[teaston]

Originally Posted by verysideways

LSD is trying to keep wheel speeds on both rear wheels the same. There is some "drag" associated with making one wheel turn more than the other in any kind of limited slip diff.
Net result is that it forces the car to understeer more than it would otherwise.

Much more noticeable on lighter cars like the Atom, but even so it's still a factor in heavier cars.

And much more noticeable in cars with Ackerman geometry at low speeds (having added limited slip diffs to various cars which came with open diffs!).

More good info! And would explain why it seems everyone suffering with this is from UK, where the M Diff is standard on the M40i/d!

I wonder if it has anything to do with the excessive outside shoulder wear on my front left tyre?

[verysideways]

Originally Posted by teaston

More good info! And would explain why it seems everyone suffering with this is from UK, where the M Diff is standard on the M40i/d!

I wonder if it has anything to do with the excessive outside shoulder wear on my front left tyre?

Do you ever hoof it around roundabouts?

[HighlandPete]

Originally Posted by teaston

More good info! And would explain why it seems everyone suffering with this is from UK, where the M Diff is standard on the M40i/d!

I wonder if it has anything to do with the excessive outside shoulder wear on my front left tyre?

I'm not seeing an 'on demand' diff being the issue for the tire skipping when on full lock, like when parking. Many vehicles do it, and are simple 2WD setups.

The one N/S tyre 'outer shoulder' wear pattern is very common (in the UK). Alignment may be in tolerance, can simply be the roads we drive. We have quite a lot of road camber on some of our roads up here. See the wear on the N/S tyre shoulder on lots of vehicles. Any need for camber correction contributes to it, as does a lot of roundabout use.

I recall my local VW dealer chatting to me about tyre wear and some locals commenting on a similar tyre wear issue. A new roundabout had be built and he'd first ask them if they were regularly using it? Due to the road system, many would be driving 270° or 360° around it. Most times that was a contributing factor.

BTW, my son's E83 X3 35d would always wear as your example. Certainly check the alignment, but don't be surprised if it is all in tolerance.

[clivem2]

Surely the skipping at trickle speed isn't the diff, it can't be doing anything when I'm going super slow reversing carefully out of my drive? Must the geometry but what do I know!

[teaston]

Originally Posted by verysideways

Do you ever hoof it around roundabouts?

Not particularly, no. I don’t really hoof it anywhere on public roads these days to be honest.

[teaston]

Originally Posted by HighlandPete

I'm not seeing an 'on demand' diff being the issue for the tire skipping when on full lock, like when parking. Many vehicles do it, and are simple 2WD setups.

The one N/S tyre 'outer shoulder' wear pattern is very common (in the UK). Alignment may be in tolerance, can simply be the roads we drive. We have quite a lot of road camber on some of our roads up here. See the wear on the N/S tyre shoulder on lots of vehicles. Any need for camber correction contributes to it, as does a lot of roundabout use.

I recall my local VW dealer chatting to me about tyre wear and some locals commenting on a similar tyre wear issue. A new roundabout had be built and he'd first ask them if they were regularly using it? Due to the road system, many would be driving 270° or 360° around it. Most times that was a contributing factor.

BTW, my son's E83 X3 35d would always wear as your example. Certainly check the alignment, but don't be surprised if it is all in tolerance.

Thanks Pete, I think I may just swap my front wheels over when the tyres are half worn, to overcome the shoulder wear issue.

Will hopefully get my winter wheels on in the next couple of weeks which should give some more info on the skipping.

[chassis]

If an on demand differential has a pre-load or "static bias", torque buildup will occur in shafts connected to said differential, when unequal rotation is present. Such as in a low speed, full lock turn. This means Ackerman geometry and torque bias are present. The differential referred to can be either front differential or center differential.

If no static bias exists, then Ackerman geometry is the only factor in play for crabbing, or stick-slip of tires.

Static bias can be achieved with a friction clutch pack and passive spring in an on demand differential, or it can be implemented using a "torsen" or helical gear differential. Looking at diagrams of the xDrive center differential, a clutch pack is used. It wouldn't surprise me if there isn't a small amount of preload on the clutch pack to improve speed of response by elminating take up travel in a non-preloaded clutch pack.

The VW 4MOTION system uses torsen differentials, and a groaning sound occurs on low speed full lock turns. This is the high friction gears sliding in the torsen mechanism, caused by torque buildup because of Ackerman geometry.

In summary: this phenomenon is caused by Ackerman geometry, and can be accentuated by driveline design and tire size, tread design, rubber compound, age and road surface

[atrueb]

I get that tire slip on my front left with my Sommer tires (p zero) but not on my winter tires (conti). It occurs on full or near full lock right at slow speeds. As both tire formats are identical, I'm guessing that it is the higher tread on the winter tires that absorb this issue. When asked by people in my car what that sound is, I would say it's the massive torque making the wheel slip:-D lol...

[clivem2]

Thanks for all the explanations. It seems there's little play in transmission line and Ackerman geometry is unlikely to 100% accurate. With the wheels at slightly differing angles to track a turn properly any minor error will be significantly amplified at full lock. Tyres may take up the error or in other cases not.

