[Charlie (Wessex)]

Been thinking....

How many xDrive vehicles do BMW sell in a year? How many people are religious about star marked? How many problems are reported?

A lot. Possibly not a huge number. A few.

And let's face it, if it was a common issue, you'd be mad to buy a used xDrive if you have no idea what tyres have been on it. Is there a big caveat about buying used xDrives?

[HighlandPete]

Tyre sizing mentioned today, definitely opens up the 'technical' debate for sure.

I try to keep up with the developments at BMW, but find the latest tyre observations and fitments do appear to contradict on many fronts.

Will need to re-read some of the data and technical details.

Issues it raises for me, is not just xDrive, but functions like DSC and ABS as well. (I remember the time when odd tyre treads, across an axle, could mess with DSC and AT gear change function).

The concept of xDrive is for variable 'front to rear' torque transfer. If the tyres are outside the 1% allowed for "tyre tolerance logic", it does pose the question what's the normal running state? How is the sensitivity of xDrive controlled with possibly wider limits?

"The tire circumference can fluctuate up to 1% or more as a result of mixed tires or wear. The tire tolerance logic decides depending on the driver's command and driving situation whether the slip is to occur in the transfer case clutch or at the contact area between tire and road.

If the slip is permitted in the transfer case clutch, the locking pressure set by the pre-control is reduced in order to keep the work loss low. In the driving dynamic control situation, the clutch is locked slightly more than normal, the four wheel drive is always guaranteed when required.

For maximum xDrive performance, tires (and wheels) of the same diameter should be installed on the vehicle."

Does a wider range of tyre sizing mean we run an "over open" clutch giving near 100% torque to the rear wheels? Only transferring to the front (or individual wheels) on demand?

Also DSC sensitivity... do BMW have to reprogram for the different tyre sets? The understanding is, all four wheels have/require a "baseline", (same for ABS), from which any deviation triggers intervention.

To me, the tyre options observed, (and as options) do muddy the waters. Definitely requires more understanding. I'm not seeing (at present) where BMW are getting the 'slack', with such sensitive systems.

For example, with different RC, front to rear, either the transfer clutch is open much more, (with the inherent heat management issues) or the tyres micro scuff.

Perhaps BMW are confident in their latest ATC13 transfer box oil flow and heat management improvements, to move to wider tolerances.

[verysideways]

Originally Posted by HighlandPete

Does a wider range of tyre sizing mean we run an "over open" clutch giving near 100% torque to the rear wheels? Only transferring to the front (or individual wheels) on demand?

Also DSC sensitivity... do BMW have to reprogram for the different tyre sets? The understanding is, all four wheels have/require a "baseline", (same for ABS), from which any deviation triggers intervention.

To me, the tyre options observed, (and as options) do muddy the waters. Definitely requires more understanding. I'm not seeing (at present) where BMW are getting the 'slack', with such sensitive systems.

For example, with different RC, front to rear, either the transfer clutch is open much more, (with the inherent heat management issues) or the tyres micro scuff.

Perhaps BMW are confident in their latest ATC13 transfer box oil flow and heat management improvements, to move to wider tolerances.

I think the resetting of the TPMS will help with the DSC calibration, which is why you need to do that reset when you change tyres or significantly alter pressures, etc.
And of course it has a steering angle sensor too so can do the maths for different rotations depending on cornering radius.

Oh, and the M sport pro pack on the M240i (with the different wheels and tyres) does also mention additional transmission cooling actually...

[HighlandPete]

Originally Posted by verysideways

I think the resetting of the TPMS will help with the DSC calibration, which is why you need to do that reset when you change tyres or significantly alter pressures, etc.
And of course it has a steering angle sensor too so can do the maths for different rotations depending on cornering radius.

Oh, and the M sport pro pack on the M240i (with the different wheels and tyres) does also mention additional transmission cooling actually...

Good point on the TPMS... I'd thought of that possibility, and is one area where I will be looking for explanation.

Interesting comment on the M240i, will try and get to the technical data.

[Charlie (Wessex)]

Originally Posted by verysideways

I think the resetting of the TPMS will help with the DSC calibration, which is why you need to do that reset when you change tyres

There was an option for Winter/All-season and summer. I put Cross Climates as sumemr, but maybe I should have put in All Season!

[bmnut]

Originally Posted by Charlie (Wessex)

There was an option for Winter/All-season and summer. I put Cross Climates as sumemr, but maybe I should have put in All Season!

I would definitely think cross climates would fall into the All Season category, on definition alone?

[Charlie (Wessex)]

Originally Posted by bmnut

I would definitely think cross climates would fall into the All Season category, on definition alone?

Yes but when you go and set the tyres, their is no 19" option for Winter?All-Season. Frankly this topic is just shambolic by BMW.

[bmnut]

Originally Posted by Charlie (Wessex)

Yes but when you go and set the tyres, their is no 19" option for Winter?All-Season. Frankly this topic is just shambolic by BMW.

'Aint that the truth!

[HighlandPete]

Originally Posted by bmnut

'Aint that the truth!

