XSeries-forum

Hi all,

So I've been mulling this one over a little with regards to how sensitive these cars are to tyre size differences, and short of asking BMW (where I'm not expecting them to have the foggiest) I thought I'd post up some thoughts first.

Now as we're (mostly) aware, using * marked tyres is recommended due to the tighter checks around sizes etc and for the protection of the transfer case, but how do we stand when it comes to pressure differences and also different rates of wear between front and back, especially with the lack of a centre diff to soak up the difference?

The tyre pressures advised are higher on the rear than the front, and couple that with potentially different rates of wear (more at the front?) and potentially a staggered setup on the M's how do we stand? 

Do we not only have to use * marked tyres but also make sure they're within a certain amount of MM tread left across all 4, and also make sure the pressures are X PSI to guarantee as close a rolling radius front to back as possible?  The ABS uses rolling radius to detect tyre pressures being out so if the differences can be big enough for that to pick it up, surely that's more than enough to be damaging to the transfer box?

This has been a topic mulled over many times so just a few thoughts and observations about it!

First, I've done 140,000 miles in my car (2007 model regitered in 2008) on several sets of tyres - Continental, Bridgestone, Goodyear * rated summer runflats and Vredestein non star winter tyres. Wheels are 255x50x19 front and 285x45x19 on the rears.  The rears of the runflats always wear out much quicker than the fronts but most of my miles have been on open dual carriageways!  After about 16000 miles over the last 3 winters, the Vredesteins are wearing evenly.  Tracking front and rear checked and adjusted regularly so now I get much less wear on the rear inside edges . Not so much wear now on the outside fronts as I don't travel to work on roads with many roundabouts since I retired a couple of years ago!

Secondly, I measured the circumference on a set of Goodyear runflats * rated waiting to be re- fitted when the winter is over. After 17000 miles over the last 2 summers the rears are 5-6mm smaller than the fronts, but really should be replaced as tread depth is down to 2.5mm, fronts only down to 5mm.  That means they started out the same but more wear on the rears has led to a difference in circumference! Vredesteins are close in circumference front and rear.

I keep pressures to those given on the plate by the door, based on the principle the manufacturers researched this and that is their recommendation.  The lower figure for general use and the higher one when towing a caravan with about 70kg on the ball.  Although it's a 7 seater, I cannot recall the last time the back seats were used now the grandchildren have grown up!

Some food for thought!!!

Graeme

Interesting!

So, even using recommended pressures all round, naturally increased wear on the rears resulted in a 5-6mm difference (reduction) in rolling radius which * rated tyres cannot prevent.  Sure, they will be fine when new (all round, assuming all 4 are replaced at the same time) but gradually get further and further apart as time/ miles go on.

So, even sticking to the * marked tyres there will always be a difference in rolling radius between front and rear - not to begin with but as time goes by, thus inadvertently adding strain/ wind-up on the transfer case?

I'm just doing some thought into the * marked vs non * marked thing, as no doubt many have previously done as when they're new I totally understand that there will be more of a stringent QA process to guarantee sizes, but as time goes by, with the wear of them (and possibly random pressures, which can also be temp dependant) this will be negated. 

Also, I'm talking front vs rear here, not left vs right, aimed at the transfer case primarily.

I keep the same tyre pressures as the plate suggests and also notice the rears wear a little faster - mainly because it is predominantly RWD no matter what the AWD/4x4 xDrive might imply.

It’s also about the tread difference front/rear being within a limit too.  2.5mm difference probably isn’t enough to cause an upset (though it might be approaching that point) but 2.5mm on one axle and brand new (8 or 9 mm?)  on the other probably is.  I have seen (and used) advice that the circumference should be within 1.5% but haven’t yet worked out what that means in reality when compared against relative tread depth.

There just doesn’t seem to be any hard and fast approach other than sticking within manufacturers guidelines that seems to work out.

So (and I'm playing devils advocate a little here I know) it's entirely possible that a combination of tyre pressures and tread wear front vs rear it's possible for the difference to be the same or maybe even beyond what would be present if non * marked tyres were used?  Could those be around 6mm out front to rear or left to right through manufacturing defects? 

Also, on the * marked tyres and a staggered setup, with a 50 profile front and a 45 profile rear, are * marked tyres compared to make sure both different profiles offer the same rolling radius, even with pressure differences taken into account? 

I'm just trying to rationalise in my own mind, the science behind the suggestion that * marked tyres are required to ensure the rolling radiuses (radii?) are close enough across front to back to keep the transfer case happy, especially with many other factors at play such as wear and pressures which can vary a fair bit with temperature too.

