Cracked rear subframe - Is this a thing?

Markos

Markos

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The spare rear subframe/diff mount is cracked. Is this common? The strange thing is that it is a newer part. It still have the orange BMW sticker on it.

I suspect that it is self inflicted. I put a floor jack under the sideloader. This is a common practice with solid axle trucks. The sideloader should press up against the body mount, and lift the car (which it does). However, there is quite a bit of leverage on this 1/8” thick mounting plate. Hard lesson to learn, but I do have the subframe mounted on the 2800CS. Full disclosure I have put the jack under that one as well, but I think that is the end of that practice for me.

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From mye28...

Rear:
Some say it's alright to lift the car by the differential, but in this writer's experience, it places a strange load on the differential mount. Because of that, I lift the car under the subframe bushing on each side. Do not lift by the subframe itself. That puts a lot of stress on the bushings and the subframe (or rear-axle carrier as described in the Bentley Manual) is not designed for that type of load. The bolt going through the bushing holds the subframe rigidly to the car, which makes this the most-suitable lifting point.

*Remember to always chock the wheel(s) opposite of the one being raised.*
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You mean use two floor jacks, one on each side? Does seem safer, but what an annoyance.

John
 
bfeng said:
You mean use two floor jacks, one on each side? Does seem safer, but what an annoyance.

John
Click to expand...

I have never done that. I have used a jack on the sideloader, which is seemingly bad news. I’m curious to know how folks add jack stands to their subframe mounts. Where do you start jacking?
 
Ouch. I'm glad I'm going to be lifting mine with my MaxJax. Is that cracked part steel or AL alloy? Hard to believe steel would crack that way from lifting the car unless it had already begun to crack.
The frame head of my '54 beetle pan had a number of long fatigue cracks that I had to weld up and it was only 26 years old at the time. Our cars are getting to be twice that age and they are heavier and have more power.
It probably can be welded back together easily enough (after a complete degreasing) though it might be good to have it baked a bit after welding to relieve stress.
 
mulberryworks said:
Ouch. I'm glad I'm going to be lifting mine with my MaxJax. Is that cracked part steel or AL alloy? Hard to believe steel would crack that way from lifting the car unless it had already begun to crack.
The frame head of my '54 beetle pan had a number of long fatigue cracks that I had to weld up and it was only 26 years old at the time. Our cars are getting to be twice that age and they are heavier and have more power.
It probably can be welded back together easily enough (after a complete degreasing) though it might be good to have it baked a bit after welding to relieve stress.
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It’s steel, at lead 1/8” thick! I’ll measure it this weekend.
 
Yep, mine was cracked as well, but further back on the side around the holes (no pics).
I drilled the end of the cracks, welded the cracks, and reinforced the sides where the cracks were. I added/welded on big 4mm plates on each side.

IMG_20200131_084753.jpg
 
Gransin said:
Yep, mine was cracked as well, but further back on the side around the holes (no pics).
I drilled the end of the cracks, welded the cracks, and reinforced the sides where the cracks were. I added/welded on big 4mm plates on each side.
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Good to know! It is definitely fixable. I will likely dump it though if the other one is better.
 
I jack my car from the frame rail before it curves into the floor pan. I put one jack stand under the forward frame rail near the steering linkage (after the rail curves up and straigntens out towards the engine) and the othe jack stand goes under a piece of wood which covers the rear sub frame bushing bolt and plate just inside the rocker cover. Repeat for the other side. Never any issues and doors open and close easily if level.
 
This photo strikes me as an important clue. This corner is too sharp and should have a radius instead.
Stresses get magnified at corners, a lesson learned by the crashing of the De Havilland Comet in 1953. The windows were square and cycle fatigue combined with a pressurized cabin would cause catastrophic failure.

I'm sure BMW thought that this sub-frame steel was strong enough, but with many cycles, it proved to not be. I for one, will take extra care when looking at all the sress points in my car and will reinforce by welding in extra metal those that are suspect or known to be weak such as the differential and rear shock mounts.

