Correct wheel offset for 986 cars

[bwdz]

I just picked up a set of Cayman wheels I want to put on the 986, they are 18s. Fronts are 8 with an ET57 and rears are 9s with ET43 I believe that is the numbers stamped on them. These are factory wheels so I would imagine they fit fine but anyone have any guesses as to how they will look? I couldn't pass up the deal, guy bought them for a 928 and they did not look good, he put new tires on them and drove a few blocks and they rubbed on the front, I got the wheels with $700 worth of tire on them for $400.

[Enginuity]

IN TERMS OF THE WIDEST WHEEL/TIRE SETUP FOR A PORSCHE BOXSTER 986 CHASIS...

INTRO:
For all-around performance and looks, go with 18x8.5" Front Wheel/Tire and 18x10.5" Rear Wheel/Tire setup. This 18" wheel/tire combination with good tires and suspension components amazingly allow the car to handle as if it's sitting on train-track rails! No spacers needed with this setup and they fill in the wheel well perfectly.

Hence, although 19" wheels work and look great, they don't perform as well, not even close. However, if you want to go with 19s and you're going to keep your car, I highly recommend that you replace all of your suspension components with race quality parts (from control arms to struts).

If you rub with either of the 18" or 19" setups I have mentioned, then you may own a car that has had front-end impact, and may require you to figure out what needs to be pulled out.

The positive aspect with going with a 19" setup is that your car ends up being a higher, which slightly helps reduce driveway/speed bump scrubs, until you need new tires.

CURRENT WHEEL DIMENSIONS (No Rub Issues):
:: Front: 19x8.5" (45 offset - 2" lip)
:: Rear: 19x10.5"****(43 offset - 4" lip)

CURRENT TIRE DIMENSIONS (No Rub Issues):
:: 
Front: 235/35/19 (25.48” Diameter)
:: Rear: 275/30/19 (25.49” Diameter)

FUTURE WHEEL DIMENSIONS (L+4" - R+4" Steel-Super-Wide-body Fabrication):
:: Front:19x9.5" (3" lip)
:: Rear: 19x13.5" (7" lip) +1" Spacers OR 1" Arm Extensions (alleviates spacers)

FUTURE TIRE DIMENSIONS (L+4" - R+4" Steel-Super-Wide-body Fabrication):
:: Front: 275/30/19 (25.5” Diameter)
:: Rear: 355/30/19 (27.4” Diameter)

THE SCIENCE BEHIND WIDER/STAGGERED WHEEL SETUPS:
In short, you typically want to stay within a 2" difference between the front and rear tires to avoid understeer/oversteer.

Based on this methodology, I should go with a 10" wheel up front; however, I would have to rework the entire steering and front end suspension system. And even then, for those who have driven a Viper with 10" wheels in the front and 13" i the rear, its challenging to turn. For cars that aren't setup for it, a similar wide-body setup would quickly wear-out relevant front and rear-end components.

Obviously, I am extending the fenders to accommodate the new wider wheels and tires; however, I sincerely do not recommend you doing what I am unless you have the funds to do it correctly (GT/RS specs).

Please be sure to also note that going larger and/or heavier will wear-out your bearings a lot quicker, but that goes for any sports car.

CONCLUSION
I truly hope this information helps one of you in some way. Such R&D knowledge doesn't come cheap and not many people I know or have ever met have produced such conversions unless they have large budgets to play around with. Personally, such R&D has cost me a few "million pennies" (so to speak), and what's left is pure knowledge, experience, and how to do things better each time around.

Best wishes,

MoMo
Self-taught Coach-builder/Industrial Engineer