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      11-22-2011, 09:23 AM   #1
Plaz
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Please help check my wheel math

OK, so I'd like to use the BBS REs I had on my E46 many moons ago as winter wheels for my 1M. Here are all the details... can you tell me if I've got my spacer math correct here?

Stock 1M wheels:
19" x 9" wide, @31mm offset (front)
19" x 10" wide, @25mm offset (rear)

BBS REs:
18" x 8" wide, @35mm offset (front)
18" x 9" wide, @45mm offset (rear)

So, if I run 20mm spacers (!) in the rear, and 5mm spacers in the front, I should be okay, right?

By reaching the same effective offset through the use of spacers, but running narrower wheels/tires, does that mean the outside face of the wheel will be at the same depth, or does it just mean the center of the contact patch will be at the same place?

Thanks for any assistance. I don't have a lot of experience calculating this crap, and it makes my brain hurt.
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      11-22-2011, 09:27 AM   #2
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You will have 12.5mm more space on the inside and outside of the wheel. By keeping the same effective offset, your wheel position remains centered, but because the wheel is narrower you'll have exactly a half inch more space on the inside and on the outside.

If you want the same fender to tire clearance (assuming the same overall sidewall shape), you would have to subtract an additional 12mm from your offset number, meaning you'd have to use a 17mm spacer in the front, and a 32mm spacer in the rear.

The spacer sizes you listed will allow you to use the REs with no issue. They may just not be an aggressive enough fit for your tastes.
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      11-22-2011, 09:33 AM   #3
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Quote:
Originally Posted by Ilia@IND View Post
You will have 12.5mm more space on the inside and outside of the wheel. By keeping the same effective offset, your wheel position remains centered, but because the wheel is narrower you'll have exactly a half inch more space on the inside and on the outside.

If you want the same fender to tire clearance (assuming the same overall sidewall shape), you would have to subtract an additional 12mm from your offset number, meaning you'd have to use a 17mm spacer in the front, and a 32mm spacer in the rear.

The spacer sizes you listed will allow you to use the REs with no issue. They may just not be an aggressive enough fit for your tastes.

Thanks so much... very helpful. I'm more concerned with maintaining the geometry than maintaining the appearance, so I think I'll go with my initial spec here. Plus the fact that I'm already a little skittish with using such an enormous spacer in the rear... adding another 12mm just for looks doesn't seem like such a great idea.

Thanks again!
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      11-22-2011, 09:54 AM   #4
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Quote:
Originally Posted by Plaz View Post
Thanks so much... very helpful. I'm more concerned with maintaining the geometry than maintaining the appearance, so I think I'll go with my initial spec here. Plus the fact that I'm already a little skittish with using such an enormous spacer in the rear... adding another 12mm just for looks doesn't seem like such a great idea.

Thanks again!
Agreed. If outer spacing is not a concern to you from an appearance standpoint, then you are right on! I would also not recommend using such a large spacer- I was more speaking hypothetically in order to explain what it would take to have equivalent outside fitment.

I'm looking forward to seeing photos once you have the REs installed! They'll look great on a 1M.
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      11-22-2011, 09:54 AM   #5
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Okay, so I changed my mind and split the difference a little... ordered 12mm spacers for the front and 25mm spacers for the rear.

Thanks again!
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      11-22-2011, 09:56 AM   #6
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No problem, feel free to PM or call with any additional questions.
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      11-22-2011, 09:56 AM   #7
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Quote:
Originally Posted by Ilia@IND View Post
Agreed. If outer spacing is not a concern to you from an appearance standpoint, then you are right on! I would also not recommend using such a large spacer- I was more speaking hypothetically in order to explain what it would take to have equivalent outside fitment.

I'm looking forward to seeing photos once you have the REs installed! They'll look great on a 1M.
I'll definitely post some pics once they're mounted.
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      11-22-2011, 10:07 AM   #8
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Your offset calculations to the BBS 18 will maintain the original 1M offsets. This will maintain the original scrub radius for the car. As mentioned, this will move the tire/wheel in a little from the fenders. Personally, I would rather maintain the scrub radius than correct the appearance. This assumes you will be using OEM diameter tires.

You can move the tire out, i.e., decrease the offset, and maintain the scrub radius to some degree, by decreasing the diameter of the tire. But, even that strategy has some effects on spindle loading which amplifies the influence of caster. Caster is significant on a 1M, over 7 degrees. Hope this helps.
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