[chassis]

Pretty sure this is the M Diff, by GKN.



https://www.gknautomotive.com/global...t/etm-engl.pdf

[HighlandPete]

Originally Posted by chassis

HighlandPete, how do you reason the Belleville washer is meant to open the clutch plates?

Look closely at the photo. One side of the Belleville bears against the plate carrier splined to one of the wheel shafts. The other side of the washer bears against the rotating half of the ball-ramp mechanism. There can only be force applied by the Belleville that separates these two pieces. When the aforementioned pieces are separated, pressure is applied to the clutch plates. Since in the photo there is no discernible space, or clearance, between the Belleville and its mating pieces, it is reasonable to think there is a preload, at least a small one, in the system. If nothing else, preload avoids rattling and unecesary wear. More favorably, preload contributes to quick reaction time, measured in milliseconds.

I also am struggling with there being 'no' preload, however small, but BMW are the ones saying it acts as a conventional open diff. That implies negligible drag.

What I have noted in the description of start up calibration, BMW state the open value = 0 lb-ft. I've have to find the reference, to see the full context of using that figure.

[chassis]

In practical terms, the preload could be small enough that there is negligible torque transferred between rear wheels. In practical terms, the stack of components in line with the clutch plates needs to be in a known position for the control system to function. In industry this is commonly achieved via a spring preload, such as a Belleville washer in this case.

To be clear, I have been talking about the rear differential for the past few posts, when the topic of M-Diff was first introduced.

In the case of full steering lock low speed parking maneuvers, left and right wheel speeds on both front and rear axles are different, and both front and rear differentials are doing just that, differentiating. There is some differentiating going on in the center differential also, because the front two wheels are not turning at exactly the same speeds as the rear two wheels.

[teaston]

I have now fitted my winter wheels/tyres, still with the 18mm spacers, and can confirm that there is absolutely no tyre skipping at all now!!

[clivem2]

Originally Posted by teaston

I have now fitted my winter wheels/tyres, still with the 18mm spacers, and can confirm that there is absolutely no tyre skipping at all now!!

Hopefully that's a good thing, thanks for the report!

[clivem2]

Maybe I should explain my "good thing" comment. If skipping is mostly to do with geometry is it better the tyres skip vs they don't and the transmission takes the stress?

[teaston]

Originally Posted by clivem2

Maybe I should explain my "good thing" comment. If skipping is mostly to do with geometry is it better the tyres skip vs they don't and the transmission takes the stress?

I see what you mean, but it is very low speed/torque so I doubt there would be much strain.

Tbh I think the Pirelli’s are just crap, could be why they’re so cheap, only £120 online, for what is a very recently introduced 20” BMW tyre size!