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      11-12-2019, 03:43 PM   #1
MalcolmT
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Replacing rear tyres only on xdrive

I have a 435d running on PS4S tyres, the fronts are still in good shape with around 5mm tread left but the rears are down to about 3mm. Am I going to be ok to change the rears only without the xdrive getting out of shape, Iím sure I read on another thread that 2mm difference between tyres should be fine
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      11-12-2019, 04:02 PM   #2
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You should be fine yeah. If you notice any hesitation, 1-2psi drop in rear pressure should help bring the rolling circumference back down again.
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      11-12-2019, 04:40 PM   #3
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If you're going to mix new tyres with roughly 8mm of tread with used tyres that are already down to 5mm that's a 3mm difference (and potentially I think that's starting to get marginal with X-Drive).

As an aside, I'm surprised you've experienced such a difference in wear between the axles - the tyres on my 335d have always worn at an almost identical rate and hence come replacement time it's been a case of changing all four and not just a pair....
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      11-13-2019, 07:22 AM   #4
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In 100k miles I have regularly replaced PS4 tyres when either front or rear has been down to the indicators and the other end has still had 3mm. That is what I am running just now - fronts brand new, rears on 3mm. Never had any issues.

BMW sold me an AUC xDrive with new tyres on the front and 3mm left on the rear.
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      11-13-2019, 08:34 AM   #5
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I didn't ever have issues on the 335d xDrive I had for 70k miles - although I kept to runflats, I didn't stick with * marked and I always needed to replace fronts sooner than rears. I was on staggered wheels where, on non * marked tyres there will already be a difference in rolling circumference (fronts will be slightly larger), so changing fronts first had a greater risk of exceeding xfer box tolerances but it was fine. Therefore I would think it very unlikely you will have issues changing rears first whether on staggered or not. A square setup will generally be even less susceptible as the RC will at least start off the same (assuming same tyres). As has been said if there is an issue first check pressures & look to increase front/reduce rear...
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      11-13-2019, 02:00 PM   #6
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Quote:
Originally Posted by Eddamoo View Post
You should be fine yeah. If you notice any hesitation, 1-2psi drop in rear pressure should help bring the rolling circumference back down again.
Quote:
Originally Posted by lethbridge View Post
As has been said if there is an issue first check pressures & look to increase front/reduce rear...
Just so I'm clear, are you chaps suggesting you should adjust tyre pressures as a means of compensating for how worn tyres are on different axles? If so I can't say I've heard of that before - I always inflate tyres to the recommended pressures regardless of whether they're brand new or just about to be replaced...
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      11-13-2019, 02:23 PM   #7
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I've just put a set of rears on my xdrive the fronts are at 4mm rears now at 8mm? (not sure of the new tread depth) no issues and it was done at my BMW dealer, across the same axle is an issue though.
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      11-13-2019, 02:27 PM   #8
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Quote:
Originally Posted by JNW1 View Post
Just so I'm clear, are you chaps suggesting you should adjust tyre pressures as a means of compensating for how worn tyres are on different axles? If so I can't say I've heard of that before - I always inflate tyres to the recommended pressures regardless of whether they're brand new or just about to be replaced...
It is a way to tweak the Rolling Circumference, front to rear. Not the way I'd want to prevent transfer box issues, but is an 'available' solution.
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      11-13-2019, 02:47 PM   #9
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If you have access to ISTA or similar you can see the speed of each wheel live as you drive, on straight road with cruise control see how much they differ front vs rear.
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      11-13-2019, 03:16 PM   #10
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Quote:
Originally Posted by HighlandPete View Post
Quote:
Originally Posted by JNW1 View Post
Just so I'm clear, are you chaps suggesting you should adjust tyre pressures as a means of compensating for how worn tyres are on different axles? If so I can't say I've heard of that before - I always inflate tyres to the recommended pressures regardless of whether they're brand new or just about to be replaced...
It is a way to tweak the Rolling Circumference, front to rear. Not the way I'd want to prevent transfer box issues, but is an 'available' solution.
I see the theory but in practice is there some sort of formula that can used to equate changes in (say) PSI to rolling circumference? Potentially all feels like guesswork to me but I could be wrong....
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      11-13-2019, 03:36 PM   #11
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I'm struggling to see how reducing the tyre pressure leads to the circumference of the tyre reducing.
Sure, the radius to the point of contact with the road may reduce as it squashes to the tarmac but, as the car rolls forward, surely the distance travelled by the tyre to rotate it 360 degrees is the same as when it was inflated to the higher pressure.
Or is it that the carcass of the tyre actually expands slightly at operating pressures (compared to being uninflated), so reducing the pressure (slightly) causes the tyre to reduce its circumference.
Thoughts please.
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      11-13-2019, 03:41 PM   #12
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Quote:
Originally Posted by JNW1 View Post
I see the theory but in practice is there some sort of formula that can used to equate changes in (say) PSI to rolling circumference? Potentially all feels like guesswork to me but I could be wrong....
I suspect you're probably right actually! Arbitrarily fiddling with tyre pressures, a safety-critical part of a car, especially in this weather, because someone somewhere had transfer box issues that may or may not have been caused by a mix of worn tyres is utter madness.

