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2Addicts | BMW 2-Series forum Technical Topics Wheels and Tires -- Sponsored by The Tire Rack 235 & 255 8.5 or 245 square on 8?

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      06-14-2022, 01:28 AM   #1
SoftShoe
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235 & 255 8.5 or 245 square on 8?

While staying otherwise stock, on RWD it seems you can fit
A. 235 on an 8.5 in front at ET 38 (I’d run 255 in back)
Or
B. 245 on 8 at ET40 (I’d go square)

(Correct me if I’m wrong)

Which option is better for handling (make your case)?

Already have the LSD.

Last edited by SoftShoe; 06-14-2022 at 10:12 AM..
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      06-14-2022, 12:52 PM   #2
Kernel Kurtz
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For RWD I would go staggered to get maximum traction on the driving wheels. For AWD I think increasing traction in the front is as important as the back (even though the car ultimately decides how to distribute the power).

Just my opinion of course.
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      06-14-2022, 01:29 PM   #3
Cos270
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Biggest you can get out back will be "better" for handling. I went square 245 because I like the ability to rotate tires.
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      06-14-2022, 01:33 PM   #4
TrboMike
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If you track the car or drive hard enough on the street you're losing traction either front or rear and already have super sticky tires, then you need to choose. Otherwise you're doing it for the look because you wont notice the difference. 255 looks amazing in the rear from the car behind you.

If you launch hard (ie: drag race) you need more meat in the rear and less weight in the front. If you autox or drive hard at low speeds on the street, IMO you want the balance of square tires so the rear can loosen the same as the fronts (ie: bigger rears dont stay planted causing the fronts to plow more).

As you drive now, is your driving style the type where you're more likely to have the rear rotate out because you're hard on the gas and need to be careful about your throttle to not spin the car, or where the fronts will plow a little and you need to worry about your turn in speed? There's your answer.
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      06-14-2022, 04:46 PM   #5
SoftShoe
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Thanks for your perspectives and (trboMike) the very practical and thought provoking real world scenarios that I can plug myself into.

Helps clarify my thinking.

Also curious about grip limits out front:
• the narrower tire on the wider rim
vs
• the wider tire on the narrower rim.

Is it possible that, between the two front setups there’d be *very little difference in available grip at that end of the car due to how the tires sit on the rims?
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      06-14-2022, 08:48 PM   #6
dradernh
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Quote:
Originally Posted by SoftShoe View Post
Also curious about grip limits out front:
• the narrower tire on the wider rim
vs
• the wider tire on the narrower rim.
Other things being (at least somewhat) equal, a narrower tire on a wider rim places more rubber on the road than does a wider tire on a narrower rim. You may be able to visualize this, as stretching a narrow tire on a wide wheel necessarily flattens the tread. Doing the opposite results in a tread profile that is rounded, leaving less of the tire's tread width on the road.

As told to me by a Hoosier tire engineer, the general rule is that a tire's tread width* should be within +/- 1" of the wheel's width. He went on to explain that the narrower tire, when placed on a wider wheel, will have its tread "flattened" due to the difference in their relative widths.

To demonstrate this for me, a race shop owner who looked after one of my cars mounted both the same model 245-width and 255-width race tires on one of the 9" wheels I was running. He placed the long side of a framing square against the side walls and the short side across the tread. It was obvious that for that model tire, the 245-width tire would place more rubber on the road than would the 255-width tire.

* = I rely upon Tire Rack for accurate-across-manufacturers tire tread width measurements (see https://www.tirerack.com/tires/tiret...jsp?techid=201) while understanding that TR doesn't measure every tire available to us owners.
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      06-15-2022, 02:31 PM   #7
marcthedark
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And for x-drive either do 245 square or 225/255 to keep the difference in circumference at close to 0%
Going from 225/40 to 235/40 or 235/35 would create a difference front to back
I would go 245 square to dial out understeer but it depends on your use case (hard launch, looking for traction, canyon carving,..)
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      06-15-2022, 07:20 PM   #8
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Quote:
Originally Posted by marcthedark View Post
And for x-drive either do 245 square or 225/255 to keep the difference in circumference at close to 0%
I'd always heard/read that you should keep the difference in circumference on xDrive cars under 1%, but when I plug the factory staggered tire sizes (225/40 and 245/35) into a calculator I come up with 1.2%. And not only that, the fronts are the bigger of the two. Calculator does not take into account the half inch difference in wheel width, but it still sure looks like BMW is not taking the prevailing advice.
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      06-18-2022, 02:59 PM   #9
marcthedark
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Quote:
Originally Posted by Kernel Kurtz View Post
I'd always heard/read that you should keep the difference in circumference on xDrive cars under 1%, but when I plug the factory staggered tire sizes (225/40 and 245/35) into a calculator I come up with 1.2%. And not only that, the fronts are the bigger of the two. Calculator does not take into account the half inch difference in wheel width, but it still sure looks like BMW is not taking the prevailing advice.
That is correct - a 255 in the back would get it close to 0 - but not really recommended on an 8 inch wheel.
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