[Pete_vB]

Originally Posted by M3 Adjuster

Clearly I'm no better than the Stig....I couldnt gather this one up.... It definitely has had me scratching my head.

Very interesting... Definitely looks nearly identical to the off the Stig had. So I don't know what's going on here, but I can theorize... It's very difficult to tell what caused it, but it's almost certainly magnified by some kind of interaction with the car that causes instability.

If you look closely at the video you can see both rear tires laying stripes, the rear end fairly loose as you'd get coming hot into a decreasing radius corner, which is expected. The main issue seems to be caused by the way the rear end "bites" when the car straightens out. Watch the skid marks and the path of the rear tires- the correction causes the rear to snap violently back in line.

In shock terms we're seeing load go onto the inside rear (bump), and off the outside rear and outside front (rebound).

In this thread I discussed what BMW appeared to be doing with the shocks and springs vs the M3: they significantly increased the stiffness of the rear shocks, probably in an attempt to promote both corner entry oversteer to help the car rotate, and corner exit understeer to help put power down.

In the case above, however, when you're not on power it looks to me like that same shock tuning probably becomes at least part of the issue. The 1M has maybe 15% higher rear low speed compression than the M3, 2x the low speed rebound, both of which increase rear end grip when the car is unloading (the table here helps explain why), so when the rear shocks start moving that way the rear gains lots of grip and "snaps" back into line and then overshoots. One part of the theory...

Originally Posted by M3 Adjuster

With track tires on my car, the car is pretty much worthless in US MDM mode, because you can get the light blinking warming the tires up in the paddock.

What sizes are you running, by the way?

Originally Posted by M3 Adjuster

I haven't noticed CBC yet.. but need more time on track and probably won't know for sure until I put track pads on again.

CBC is certainly another possible explanation, for this spin at least, but I now suspect the diff more:

Originally Posted by Advevo

If you're in a drift never back off completely off throttle. What happens then. You're diff is going from lock to open again and you get instant grip in you're counter steering. The car wil go very hard the other way and you loose the back end.

Thanks for jumping in here, very interesting also...

This is the other possible explanation for M3 Adjuster's spin, or at least that initial violent correction when the rear catches. That unlocking might also significantly increase rear end grip. The question is, was the rear end catching also accompanied by lifting out of the throttle?

The video shows the rear out of shape four separate times, and I don't know that every one of the slides could have been accompanied by a throttle lift because I suspect at some point you backed out? Thus I think there is more than just the diff going on, but it could certainly have contributed to the first jump.

Originally Posted by Advevo

When you start a drift you have half a second inside wheelspin and then diff locks quite extreme you need to be fast on the counter steering otherwise you loose the car.

This sounds exactly like what Scott originally described. That event sounds different than the Stig or M3 Adjuster's event above. Power on, then violent lockup punts the car sideways.

On balance I'm suspicious that in both of these cases we have two or more events conspiring together, as usual in an accident. It would make sense that in both cases the diff acted violently, either locking or unlocking, and then the stiff rear shock turning contributed to making the resulting slide much harder to catch.

The factory seems to know the diff can be an issue. Not only Dackelone's info from the factory guy, but also the M5's diff and new M3: they add an electric actuator to make the diff function active and under computer control. They would only add this weight, complexity and cost if they felt passive was a problem, letting them regulate the locking and unlocking.

It's just a theory, but it does seem to fit the facts- violent diff lockup and stiff rear shock tuning together could be expected to cause both issues.

So, Scott, my current best guess, for your use, is that you should be looking for another diff, though I suspect it's not a cure-all.