I fail to accept that the tolerances are so tight in the transfer box that they can't cope with slight variations... Every flippin bend means the wheels are all rotating at different speeds all the time anyway! So for everyones sake, please don't adjust tyre pressures, especially for this nonsensical reason. Pretty please!
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      11-13-2019, 04:29 PM   #13
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Quote:
Originally Posted by JNW1 View Post
I see the theory but in practice is there some sort of formula that can used to equate changes in (say) PSI to rolling circumference? Potentially all feels like guesswork to me but I could be wrong....
Definitely guesswork. Simple physics, lower the pressure the RC reduces.

You could measure the change... roll the wheel onto a flat plate, measure to the wheel centre (or rim lip). Lower the pressure in increments and measure again. That would give dimensions of the static radius at defined pressures, something to do the maths on.
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      11-13-2019, 04:35 PM   #14
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Quote:
Originally Posted by Techno 9000 View Post
I'm struggling to see how reducing the tyre pressure leads to the circumference of the tyre reducing.
Sure, the radius to the point of contact with the road may reduce as it squashes to the tarmac but, as the car rolls forward, surely the distance travelled by the tyre to rotate it 360 degrees is the same as when it was inflated to the higher pressure.
Or is it that the carcass of the tyre actually expands slightly at operating pressures (compared to being uninflated), so reducing the pressure (slightly) causes the tyre to reduce its circumference.
Thoughts please.
RC reduces with lower pressure. It is the principle that is used by the 'indirect' tyre pressure warning system, which uses (ABS) wheel speed sensors. As we lose pressure the wheel speed sensor picks up the change in rotational speed. When the threshold is reached, that pressure has dropped something like 20%, triggers a low pressure warning.

Remember there is a lot of 'scuffing' that goes on, as tyres rotate through the contact patch, low pressures making wear rates (and heat) worse.
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      11-13-2019, 04:54 PM   #15
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Quote:
Originally Posted by JNW1 View Post
If you're going to mix new tyres with roughly 8mm of tread with used tyres that are already down to 5mm that's a 3mm difference (and potentially I think that's starting to get marginal with X-Drive).

As an aside, I'm surprised you've experienced such a difference in wear between the axles - the tyres on my 335d have always worn at an almost identical rate and hence come replacement time it's been a case of changing all four and not just a pair....
Ok have a feeling the PS4 tyres start at around 7mm anyway, so that would only be a difference if 2mm between front and rear.
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      11-13-2019, 05:25 PM   #16
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I think summer tires usually start with around 8mm.

