A thought regarding diff subframe issues

[Sailor]

There has been substantial discussion of body cracks in the rear subframe mounting points, as most of you well know. This had been a problem for E36 as well as E46 cars, and seems to be correlated with how hard they are driven and/or tracked. I wonder if there is a correlation with whether cars have a limited slip differential as well. Wouldn't that put more torque on the bushings/bolts/mounting points? Is it my imagination that M cars with LSD seem to exhibit this problem more frequently - is some of this due to the combo of more power and less slippage as well as the likelihood that these cars are driven more aggressively? Going a step further, would swapping to an LSD without beefing the subframe mounting area possibly be asking for problems later, since those who do so are more likely to push their cars a little harder?

[Sockethead]

Yes to all of the above... anything you do to your car back there will exacerbate the problem...

[ZHPizza]

@slater gonna come up in here and throw down a sermon from the mount

[Sockethead]

That's because the Vincebar is the best solution out there right now. I can think of two other that are also good solutions but all three of them require structural modification

[BMWCurves]

@slater gonna come up in here and throw down a sermon from the mount

More like a WWE smack down of knowledge.

[slater]

There has been substantial discussion of body cracks in the rear subframe mounting points, as most of you well know. This had been a problem for E36 as well as E46 cars, and seems to be correlated with how hard they are driven and/or tracked. I wonder if there is a correlation with whether cars have a limited slip differential as well. Wouldn't that put more torque on the bushings/bolts/mounting points? Is it my imagination that M cars with LSD seem to exhibit this problem more frequently - is some of this due to the combo of more power and less slippage as well as the likelihood that these cars are driven more aggressively? Going a step further, would swapping to an LSD without beefing the subframe mounting area possibly be asking for problems later, since those who do so are more likely to push their cars a little harder?

great question/post. i've already written a ton on the topic on here, so i'm not going to reinvent the wheel... but to address the LSD question specifically, i'm not sure. you would think that it would provide more even power application, but that does not change the fact that when the rear diff is loaded, it pulls down on the left rear mount and pushes up on the front right. it will do this whether you have an LSD or not - and there have definitely been non-M, non-LSD cars that have had problems. however, if you have an LSD, as you mentioned... you likely are inducing some wheelspin. ;)

some RACP info/repair reading for you...

the Correct Solution:
http://www.zhpmafia.com/forums/showthread.php?18871-VinceBar-Sub-Frame-Reinforcement-Kit!

pics of said Correct Solution in my own car:
http://www.zhpmafia.com/forums/showthread.php?19240-peter-s-Touring-project/page87



@slater gonna come up in here and throw down a sermon from the mount

:hi

good pun, nate... ;)



That's because the Vincebar is the best solution out there right now. I can think of two other that are also good solutions but all three of them require structural modification

are you thinking of the mason 'bar'? (maze of tubes) it does not tie into the frame rails.... but if you were going to that type of installation, you might as well tie it into the frame rails. but that solution doesn't tackle the front mounts.



More like a WWE smack down of knowledge.

ha. i referred to previous rantings above, by linking. ;)

[Rovert]

:hi I'm Vincebarred and Vinceskinzed. But I haven't driven the car yet as I'm just waiting for a silly calliper boot to come in before I can install the callipers and put the car on the ground.

My friend's 2002 M3 has similar milage than mine, fully modded for handling that's AutoX'ed and driven hard showed no signs of cracks.

Meanwhile my car with far less mileage that has more RACP spot welds as it's a facelift, looked like it was slowly tearing itself apart on the bottom and top side. I do get the tail out when it's wet or snow out but I try hard not to just snap it as that shock isn't good for anything. But my car took some work to sew back together. The bushings and bolts and subframe were all fine. They're all built to handle it but the RACP is the weak point. Welding the RACP shut took very low power even on the 120v welder or else you'd punch a hole through the sheet metal. Might as well just call it thick cardboard!

I have a friend with a 2001 M3 with double the milage of me and he hasn't had any issues. He runs with traction control disabled permanently and so he's always playing but he hasn't any issues.

It seems to be random. I'm guessing once you get the right amount of forces going the right way, meaning Venus lines up with the Sun, the moon, Earth, and Uranus, your chassis gets ripped a new one and it just gets worse from there.

The engine turns the transmission which turns the driveshaft connected to the rear differential that is housed in the subframe which is bolted to the RACP. No matter what kind of diff, the driveshaft is still spinning the same way and putting a similar twisting force on the subframe thus attempting to twist it off of the RACP as your accelerate hard. The RACP will fail before the bolts break and shear. Hope that kind of helps.

[Sailor]

My thinking contained in the original question was that the LSD allows additional torque to be transmitted to the mounts, as compared to an open diff that can "bleed off" some torque through wheelspin on one side. That's my intuitive take on the forces involved - might not be valid.