gaz_gaz
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| posted on 5/8/15 at 07:30 PM |
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opinions on this failure
rear lower wishbone failure on a friends car.
Any idea of cause?
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wylliezx9r
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| posted on 5/8/15 at 07:40 PM |
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Seized ball joint ?
I spent a lot of money on booze, birds and fast cars. The rest I just squandered.
George Best
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Smoking Frog
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| posted on 5/8/15 at 07:48 PM |
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Partial seizure or binding in joint causing friction. Can't think what else it could be.
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Bluemoon
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| posted on 5/8/15 at 07:48 PM |
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odd you need to get some bending moment in there. binding bearing? look's like a crack was there for a while then propagated (top right tube
section not bright).
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gaz_gaz
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| posted on 5/8/15 at 07:53 PM |
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car was stripped and checked a couple of weeks ago.
rod ends where all in good shape and a crack wasn't visable on the wishbone. a crack was detected on the opposite side wishbone but in a
different location.
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MikeRJ
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| posted on 5/8/15 at 07:55 PM |
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Fatigue failure due to the brittle heat affected zone next to the weld on the reinforcing tube that runs to the other side of the wishbone.
Assuming the rod end sits at about 90 degrees to the fixing bolt on the chassis, that point will naturally have bending forces applied to it under
acceleration and braking
[Edited on 5/8/15 by MikeRJ]
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Bluemoon
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| posted on 5/8/15 at 08:03 PM |
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quote:
Originally posted by MikeRJ
Fatigue failure due to the brittle heat affected zone next to the weld on the reinforcing tube that runs to the other side of the wishbone.
Assuming the rod end sits at about 90 degrees to the fixing bolt on the chassis, that point will naturally have bending forces applied to it under
acceleration and braking
[Edited on 5/8/15 by MikeRJ]
missed the cross tube sounds spot on.. wounder why the tube is there at all?
hard to tell but it looks diagonal i.e to take braking force, mk indy is simular but not seen that part fail (yet!).
[Edited on 5/8/15 by Bluemoon]
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Slimy38
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| posted on 5/8/15 at 08:03 PM |
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That threaded insert looks to be the wrong size, could that have applied some bending force internally? Combine it with a HAZ from the crossbar and
potentially a focussed load point?
I've only got threaded inserts on the front suspension, but they're a tight fit in the tube and fully welded (rather than just end
welded).
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gaz_gaz
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| posted on 5/8/15 at 08:06 PM |
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mark chandler
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| posted on 5/8/15 at 08:16 PM |
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It's a common design fault exasperated with that brace.
The threaded part of the Rose joint should be pointing straight towards the hub putting all the load directly into the ball joint instead of trying to
push it out of its socket under acceleration and braking.
Better still turn the bracket by 90 degrees on the chassis instead of using the rubber bush orientation.
Regards Mark
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Talon Motorsport
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| posted on 5/8/15 at 08:44 PM |
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I would go with the less complicated answer of the tube wall just looks too thin to be used in a lower wishbone. There are people that say the
wishbones should be expendable in a crash to save the chassis hence using 16-18g/1.6-1.2mm wall tube but I stand by the outer tubes of the wishbone
should be 14g/2mm minimum.
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liam.mccaffrey
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| posted on 5/8/15 at 09:23 PM |
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That threaded insert doesn't fill that tube it should be a much tighter fit. Its not parallel with the wishbone tube and will impose a bending
moment/couple on the outer tube every time the force cycles.
Rose joints should feed directly into the chassis, again every time the wheel is braked or accelerates it will try to effectively snap the threaded
insert off the wishbone, exactly where it failed.
The cross link tube weld has acted as a stress concentration and initiated a fatigue fracture which has propagated around the tube.
Also as Talon said tube might be a bit thin.
Just my opinion 
Build Blog
Build Photo Album
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Neilwilson
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| posted on 5/8/15 at 09:34 PM |
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Not much of an opinion but this link http://met-tech.com/preheater-tube-failure.html is an interesting read. The website also has some other examples
of failure types and analysis of all kinds of components. Just some literature to trigger thought processes.
