Keep it coming Gents!! This is one of the most informative threads I've read in years. Pros and cons from all sides, very interesting!!! Thanks all for sharing your knowledge! JIM
Differential Mount Cover Bushing
- Thread starter Wasatch7
- Start date
We had an E36 3 series in at work for an m.o.t. a few months ago and it had ripped the boot floor out at the diff too. It was "only" a 4 cyl but I can't just remember the year.
I worked the root cause out years ago and have posted about it on .net and bimmerforums. Randy usually gets tetchy with me pretty quickly since he obviously knows a less on the topic. Cant fault him for making so much cash from the problem though.
The cause is torque reaction going through the diff pulling downwards on the rear carrier bush. The load is proportion to tractive effort plus an effect from car mass. I have done the calculations. Worst risk of failure is for heavy footed drivers spinning the rears, where the most T.E goes down. The M fairs worse due to the LSD with which more peak T.E can be achieved.
The cross member in the boot floor is a bit weak vs this load and there are insufficient welds used.
Solid carrier bushes (all 3) should help reduce loads a bit since less movement is less dynamic loads. My calcs looked at this statically so I cant substantiate the comment, but its an engineering no-brainer.
The cause is torque reaction going through the diff pulling downwards on the rear carrier bush. The load is proportion to tractive effort plus an effect from car mass. I have done the calculations. Worst risk of failure is for heavy footed drivers spinning the rears, where the most T.E goes down. The M fairs worse due to the LSD with which more peak T.E can be achieved.
The cross member in the boot floor is a bit weak vs this load and there are insufficient welds used.
Solid carrier bushes (all 3) should help reduce loads a bit since less movement is less dynamic loads. My calcs looked at this statically so I cant substantiate the comment, but its an engineering no-brainer.
Last edited:
Z
zedonist
Guest
Deano that's not the root cause though mate that's a contributing factor to the failure of the root cause, in other words it's what exposed the root cause failure. The OEM engineers would have done the same if not more precise calculations, that lead them to the material used and welding configuration used in the design.
The root cause is actually what caused the boot floor and diff carrier to become weak, that they were then exposed to the forces of mass at the rear end, the root cause was poor execution of the welding process, and incorrectly supplied products from the supplier, even these are not the true root cause, because then you have to ask why, the process was poor,and why the supplier had an issue, this later part was my main role on working with the Gate Hold Team.
I don't fault anyone from blaming the design, and if they've made money from it well that's all good too, but if you don't have to spend the money, then it's good to understand the issue and make an informed decision, but you have to accept that the design was correct and fit for purpose, it would not have been safe otherwise, and would certainly manifested itself during homologation testing and crash testing, and the launch would have been delayed.
I watched the head of Ford at the time delay the new build of the X350 because it did not have enough rear leg room, so I am sure that a significant structural issue, would have been a cause of delay also.
The root cause is actually what caused the boot floor and diff carrier to become weak, that they were then exposed to the forces of mass at the rear end, the root cause was poor execution of the welding process, and incorrectly supplied products from the supplier, even these are not the true root cause, because then you have to ask why, the process was poor,and why the supplier had an issue, this later part was my main role on working with the Gate Hold Team.
I don't fault anyone from blaming the design, and if they've made money from it well that's all good too, but if you don't have to spend the money, then it's good to understand the issue and make an informed decision, but you have to accept that the design was correct and fit for purpose, it would not have been safe otherwise, and would certainly manifested itself during homologation testing and crash testing, and the launch would have been delayed.
I watched the head of Ford at the time delay the new build of the X350 because it did not have enough rear leg room, so I am sure that a significant structural issue, would have been a cause of delay also.
B
badman gee
Guest
If you drive the car properly it shouldn't be an issue.
It's how you look after it.
It's how you look after it.
Z
zedonist
Guest
You are correct Mark, but Deano was partially right, its not a torque issue that helps manifest the issue, but actually what in the old days we used to call tramping, where by the rear tyres lose traction and the motion of heavy wheel spin causes the axle to bounce up and down, quite noticeably on a live rear axle, on A Z3 it would be less, but any bouncing moment with that weight could contribute to weakened areas failing. The answer is in reality either not to spin the wheels, or fit an anti-tramp kit, don't know how you would fit one to a Z3 though, must be possible, because there still used in motor sport today.
And as an aside, solid bushes won't reduce the effects of tramping.
And as an aside, solid bushes won't reduce the effects of tramping.
It seems logical to me that solid bushes would only transmit the forces applied even more rigorously and amplify the damage. That's just coming from a mechanic's point of view, not an engineer.
Z
zedonist
Guest
Quite right Tony, ever wondered why rally cars sound like a bag of bolts in a cement mixer, it's because they have solid bushes and all the noise transfers through the body, it would be the same effect for any forces applied.
