[alij]

Hi guys , I have 20” rims on my Xdrive 320d.
The fronts are currently Michelin ps4s and backs are Pirelli pzero R01 ( given by dealer like this )
All tyres currently have around 4mm tread - my front right has got a bulge in the tyre, so I wanted to ask, is it better if I :

A) buy a new Michelin ps4 and shave it down ( if there’s any shops in London ! ) which will cost around £250 all in

B) or, I can buy these : https://www.ebay.co.uk/itm/285441665...mis&media=COPY

If I put the new Pirelli’s on , at 7/8 mm, will I have to change the backs at 4mm? Are these tyres fine to use for the front? Or what would you guys recommend

Not looking to fork out £800ish pound for a whole new 4 tyres!

Thanks

[AlfaM]

I’m sure the tread depth just needs to be within 2mm on each axel not across all four wheels, and at 4mm you’ll be changing them all soon anyhow, so personally I’d just put a new one on.

[alij]

Originally Posted by AlfaM

I’m sure the tread depth just needs to be within 2mm on each axel not across all four wheels, and at 4mm you’ll be changing them all soon anyhow, so personally I’d just put a new one on.

In that case am I good to just replace the fronts and leave the backs?

[AlfaM]

Just looked at the manual for a 340, it does actually state that it should be 2mm across all for 4-wheels drive - but its for control system and alignment rather than risking damage to the drive system.

I've got 4-5mm on the front and BMW have just fitted new 8-9mm on the rear - so cant be that much of an issue?

[alij]

Originally Posted by AlfaM

Just looked at the manual for a 340, it does actually state that it should be 2mm across all for 4-wheels drive - but its for control system and alignment rather than risking damage to the drive system.

I've got 4-5mm on the front and BMW have just fitted new 8-9mm on the rear - so cant be that much of an issue?

With that logic , 4mm on back and 7 on front would be perfect!
However, as my lower thread is on back is that an issue?

[Bianchi Dave]

It's the transfer box of the X drive system that can become damaged, due to stresses caused by tyres of different rolling circumferences.

BMW state 2mm for a reason (yes, probably to protect themselves also) but you'll likely find if an issue develops you can kiss goodbye to any warranty claim (its about a £4k job iirc).

Personally I wouldn't take the risk, but ETTO.

Loads of info on the various forums about this....

[FluffyCat]

The fact they supplied the vehicle with different manufacturer tyres front and rear is shocking.

On a separate note... Although it's the safe bet to follow their guidelines on differences in tread depth. Under 'normal' driving in comfort mode, I believe the newer models are RWD until AWD is needed and not the traditional 60/40-R/F split. Finding official info on this is proving difficult, but if true the stress on the transfer box should be a lot less?

[bmnut]

Originally Posted by alij

With that logic , 4mm on back and 7 on front would be perfect!
However, as my lower thread is on back is that an issue?

Well not really when the manual is saying 2mm difference maximum between any of the tyres?

[bmnut]

Originally Posted by Bianchi Dave

It's the transfer box of the X drive system that can become damaged, due to stresses caused by tyres of different rolling circumferences.

BMW state 2mm for a reason (yes, probably to protect themselves also) but you'll likely find if an issue develops you can kiss goodbye to any warranty claim (its about a £4k job iirc).

Personally I wouldn't take the risk, but ETTO.

Loads of info on the various forums about this....

I personally would agree with all of this ^^^^.
Why risk a potential £4k repair bill for the sake of the cost of replacement tyres even if that cost was £1k? That's a hell of a difference in financial outlay by my reckoning.

[bmnut]

Originally Posted by FluffyCat

The fact they supplied the vehicle with different manufacturer tyres front and rear is shocking.

On a separate note... Although it's the safe bet to follow their guidelines on differences in tread depth. Under 'normal' driving in comfort mode, I believe the newer models are RWD until AWD is needed and not the traditional 60/40-R/F split. Finding official info on this is proving difficult, but if true the stress on the transfer box should be a lot less?

That's a disgrace letting a car go out with that mix of tyres on it when it's X-drive. They either don't understand the complexities of BMW's X-drive system or else they don't care?

Under 'normal' driving I suspect you're correct when you say you don't think the X-drive system would be invoked and the car is probably running in a RWD configuration but I feel sure there are occasions when the system is invoked but we as drivers are unaware of it happening?
Unless you are really driving the car hard then can you tell when the front wheels are being 'driven' ? Also the system is designed to react to the amount of grip any individual wheel is experiencing and feeding back to the control unit a computation of the levels of traction and applying or reducing drive to each wheel independently as well as collectively for the sake of vehicle stability. All very clever stuff to keep the vehicle on the road and out of the scenery and a far cry from my early days in E27s and E30s when to keep it in a straight line sometimes meant an armful of 'opposite lock' and the car going sideways occupying a scary amount of road!

