[Tambohamilton]

But either with or without the brace, the differential is wholly mounted to the rear subframe; 100% of the drive load and torque is routed in the same direction through the subframe. One way the diff can flex a bunch, the other way it can't (as much)...but the force still goes through the subframe.

...what gives? Detailed explanations welcome!

[feuer]

Originally Posted by Tambohamilton

But either with or without the brace, the differential is wholly mounted to the rear subframe; 100% of the drive load and torque is routed in the same direction through the subframe. One way the diff can flex a bunch, the other way it can't (as much)...but the force still goes through the subframe.
...what gives? Detailed explanations welcome!

Incorrect. Study how the diff is mounted. Where is mounted. Length of half axles. Wheel hop. Broken diffs. Broken half axles and etc....

[Tambohamilton]

Care to explain?

[feuer]

Originally Posted by Tambohamilton

Care to explain?

You can study. Start with this:

[Angel67]

Unequal shaft length = unequal torque angle of twist, assuming same shaft radius ...

But that will be true with or without brace.

a

[Angel67]

Strange picture. Wasn't the 335is first produced in 2011, not 2007?

a

[Tambohamilton]

Originally Posted by feuer

You can study. Start with this:

Come on, spell it out already. I know what the underside of our cars looks like.

[feuer]

Originally Posted by Angel67

Unequal shaft length = unequal torque angle of twist, assuming same shaft radius ...

But that will be true with or without brace.

a

Are you talking about twisting the body of the half axles themselves? I'm talking about the diff wanting to twist (rotate). If the diff is mounded solid via 4th mount (diff brace or diff lockdown) that will provide lets's assume at least equal torque in opposite direction which will stop the diff rotation. If the diff is moving, wanting to twist (rotate) it will transmit all of these movements to the rest of the vehicle.

[feuer]

Originally Posted by Tambohamilton

Come on, spell it out already. I know what the underside of our cars looks like.

The diff isn't mounted solid. Has only 3 mounts that are also not solid but rubber bushings allowing flex. So is lacking 4th mount, thus wanting to twist. This is an action. Every action has a reaction. So when diff is twisting is loading the tires differently. It will lift one side while push down the opposite. There are different remedies such: different shock setting side to side, different suspension pre-load side to side, or a diff brace.

[feuer]

Guy who set up the rear end properly:

[feuer]

Guy who did not:

[Tambohamilton]

Originally Posted by feuer

The diff isn't mounted solid. Has only 3 mounts that are also not solid but rubber bushings allowing flex. So is lacking 4th mount, thus wanting to twist. This is an action. Every action has a reaction. So when diff is twisting is loading the tires differently. It will lift one side while push down the opposite. There are different remedies such: different shock setting side to side, different suspension pre-load side to side, or a diff brace.

Yep, the diff twists in the subframe, but how do the driveshafts transfer this movement of the diff into the suspension? That's the bit that doesn't add up for me.

You're posting pictures of cars which probably have solid rear axles, which is completely different from the independent rear suspension on our cars. On those cars the diff is rigidly connected to the rear hubs; any movement of the diff is movement of the hub. On ours, there's no rigid connection between them.

[feuer]

Originally Posted by Tambohamilton

Yep, the diff twists in the subframe, but how do the driveshafts transfer this movement of the diff into the suspension? That's the bit that doesn't add up for me.

You're posting pictures of cars which probably have solid rear axles, which is completely different from the independent rear suspension on our cars. On those cars the diff is rigidly connected to the rear hubs; any movement of the diff is movement of the hub. On ours, there's no rigid connection between them.

I posted pictures so people can visualize what is happening, not to make direct correlation or compassion between the two rather obviously diferent rear ends. Diff will generate torque twist in both, hen se both setups utilizing sway bars. To your question: Where is the diff mounted to? Subframe. Where is the subframe mounted to? The unibody. Where are the suspension components mounted to? Subframe and unibody. Is all connected. The torque (twist) will affect all. With fixed diff (diff brace (or solid rear axle)) you have one less viriable to sort out. People use diferent tire pressure side to side, diferent dampening settings, some even diferent spring rate, but after the diff twist movement issue is addressed.

[Angel67]

Originally Posted by feuer

Are you talking about twisting the body of the half axles themselves? I'm talking about the diff wanting to twist (rotate). If the diff is mounded solid via 4th mount (diff brace or diff lockdown) that will provide lets's assume at least equal torque in opposite direction which will stop the diff rotation. If the diff is moving, wanting to twist (rotate) it will transmit all of these movements to the rest of the vehicle.

I think the other factors you mention are more important than a brace. Brace or no brace the action will provoke a reaction. The question is how are those forces spread. The brace probably helps translate the forces better to the subframe and wider and everything adds up so it's good but probably not enough on it's own.

[feuer]

Originally Posted by Angel67

I think the other factors you mention are more important than a brace. Brace or no brace the action will provoke a reaction. The question is how are those forces spread. The brace probably helps translate the forces better to the subframe and wider and everything adds up so it's good but probably not enough on it's own.