We have that problem with drag race vehicles known as wheel hop which is your tramping. This takes the exerted pressures to the n th degree and results in pieces scattered all over the track. It's a fine line in tuning the suspension to work at it's limit while still preventing the tire hop. After reading my manual many times, I agree there is little that can be done other than re doing the complete rear structure to utilize a different form of suspension. The model A that I'm having the school kids build will be a challenge for traction and we've decided on a 4 link system that's very adjustable with a pan hard bar and hopefully can put the power to the track instead of smoking the tires and breaking parts. Wish you were closer Zedonist!! JIM
Z
zedonist
Guest
You don't need to be close with the internet Jim, the art is stopping the spin, and that's why you see larger rear diameter wheels on drag cars than normal, so you may want to look at this also.
Yup! Have two sets of Hoosier slicks sitting in my cousin's shop in MO, USA Along with a trailer load of parts and a Harley sidecar body. Just need the time to get down for a visit. We're running a stock GM 350 4bbl with stock exhaust , auto and ten bolt rear with unknown ratio to get a baseline before we get too crazy. This is a very low buck teaching/ learning experiment with me supplying parts and the students doing the labour. They'll then use the car in the BC high school drag race program against other schools. Once this car is done I'll take my second body out there where they'll build an old style with the I beam and hopefully a flathead v8 manual shift with a narrowed ford 9" rear with 35 spline Strange axels, etc.
Z
zedonist
Guest
Made me laugh Jim, the film grease popped straight into my mind, your not married to Sandy are you?
No, but if she's available my wife's due for an accident I think. just kidding, wouldn't trade her for anything, not even Baaabs!!. My idea is to give the students a taste of the newer vs old in technology. I think they've really grasped on to the challenges my friend and I have thrown at them and have learned a lot about fabrication , welding and design, etc. this will be the third year and covers grades seven to twelve all working to a common goal. I plan on involving all grades when it comes time for painting and decorating. With it being A Christian School the paint scheme should reflect that and all of the school should be involved in picking the theme.
I'm reviving this thread (also my first post here) because this is the only thread I've read anywhere that challenges the "conventional wisdom" and "standard practice" for the much discussed "subframe failures" of Z3 series cars. Having participated in a number of web forums covering several interests over the years, one often sees a herd mentality develop on popular topics. But, the herd is not always right, just ask any wildebeast 
I've never been a car collector but I do lust for the old days when it was easy to find cars with manual transmissions that were fun to drive. I realize that I am myself becoming a fossil, just like those cars I so fondly remember. Having owned two different small, historic BMWs, a 2002 and a 325i (vintage 1994), both owned for 8-10 years, I became disgusted with what BMW was producing and shifted to other brands (mostly Subaru turbos) for daily drivers. But I decided it was time to buy a piece of automotive history, and hence I have a newly purchased (from online auction) Z3 Coupe enroute as I type this.
Not having done very much research on this foolish decision to buy such a 16 year old car, I then stumbled onto all the verbiage associated with this frame failure topic, and without having laid eyes yet on my soon to be playtoy, I'm having all sorts of second thoughts and potential regrets about having decided to buy such a car at a distance, even from a seller with very high ratings who describes the car as being more or less immaculate and rust free.
To begin with, this thread has helped me to put this whole thing into perhaps better perspective, than those I read over on that other web forum. For that I thank you. I am however trying to come up with the best strategy to deal with whatever I find in this vehicle on delivery. Currently, I have an appointment scheduled at a local garage (not a specialty garage, but probably the best one in my area and one that does work on some older BMWs) to occur a day or two after the vehicle is delivered. I have spoken to the head mechanic and aside from being able to deal with esoteric electronic issues, he sounds fully competent to evaluate and work on this vehicle, and sounds like he has a lot of experience with vehicle corrosion issues.
My question, if I really have one, is what I should do at this juncture if when we put the car up on a lift and examine the trunk floor, we find evidence of early (popped welds) or later failure of the aforementioned "subframe." I am not expecting major mechanical issues with the car on delivery -- obviously, I could be wrong, but I think the seller would have been aware and honest about those, which is not to say that the water pump and a few other common parts might not need replacing in the near term.
Simply stated, should I try to send the car back (at what will be great expense) or press the seller for a refund of some sort if these subframe issues are present, and what should be the threshold for demanding this? And, if I end up being stuck with such a vehicle, how should I proceed in trying to deal with these issues? I'm somewhat reluctant to involve the nearest BMW dealer, who has become rather impressed with itself over the last 11 years since I stopped being a customer. They are also 2.5 hours drive away from me, although there are several well-regarded non-authorized foreign/German car garages in that major city that also work on BMWs, apparently with good results. For the record, I live in a rural area in the Western USA.
Thanks very much in advance for any suggestions.
I've never been a car collector but I do lust for the old days when it was easy to find cars with manual transmissions that were fun to drive. I realize that I am myself becoming a fossil, just like those cars I so fondly remember. Having owned two different small, historic BMWs, a 2002 and a 325i (vintage 1994), both owned for 8-10 years, I became disgusted with what BMW was producing and shifted to other brands (mostly Subaru turbos) for daily drivers. But I decided it was time to buy a piece of automotive history, and hence I have a newly purchased (from online auction) Z3 Coupe enroute as I type this.
Not having done very much research on this foolish decision to buy such a 16 year old car, I then stumbled onto all the verbiage associated with this frame failure topic, and without having laid eyes yet on my soon to be playtoy, I'm having all sorts of second thoughts and potential regrets about having decided to buy such a car at a distance, even from a seller with very high ratings who describes the car as being more or less immaculate and rust free.
