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      12-08-2015, 02:29 PM   #1
Hujan
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Load ratings on OEM tires on 325M wheels?

I am trying to figure out what air pressures to run on some aftermarket tires (Dunlop ZII) on my OEM 325M wheels. These are the 18" wheels that come on the M Sport package.

I have a calculator that will tell me what pressures to run based on the load ratings of the new tires, but I need to know the load ratings on the OEM tires. This is typically a two- or three-digit number followed by a letter (e.g., 99W or 101Y).

If you have these wheels on your car with OEM tires, I would greatly appreciate getting that information from you. Thanks!
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      12-10-2015, 09:43 AM   #2
paris1
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I run the same BMW recommended pressures on my non-RFT Bridgestones as the originals. See this thread http://e89.zpost.com/forums/showthre...re+for+non+rft.
I believe TireRack recommends the same. You may be over-thinking this issue.
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      12-10-2015, 03:17 PM   #3
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Hi Hujan, I think this is the same conversation we had a few days ago, but to answer your specific questions

Here are the specs of the OEM RFT that came on those wheels (88W and 90W load ratings) when they were mailed to me by CedarZ4

I ran the owner's manual pressure then landed on 32f/32r cold psi based on track experience (less grip in rear when using the the owner's manual pressures)
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      12-10-2015, 03:39 PM   #4
Hujan
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Quote:
Originally Posted by paris1 View Post
I run the same BMW recommended pressures on my non-RFT Bridgestones as the originals. See this thread http://e89.zpost.com/forums/showthre...re+for+non+rft.
I believe TireRack recommends the same. You may be over-thinking this issue.
Thanks for pointing that thread out. However, there is a well-established belief that the ideal PSI is a function of the load rating on the tire. Thus, when moving to aftermarket tires, the ideal PSI is generally a function of the load rating of the OEM tires and factory recommended PSI, then making adjustments for the load rating of the new tires. There are tables and formulae for this purpose.

Quote:
Originally Posted by nicknaz View Post
Hi Hujan, I think this is the same conversation we had a few days ago, but to answer your specific questions

Here are the specs of the OEM RFT that came on those wheels (88W and 90W load ratings) when they were mailed to me by CedarZ4

I ran the owner's manual pressure then landed on 32f/32r cold psi based on track experience (less grip in rear when using the the owner's manual pressures)
Thanks, Nick. You are correct that this stuff is in the owner's manual. I was able to find that and use it for my purpose. I landed on 32 front and 38 rear based on my pressure/load rating table. I will keep an eye on the rear pressure and adjust as necessary.
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      12-10-2015, 08:54 PM   #5
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Quote:
Originally Posted by Hujan View Post
I landed on 32 front and 38 rear based on my pressure/load rating table. I will keep an eye on the rear pressure and adjust as necessary.
Hujan, I trialed 36f/42r at the start, and found there was a substantial part of the tread block (particularly the outer edge) that will always appear clean (i.e. not making contact with the road) and when the car gets driven harder and the rear tire pressure go up it will feel "greasy" / "slippery" in the rear

I interpreted this as an "over inflation" situation and reduced the pressures until I got a more consistent contact (i.e. scuff marks on tire tread went to the edge of the sidewall with the little triangles, and not past the triangles). This resulted in the 32f/32r recommendation. The grip levels of the car were also more consistent when hot with those cold psi. This was easier for me to notice, since there tires were brand new and it was obvious which part of the tread was being scuffed and which wasn't.

also, in commuting, I didn't notice a difference either way. The grip level of those ZII is huge compared to the RE050A I (which I took down to wear bars before replacing, including doing a DE on them)

Lastly, do you have access to a pyrometer? I suppose if you wanted to be really quantitative, you could look at the pressures that would result in even temps across the full tread block, rather than the eyeballing/guess and check method I used

Please post some pics of your build when done!
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