gaz_gaz
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| posted on 22/8/11 at 08:04 PM |
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what could/would/might have caused this?
i was out on Saturday giving the car a good old blast when i suddely got a sensation of steering from the rear,
i pulled over expecting to find a puncture and was shocked to find this!
as you can see the offside rear lower wishbone, rear mounting point has torn away.
what i would like your opinions on is why this failure has taken place and what i can do to make sure it never happens again as its totally removed
any confidance i had in the car at the moment,
i've had the mount replaced but i'm now stressing about all the other mounts suffering the same fate.
looking forward to your opinions
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Mark Allanson
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| posted on 22/8/11 at 08:13 PM |
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I think I would be pretty safe in saying it is metal fatigue, but why I cannot say. What car is it from? so we can see the geometry.
If you can keep you head, whilst all others around you are losing theirs, you are not fully aware of the situation
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gaz_gaz
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| posted on 22/8/11 at 08:14 PM |
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its from an outboard MNR with sierra geo.
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zilspeed
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| posted on 22/8/11 at 08:15 PM |
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Like this ?
[Edited on 22/8/11 by zilspeed]
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gaz_gaz
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| posted on 22/8/11 at 08:21 PM |
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similar except the brackets are on the bottom of the box section where there on the top in your photo,
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david_hornet27
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| posted on 22/8/11 at 08:26 PM |
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I had a similar thing happen on my Seven, although it was the weld that had come apart on the rear suspension arm mount rather than metal fatigue
which seems to have caused your problem as the weld seems to still be in place.
I cleaned and welded it back up and thoroughly checked every other weld on the car for any dubious signs to try and put my mind at rest as I had the
same feelings of lack of confidence. Everything else looked and felt nice and strong...
Once I had satisfied myself the other welds looked good I booked myself a trackday at an airfield so I could thrash the hell out of the car without
fear of meeting a big heavy car coming at me the other way or crashing into something hard if something similar went wrong. That was 900+ miles ago,
I still regularly check the car for signs of metal fatigue or dodgy welds in the places I would expect to find problems but have restored my
confidence totally knowing it has taken the load of a trackday thrashing without probs...
Hope you start enjoying the car again sooner rather than later!
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Mark Allanson
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| posted on 22/8/11 at 08:29 PM |
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Could the bush have been seized or partially seized causing the bracket to be taking the twisting action instead of the bush?
If you can keep you head, whilst all others around you are losing theirs, you are not fully aware of the situation
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gaz_gaz
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| posted on 22/8/11 at 08:33 PM |
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thank you David,
i've had the brakcet re-placed by a pro and everything else checked over to make sure it looks ok,
i actually have a trackday booked tomorrow at Brands Hatch and another on the 8th at Snetterton but this has me thinkin about taking the M3
instead.
Mark, as far as i'm aware the bush was moving freely, i'm fairly sure as i had to measure the motion ratio etc for my new dampers and
every seemed to be moving freely enough.
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david_hornet27
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| posted on 22/8/11 at 08:34 PM |
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Just a thought... As the mounting is on the bottom of the box section and seems pretty close to the road could it have hit something? I cant see any
signs of it taking a whack though.
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gaz_gaz
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| posted on 22/8/11 at 08:39 PM |
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there are certainly no scrapes or chunks out the braket to suggest it has taken a hit of any kind,
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zilspeed
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| posted on 22/8/11 at 08:39 PM |
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I have to say, I'm more a fan of a single mounting point between each wishbone and the upright in conjunction with a single adjustable toe link.
It will allow for a degree of, how shall we say, "weirdness" in the rear geometry without putting stresses on the mounting points caused
just by the geometry itself.
With the damper off the car, does it move throught its whole range of travel without any tightness or other irregularities ?
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ReMan
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| posted on 22/8/11 at 08:43 PM |
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I'd have thought seised or incorrectly set up bush/tube/bracket relationship?
www.plusnine.co.uk
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Liam
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| posted on 22/8/11 at 08:48 PM |
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Is it welded onto a piece of round tube, or just looks that way in the picture? Round or not you can see that the bracket stuck out a long way either
side of the tube it was welded to - plenty of scope for flexing and therefore fatigue. Not a good design at all, imho. You can see in the second pic
the bit of the back/bottom of the bracket that is left how much it has bent (unless it got that way by taking a knock after the bracket failed).
I'd be checking the other side at least, and any similarly mounted brackets for cracks, and considering reinforcing gussets wherever you have
edges/corners of brackets sticking a long way out from the tubes they are attached to to stop flexing.
Edited to add: Also looks to be significant rust and missing paint/powder coat around the failure suggesting corrosion could have been a factor. Could
have had a crack there for a while before the major failure.
[Edited on 22/8/11 by Liam]
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gaz_gaz
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| posted on 22/8/11 at 08:48 PM |
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i can and could before the failure, move the hub up and down the full travel freely without any frction.
i know this for certain as i only checked the same thing a couple of weeks ago while measuring for my new dampers
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907
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| posted on 22/8/11 at 08:53 PM |
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Possibly the crush tube being too short and friction between the bush and the bracket causing the bracket to twist,
or corroded steel crush tube stiff in the bush i/d ??? (As Mark says)
Are the crush tubes mild or stainless?
