[Cafe.Racer]

I'm sure the owner and everyone else is a bit peeved (that's a understatement - thank god you're okay, sounds a right nightmare), but I agree with previous comments - earlier impact damage weakened the part, IMHO (although on thing that puzzles me is how "bright" the crack is).

As for comments about it being a weaker, cheapo part - I think that unless any of you have a background in stress in materials, FMEA and design engineering, well - best to assume that BMW know a little bit more than the rest of us, eh? And that's before they start using fancy software to analyse the stress suspension components are under.

As for the part failing at the weakest point where there is no lip, clearly (for the reasons given above) it was meant to be like that. It appears that that section is roughly in the middle (halfway along the span) which is the point at which there is the most stress.

Why did BMW include a cheap lightweight part in their suspension design, and then include a design feature that would, apparently, promote failure at point of highest stress?

The answer is - on purpose. My guess is that the arm bends under impact damage, but the wheel remains attached. A heavier, beefier part would pass the stress loading caused by impact damage to it's sister part - cracked upright, or even bent chassis.

Anyway, see what Soper say (or more likely what BMW say to Soper). Bearing in mind their stance on cracked alloys, I would practice counter arguements...

[F31-340i]

on further reflection:

i) the implied weakness on the top of the part could be deliberate on the part of the designers to ensure a heavy knock to the wheel leads to this part failing and not distorting the chassis. This is arguably good design.

ii) most likely scenario is that the car has hit a curb hard leading to a partial (potentially unnoticable) deformation of the arm - but seriously weakening it - this has then failed during high but normal loading over a period of time until it let go. It is likely this knock was hard enough to damage the rim - which may have been replaced - but with the seemingly undamaged suspension left in place. You'd have known if you did this, so assume a previous owner - who may not have admitted it to the dealer. Could have been the previous owners son who did it etc. This could easily have been missed on an AUC check as I've very little faith in them!

iii) whilst shocking, this failure in itself was probably reasonably "fail safe", and again potentially a feature of the design.

EDIT - phil - I agree - and wrote the above before you posted. I do have a history in design engineering - I posted my earlier comments when I was really watchhing Jenson become champ.

[beemerbird]

I realise that when BMW repair your car you automatically get the 'assurance' that the car has been repaired satisfactorily and is in road worthy condition.

Depending on how things progress with this Mike, I would consider requesting a certificate stating that suspension, wheels, geometry etc are all alligned correctly.

The only reason I say this is when the power steering pump on the Porka T was replaced under warranty, I was given a report stating that the associated mechanical and electronic components that work in tandem with the power steering pump had all been checked and verified to be in working order. The report had been signed by the OPC and an independent company as a second opinion.

[MIKEYBMW1]

surely fast cornering can't of done this though and I have only driven the car a handfull of times and I can promise you that I have never had any kind of impact in the car and why is the arm facing straight down to the ground , the Bmw Guy didn't even look suprised that it had happened , have they had this happen before ??????????????/ all questions I know but I need to know why this has bent/ snapped like this.

[MIKEYBMW1]

Originally Posted by ///ajd

on further reflection:

i) the implied weakness on the top of the part could be deliberate on the part of the designers to ensure a heavy knock to the wheel leads to this part failing and not distorting the chassis. This is arguably good design.

ii) most likely scenario is that the car has hit a curb hard leading to a partial (potentially unnoticable) deformation of the arm - but seriously weakening it - this has then failed during high but normal loading over a period of time until it let go. It is likely this knock was hard enough to damage the rim - which may have been replaced - but with the seemingly undamaged suspension left in place. You'd have known if you did this, so assume a previous owner - who may not have admitted it to the dealer. Could have been the previous owners son who did it etc. This could easily have been missed on an AUC check as I've very little faith in them!

iii) whilst shocking, this failure in itself was probably reasonably "fail safe", and again potentially a feature of the design.

EDIT - phil - I agree - and wrote the above before you posted. I do have a history in design engineering.

the first thing the Bm guy said once he had seen the damage was that the wheel is in perfect condition and no sign of impact with a curb or anything else for that matter and I would know if I had hit anything so yeh maybe the previous owner but they should check them before selling on surely?

[Penyfan1960]

MikeyBMW1 are you sure it had only done 12k or so miles before purchase?
With all the rust and decay I can see in those few pics it looks as if's done a few more miles than that!
I got under my BM after 23k and it looked a lot better than your pics!
Where has your car been?
Cheers!

