DONT DO WHAT I DID

[omodos]

Originally Posted by Homerlovesbeer

I still think that having this issue simply due to worn tyres on one end is truly bizarre!

A diff is supposed to allow one side/end to spin faster than the other so why is this such a drama?

I attach link to post by grounch that prompted me further to go get a new set all round despite fact that this fella had the opposite he replaced rears only, and I replaced fronts. Symptoms we both had pretty identical....I will wake his post and see if he did sort his out by replacing all 4 or not...

https://xoutpost.com/bmw-sav-forums/...t-happy-2.html

[Bdc101]

Originally Posted by Homerlovesbeer

I still think that having this issue simply due to worn tyres on one end is truly bizarre!

A diff is supposed to allow one side/end to spin faster than the other so why is this such a drama?

To make it easier to think about. Consider a 2wd car. Say you put a bigger tire on one wheel than the other. Since that tire is bigger, if it turns at the same RPM, it will be traveling a longer distance, thus it would try to turn the car in one direction all the time.



On an AWD car, instead of trying to turn the car, the front axle was trying to pull the car faster than the rear was, even though the two axles were rotating at the same RPM. The only thing that can give when that happens, is the tire sidewalls will deflect a little, because there is a great deal of force being transmitted through the differential and driveshaft, due to the fact that each axle is trying to move the car at a different linear speed. Eventually the force becomes too great, the tire skids and then the sidewall unloads and the process starts again, all the time, every second the car is moving. The differential will overheat doing this very quickly.

[andrewwynn]

I think this is an x-drive issue more than pre x-drive.

Original e53 had open diff planetary gear that should allow any speed front or back. The x-drive uses a variable clutch that can/will lock the front and back axles. I'm not sure what the limits are for differences in rolling circumference on the original.

[Attacking Mid]

Originally Posted by cn90

Tread DEPTH is what matters. For AWD vehicles, if you replace only 1-2 tire(s), as long as the depth is the same as the existing tires, you *should* be fine.

The only time this would be correct is if the tires are the exact same make/model/size. Tread depth when comparing different tires is meaningless, as they can have significantly different circumferences even though the stated "size" is the same. Rolling circumference is what affects the transfer case. Some tire mfgrs will state this as revolutions per mile.

AM.

[Bdc101]

Sorry. I should have stated that my interpretation is for Xdrive cars when the diff is locked. Pre-xdrive cars will still overheat the diff with mismatched tires, but will tolerate a lot more RPM differential from one axle to the other before they have trouble.



Some old AWD racecars actually used viscous LSDs with slightly different sized tires front to rear in order to keep the viscous fluid hot enough. The Porsche 959 is one example.

[omodos]

Originally Posted by Bdc101

To make it easier to think about. Consider a 2wd car. Say you put a bigger tire on one wheel than the other. Since that tire is bigger, if it turns at the same RPM, it will be traveling a longer distance, thus it would try to turn the car in one direction all the time.



On an AWD car, instead of trying to turn the car, the front axle was trying to pull the car faster than the rear was, even though the two axles were rotating at the same RPM. The only thing that can give when that happens, is the tire sidewalls will deflect a little, because there is a great deal of force being transmitted through the differential and driveshaft, due to the fact that each axle is trying to move the car at a different linear speed. Eventually the force becomes too great, the tire skids and then the sidewall unloads and the process starts again, all the time, every second the car is moving. The differential will overheat doing this very quickly.

I have no idea but don't want to risk the same move again, had the car since 2008 ...and nearly any change in the driving manner I can feel.....and the change could be felt when I whacked on 2 different branded and different tread used tires at rear...car was not 'normal' so I swapped all 4 out with identical tires and all good PS this was also the first time during ownership that I didnt replace all 4 corners....so cannot be a coincidence that when i replaced just 2 things went pear shaped....

[Spartan]

Originally Posted by andrewwynn

I think this is an x-drive issue more than pre x-drive.

Original e53 had open diff planetary gear that should allow any speed front or back. The x-drive uses a variable clutch that can/will lock the front and back axles. I'm not sure what the limits are for differences in rolling circumference on the original.

You know me, the one of little knowledge, but I did get some info re the TFC to 2003 and the Xdrive 2004 onwards.
Pre-face lift X5s use a permanent 4wd system with 35% front and 65% rear split. All auto gearboxes are 5-spd and the TFC uses ordinary ATF oil as you would in the transmission.


Post face lift X5s use a completely new 4wd system based around a clutched transfer box. It can send all the power to either the front or rear based on various inputs. All cars use a ZF 6-spd box which is much more robust than the 5spd unit.

All X5s have proper traction control.



Based on this concept I am only making a generalized assumption that the Xdrive system should cope better if the rolling diameter of a pair of wheels was slightly different, due to the ability to transmit the power accordingly.


All E53's carry a 17" spare wheel, while they are fitted with rim sizes of 17", 18" and 19" from point of sale. This is not delving in to the dreams of wild custom builders, but everyday people. Shouldn't the Xdrive cope with the variation better?


I am always ready and willing to be corrected if this is incorrect information

[andrewwynn]

The x-drive system uses a variable clutch to change how much power goes to the front it’s a bit of a misnomer to say it can transfer full power to one axle that’s only true if the other axle has zero traction and can be dragged with the abs locking the wheels.

The pre Lci uses a pure planetary gear system that uses gear ratio to divide torque. No slipping of clutch plates and can handle any speed differential. I think that the problem with different size tires will come from dsc detecting some wheels going faster than other and using abs to attempt to correct this.

The wheels and tire sizes are very carefully selected so the rolling circumferences are almost identical if 17-18-19" wheels

Eg: 235/65/17 255/55/18 255/50/19 all have nearly identical rolling circumference so the 17 can be used as a spare.

[Spartan]

Oh, this seems very logical and does make sense as far as the 17" rolling diameter against 19" rolling diameter. I tried this after I woke up from writing that...damn nanna naps! I happen to have two new tyres on 9x19" and one ? x17" and they roll exactly the same.
Yes, you are correct in what you said about the transfer of power bit...It did say that the Xdrive can transfer all the power to the one wheel if it was necessary. Guess you can;t believe everything BMW say


Cheers mate. Jeff

[daveking]

Hi
I had the same problem.
Had 2 part worn tyres fitted on the front.
Had the rumble and vibrations at low speeds.
Thought it was prop or drive shaft.
As it happened after tyres were fitted i checked and found out
they put snow tyres on..

Dave king