To begin with, this thread has helped me to put this whole thing into perhaps better perspective, than those I read over on that other web forum. For that I thank you. I am however trying to come up with the best strategy to deal with whatever I find in this vehicle on delivery. Currently, I have an appointment scheduled at a local garage (not a specialty garage, but probably the best one in my area and one that does work on some older BMWs) to occur a day or two after the vehicle is delivered. I have spoken to the head mechanic and aside from being able to deal with esoteric electronic issues, he sounds fully competent to evaluate and work on this vehicle, and sounds like he has a lot of experience with vehicle corrosion issues.
My question, if I really have one, is what I should do at this juncture if when we put the car up on a lift and examine the trunk floor, we find evidence of early (popped welds) or later failure of the aforementioned "subframe." I am not expecting major mechanical issues with the car on delivery -- obviously, I could be wrong, but I think the seller would have been aware and honest about those, which is not to say that the water pump and a few other common parts might not need replacing in the near term.
Simply stated, should I try to send the car back (at what will be great expense) or press the seller for a refund of some sort if these subframe issues are present, and what should be the threshold for demanding this? And, if I end up being stuck with such a vehicle, how should I proceed in trying to deal with these issues? I'm somewhat reluctant to involve the nearest BMW dealer, who has become rather impressed with itself over the last 11 years since I stopped being a customer. They are also 2.5 hours drive away from me, although there are several well-regarded non-authorized foreign/German car garages in that major city that also work on BMWs, apparently with good results. For the record, I live in a rural area in the Western USA.
Thanks very much in advance for any suggestions.
Welcome Tartufo,
First of all I think you have summed everything up pretty well, and your planned course of action is pretty sound - if not nail biting till you get the thing up on a ramp. Obviously in hindsight requesting some extra pictures of the boot interior and underneath would have been useful to put your mind at rest.
Which coupe did you purchase - 2.8, 3.0 or M? Just to give us a better idea of what power has been going through the rear end.
First of all I think you have summed everything up pretty well, and your planned course of action is pretty sound - if not nail biting till you get the thing up on a ramp. Obviously in hindsight requesting some extra pictures of the boot interior and underneath would have been useful to put your mind at rest.
Which coupe did you purchase - 2.8, 3.0 or M? Just to give us a better idea of what power has been going through the rear end.
I think, personally you might be meeting trouble halfway. If the car is as described in all others aspects it would seem to me that it's more than likely to be in good 16 year old condition. Add to that it is " only" a 2.8 so less likely to have the problems you're envisaging than, say, a 3.0 or a 3.2M. Cross that bridge if it arises not before. If the seller has previous good feedback you have to trust that if you buy at a distance, there is very good buyer protection on auction sites now too.
Tony.
Tony.
I more than most can understand your aprhension having bought both my cars from 6500 miles away. I agree with Tony, wait until you actually have a problem then deal with it otherwise every time you start your car the little devil in your head will be chattering away " is that a waterpump noise, isn't the temp gauge a hair higher, etc". Doesn't sound like your PO was doing donuts or racing on a regular basis so I wouldn't be worried too much. Enjoy the forum and the car. JIM
But of course there is more!
I originally intended to buy an "M" version but could not find a suitable one at a price I wanted to pay.
Now, before even taking delivery of this 2.8L "non-M" version I stumbled onto an M (S52) with low miles and a favorable history. So, I am going out to look at it (involves a short plane ride) and probably buy it.
Which means I may soon have two of these things, a 2.8L Z3 and an S52 Z3M. I will likely keep both of them, at least for a while, as I have places to park them, although this may make life a little inconvenient for my "real" car. If they are both good cars, it seems to me that the smartest thing would be to keep both of them, at least until I decide to get rid of one for whatever reason.
These things don't appear to be depreciating in value right now, at least not in the USA.
I originally intended to buy an "M" version but could not find a suitable one at a price I wanted to pay.
Now, before even taking delivery of this 2.8L "non-M" version I stumbled onto an M (S52) with low miles and a favorable history. So, I am going out to look at it (involves a short plane ride) and probably buy it.
Which means I may soon have two of these things, a 2.8L Z3 and an S52 Z3M. I will likely keep both of them, at least for a while, as I have places to park them, although this may make life a little inconvenient for my "real" car. If they are both good cars, it seems to me that the smartest thing would be to keep both of them, at least until I decide to get rid of one for whatever reason.
These things don't appear to be depreciating in value right now, at least not in the USA.
Getting back on topic its my view bmw engineers made a mistake in the design of the boot floor. At limit of tyre adhesion the static stress in the parent metel in the boot floor cross member is around 160MPa. A bit high? Above the fatigue endurance limit for sure. Dynamic stresses higher due to soft bushes letting the heavy differential move around and its inertial effects adding more load.
I bet many cars with no visual problems with the boot floor will have deformation in the boot floor structure. Stick a straight edge across the top of the cross member in the boot floor and see if its flat. On my car i had a couple of failed welds but the whole cross member was bent down 5mm.