[MIKEYBMW1]

Originally Posted by Cafe.Racer

I'm sure the owner and everyone else is a bit peeved (that's a understatement - thank god you're okay, sounds a right nightmare), but I agree with previous comments - earlier impact damage weakened the part, IMHO (although on thing that puzzles me is how "bright" the crack is).

As for comments about it being a weaker, cheapo part - I think that unless any of you have a background in stress in materials, FMEA and design engineering, well - best to assume that BMW know a little bit more than the rest of us, eh? And that's before they start using fancy software to analyse the stress suspension components are under.

As for the part failing at the weakest point where there is no lip, clearly (for the reasons given above) it was meant to be like that. It appears that that section is roughly in the middle (halfway along the span) which is the point at which there is the most stress.

Why did BMW include a cheap lightweight part in their suspension design, and then include a design feature that would, apparently, promote failure at point of highest stress?

The answer is - on purpose. My guess is that the arm bends under impact damage, but the wheel remains attached. A heavier, beefier part would pass the stress loading caused by impact damage to it's sister part - cracked upright, or even bent chassis.

Anyway, see what Soper say (or more likely what BMW say to Soper). Bearing in mind their stance on cracked alloys, I would practice counter arguements...

also wouldn't the arm bend upwards if the wheel was hit against something and surely it would have to be some impact ? I keep going outside and looking at it and its not just a flimsy bit of metal but the sag in it must be 4" out of line

[dxb335d]

Originally Posted by ///ajd

on further reflection:

i) the implied weakness on the top of the part could be deliberate on the part of the designers to ensure a heavy knock to the wheel leads to this part failing and not distorting the chassis. This is arguably good design.

ii) most likely scenario is that the car has hit a curb hard leading to a partial (potentially unnoticable) deformation of the arm - but seriously weakening it - this has then failed during high but normal loading over a period of time until it let go. It is likely this knock was hard enough to damage the rim - which may have been replaced - but with the seemingly undamaged suspension left in place. You'd have known if you did this, so assume a previous owner - who may not have admitted it to the dealer. Could have been the previous owners son who did it etc. This could easily have been missed on an AUC check as I've very little faith in them!



iii) whilst shocking, this failure in itself was probably reasonably "fail safe", and again potentially a feature of the design.

EDIT - phil - I agree - and wrote the above before you posted. I do have a history in design engineering - I posted my earlier comments when I was really watchhing Jenson become champ.

if previous owners son did it, then the owner would know.

[MIKEYBMW1]

Originally Posted by Penyfan1960

MikeyBMW1 are you sure it had only done 12k or so miles before purchase?
With all the rust and decay I can see in those few pics it looks as if's done a few more miles than that!
I got under my BM after 23k and it looked a lot better than your pics!
Where has your car been?
Cheers!

oh don't say that mate , I know it came down here from Scotland area it is 2 years old now though

[M3-FAST]

Sh!t Mikey, thats shocking buddy

Can't believe the pics, how bent that arm is. I note you said that the BMW tech did not look shocked? Surely a tech looking under an M3 and seeing that would look shocked if it was the first one he had seen. Doubt there has been any failures on this UK forum due to the limited number of owners, you would have heard about one by now.

I would try posting on the US M3 forum, see if anyone there has any thoughts or experiences, will help when addressing the dealer in case it is a known problem.

Good luck in getting it sorted matey and thank god you are still here to tell the story

[beemerbird]

Originally Posted by Cafe.Racer

Why did BMW include a cheap lightweight part in their suspension design, and then include a design feature that would, apparently, promote failure at point of highest stress?

The answer is - on purpose. My guess is that the arm bends under impact damage, but the wheel remains attached. A heavier, beefier part would pass the stress loading caused by impact damage to it's sister part - cracked upright, or even bent chassis.

So in your opinion BMW are purposefully prepared to put the drivers/occupants of their vehicles at risk by designing parts which when damaged or broken cause the car to become uncontrollable as Mike stated in his original post.

Doesn't matter a jot who the manufacturer is to me, BMW build mass produced cars like Audi, like Mercedes etc, etc if that is the case then it is totally wrong. None of them are gods imo.

I don't have any history of engineering but I've spoken to a man who certainly does and has.

[MIKEYBMW1]

thanks mate ,what would you all call this part ?