Reading elsewhere, a diff usually sorts this sort of stuff out so with AWD there's no wind up on tarmac and these kinds of things are just not an issue - tyre places insisting you replace all 4 tyres at the same time seems to be old hat.  If the XDrive system simply can't cater for that due to BMW's choice(?) to omit a diff is surely a massive oversight?

Para 1 - pass but I doubt t if (and it’s a big assumption) proper quality control is adopted.

Para 2 - yes because it’s the staggered set up that seems to suffer.  But of course they only properly match when new.  My X6 tyres are the same pressure all around.  However the transmission does not like the high pressure end when fully loaded even on ‘*’ marked tyres.

Para 3 - agreed on wear, but pressure differentials at temperature are not enough to make any significant difference.

Para 4 - and that may in fact be the crux of this.  Certainly some of the engineering decisions made by bean counters rather than Engineers may have compromised the system.  It should not be that a system is that sensitive.  BMW may have decided that as the ‘life’ of one of their cars ends when the warranty does, they are not bothered about it.  Both Audi & Merc have their own tyre marking scheme, but I’m not sure if they have the same issues?

Last year another member on here checked the difference of none star rated and star rated which was quite shocking , the none star rated were a cheap ditch finder but were 31mm different in size .
  At the end of the day the cars are known to have problems with none star marked tyres and no matter what tests or calculations you do that won't change , transfer boxes and gearboxes are expensive so it is just down to if you want to chance it to save some money !
The question most people ask is runflat or none runflat but that is another can of worms .
    Good luck with your choice .
          Sid10

To clarify I'm not considering going for non * marked tyres but wished to discuss the logic behind the claims that they protect the transmission based on rolling radius tolerances etc. The information around cheapo ditch finders being that far out is by far the most telling and very interesting - I'd be curious to see how decent non * marked tyres fare on that front too, and then non RFT and so on.

Audi with the Haldex diff I think won't have such issues, not sure on the Mercedes front with the 4matic system. Would need to Google how that works.

As I say, just curious as to the actual real world findings re this, and it came to me when I was pumping my tyres up the other day and wondered about rolling radius etc.

BMW do offer non-RFT and a number of folks in here have found the ride far more forgiv8ng on our rubbish roads.  I have RFT on both the 19” & 20” sets and the 19” is noticeably easier on the teeth!  I would imagine the non-RFT is even smoother.  Not having air springs (they were a cost option and tother half suffers motion sickness with them) on the X6 gives a different ride to the E70.

Your best way forward would be to check how the tyres receive the star rating as Michelin , Continental , Pirelli , Bridgestone , Good year & Dunlop seem to be the only choice that you can buy with the star rating obviously they have spent the money to get the star rating being tested to the limits .
  Just amazing that lower brands haven't thought that it is worth investing the money in to get the star rating , unfortunately most people pick the tyres via price , the transfer problem is a get out of jail card for BMW as it is the first thing they check if problems occur , if star marked tyres are not on the car the warranty is voided saving them a fortune .
I also found out that all the tyres are graded BMW only put A grade on their new cars , main garages get B or C grade going down to F/G grade for some online tyre dealers , this was explained to me by a main dealer hence why there is a huge difference in price although they now price match .

I went from Rft to NON Rft and the difference in ride and tyre roar was significant.
The vehicle came with 245/45/19 front and 275/40/19 on Pirelli P zero rft's.
I swapped to Pirelli Rosso P zero's once the originals were down to 3mm.

Had 3 punctures in 6 months with rft's.
Same mileage and same roads, NO punctures with non rft's

Both sets are * rated and I keep the tyres with 3 psi of the nominal value given on the plate.

Would I go for non * rated? No way, as looking for problems is not in my nature, the car is under warranty and don't wish to invalidate it.
The cost of 4 non Rft tyres was £660 fitted from blackcircles. If the rears do wear by +2mm to the Fronts, I will have no hesitation in Swapping all 4.
On the X3 I find all 4 wear evenly.

Sent from my SM-G930F using Tapatalk

Again the leap to non RFT's had interested me after I did the same in the E91 320d - the ride quality, and most noticeably the lack of tramlining afterwards. It was like night and day.

Assuming tyre pressures are all OK, with the softer sidewalls and with the use of * marked tyres I'm thinking it'd a safe enough bet that the radius of the tyres would be consistent enough to be safe on the transfer case. 

Got a good 6mm plus on all 4 at the moment (Bridgestone Duellers) so I'm a way off replacing them but as I said before, *marked was always going to be on the cards - I just wanted to know the science behind the difference and what other factors may unintentionally undo any benefits. I'll be re-checking the pressures at the weekend me thinks to make sure I'm close to the ones on the B pillar.