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Hello all,
thanks for your responses.
Last week, I bought some taller tyres to put put on the front end (235 40 rather than a 245 35). As mentioned in my previous post, this brought me to a 0.37% difference in diameter with respect to the rears.
I wanted to make sure that the "right" size tyres did fix the handling issues and did not just ensure the xDrive was no longer under stress (my thought process being that if the car still handled poorly in comparison to the 400M 18s, I would sell the 437Mss and go back to 18s). I therefore did not fancy buying some new 235 40 R19s for the front, only to find out the handling was not back to normal and that I needed to shift 2x brand new tyres as well as the rims. The tyres that I bought were a pair of Continental ContiSport 5s, second hand on eBay with 5mm of tread left (conveniently, my rears are at 6.25mm of tread and hence my diameters are now identical!). These tyres are not the same as the rear (Michelin PSS), but given that BMW fit both the Conti and the Michelin to the M4, I figured they would be good enough for my test (even though the forum has many posts saying that different tyres front to rear is never a good idea).
I went for a 5 min drive to see “what happened”...what happened was that the new fronts were in need of some serious scrubbing in! The traction difference front-rear was so great that the traction control kicked in every single input I gave it (even when going in a straight line and breathing lightly on the throttle). I suspect the tyres had been sitting in a warehouse for a year and were in need of a decent clean.
I took the car back out for a 2hr drive in which I got the tyres warm and gave them a proper scrubbing with some heavy braking and slow speed corner, aggressive throttle stuff (with everything turned off) to get them spinning and cleaned up. After I got through that initial part of the drive, I tried all modes (Comfort, Sport, Sport+, DSC off) and can report the following:
The car is pretty much back to how it was before the new rims went on...it is not identical, in that it is pointier and the initial rotation when entering a corner is much quicker than it used to be, however the weight transfer is back to being a truly joyous occasion, and the mid-corner composure and grip is just sublime. I can feel the TC working in all modes (bar DSC off) when I give it huge bootful at the apex, but I think that is down to the masses of torque (the engine has been chipped) and the difference in rubber front-rear. The next set of tyres that go on will be all matching.
I still need to test it in the rain, and on a motorway, but the signs are good...
Now that the car is behaving as I had hoped, I was wondering whether anyone would like to help me make a definitive F3X xDrive 437M alignment guide? I have scoured the forums; unfortunately several people have asked for help with few replies.
I have attached my current alignment data, I am thinking a touch more toe on both front and rear (rear with circa 0.1deg toe than rear total) + a little bit more rear camber might make the car a little more stable on corner entry, even if the initial rotation is slightly slower.
Sorry the alignment document is in Italian, FYI:
Campanatura = Camber
Convergenza = Caster
One final thing...with the 235 40s on the front, when manoeuvring around a carpark, the increased diameter means that they rub slightly on the arch lining at the front. It is not excessive, but it is slightly annoying.
Thanks again.
Yours,
Dom
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