I'm working through some of the technical data (when I've got a few minutes spare). BMW are continuously improving the designs (as I've previously commented on) and are clearly changing the xDrive working parameters.

I've not got the complete picture across the models, but some of the M-Lite and M-cars (even in AWD mode) are definitely not using xDrive in the same way it was used in past generations of vehicles. Still piecing together how this is implemented in other xDrive models.

Just one example, which indicates how BMW can offer what appears 'odd' wheel combinations, is the new F42 M240i:

The rear-wheel drive bias of BMW xDrive optimizes both the efficiency and the dynamic properties of the compact BMW M model. In driving situations where all-wheel drive is not necessary, the power from the inline 6-cylinder engine is sent to the rear wheels only.

More to come...

[bmnut]

Originally Posted by HighlandPete

I'm working through some of the technical data (when I've got a few minutes spare). BMW are continuously improving the designs (as I've previously commented on) and are clearly changing the xDrive working parameters.

I've not got the complete picture across the models, but some of the M-Lite and M-cars (even in AWD mode) are definitely not using xDrive in the same way it was used in past generations of vehicles. Still piecing together how this is implemented in other xDrive models.

Just one example, which indicates how BMW can offer what appears 'odd' wheel combinations, is the new F42 M240i:


More to come...

This makes very interesting reading Pete and I'm looking forward to hearing more from the outcome of your further research!

[HighlandPete]

Originally Posted by bmnut

This makes very interesting reading Pete and I'm looking forward to hearing more from the outcome of your further research!

We've become accustomed to the BMW blurb, like the following.

If the multidisc clutch in the transfer case is activated, the drive torque will normally be distributed between the front and rear axle according to the BMW-typical characteristic (40/60%).

Or

In normal driving, the clutch is operated with minimum slip so that permanent four-wheel drive with a driving torque distribution of 40% on the front axle and 60% on the rear axle is available.

And

During normal straight-ahead driving, the front and rear axles are powered with the normal 40 to 60 force distribution.

With the ability...

...the drive torques are distributed variably in the range between theoretically 0 to 100 and 100 to 0 according to the driving situation.

Which means...

Continuous ongoing calculation of thermal loading models in the control unit of the transfer case (VTG) protect the transfer case from destruction as a result of overheating.

This is superseded, in some setups, with descriptions like.

Efficiency Mode is always used when there is no all-wheel drive request from the DSC control unit and the multidisc clutch is therefore open.

The torque distribution towards the front wheels can be completely cut-off in driving situations where all-wheel drive is not needed.

Meaning...

With an open multi disc clutch, the total torque is transmitted to the rear axle. In order to transmit the torque to the front axle, the multi disc clutch must be closed.

Are you getting the drift of where this is going?

More to follow on heat control, and the ability to have an open clutch pack, in more driving situations.

[bmnut]

I think so!........The current xDrive systems under 'normal' everyday driving conditions are operating the car as a RWD in effect and only when certain criteria are met due to changing conditions then the FWD capability is brought in to play?
Presumably meaning that the concerns regarding transfer boxes being destroyed as a consequence of unapproved tyre choices are becoming more and more infrequent?

[HighlandPete]

Originally Posted by bmnut

I think so!........The current xDrive systems under 'normal' everyday driving conditions are operating the car as a RWD in effect and only when certain criteria are met due to changing conditions then the FWD capability is brought in to play?
Presumably meaning that the concerns regarding transfer boxes being destroyed as a consequence of unapproved tyre choices are becoming more and more infrequent?

Yes, that's how I'm understanding it. Whereas the older xDrive systems (including into the F-Series ranges), were more a permanent AWD setup, with a closed or 'slipping' clutch pack most of the time.

It is clear with the G12 transfer box (ATC13) advances, and applying them to the F90 M5. requiring xDrive with a RWD (only) mode, BMW have tested the waters for running an open clutch pack in many more driving situations. This is possible with the design changes to heat management in the transfer box, 2-part oil reservoir, lower churning losses and much better heat control. The open or 'over-open' clutch pack as mentioned by BMW, appears to enable a greater rolling tolerance in motion, in many more driving situations.

Although BMW still add the caveat in many technical documents, describing the improvements, with something like...

Warning Symbol!
The tire tread depth, tire rolling circumference as well as the tire manufacturer should be the same on the front and rear axles if possible, in order to ensure proper functioning of the xDrive. It is also recommended to use only tires that have been approved or recommended by BMW.

Still a bit more to do to see what models really have the latest functions. BMW are still using the 60/40 definition in a lot of the xDrive blurb.

[bmnut]

Originally Posted by HighlandPete

Yes, that's how I'm understanding it. Whereas the older xDrive systems (including into the F-Series ranges), were more a permanent AWD setup, with a closed or 'slipping' clutch pack most of the time.

It is clear with the G12 transfer box (ATC13) advances, and applying them to the F90 M5. requiring xDrive with a RWD (only) mode, BMW have tested the waters for running an open clutch pack in many more driving situations. This is possible with the design changes to heat management in the transfer box, 2-part oil reservoir, lower churning losses and much better heat control. The open or 'over-open' clutch pack as mentioned by BMW, appears to enable a greater rolling tolerance in motion, in many more driving situations.