[verysideways]

Originally Posted by HighlandPete

I'm not seeing an 'on demand' diff being the issue for the tire skipping when on full lock, like when parking. Many vehicles do it, and are simple 2WD setups.

My rear diff isn't "on demand", it's the M diff (i.e. a plate diff), which means it is ALWAYS inclined to go straight.
Certainly the issue is mostly because of the Ackerman geo, but a limited slip diff exaggerates the issue.

[teaston]

Also, I'd like to add that since fitting spacers the crabbing has been happening more. I guess because widening the track means there needs to be even more difference in rotation speed between the opposite wheels?

[chassis]

Spacers significantly affect Ackerman geometry, or geometry error. Spacers increase the tread width, or track width if you like.

Wheel cut (steer) angle is a permanent feature of the vehicle, and cannot be changed, unless the steering rack, tie rods/track rods, and knuckle/spindle/upright are changed.

With no change wheel cut angle, or in the steering or suspension components, adding spacers deviates from the manufacturer's Ackerman design. Any errors or shortcomings in the manufacturer's design will be increased with the addition of spacers.

In any turn, each of the four tires rotates with its own angular velocity, and because of this its own angular displacement over time and distance. At full lock, or minimum turning radius, this is the case where individual wheel angular velocity differences are the greatest. The accumulation of different angular displacement over time and distance is absorbed by open differentials, all three of them.

However, joints (cardan or CV), non-open differentials and clutch packs affect this, so torque buildup in the shafts can occur. Torque buildup is felt by the tire, and when the built up torque exceeds the capacity of the contact patch to contain it, the tire slips/skips/crabs/scrubs.

[X3M40i_Nev]

Originally Posted by clivem2

It is with summer tyres. Temp outside is 7C to 8C. I can imagine that at 15C there would be more grip.

7 deg C, time for winter tyres ❄️

[clivem2]

Originally Posted by X3M40i_Nev

7 deg C, time for winter tyres ❄️

Depends on how winter goes, it barely goes lower at the times I drive. I don't commute and mostly drive in the day time.

If we get a hard winter then I'll have wished I got a winter setup. Probably needed in my situation 1 year in 8 or 10. Sods law it'll be a hard winter!

[HighlandPete]

Originally Posted by verysideways

My rear diff isn't "on demand", it's the M diff (i.e. a plate diff), which means it is ALWAYS inclined to go straight.
Certainly the issue is mostly because of the Ackerman geo, but a limited slip diff exaggerates the issue.

If you are talking of the M-Diff (GHAS) as in the M2/3/4/5 and optioned on the G01, it is demand-controlled (on-demand). The control motor can 'back wind' to open the diff. It can function as a conventional open diff. What is the demand in normal 'full lock' parking manoeuvres?

The M-diff has specific working parameters, a bit like the xDrive transfer clutch.

[chassis]

Looks like the M-diff in the photo is a typical ball and ramp actuated clutch pack differential. A conical spring washer can be seen, which suggests a static bias. If a static bias is present, friction and therefore some torque will be transferred at all wheel speed differences, including parking speed at full steering wheel lock. Torque transfer above the static bias would depend on the electric motor actuation, which is software controlled.

[HighlandPete]

Originally Posted by chassis

Looks like the M-diff in the photo is a typical ball and ramp actuated clutch pack differential. A conical spring washer can be seen, which suggests a static bias. If a static bias is present, friction and therefore some torque will be transferred at all wheel speed differences, including parking speed at full steering wheel lock. Torque transfer above the static bias would depend on the electric motor actuation, which is software controlled.

It is ball and ramp. BMW state the active M-diff can behave as a conventional open diff. How can it with a static bias?

[chassis]

Look closely at the photo. There is what appears to be a conical "Belleville" washer in the assembly. This keeps the clutch plates in contact, or nearly so. This can be done to reduce response time and to account for wear over time.

I don't know for certain if there is static bias. However, in looking at the photo, the design by virtue of the conical washer lends itself to static bias.

[HighlandPete]

Originally Posted by chassis

Look closely at the photo. There is what appears to be a conical "Belleville" washer in the assembly. This keeps the clutch plates in contact, or nearly so. This can be done to reduce response time and to account for wear over time.

I don't know for certain if there is static bias. However, in looking at the photo, the design by virtue of the conical washer lends itself to static bias.

According to the BMW schematic and description, the Belleville disc spring is to open the lock if the engine is not supplied with power.

[teaston]

Some serious technical discussion going on here guys, I like it, keep it going!

It's way beyond my level of knowledge now though!!

[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.

[HighlandPete]

Originally Posted by teaston

Some serious technical discussion going on here guys, I like it, keep it going!

It's way beyond my level of knowledge now though!!

What I'm looking for, is whether the diff is open at the speeds and in the conditions where you guys are on full lock and get the crabbing and skipping of a front wheel.

BMW don't seem to spell it out, if the diff is open in those conditions, in any documents I can source. At the same time, an active diff (in those conditions), doesn't specifically fit any of the parameters for it being active. I'm minded the speed being too slow for typical DSC function and intervention. DSC feedback is a main source of data collection, which is used for determining the diff locking forces.

We know the transfer clutch is open, BMW do spell that out. So no drive torque to the front wheels, would be logical for the diff to be open as well, in such slow speed turning manoeuvres.