[hedditch]

Please excuse me for Preaching to the Choir, but this will take a little wind-up to get going , so please spare me a moment!

There are two agreed "truths" which although cause me to scratch the old head, I am happy to accept without issue. However they both, lead me to a third question.

Truth #1:
The First is the accepted advice from VW that one should disable the ESP/ABS when driving in Sand or Mud to enable to different amounts of wheel spin without the Traction Control slamming the brakes on and helping you get bogged.

Truth #2:
The other one is the understanding that running tyres of different circumferences (sizes) on the same axle can cause catastrophic transmission damage.


Now here is where the fun starts. On most 4XMotion Touaregs, the exception being units equipped with rear diff locks, both from and rear differentials are "Open" units, consisting of a simple differential without any slip limiting capability. All Slip control on a 4XMotion ( I am not talking 4Motion, which are Torsen Diffs), is controlled through the ESP/ABS using the brake system to limit or restrict wheel spin. The exception being the centre differential which is not only controlled by stepper motor controlled by the ESP/ABS but also manually lockable to provided a fixed 50/50 front rear split.

When you turn of the ESP/ABS ( Traction Control ) all four wheels are free to spin their little hearts out, and the Transmission has no idea which wheel is doing what, nor (as the the ESP is off) does it give a tinkers cuss. As at this stage it is simply spinning a prop shaft (tail shaft) which divides into two at the Transfer Case.. In fact you should be able to turn of the ESP, jack up one wheel, and nothing should happen except for that wheel to spin freely. As it would if you were up to your tits in mud.

Bare with me a little more...... sorry

With the ESP off you can have one wheel rotating many times faster than the partner on the same axle with no logical reason why it would do anything more that make the differential do what it was engineered to do in the first instance.

Finally ....My Point...again a bit tedious...

I should be able to use any size tyre or combinations of tyres I care to choose irrespective of the rolling size of the wheel as long as I turn of the ESP/ABS so as not to have the electronics have a hissy fit and demolish my Bank Account....

If that holds then any full size wheel would work as an off road spare...

Any thoughts on the logic..??????

Stuart.......

PS: have there actually been any truly documented records of Treg Transmission committing suicide after being insulted by an in appropriate tyre, or are we only going on engineering advice from VW. Just interested to know the whole story???

 

[schootsie]

Stu, a little more info to add to the confusion. We bought the R50 as a demo from the dealer with just under 8000km on the clock. Front tyres were not real good to the point I asked for new ones before signing up. They replaced 1 and put it on the back with the other ex front one and put the rears up front. This left me with 1 brand new tyre and one 50% worn tyre on the rear and 2 40% worn tyres on the front. Totally sanctioned by the dealer (sales staff naturally). Drove with this combination for the 22000km before taking the wheels off to replace them with 19" and more recently 18". Had no transmission issues then or now from the different diameters and that was with ESP on almost totally, just took it off to play every now and then on wet roads. John

 

[noobytoogy]

A full sized spare will probably work up to 50mph/80kph in the same way as the space saver spare [which is a different overall diameter to regular road wheel/tire combinations] works up to that speed inasmuch that the electronics and the transmission can take care of variations in wheel rotation up to that speed but not beyond it.

Above 50 mph/80 kph it will probably still work okay PROVIDED THAT the wheels can slip as required on the surface to unwind any wind-up so travelling on ice, snow, mud, gravel or sand would be probably be okay.

In both instances you could leave the ESP on as you would when running on the space saver.

I'd be happy to run my car in the first scenario above, but probably not in the second!

And regardless of anyone else's experience, I would always stick to the four identical tires advised in the handbook and by Touareg technicians.

 

[hedditch]

Thanks John,

The interesting thing is the design of he Treg's driveline is extremely basic ( 4xMotion) and relies on the electronics. The electronics must have a threshold value below which it ignores the different spin rates. My experience with Traction controls on other equally electronic controlled cars is the difference needs to be significantly more than a few percent in rotational speed. I have heard figures of only millimetres difference as a maximum..I need to do a little maths here as I suspect the delta in rotation doing a sharp turn would probably amount to a larger percentage?