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theprisioner
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| posted on 5/8/15 at 09:58 PM |
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I have seen this before when a 7style car wishbones, it had slick tyres pulling a lot of G and a lot of power. I saw it at Forrestburn Hill Climb. In
that case the rose joint thread pulled out of the wishbone. I have also seen cracked wishbones with slicks.
http://sylvabuild.blogspot.com/
http://austin7special.blogspot.co.uk/
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twybrow
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| posted on 5/8/15 at 11:54 PM |
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I know the owner, and I can tell you that this occurred in a straight line, on the road! No slicks, and no high cornering loads. Very scary.
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Staple balls
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| posted on 6/8/15 at 09:36 AM |
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I'm possibly being really slow here, but surely that threaded insert is too small a diameter, and too short in length? I'd think
you'd at least want the inevitable stress riser the other side of the brace, to avoid any weakening from the weld?
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sdh2903
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| posted on 6/8/15 at 10:27 AM |
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What car is it?
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britishtrident
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| posted on 6/8/15 at 04:07 PM |
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quote:
Originally posted by Bluemoon
quote:
Originally posted by MikeRJ
Fatigue failure due to the brittle heat affected zone next to the weld on the reinforcing tube that runs to the other side of the wishbone.
Assuming the rod end sits at about 90 degrees to the fixing bolt on the chassis, that point will naturally have bending forces applied to it under
acceleration and braking
[Edited on 5/8/15 by MikeRJ]
missed the cross tube sounds spot on.. wounder why the tube is there at all?
hard to tell but it looks diagonal i.e to take braking force, mk indy is simular but not seen that part fail (yet!).
[Edited on 5/8/15 by Bluemoon]
Lotus used a similar cross tube on the Elan which seems to have been copied on a lot of kits and homebuilds. but 1960s/70s Lotus rear wishbones were
brazed/bronze welded and they used fairly compliant rubber bushes.
I never liked the arrangement See this
page
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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MikeRJ
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| posted on 6/8/15 at 05:00 PM |
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quote:
Originally posted by Bluemoon
quote:
Originally posted by MikeRJ
Fatigue failure due to the brittle heat affected zone next to the weld on the reinforcing tube that runs to the other side of the wishbone.
Assuming the rod end sits at about 90 degrees to the fixing bolt on the chassis, that point will naturally have bending forces applied to it under
acceleration and braking
[Edited on 5/8/15 by MikeRJ]
missed the cross tube sounds spot on.. wounder why the tube is there at all?
It's to stop the wishbone trying to "spread" open under stress, though normally that's only a problem if you have compliant
bushes rather than rod ends.
The undersized threaded insert is a rubbish bit of engineering, but I suspect not the root cause in this case.
The wishbone design is imposing significant bending forces on the tubes instead of primarily tension/compression. Much like the many Locost book
front wishbones that have bent where the shock mounting platform has been welded. Making the tube thicker is the brute force solution, but makes
things heavier than they need to be.
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Petrolheadlukey
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| posted on 6/8/15 at 06:28 PM |
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It's an Mnr vortx rt+
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gaz_gaz
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| posted on 6/8/15 at 07:00 PM |
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the end result. check your wishbones folks
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pewe
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| posted on 6/8/15 at 07:18 PM |
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Ouch bet that hurt - trust you're OK?
Similar thing happened to a Westie in which No1 son was passengering at a track-day at Combe last year.
Front lower wishbone collapsed at high speed shunting them into the tyre wall.
Owner reckoned the tubing had been weakened by Westfield welding a support plate to the underside of the wishbone just near the outer pivot.
My own take was that the tubing was under-specced - why else re-inforce it by welding on a support plate?
Also I suspect they'd encountered the problem before, hence the plate.
Unsurprisngly they weren't overkeen to admit liability - preferring instead to supply parts at a discounted price.
On the F27 I used 25mm diam x 3mm wall tubing for the wishbones - heavy but strong....
Cheers, Pewe10 
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sdh2903
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| posted on 6/8/15 at 07:18 PM |
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 wow. Was everyone ok?
That's the car that's recently been overhauled and up for sale on here isn't it?
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gaz_gaz
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| posted on 6/8/15 at 07:20 PM |
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luke posted just above here but considering he's not 85 years old he's terrible on computers so I put the pictures up for him.
he walked away with only bruising and stiffness thankfully.
it is the car that recently had a massive overhaul unfortunately.
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