big nail : flat tyre

[Mujahid]

Originally Posted by ard

Yes, that is PRECISELY what I am saying.

Tell you what, go to tirerack, look up your car. Select tires for "OE"..look at each of the OE tires, and write down the DIAMETER of the OE 19", the OE 20" etc...all of them will be the same diameter.

Why?

BECAUSE BMW DOESNT WANT TO HAVE TO RECALIBRATE CARS AS THEY ARE BUILT, ordered with different tire/wheel combos!

So the OE donut spare will work on and OE wheel size.

And I do have and idea what BMW recommends: the OE donut spare

100%, i get it now. thanks again. (Did not realize that 18'", 19" and 20" had teh same OD)

[ard]

Originally Posted by Mujahid

To the Godfather, when a vehicle is turning a tight corner is the outer wheel not turning faster than the inner wheel. And therefore the reason for a differential in the axle. Does this not then mimic the different speeds of two wheel sizes? If different wheel speeds can damage the AWD system, then too much cornering would or should have the same effect.

The issue with damage to a diff is TIME and SPEED. Effectively mismatched tires will heat the diff and diff fluid. On turns you simply CANNOT geenrate enough continuous mismatch at speed to damage the diff. Put mismatched tires on and drive 70mph in a straight line for 2 hours and you can.

[Mujahid]

Originally Posted by ard

The issue with damage to a diff is TIME and SPEED. Effectively mismatched tires will heat the diff and diff fluid. On turns you simply CANNOT geenrate enough continuous mismatch at speed to damage the diff. Put mismatched tires on and drive 70mph in a straight line for 2 hours and you can.

ok, this makes sense - thanks!

[TerminatorX5]

i have factory 20" rims, staggered... the spare is 19" donut... from the factory... as all 4.6/4.8iS (E53) - nothing smaller than 18" will not fit over the brakes...

outside diameter is the key, and there is speed limit, about 50MPH/80kmh on a donut


PS - some non-US distribution "iS" had 19" rims from the factory...