[alij]

Originally Posted by bmnut

That's a disgrace letting a car go out with that mix of tyres on it when it's X-drive. They either don't understand the complexities of BMW's X-drive system or else they don't care?

Under 'normal' driving I suspect you're correct when you say you don't think the X-drive system would be invoked and the car is probably running in a RWD configuration but I feel sure there are occasions when the system is invoked but we as drivers are unaware of it happening?
Unless you are really driving the car hard then can you tell when the front wheels are being 'driven' ? Also the system is designed to react to the amount of grip any individual wheel is experiencing and feeding back to the control unit a computation of the levels of traction and applying or reducing drive to each wheel independently as well as collectively for the sake of vehicle stability. All very clever stuff to keep the vehicle on the road and out of the scenery and a far cry from my early days in E27s and E30s when to keep it in a straight line sometimes meant an armful of 'opposite lock' and the car going sideways occupying a scary amount of road!

Thanks for all the help guys!

Will defo be replacing all 4 😛

I found 2 sets of Pirelli PZ4s that I’ve purchased, but one is MC rated and one isn’t. There’s a slight difference in tread ( pics below ). They are both Pzeros PZ4S, but seem to have a different look on tread so unsure if I should purchase or not

[bmnut]

The tyre in your first photo looks very much like the tread pattern on my factory fitted Pirelli P Zero PZ4's that are on my car if that helps any?

[alij]

Originally Posted by bmnut

The tyre in your first photo looks very much like the tread pattern on my factory fitted Pirelli P Zero PZ4's that are on my car if that helps any?

Is that on the 255/30/20? Can’t seem to find that tread anywhere on that size!

[Bianchi Dave]

BMW state that all four tyres should be identical, including make and model and tread.

Tbh I'd just be biting the bullet and buying four new tyres, unless you can get a used set of identical ones, with very similar tread depths, which may be difficult to come by.

The odd newish set pops up on the forums from time to time, normally be people switching to PS4S and ditching their stock runflats.

As well as this (G2*) side of the forum, also check the F3* uk side, as the tyre sizes are the same between the generations.

[bmnut]

Originally Posted by alij

Is that on the 255/30/20? Can’t seem to find that tread anywhere on that size!

The tyre sizes are 225/40 R19 front and 255/35 R19 rear, but as Bianchi Dave has said above I'd agree with him that you'd be far better off just getting a set of new all round in the BMW recommended sizes then you won't have any problems and on the off chance you do then you're covered with warranty.

[Bianchi Dave]

Originally Posted by bmnut

The tyre sizes are 225/40 R19 front and 255/35 R19 rear, but as Bianchi Dave has said above I'd agree with him that you'd be far better off just getting a set of new all round in the BMW recommended sizes then you won't have any problems and on the off chance you do then you're covered with warranty.

OP says he is on 20s in first post. If OEM wheels (795Ms etc), the sizes are 225/35 R20 front and 255/30 R20 rear, so same width as the 19s but different aspect ratio due to the larger diameter rim.

Non OEM wheels may have wider tyres fitted though.

[bmnut]

Originally Posted by Bianchi Dave

OP says he is on 20s in first post. If OEM wheels (795Ms etc), the sizes are 225/35 R20 front and 255/30 R20 rear, so same width as the 19s but different aspect ratio due to the larger diameter rim.

Non OEM wheels may have wider tyres fitted though.

Yep Dave, missed that bit!
Bloody hell even my eyesight's failing now, I must be getting old!

[Bianchi Dave]

Originally Posted by bmnut

Yep Dave, missed that bit!
Bloody hell even my eyesight's failing now, I must be getting old!

😄 welcome to my world!

[bmnut]

Originally Posted by Bianchi Dave

😄 welcome to my world!

I wouldn't care I'm still several weeks away from my first state pension.....could be spending it at Specsavers!!!

[Assassin]

Inside the transfer box is a set of several clutch friction plates. When the car is travelling in a straight line the front and rear wheels are rotating at the same speed and the tyre circumferences are the same. The clutches are fully engaged so no slippage.

Simplistically, when a wheel(s) is losing traction, all the wheels are not rotating at the same speed so the transmission has to redistribute the drive to the relevant gripping front / rear wheels. Part of that process involves controlling the amount of slippage of the clutch friction plates to take account of the different axle speeds required to maintain drive. These clutches slipping causes wear over time, but they are durable so its not really an issue.

The problem arises when the rolling circumference of the wheels and tyres is significantly different as can be caused by different sizes, tyre types/brands, tyre pressures. In that case, the axles need to turn at different speeds all the time causing the clutches to slip constantly and eventually they degrade and can fail themselves and/or cause premature wear on other transfer case components.

But as stated above ETTO, but I'll be keeping mine within the suggested BMW spec.