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2Addicts | BMW 2-Series forum Technical Topics Wheels and Tires -- Sponsored by The Tire Rack The quest for 245/40-18 square: success

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      03-04-2021, 11:33 AM   #1
TrboMike
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The quest for 245/40-18 square: success

Car:17 M240ix

After much research here (thanks too all who posted their various tire/rims sizes with offset!!) IMO the perfect offset to center a tire (to mount the widest tire possible) in the front is 38 to 40mm. Every brand/model tire is slightly a different width even at the same numbers, so you can push the envelope but one tire may tire and the next one in the same size suddenly will be wider and not fit. I bought a 245/40-18 Mich PS4S and any other tire might be slightly wider at the same size so you need to watch for that.

I drove 2 days on 245/40-18 with 40 offset on 18x8.5 with no rubbing. Put the tire on the opposite side and two more days of driving and no signs of rubbing. I think you could get away with 42offset but will be very close to the suspension, and probably around 35offset and stick out a little but not enough to hit the fender. If someone really wanted a 255, it would work on the suspension side at 35 to 38mm (pick a narrow model) but who knows about rubbing on the fender. I bet it would work.

My rims are 45offset, and the tire was only about 2mm from the suspension at 45mm. It might be ok but was too close for my blood so I put on 5mm spacers and now I have about 6mm of room which I'm comfortable with.



Pic WITHOUT spacer (45mm offset):






Pic WITH 5MM spacer (40mm offset):

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      03-04-2021, 07:11 PM   #2
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I run 245/40r18 square as well. My wheels are 18x8.5+40, but I need at least a 5mm spacer on the front to clear the strut.

I am RWD. I wonder if there is a small difference with the strut/knuckle with xdrive that pushes the wheel further away from the strut.

I run a 10mm spacer front because I'm running -2.5* front and rear camber. I also run a set of 17x9 +42 with a 10mm spacer front and 255/40r17.
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      03-13-2021, 01:50 AM   #3
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These pics and numbers are amazing thank you! Fitting my 18x8.5 ET35 setup in a few weeks and I have some PS4S in both 225 and 245 and I can't decide which to mount with the new wheels. You said 35 offset would probably stick out a little but not enough to hit the fender, how much clearance do you have currently to the fender with your +40 effective offset?
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      03-15-2021, 12:09 PM   #4
TrboMike
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With my front PS4S 245/40-18 +45 wheels and the car jacked up the tire is almost touching the strut I'd say a mm or two away. I can easily see it rubbing with aggressive driving, possibly even with the car off the floor jack. With the 5mm spacer (so now I'm at 40 offset) its about 6mm away.

With your 35 offset wheels you should have about 10mm of space from your suspension, more than enough. Mine are not rubbing the fender at 40mm, but your 35mm will be 5 mm closer to the fender than me. You might have a problem there. I've driving over some bumps and done some high speed turns and saw/heard no signs of rubbing. You might.

Also, if you have 245/35-18 (diameter slightly less than mine) you will get an extra 2mm or so on both sides.
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      03-15-2021, 01:27 PM   #5
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Quote:
Originally Posted by TrboMike View Post
With my front PS4S 245/40-18 +45 wheels and the car jacked up the tire is almost touching the strut I'd say a mm or two away. I can easily see it rubbing with aggressive driving, possibly even with the car off the floor jack.
It's my understanding that tires move away from the struts when driving aggressively.
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TC Kline Coilovers; H&R Front Bar; Wavetrac; Al Subframe Bushings; 18X9/9½ ARC-8s; 255/35-18 PS4S (4); Dinan Elite V2 & CAI; MPerf Orange BBK; Schroth Quick Fit Pro;
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      03-15-2021, 04:19 PM   #6
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Quote:
Originally Posted by dradernh View Post
It's my understanding that tires move away from the struts when driving aggressively.
Yes, with the forces being significantly higher on the tyre on the outside of the turn compared with the inside, the slight bearing play would cause the bottom of the wheel to pivot inwards and the top outwards. Should only be sub-millimetre, though. The lateral tyre flex under hard cornering only extends about the length of the contact patch fore and aft of the contact patch itself, so has minimal movement at the top of the tyre (mainly radial at high speeds due to centrifugal force on the belts and tread).
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