[MIKEYBMW1]

Originally Posted by beemerbird

So in your opinion BMW are purposefully prepared to put the drivers/occupants of their vehicles at risk by designing parts which when damaged or broken cause the car to become uncontrollable as Mike stated in his original post.

Doesn't matter a jot who the manufacturer is to me, BMW build mass produced cars like Audi, like Mercedes etc, etc if that is the case then it is totally wrong. None of them are gods imo.

I don't have any history of engineering but I've spoken to a man who certainly does and has.

I agree Helen surely this can't be right ?

[dxb335d]

It must have had an impact to weaken it previously.

[SiKkBaSs]

Originally Posted by dxb335d

It must have had an impact to weaken it previously.

All questions point to this I'm afraid.

[willhollin]

That's bad mate! Hope you have recovered, did you have to swiftly change your underpants when you got home?

Originally Posted by MIKEYBMW1

thanks mate ,what would you all call this part ?

Trailing arm.

[F31-340i]

Originally Posted by MIKEYBMW1

the first thing the Bm guy said once he had seen the damage was that the wheel is in perfect condition and no sign of impact with a curb or anything else for that matter and I would know if I had hit anything so yeh maybe the previous owner but they should check them before selling on surely?

The wheel could have been replaced. Maybe useful to check whether its one of the originals (not sure whether wheels have date of manuf stamped on them as well as part no. etc., but if so you could cross check with the others). The previous owner could have got one outside the main BMW network and perhaps not realised they'd damaged the suspension - if it had been me I'd certainly be trying to avoid main dealer prices for the wheels! (used to be half the price in Europe etc.).

The AUC should have identified any damage, but of course it was almost certainly not fully bent when you bought and may have only shown very minor signs of damage, if any. If they admit impact damage, I think you have every right to demand it should have been picked up. I don't hold much faith in AUC checks - I think they are a quick once over, check the tyres etc. - nothing comprehensive.

A possible impact scenario would be heavy side loading on the wheel - i.e. getting the tail too far out and clobbering a curb - rather than just curbing a wheel tangentially. This is much more likely to create the damage you have.

[MIKEYBMW1]

thats it trailing arm and yes I did just about sh*t myself at the time ,just don't know how I kept the thing on the road.

if it has had previous damage why has it just bent downwards I just don't understand this?

[F31-340i]

Originally Posted by beemerbird

So in your opinion BMW are purposefully prepared to put the drivers/occupants of their vehicles at risk by designing parts which when damaged or broken cause the car to become uncontrollable as Mike stated in his original post.

Doesn't matter a jot who the manufacturer is to me, BMW build mass produced cars like Audi, like Mercedes etc, etc if that is the case then it is totally wrong. None of them are gods imo.

I don't have any history of engineering but I've spoken to a man who certainly does and has.

No I think cafe racer has a valid point - which also belatedly occured to me. This maybe the safest failure mode. If it was stronger it may punch through the chassis and into the fuel tank etc. It is almost certainly designed to be as strong as it is, and fail exactly as it has.

I haven't done any FMEA for 15 years but I am an engineer (i.e. proper engineer).

[dxb335d]

It should then have been replaced when the wheel was. (Presuming the wheel has, Im quite sure it has anyhow)

[F31-340i]

Originally Posted by MIKEYBMW1

thats it trailing arm and yes I did just about sh*t myself at the time ,just don't know how I kept the thing on the road.

if it has had previous damage why has it just bent downwards I just don't understand this?

The damage is not, as I previously hinted, most likely to have been caused by impact from below (though not ruling that out). It is likely to have been caused by forces transmitted through the part - i.e through the bush. Hence it has failed in compression, and due to its design it will nearly always fail the same way - in this case by the top parts of the U-web creasing. This will always lead to it bending down. Indeed, even if it was caused by damage from below - it could still ultimately deform in the same way - the damage would just weaken the part leading to failure, not necessarily change its failure mode.

[beemerbird]

Originally Posted by ///ajd

No I think cafe racer has a valid point - which also belatedly occured to me. This maybe the safest failure mode. If it was stronger it may punch through the chassis and into the fuel tank etc. It is almost certainly designed to be as strong as it is, and fail exactly as it has.

I haven't done any FMEA for 15 years but I am an engineer (i.e. proper engineer).

Good point.

As you state, this may be a safe failure mode. Does that then mean whenever a wheel is kerbed heavily the trailing arm should be inspected and checked for stress fractures and damage as a matter of safety?