As said by Highlandpete, playing with pressure is just a tweak, just in case. It fixes a problem with my Mrs 1series. Tyre pressures aren't an exact science, especially changing away from manufacturer load rating and to a non RFT. Most people here are probably still running their MPS4S on RFT pressures when they should be dropped 3 to 4 pounds! Id hardly put it in safety affecting territory!
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      11-13-2019, 05:56 PM   #17
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Quote:
Originally Posted by HighlandPete View Post
Definitely guesswork. Simple physics, lower the pressure the RC reduces.

You could measure the change... roll the wheel onto a flat plate, measure to the wheel centre (or rim lip). Lower the pressure in increments and measure again. That would give dimensions of the static radius at defined pressures, something to do the maths on.
But would that work with run flats ??

Presume the stiffer side walls would/should negate the extra low pressure bulge
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      11-13-2019, 06:04 PM   #18
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Quote:
Originally Posted by Eddamoo View Post
I think summer tires usually start with around 8mm.
Unfortunately a lot of tyres are now only around 7mm new. This review on the tyre reviews website measured new PS4 tyres at 7.2mm http://www.tyrereviews.co.uk/Tyre/Mi...ot-Sport-4.htm
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      11-14-2019, 03:06 AM   #19
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Quote:
Originally Posted by Kal101 View Post
But would that work with run flats ??

Presume the stiffer side walls would/should negate the extra low pressure bulge
Run-flat tyres still deform with lower pressure, much less of course. Indirect TPWS works with run-flats, (my car uses it) measuring the change in RC as pressure drops.
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      11-14-2019, 08:48 AM   #20
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Remember that if you run a staggered set up, 255 rear and 225 front they don't have the same rolling circumference from the start (in theory) The 225''s have a 5mm bigger rolling circumference and are 3mm bigger in diameter, so in theory the more the rears wear the bigger that gap gets between front and rear....so there must be a tolerance in the transfer box to allow for that. They don't keep failing just because you have worn off some rear tread.

Fronts and rears will only truly be running at the same speed when new tyres are fitted to the rear at say 7.0 mm of tread and the fronts have worn down to 5.5 mm giving the same diameter tyres and allowing for the 3mm diameter difference.

I think the box can handle new rear tyres....and in the OP's situation will have a nearer rolling circumference than when the tyres were all new....in theory. In practice tyre sizes can vary massively due to allowable tolerances.

Last edited by sensible; 11-14-2019 at 10:33 AM..
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      11-14-2019, 09:31 AM   #21
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Quote:
Originally Posted by sensible View Post
Remember that if you run a staggered set up, 255 rear and 225 front they don't have the same rolling circumference from the start. The 225''s have a 5mm bigger rolling circumference and are 3mm bigger in diameter
Where did these numbers come from? In any case the tyre markings are not accurate to the millimeters, you can have significant differences between different models and brands no matter what a calculator says. That is why the safest bet for staggered setup is to use BMW star marked tyres as the assumption then is that they will be matched.
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      11-14-2019, 10:02 AM   #22
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Quote:
Originally Posted by zeikei View Post
Where did these numbers come from? In any case the tyre markings are not accurate to the millimeters, you can have significant differences between different models and brands no matter what a calculator says. That is why the safest bet for staggered setup is to use BMW star marked tyres as the assumption then is that they will be matched.
The figures were from a tyre calculator comparing tyre sizes, but I get your point about star marked tyres.

I did a Tyre Technical course a few years back at Michelin and you would be surprised at the tolerance of tyres, they have quite large margins +\- . Tyre's of the same size can vary massively from one manufacturer to the next. (One reason not to mix brands). I will dig out my book and post the variance of say a 225 tyre for example.

My point is that it is not an exact science, and it is really splitting hairs to worry about replacing the rear tyres and modifying tyre pressures to compensate. (Even the 255 rear tyre is on an 8.5j wheel which isn't the ideal size for that wheel.)
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