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
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.
its from an outboard MNR with sierra geo.
Like this ?
[Edited on 22/8/11 by zilspeed]
similar except the brackets are on the bottom of the box section where there on the top in your photo,
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!
Could the bush have been seized or partially seized causing the bracket to be taking the twisting action instead of the bush?
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.
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.
there are certainly no scrapes or chunks out the braket to suggest it has taken a hit of any kind,
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 ?
I'd have thought seised or incorrectly set up bush/tube/bracket relationship?
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]
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
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
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
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,
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]
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]
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.
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
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.
quote:
Originally posted by ReMan
I'd have thought seised or incorrectly set up bush/tube/bracket relationship?
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.
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?
Hi
It's just bad design having the bracket left unsupported hanging of round tube. Again not the first one There was Colin's and the back end
of Matt Lewis's car has fallen apart spectacularly twice due to incompetent design and poor welding. Just get it back to MNR and get them to
redesign and repair.
You can see on the pic that it hasn't just happened due to the rust present on the fractured section on the one edge.
Cheers Matt
What it's welded to isn't quite wide enough - the loads are being transferred from the bush to the sides of the bracket & into the base
of the bracket before they reach the chassis. The base of the bracket is twisting causing the fatigue.
Ordinarily, loads are transferred fromt he sides of the bracket straight into the chassis via the welds - more a compression/tension arrangement with
little bending.
Just my 2p.
How it should be:
----|--------|---
| |
----|--------|---
How it is:
|-/------/-|
|/ / |
|------/---|
Wishbones don't just pivot up and down on the mountings. They pull, push and twist at them as a result of cornering forces and the torque effects
of accelerting and braking. As the bushes were recently checked and free then it's most likely fatigue from the bending caused by those
forces.
Several Fury's have suffered from similar failure, although it was the tube wall that gave in those cases (KitCarWorkshop produced chassis). The
brackets on mine were reinforced with fillets to prevent/reduce the flexing after it had happened on a couple of race weight chassis in RGB. New
chassis by FurySportsCars are being made with heavier guage tube I believe.
Hi
Yea the Fury failures where all down to using 18G tubing that was down to Min tolerance IE 19.5G. And was tube fracturing / failure. This is purely
down to having brackets attached to the chassis with insufficient bracing and poor design.
Cheers Matt
anyone know what guage the bgh chassis were made of?
Think they were 16g but Steve has re-speced them since taking over and bringing the chassis welding in-house so not sure if that is what they are now
or maybe thicker.
The fillets on the front lower brakets of mine can just be seen in this photo. The rear ones are the same but inward along the edge of the diff cage.
Mine was KCW produced. Would be easy enough to modify like mine. Matt might comment on what he thinks as I believe he has seen/fixed at least one of
the broken ones.
thank you for all the replies,
Matt is this something you'd be able to remake and make properly for me?
i decided to give the car a go at Brands and it rained non stop before a broken clutch cable ended play..
all in all i'm loving kitcar life
Hi
Yea it's the sort off thing we do. Will need those 4 brackets removing and re mounting with appropriate strengthening to ensure they wont fall
off again. Just give me a U2U if interested and we can exchange details Etc.
Cheers Matt
I did speak to steve re fury chassis, and KCW was made of 1.2mm steel, later cars were 1.6mm steel, apparently without problems.
quote:
Originally posted by procomp
Hi
Yea it's the sort off thing we do. Will need those 4 brackets removing and re mounting with appropriate strengthening to ensure they wont fall off again. Just give me a U2U if interested and we can exchange details Etc.
Cheers Matt