[clivem2]

When I'm manoeuvring at very low speed on full or close to full lock I find the front tyres "skip" or jump a little as though the amount of lock is too much for them.

Is this typical? I hope it's not like the Merc GLC right-hand drive issue....

[dylan1357]

Happens with my X3MC... I assumed it was normal due to the weight of the car, only happens when I'm on an incline from standstill whilst turning

[teaston]

Yes I get a bit of "crabbing" on full lock sometimes, I think it depends on the angle of the road and also if it's wet.

[clivem2]

Thanks, that's sort of reassuring. At least it's not unusual.

[rajp]

Originally Posted by clivem2

When I'm manoeuvring at very low speed on full or close to full lock I find the front tyres "skip" or jump a little as though the amount of lock is too much for them.

Is this typical? I hope it's not like the Merc GLC right-hand drive issue....

I had a GLC 43AMG and it loved to Crab.... after a year or so, they replaced the tyres and it pretty much fixed it.. my X3M40i does not seem to show any of those characteristics... even on full lock..

[chassis]

Crabbing is related to drivetrain differential locking strategy and steering Ackerman geometry. Two different mechanical phenomena with similar symptoms and both related to vehicle design.

[teaston]

Originally Posted by rajp

I had a GLC 43AMG and it loved to Crab.... after a year or so, they replaced the tyres and it pretty much fixed it.. my X3M40i does not seem to show any of those characteristics... even on full lock..

I think it could be to do with the tyres, the P Zeros are great anyway, but I think the crabbing has been worse since the tyres have been more worn.

[clivem2]

Originally Posted by teaston

I think it could be to do with the tyres, the P Zeros are great anyway, but I think the crabbing has been worse since the tyres have been more worn.

Mine are P Zeros (non RFT). Brand new though...

The P Zeros on my F25 didn't do this.

[teaston]

Originally Posted by clivem2

Mine are P Zeros (non RFT). Brand new though...

The P Zeros on my F25 didn't do this.

Yep mine are the non RFT too. I'll be putting my winter wheels on soon, so we'll see if they also do it.

[verysideways]

Originally Posted by chassis

Crabbing is related to drivetrain differential locking strategy and steering Ackerman geometry. Two different mechanical phenomena with similar symptoms and both related to vehicle design.

This.
Anyone want more details?
Google "Ackerman geometry".

[Jurgen Belgium]

Originally Posted by clivem2

When I'm manoeuvring at very low speed on full or close to full lock I find the front tyres "skip" or jump a little as though the amount of lock is too much for them.

Is this typical? I hope it's not like the Merc GLC right-hand drive issue....

Hi,
Can i ask at what outside temp this happens?
Greetz,
Jurgen

[clivem2]

Originally Posted by Jurgen Belgium

Hi,
Can i ask at what temp this happens?
Greetz,
Jurgen

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

[teaston]

Originally Posted by verysideways

This.
Anyone want more details?
Google "Ackerman geometry".

So the inside wheel turns more than the outside wheel! I didn't know that.

[HighlandPete]

Originally Posted by teaston

So the inside wheel turns more than the outside wheel! I didn't know that.

Clearly a few compromises with the G01 setup. We won't go down the double wishbone discussion route. BMW have to save money somewhere.

You mentioned in an earlier post you notice crabbing a more with worn tires. Less tread flex, (different tires will respond with more or less skip anyway) to absorb the less than perfect geometry. Same as tires could make a difference on the Mercs.

[verysideways]

All my 911's have done this too - when doing full lock turns, there is always some skip. Combination of Ackerman and limited slip diff.

[clivem2]

Originally Posted by verysideways

All my 911's have done this too - when doing full lock turns, there is always some skip. Combination of Ackerman and limited slip diff.

Does it chew up the tyres? Thinking about it my F25 did it on occasion. The G01 does it every time I reverse out of my drive. Twice...in reverse and then when driving off.

[teaston]

Originally Posted by verysideways

All my 911's have done this too - when doing full lock turns, there is always some skip. Combination of Ackerman and limited slip diff.

How does the LSD contribute to it?

[clivem2]

Originally Posted by teaston

How does the LSD contribute to it?

Presumably the LSD doesn't allow one wheel to "slip" sufficiently, something has to give so it's the tyre. That's my uneducated guess.

[teaston]

Originally Posted by clivem2

Presumably the LSD doesn't allow one wheel to "slip" sufficiently, something has to give so it's the tyre. That's my uneducated guess.

But it's the front wheels that skip, and the LSD is only on the rear isn't it?

[clivem2]

Originally Posted by teaston

But it's the front wheels that skip, and the LSD is only on the rear isn't it?

I wondered about that, I don't know what the connection is the rear...

[HighlandPete]

Originally Posted by clivem2

I wondered about that, I don't know what the connection is the rear...

Shouldn't be any 'traction' connection during the manoeuvres where the tires are noted to skip.

According to BMW blurb on xDrive function, permanent all-wheel drive is cancelled in 3 control cases:

This is one of them.... (the other situations are high speed and severe understeer).

during very tight cornering manoeuvres with little engine torque to permit speed compensation between the front and rear axles (e.g. parking)

Tendency to skip, points to the steering geometry on full lock.

[verysideways]

Originally Posted by teaston

How does the LSD contribute to it?

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!).