) the damper mounting is so much closer to the balljoint, reducing the bending moment compared
to the locost design and 2) It is much closer to a pure wishbone design minimising any bending forces in the front and rear members. The book Locost
design has a large triangular plate to mount the damper bracket on which effectively turns the triangular wishbone into a quadrilateral. This means
the front and rear members experience bending as well as tensile forces, and the change in section due to the plate also introduces a stress raising
point.
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