Yes and those forces are powerful. Look at the the issue e36 and e46 had. The subframe was getting the unibody ripped due to weak and narrow (not spread out further apart) subframe mounting points. Stiff suspension was a contributor too but ussually was happening during hard launches.

[Angel67]

Makes sense. That's why outboarding shocks works. But also why I asked what engine/tranny mounts are used. If diff brace and stock tranny/diff mounts then forces go there and rear end will be more stable but torque will stress engine/tranny mounts and not translate to optimum power to the wheels.

I have a 335xi and have used poweflex subframe bushings, with 335is engine mounts and stock diff bushings. I am sacrificing some power delivery to protect the unibody...

a

[Angel67]

Sorry, meant stock engine/tranny mounts.

Note that xi does not really have soft tranny mounts, so a bit different...maybe better in some ways.

a

[feuer]

Originally Posted by Angel67

Makes sense. That's why outboarding shocks works. But also why I asked what engine/tranny mounts are used. If diff brace and stock tranny/diff mounts then forces go there and rear end will be more stable but torque will stress engine/tranny mounts and not translate to optimum power to the wheels.

I have a 335xi and have used poweflex subframe bushings, with 335is engine mounts and stock diff bushings. I am sacrificing some power delivery to protect the unibody...

a

Outboard shocks are longer and experience less flex so they can get away with less dampening which means less heat and less wear thus more longevity. The IS got defferent right engine mount probably because of how the dual clutch transmission transmits the torque during gear shifts. I have two right IS engine mounts on my 335i and 635csi transmission mounts. It has been ok. I know xDrive transmission mount is different. Some people fill the void with polyurethane. I would worry more about the front diff breaking than damaging the chassis if I have xDrive.

[Tambohamilton]

Originally Posted by feuer

I posted pictures so people can visualize what is happening, not to make direct correlation or compassion between the two rather obviously diferent rear ends. Diff will generate torque twist in both, hen se both setups utilizing sway bars. To your question: Where is the diff mounted to? Subframe. Where is the subframe mounted to? The unibody. Where are the suspension components mounted to? Subframe and unibody. Is all connected. The torque (twist) will affect all. With fixed diff (diff brace (or solid rear axle)) you have one less viriable to sort out. People use diferent tire pressure side to side, diferent dampening settings, some even diferent spring rate, but after the diff twist movement issue is addressed.

We're still no further forward. The pictures demonstrate perfectly that even with the diff solidly mounted to the rear axle/subframe, there are still forces going in unfavourable directions, which need to be resolved...so why does solidly mounting our diff's seem to fix anything? I still need an explanation for how any force from the diff flexing in the subframe is transferred into the suspension or drivetrain.

[streetpro]

Most if not all diffs have 3 point mounting pattern because of freedom of movement, vibrations etc. I am sure that Bmw could easily do 4 point pattern if needed/beneficial. That is where I am puzzled with our brace thing, or we could call it poor mans band aid to something else that needs to be addressed...
Anyway when you take a look at these few examples - f.ex. e36 platform I am familiar with, lesser machinery is fine with upper style brace however M3 3,2 (Euro version was much like a racing car, dry sump, 100PS/litre engine, refined little details all over the vehicle incl. this diff)... brace is down, unequal arms, front bolt in a weird (actually in a direction of reaction forces) angle...

This all makes me wonder why they went back to that "e30" style mount - 2 bolts up front, one rear silentblock... to save money?

[feuer]

Originally Posted by Tambohamilton

We're still no further forward. The pictures demonstrate perfectly that even with the diff solidly mounted to the rear axle/subframe, there are still forces going in unfavourable directions, which need to be resolved...so why does solidly mounting our diff's seem to fix anything? I still need an explanation for how any force from the diff flexing in the subframe is transferred into the suspension or drivetrain.

omg I don't know. Why don't you tell us? I'm obviously clueless.

[feuer]

Originally Posted by streetpro

Most if not all diffs have 3 point mounting pattern because of freedom of movement, vibrations etc. I am sure that Bmw could easily do 4 point pattern if needed/beneficial. That is where I am puzzled with our brace thing, or we could call it poor mans band aid to something else that needs to be addressed...
Anyway when you take a look at these few examples - f.ex. e36 platform I am familiar with, lesser machinery is fine with upper style brace however M3 3,2 (Euro version was much like a racing car, dry sump, 100PS/litre engine, refined little details all over the vehicle incl. this diff)... brace is down, unequal arms, front bolt in a weird (actually in a direction of reaction forces) angle...

This all makes me wonder why they went back to that "e30" style mount - 2 bolts up front, one rear silentblock... to save money?

Yes, freedom of movement, comfortable ride, cost, packaging...all are probably factors. Newer M diffs in F chassis, like in m3/4 and m4 come with additional mounting bracket that mount the diff to further apart points to combat the twist forces. Older M5 diff are know to break the subframe mounting tabs. Newer M5 are braking the diff mounting brackets. Here are some posts:
https://www.m5board.com/threads/rear...repair.599470/
https://f10.m5post.com/forums/showthread.php?t=1542282
This is not only BMW issue. Plenty of other vehicles experience the same.