BMW-E9-Subframe-crack-Markos.jpg
 
Hi @mulberryworks,

I have been noodling on this one. If you Jack from the diff, you are lifting about 1,000lbs, a majority of that is pressing on the rear upper diff mount. That steel shouldn’t give like this. My suspicion is that this is caused by the bracket flexing over years of driving with a bad rear diff isolator and/or cracked rear diff body mount (the part that gets reinforced). Every time you launch the diff wants to rotate, and causes stress on that housing. Combine this with your design concerns and CRACK! I’m still guessing that I cracked it. I’ll check the crack for sans and oil. If it is clean we have our answer.

I feel a little less guilty about the mistake but I would advise folks not to jack from the sideloader. Again it is common practice with many vehicles, but the BMW design puts a lot of leverage on the subframe.
 
HB Chris said:
Hmmm, I have always used the diff to raise the rear of the coupe.
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@HB Chris and @Ohmess ,

To be honest, it is reassuring to me that seasoned BMW veterans are using the same approach as I have. This metal must have been compromised. I am going to inspect for repairs also because my pic above looks like there is a horizontal welded seam near the cracked section.
 
Gransin said:
Yep, mine was cracked as well, but further back on the side around the holes (no pics).
I drilled the end of the cracks, welded the cracks, and reinforced the sides where the cracks were. I added/welded on big 4mm plates on each side.

View attachment 85569
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Brilliant!! How many of us reinforce the diff mount-to-body piece but neglect this part? Me :-(
 
Peter Coomaraswamy said:
Brilliant!! How many of us reinforce the diff mount-to-body piece but neglect this part? Me :-(
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Judging from all the cracks I found on my car around the rear end, cracked diff bushing mount in the chassis, cracks in and around the whole long "box" where the diff bushing attaches + cracks in the chassis going all the way in under the rear seat, and the cracked rear subframe, I believe that my car had to have been driven hard, with bad bushings and maybe even a loose diff at the end of its "previous" life 20 years ago. The totally rusted out subframe mounts in the body could have played a significant role in the whole mess as well.

With good bushings and everything tightened down properly, avoiding any exessive play in and between all the parts involved, I don't think anyone needs to be afraid of cracking a subframe, but they are getting about 50 years old, and could have been through a lot in their lifes so who really knows.
 
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I don't think lifting caused this defect
From a material perspective I must add that steel subjected to a single case of high load does not rupture like in the pics. Steel is too ductile to break like this. Steel can snap, but first it will deform (elongate) . Only when then the load is not taken away or released by deformation and subsequently taken by other components, will it snap.
In the pictures I do not see any deformation (waves/stretchmarks) in the part, nor in surrounding parts.
To my eye, it strongly resembles fatigue stress behavior. As pointed out by Mulberryworks, the crack starts in the corner, which is also indicative.

The 'half cilinder' in which the diff hangs, is subjected to rotational forces around the car's axis. After the crack appeared, the cilinder opened up by the trans pulling the side of the cilinder upwards.

Check for people that actually have a driving car: does the driveshaft (propshaft) turn anti clockwise when looking back to front? If so then my theory would hold. If not, then I'd better stick to welding...
I won't hesitate and lift it next time in the same way.
 
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eriknetherlands said:
I don't think lifting caused this defect
From a material perspective I must add that steel does not rupture like in the pics. Steel is too ductile to break like this. Steel can snap, but first it will deform (elongate) . Only when then the load is not taken away, released by deformation and subsequently taken by other components, will it snap.
In the pictures I do not see any deformation (waves/stretchmarks) in the part, nor in surrounding parts.
To my eye, it strongly resembles fatigue stress behavior. As pointed out by Mulberryworks, the crack starts in the corner, which is also indicative.

The 'half cilinder' in which the diff hangs, is subjected to rotational forces around the car's axis. After the crack appeared, the cilinder opened up by the trans pulling the side of the cilinder upwards.

Check for people that actually have a driving car: does the driveshaft (propshaft) turn anti clockwise when looking back to front? If so then my theory would hold. If not, then I'd better stick to welding...
I won't hesitate and lift it next time in the same way.
Click to expand...

Prop shaft turns in both directions depending on whether you are in one of the forward gears or reverse.
 
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