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      05-01-2020, 03:57 PM   #1
PeterWT
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RWD F22 OE Scrub Radius?

I'll be mounting new wheels/tires on my OE suspension RWD M240i that will result in +4mm front and -4 mm rear scrub radius.
Front:
https://www.wheel-size.com/calc/?whe...cl=50mm&sr=0mm
Rear:
https://www.wheel-size.com/calc/?whe...cl=50mm&sr=0mm

I'm wondering what the OE scrub radius is, so that I know what the resulting scrub radius (F&R) will be. I realize it's not a big difference, but I'm curious.

Does anyone know the OE scrub radius specs, front & rear? TIA!
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Last edited by PeterWT; 05-01-2020 at 04:19 PM..
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      05-01-2020, 06:13 PM   #2
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Quote:
Originally Posted by PeterWT View Post
Does anyone know the OE scrub radius specs, front & rear? TIA!
Sorry, can't help you with an actual number. But I've ran the exact specs you are going for and couldn't feel a difference in steering. The electronic steering on our cars is already so disconnected that a few mm doesn't bother it all

I've also ran a setup, which this calculator says would be +42mm scrub radius at the front, still no trouble. (this calculator doesn't account for camber, so it was probably less in reality)
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      05-03-2020, 07:56 AM   #3
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Quote:
Originally Posted by msendit View Post
Sorry, can't help you with an actual number. But I've ran the exact specs you are going for and couldn't feel a difference in steering. The electronic steering on our cars is already so disconnected that a few mm doesn't bother it all

I've also ran a setup, which this calculator says would be +42mm scrub radius at the front, still no trouble. (this calculator doesn't account for camber, so it was probably less in reality)
Thanks for sharing your experiences msendit.
I realize I probably won't feel any difference due to the small scrub radius changes, but I'm curious to know what the OE specs are.

Does anyone know what they are, or know where I could find them?
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      05-03-2020, 10:49 AM   #4
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Quote:
Originally Posted by PeterWT View Post
Thanks for sharing your experiences msendit.
I realize I probably won't feel any difference due to the small scrub radius changes, but I'm curious to know what the OE specs are.

Does anyone know what they are, or know where I could find them?
It is unlikely to be published by BMW, but measuring the position of the upper and lower ball joints relative to the hub face with a straight edge against the hub will give you the ball joint offsets. With the ball joint offset you can then calculate the projection of a virtual line to the ground and then determine it's offset relative to the centre of the tyre contact patch, to give you the scrub radius.
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      05-03-2020, 11:16 AM   #5
PeterWT
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Quote:
Originally Posted by aerobod View Post
It is unlikely to be published by BMW, but measuring the position of the upper and lower ball joints relative to the hub face with a straight edge against the hub will give you the ball joint offsets. With the ball joint offset you can then calculate the projection of a virtual line to the ground and then determine it's offset relative to the centre of the tyre contact patch, to give you the scrub radius.
Thanks aerobod. Yeah, it does seem unlikely BMW would publish that.
I thought maybe one of the folks that have done suspension mods might know.
I may try taking some measurements when I swap the wheels, to calculate an approx value.
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