? Regarding widening my BBS RK 18X8.5 to 18X10 in the rear? Backspacing

Jomo1994z71

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Need some input for my 2003 Ford Mustang Cobra with IRS

I have a (4) 18X8.5 BBS RK that have ET25 (backspacing).

I really want 18X10 BBS RK wheels that can accept a wider tire such as a 315/30/18Dr but these aren't made anymore, but factory spec on the 18X10 that they used to make was ET20.

My question is if I widened the inside of the rim to 10",
what would my backspace for the newly widened (from the inside) 18X10 rims by Weldcraft be?
What would the rim look in layman's terms?

Thanks for the input SVT as this is all a little new to me.:rockon:
 

ripnesscobra

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The specs ET25 and ET20 you are referring to are actually wheel offsets. From my calculations, if you widened an 18X8.5 wheel with ET25 offset to 18X10 the widened wheel would have ET44 offset (25 mm + (.75 in*25.4 mm/in) = 44 mm). So the widened 18X10 wheels with ET44 offset would stick almost an inch closer to the IRS bracket/bolt than the 18X10 wheel with ET20 offset. The tire would sit even more inside the fender than it did from factory as well. To solve both of these issues, you could run 25 mm hubcentric spacers.
 

Jomo1994z71

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The specs ET25 and ET20 you are referring to are actually wheel offsets. From my calculations, if you widened an 18X8.5 wheel with ET25 offset to 18X10 the widened wheel would have ET44 offset (25 mm + (.75 in*25.4 mm/in) = 44 mm). So the widened 18X10 wheels with ET44 offset would stick almost an inch closer to the IRS bracket/bolt than the 18X10 wheel with ET20 offset. The tire would sit even more inside the fender than it did from factory as well. To solve both of these issues, you could run 25 mm hubcentric spacers.

Thanks for the input...never liked the idea of running a spacer.
If you were in my situation, would you just hold out and try to buy some 18X10s down the road or widen the 18X8.5 and run a spacer?
 

ripnesscobra

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If you have the patience to wait it out, I would definitely go with the factory 18X10s with ET20 offset. You are eliminating 2 additional factors/components that way (widening and spacers).
 

ripnesscobra

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Having said that, I have been running 30 mm spacers on my factory 17X9 wheels for a few years with no issues. I couldn't stand the tucked in look. I now have AFS 18X9 and 18X10.5 chrome replicas with 275/35/18 and 315/30/18 Toyo Proxes R888s waiting to go on.
 

carrew

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Talk to weldcraft about welding a hub spacer on to the wheel. One of the local guys has the 17x9 BBS RX and he had the rear widened to 11" and they also welded a hub spacer on as well to his backspace specs. He still had to run the longer ARP stud.

I have the 18x8.5's and been thinking doing the same.

I run 275/35/18 on the rear now and I would say it would be the maximum. No bulge and the Rim Saver on the BFG makes it look clean.
 

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