If you take the wheels off your Boxster, you will notice that the center of the wheel is not lined up at the halfway point of the width of the wheel. The center is towards the outside of the wheel. This is called “Offset”

Offset if a figure expressed in millimeters of the amount the center of the wheel (where it attaches to the axle hub) is offset from the midpoint of the width of the wheel. It is designed to push the center of the wheel towards the outside of the car for purposes of suspension geometry and the fitment of brakes, shocks etc.

So how do you figure out what the effect of a given size wheel in inches and its offset in millimeters will be relative to Porsche stock wheels that are known to fit?. How do you make sure the three clearances shown in the second figure in red clouds will be OK and the wheel won’t rub on the fender or the shock tower or the control arm? And you have to figure this out for both the front AND the rear wheels! The process goes as follows:

HOW FAR WILL THE PROPOSED WHEEL STICK OUT

Step 1 Get the figures for the stock wheel from page 239 in the Owner’s Manual that are closest in size to the one you are proposing to use. Example: for a 2001 S 17” 7” 55mm offset front, 8.5” 50mm offset rears.

Step 2 Compute the front wheel offset to the outside of the wheel provided by the stock wheels. Divide the wheel width by 2. Multiply that figure by an inches to millimeters conversion factor of 25.4. You now have the width of the wheel in millimeters. Subtract the offset. Example: (7/2)*25.4=88.9 88.9-55= 33.9 mm

You now have the distance in mm from the hub that the stock wheel will extend to the outside.

Step 3 Do Step 2 again for your proposed wheel. Example for the proposed 18 front wheel. 8” 50mm offset fronts. 10” 47mm offset rears. (8/2)*25.4=101.6 101.6-50 = 51.6 mm

Step 4 Calculate difference from the stock wheel 51.6 minus 33.9 yields a 17.7 mm difference. Dividing by 25.4 and multiplying by 16 and rounding for us non-metric types to better visualize gives an 11/16” difference in the distance from the hub to the outside of the wheel. Remember, if the proposed wheel minus stock wheel is a positive number, it will stick out further. If a negative number, it will not stick out as far.

WILL THE NEW WHEEL BE CLOSER OR FURTHER FROM THE STRUT

Step 5 Compute the stock front wheel offset to the inside of the wheel. Divide the wheel width by 2. Multiply that figure by an inches to millimeters conversion factor of 25.4. You now have the width of the wheel in millimeters. Add the offset. Example: (7/2)*25.4=88.9 88.9+55= 143.9 mm

You now have the distance in mm from the hub that the stock wheel will extend to the inside.

Step 6 Do Step 5 for your proposed wheel. Example for the proposed 18 front wheel. 8” 50 offset fronts. 10” 47 offset rears. (8/2)*25.4=101.6 101.6+50 = 151.6 mm

Step 7 Calculate difference from stock wheel 151.6 minus 143.9 yields a 7.7 mm (5/16”) difference in the distance from the hub to the inside of the wheel. Remember, if the proposed wheel minus stock wheel is a positive number, it will come closer to the strut. If a negative number, it will be further away from the strut.

Do the same calculations for the rear wheels as well. As they said in school, this is left as an exercise for you to do.

Once you have the decreases in clearances established, go out and measure on your car what moving the wheel both in and out would do at the 3 clearance points. In addition, turn the front wheels to the both left and right steering stops and check how far from the tire ends up from hitting the inside of the wheel wells. This will also change with new wheels.

Typically, if the proposed wheel is too close to the cars centerline and may interfere with the strut , you can move the center line of the wheel away from the center of the car by using spacers. Spacers sit between the hub on the axle and the wheel and move the wheel out so that it clears the inner suspension pieces. Remember that while the spacers move the wheel away from the struts, it also will widen the stance of the wheels. Widening the stance increases the stresses particularly on the wheel bearings.

There are two kinds of spacers typically used

a. Smaller ones that consist of just a ring and which use longer wheel bolts

b. Larger ones that require additional bolts between the spacer and the hub.

I’ve seen spacers of 5, 7, 10, 14, 15, 17, 20 and 23 mentioned. Once you start using spacers, the wheel locks no longer fit!