Although BMW still add the caveat in many technical documents, describing the improvements, with something like...



Still a bit more to do to see what models really have the latest functions. BMW are still using the 60/40 definition in a lot of the xDrive blurb.

I’d be very interested to find out which transfer box is used in my M440i.
I reckon the point you make about the M5 is very valid. With that car, traditionally a ‘dyed in the wool’ RWD to be given AWD capability, much to the understandable horror of ‘M’ car purists, clearly required BMW to make a change in the way they thought about xDrive and approach it from a different direction. In other words not “how do we make an AWD feel like a RWD” but rather a case of “how do give a RWD car an AWD capability as well”?

[xenon]

Interesting reading. Might become as good as Quattro one day. <runs for cover>

[Scottish0177]

As someone new to the forum I just wanted to give a big shout out to everybody who contributes to these threads and posts.

I am a complete layman in terms of my technical and mechanical knowledge, and I guess would be your typical consumer buying a car having read several reviews and maybe having test drove the model I’m interested in.

Certainly this forum has thrown up some questions that I would never have known to even think about, let alone try and find answers too, from star or non star marked tyres to the mechanicals of the xdrive system to over the air updates.

Obviously a very knowledgeable bunch and it’s refreshing being on a forum where all levels of knowledge and ability can post freely and get involved in the discussions.

To everybody who posts, keep it up and to those ( you know who you are) that generally go above and beyond researching topics in their own time then a big thankyou.

[Charlie (Wessex)]

Warning Symbol!
The tire tread depth, tire rolling circumference as well as the tire manufacturer should be the same on the front and rear axles if possible, in order to ensure proper functioning of the xDrive. It is also recommended to use only tires that have been approved or recommended by BMW.

Taking this bit from HPs post, there seems to be quite of "ifs" and "maybes" involved & BMW appear to be covering their backsides as much as possible. Maybe I am overly cynical, but don't all manufacturers want to try and cover themselves for all eventualities even if they don't/won't exist?

Looking at the 2 sentences of the quote.....

The first one is partly meaningless. We all know that the general advice is to use same tyres on both axels (and especially so on individual axles). I do wonder about tyre wear on a RWD bias xDrive, though maybe things even themselves out with steering/weight on the fronts. Not had the car long enough to find out.

Would it be too much reading into the use of the word "recommended" on my part? Star marked = approved, so what are "recommended" tyres?

[bmnut]

Originally Posted by Scottish0177

As someone new to the forum I just wanted to give a big shout out to everybody who contributes to these threads and posts.

I am a complete layman in terms of my technical and mechanical knowledge, and I guess would be your typical consumer buying a car having read several reviews and maybe having test drove the model I’m interested in.

Certainly this forum has thrown up some questions that I would never have known to even think about, let alone try and find answers too, from star or non star marked tyres to the mechanicals of the xdrive system to over the air updates.

Obviously a very knowledgeable bunch and it’s refreshing being on a forum where all levels of knowledge and ability can post freely and get involved in the discussions.

To everybody who posts, keep it up and to those ( you know who you are) that generally go above and beyond researching topics in their own time then a big thankyou.

Second that!

[bmnut]

Originally Posted by xenon

Interesting reading. Might become as good as Quattro one day. <runs for cover>

[HighlandPete]

Originally Posted by bmnut

I’d be very interested to find out which transfer box is used in my M440i.

I reckon the point you make about the M5 is very valid. With that car, traditionally a ‘dyed in the wool’ RWD to be given AWD capability, much to the understandable horror of ‘M’ car purists, clearly required BMW to make a change in the way they thought about xDrive and approach it from a different direction. In other words not “how do we make an AWD feel like a RWD” but rather a case of “how do give a RWD car an AWD capability as well”?

Regarding the M440i, it will be a version of the ATC13 transfer box. There are several part numbers being used currently across G-series models.

What also appears possible, is different software adaptations for different models. When I read comments that Sport and Sport+ can also reduce transferring torque to the front wheels on some models, makes it difficult to know exactly what 'finer' functions do exist on a given model. M240i, data spells out the limited front wheel drive connection in normal driving. RWD bias is being emphasised. M-cars, definitely have reduced AWD function, even the switch to 2WD

Current M-cars (for example, the G80/2 M3/M4) are getting a technically revised version of the ATC13, with added oil control/cooling features. These improvements also indicate the 'weak link' is still controlling heat and wear in the multi-clutch pack.

I'll keep digging.

[HighlandPete]

Originally Posted by xenon

Interesting reading. Might become as good as Quattro one day. <runs for cover>

Do you want me to dig into Quattro limitations? VAG don't have a good record for gearbox reliability... DSG anyone?

[Charlie (Wessex)]

My DSG in my Golf 7.5R was fine but sold with 32k on the clock. Cleaned the haldex yearly. First time at 14k it was horrendous.