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Cracks / Fractures appearing in Body Behind Rear Folding Seats

7K views 36 replies 17 participants last post by  Enrico 2000 
#1 ·
Hi There,

Wondering if anyone has ever seen this on their e39 M5. Found these behind the rear fold down seats when they were removed. Cracks are appearing on both sides of opening behind the seats. They look like stress fractures due to torsion / body flex... Thoughts?

The car has a rear strut brace, ground control coilovers, and the bushings and the rear subframe are examined regularly. No signs of an issue in those components. No rust anywhere on the car (CA / Washington for its whole life).

If you have seen this before, what was the solution? An X Brace and reinforcement plates?
 

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#2 ·
Can you take a shot from further back to show where these areas are?

What spring rate do you have?
 
#4 ·
#3 ·
Photo is attached. The sun in this photo makes it hard to see on the right side, but the placement on that side is exactly the same as the left which can be seen clearly.
 

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#5 ·
I hven't seen that, but I haven't seen enough e39s with the rear seats removed to know if it common. At least it will be easy to fix :)
 
#6 ·
Wow. How many miles? How many hard track miles??
 
#7 ·
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#8 ·
Could be. I think the first step is to clear those areas of paint/coating. It could just be cracked paint but that does mean there was significant flex there which is interesting to say the least.
 
#9 ·
Driver side I cannot see if it goes beyond paint. On the passenger side, however, it clearly goes through the metal and the weld when you look at it closely.
 
#11 ·
Have you inspected other welds around the car (e.g. subframe, diff mounts, etc.)? I'm very curious how hard you're throwing the car around the track. It's almost inconceivable this would be due to daily driving.
 
#12 · (Edited)
Absolutely related to track driving. Thousands of track miles / laps, countless berms, tight hairpins, steep cork scews, and wide sticky tires. I usually run pretty evenly with high horsepower gutted CTS-Vs, STIs and S4s running race fuel so I guess I am throwing it around pretty hard.

Car is inspected for this stuff regularly. Will reinspect diff mounts and sub frame in light of these findings...
 
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#15 ·
True. I think I worked out a plan tonight. Reenforcement plates and a diagonal brace to square up the box.

I will also get an opinion from a body shop welder about the actual repair of the cracks.

Worst case I can always cage the car :)
 
#16 ·
I wonder if the e39 ring taxi was scraped only after 5,000kms due to similar chassis issues. Your car since it has the fold-down rear seat option, would have slightly less torsional rigidity then a non fold down rear seat equipped M5. Combined with the rear strut brace and track good times, the chassis flexing tendencies are off set by the added rigidity of the rear strut brace. In turn that area where the cracks have started looks to be a weak point in the chassis. At least its an easy fix, unlike the e46!
 
#17 ·
I wonder if the e39 ring taxi was scraped only after 5,000kms due to similar chassis issues. Your car since it has the fold-down rear seat option, would have slightly less torsional rigidity then a non fold down rear seat equipped M5. Combined with the rear strut brace and track good times, the chassis flexing tendencies are off set by the added rigidity of the rear strut brace. In turn that area where the cracks have started looks to be a weak point in the chassis. At least its an easy fix, unlike the e46!
Oh really? I am interested in hearing about this.
 
#19 ·
It is possible but I doubt that was the root cause. Wish I new precisely when it started happening.
 
#21 ·
Ok. An update.

I have a new M5 (long story)... Car has 160K miles of which about 500 so far are track miles.

600# front and 400# rear spring rates.
No rear strut brace
Fold down rear seats removed


After just three track events the car has developed the same cracks.

Oh well - will fix these too.
 
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#23 · (Edited)
Ok. An update.

I have a new M5 (long story)... Car has 160K miles of which about 500 so far are track miles.

600# front and 400# rear spring rates.
No rear strut brace
Fold down rear seats removed


After just three track events the car has developed the same cracks.

Oh well - will fix these too.
Wow, that's amazing. I've not heard of this issue in all these years on the board. Fold-down rear seat bodies are clearly less stiff in torsion (although I've only seen the data for the E46, but it stands to reason the E39 would be no different in a relative sense), but simply being slightly less stiff in torsion wouldn't be the cause of this imo. The more likely suspect is that the load distribution in the rear chassis area with the fold-down seats is (obviously) different and is over-stressing the areas identified with the stiffer rear springs/shocks/sway bar/etc coupled with track use.

Do you need the opening to the trunk in the car? You might want to consider welding in a stiff panel across that opening, returning it to non-fold-down status, when you fix the cracks.

Chuck
 
#22 ·
Damn... I wanna see some footage of this track time. Maybe you should try and drive a non split fold seat optioned car similarly see if you develope similar issues.
 
#26 ·
Did you ever take pictures of the repairs you made to the last car? I'd be interested to see.

Also, I have a fold down seat optioned M5. I have 218,000 miles on it. I will be installing new struts and springs soon so my rear seat will be out. Hopefully I'll remember to check these... more from a curiosity perspective.
 
#27 ·
Me too, well, not the 218k part and my coil overs are already new-ish. I have never used the folding option but I see the utility of it. I never thought it might lead to structural failure or breakdown, though. I don't track my car so I'm not too worried in any case, but I will be checking anyway.
 
#35 ·
I love my fold down seats :)

With that said, I need rear strut bar!

Tire Automotive tire Motor vehicle Auto part Wheel



Paul :)
 
#36 ·
Yes turning the key fully in one direction (I can't remember which) will manually unlock the trunk and driver's door. I also love the utility of the fold down seats. I've used it many times doing exactly what Paul did, as well as trips to Home Depot, etc.

Back on topic, hopefully the OP has old photos or will update the thread with new photos of the current car and repair. Can you see this area by simply folding the seat down or pulling out the bench or is it only visible after removing them both?
 
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