Stu..

 

[hedditch]

A full sized spare will probably work up to 50mph/80kph in the same way the space saver spare [which is a different overall diameter to regular road wheel/tire combinations] works up to that speed inasmuch that the electronics and the transmission can take care of variations in wheel rotation up to that speed but not beyond it.

Above 50 mph/80 kph it will probably still work okay PROVIDED THAT the wheels can slip as required on the surface to unwind any wind-up so travelling on ice, snow, mud, gravel or sand would be probably be okay.

In both instances you could leave the ESP on as you would when running on the space saver.

Understood, the point is if I turn the ESP off , I then should be able to do nay speed I like, as all duffs are open and therefore Transmission bind if impossible. Due to the presence of the centre diff.

Thanks..noobytoogy this could goes somewhere....

 

[noobytoogy]

With regard to turning ESP off and then being able to do any speed you like on any surface without getting wind-up, I simply don't know.

You MAY be right, but it could be expensive if you're wrong!

As for recorded damage reports, I cannot recall anyone coming on either of the Touareg forums over the past 5 years and saying their gearbox/transfer case/front diff/rear diff/stepper motor/half shaft/girl friend's finger nail* has bust because they'd been running odd tires.

But then again that may be because everyone listens to me and my mantra: "Always fit 4 IDENTICAL tires: same make, same model, same tread pattern, same size" [though I doubt it!]!

[*Just checking to see if you are reading it all!]

 

[hedditch]

Understood. I am not on a mission to promote using dissimilar tyre combination rather trying to understand the dynamics involved and the risks. Personally I would always prefer to be running four identical tyres. Just thought it could be a good conversation to kick off!

Stu....

 

[noobytoogy]

I think it's a perfectly valid conversation.

Someone questioned the "fit four identical tires" thing a few years back over the use of the differently sized space saver spare which is why I then explored what ESP and the other electronic aids do to cope with that particular scenario.

 

[Niner]

Nooby how are the T3 North american versions with 4 Motion affected by tire size variance with their ESP turned on and off?

 

[noobytoogy]

No idea!

 

[noobytoogy]

. . . I'm going to leave that to a T3 owner to sort out!

 

[Dubby]

Stu, I missed this thread as I was in the Kimberley when you posted.
I have always noted - with some trepidation - the comments by Noobs and others about the necessity of having identical donuts on the ground.
(This has been prompted now by my daughters recent purchase of a Subaru Forester which suffered centre diff. shudder/clunking and was fitted with tyres which were not all the same, ie different load/speed ratings/brands and the spare was a different aspect ratio) 112K klms on the clock.
Hmmm.
Now, why was I previously concerned?
Like Schootsie, my Treg was purchased -from a VW dealer- with different tyres front to rear. Front were Pirellis almost new, rears were Bridgestones half worn. This at 17K klms.
About 6K later, I replaced the Bridgestones with new B'stones (I had previously bought one as a full-size spare), so now the situation was reversed with new on the rear, and half-worn on the front.
At 23K I purchased my 18's and fitted the silent armors. Probably at 33K or thereabouts, I had a major failure and fitted the new sa spare which remained on the car for another 8K or so. (So 1 new, and 3 probably 1/4-worn).
For the last 3K I have returned to my 19's with the Pirelli/Bridgestone combo.
My Treg now shows 46K klms.
I have not at any time experienced any suggestion from the Treg that all is not well.
(If there was, I would expect it to tell me - it complains if one solitary LED is out on the van).
So, have I been extremely fortunate or is there such a degree of mechanical/electrical tolerance that the doomsayers may be being overcautious ?

(The Sooby by the way was 'fixed' by replacing the oil in the centre diff and is about to be fitted with a set of new tyres).
Dub