Paul G
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davidimurray
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| posted on 22/8/11 at 09:06 PM |
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From the pictures it looks like the bracket has failed along the edge of the heat affected zone of the weld. This document is interesting reading -
http://www.welding-consultant.com/WeldCracks.pdf
Gallery 1 http://www.facebook.com/media/set/?set=a.116893465324.130778.601005324
Gallery 2 http://www.facebook.com/media/set/?set=a.245243755324.181913.601005324&l=a9831a9319
Gallery 3 http://www.facebook.com/media/set/?set=a.440671625324.232627.601005324&l=3f0d42c523
Gallery 4 http://www.facebook.com/media/set/?set=a.490098255324.297598.601005324&l=efb083b7df
Gallery 5 http://www.facebook.com/media/set/?set=a.10150244028550325.366987.601005324&l=583fd5cd3a
Gallery 6 http://www.facebook.com/media/set/?set=a.10150550640070325.430417.601005324&type=3&l=fe779b358c
Duratec Engine Swap https://www.facebook.com/media/set/?set=a.10152527759580325.1073741828.601005324&type=1&l=40aae5e72f " target="_blank"> https://www.facebook.com/media/set/?set=a.10152527759580325.1073741828.601005324&type=1&l=40aae5e72f
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gaz_gaz
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| posted on 22/8/11 at 09:12 PM |
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the crush tubes are mild steel Paul. but as said the suspension was moving freely up and down without the dmaper attached only a fortnight ago and the
car hadnt moved untill Saturday when the failure happened,
i cant remember if its a round tube or not right now but i'll check in the morning,
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mark chandler
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| posted on 22/8/11 at 09:34 PM |
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Looks like fatigue to me, my mates Westfield did the same thing, you can see by the way the bracket has twisted off.
Just weld back, if worried then make a cross brace between the suspension brackets on both sides (like a strut brace), you do not need to modify the
frame just use slightly longer bolts and make U brackets to fit outside of the existing brackets on the brace (drawn on the top as its easier to
see.
[img] [/img]
Give it a good wiggle and look for cracks on the others, if all okay then track it 
[Edited on 22/8/11 by mark chandler]
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Strontium Dog
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| posted on 22/8/11 at 10:53 PM |
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quote:
Originally posted by Liam
Is it welded onto a piece of round tube, or just looks that way in the picture? Round or not you can see that the bracket stuck out a long way either
side of the tube it was welded to - plenty of scope for flexing and therefore fatigue. Not a good design at all, imho. You can see in the second pic
the bit of the back/bottom of the bracket that is left how much it has bent (unless it got that way by taking a knock after the bracket failed).
I'd be checking the other side at least, and any similarly mounted brackets for cracks, and considering reinforcing gussets wherever you have
edges/corners of brackets sticking a long way out from the tubes they are attached to to stop flexing.
[Edited on 22/8/11 by Liam]
From the pictures that is spot on I'd say. I would never weld a bracket on like that without some sort of extra support as it's just
asking for a stress fracture especially along the edge of the weld as has been mentioned too. I would definitely add some gussets or other form of
bracing and then you'll be fine, not a big job!
The fact that the paint or powder coat has flaked off at that point allowing corrosion is just a witness to the flexing that has been going on. I
wouldn't be at all surprised if the other brackets weren't shedding there paint too!
Something for other MNR owners to have a look at here as well for safety's sake!
[Edited on 22/8/11 by Strontium Dog]
http://s187.photobucket.com/albums/x319/zephyr2000/General%20forum%20uploads/?action=view¤t=3DEngine.mp4
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norfolkluego
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| posted on 22/8/11 at 11:33 PM |
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quote:
Originally posted by david_hornet27
Just a thought... As the mounting is on the bottom of the box section and seems pretty close to the road could it have hit something? I cant see any
signs of it taking a whack though.
I whacked mine sprinting last year (sideways into a tyre wall), all of the force went down the wishbone, it bent the chassis member the bracket was
attached to, it didn't snap the bracket off, can't see grounding alone would cause that failure
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snapper
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| posted on 23/8/11 at 04:07 AM |
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Worth checking that the bush tube is a few mm longer than the poly bush.
The tube should clamp in the bracket then the poly bush moves on the stationary tube if not then the poly bush side faces are twisting on the bracket
I eat to survive
I drink to forget
I breath to pi55 my ex wife off (and now my ex partner)
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Theshed
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| posted on 23/8/11 at 06:35 AM |
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Whist you can eliminate the loads from the bushes you cannot do that with the load through the wishbone . The bracket failed along the weakest part,
the weld. Welding to a round tube like that is asking for trouble. There is plenty of room to mount a more robust bracket. If you started with a flat
plate joining the round and square tubes in your photo you would spread the load.
Probably a good idea to check the rest of the suspension brackets. You should be able to see any cracks.
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britishtrident
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| posted on 23/8/11 at 07:10 AM |
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quote:
Originally posted by ReMan
I'd have thought seised or incorrectly set up bush/tube/bracket relationship?
Exactly my conclusion.
Bush binding -> flexing -> crack at defect or sharp corner -> crack growth -> fracture.
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Liam
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| posted on 23/8/11 at 10:42 AM |
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quote:
Originally posted by britishtrident
quote:
Originally posted by ReMan
I'd have thought seised or incorrectly set up bush/tube/bracket relationship?
Exactly my conclusion.
Bush binding -> flexing -> crack at defect or sharp corner -> crack growth -> fracture.
How many times does he have to say his suspension moved perfectly freely?? 
Absolutely definately, imho, fatigue failure due to excessive flexing due to crap bracket mounting.
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JAG
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| posted on 23/8/11 at 10:55 AM |
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This isn't the first time I've seen a suspension bracket failure on an MNR.
SMART51 of this forum had a similar failure on his MNR Vortex (with an R1 engine). You might find the thread if you do a search.
Have you spoken to MNR?
Justin
Who is this super hero? Sarge? ...No.
Rosemary, the telephone operator? ...No.
Penry, the mild-mannered janitor? ...Could be!
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