Will Those Wheels You Want Fit?
Wheels you are thinking of buying may come in sizes and offsets specifically tailored to the Boxster, or they may not. If they don’t and if the seller doesn’t provide recommendations for fitment, how do you know if they will fit? If you want to use wheels designed for a 911-varient, can you? And how?
Helpful Illustrations
The point of the above illustration is that the overall diameter (and thus circumference) of the wheel/tire combination should not change as you change wheel widths. Thus the speedometer and odometer will remain accurate. Many tire sellers can give you the rolling diameter of the tire for you to compare against your existing tire so that you can tell the effect on the speedometer and odometer. Changing diameters also affects the calculations the ABS and stability control systems have to make. Keep it awfully close to the original design or you are asking for trouble.
As you change wheel sizes, to keep the total rolling circumference the same, the amount of the radius taken up by the tire must change as the wheel portion gets larger. However, the stiffness of the sidewall changes as there is less crush space to prevent contact by the wheel against the bump or pothole you hit. This affects the comfort of the ride (every bump gets transmitted to the suspension and thus into the cabin), the handling (more responsive) and the survivability of the tire. I wouldn't have super low profile tires on a Porsche without road hazard insurance.
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!
A quick online calculator is here. And here. If it passes the quick test, then do the following.
Step 1 Get the figures for the stock wheel (from page 239 in the 986 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.
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 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.
Even Porsche themselves used spacers (5 mm) on the 2004 50th Anniversary Edition Boxsters to move the wheels out further towards the body in the wheel wells.
I’ve seen spacers of 4, 5, 6.5, 7, 10, 14, 15, 17, 18, 20, 21, 23, 27 and 30 mentioned.
Once you start using spacers, the wheel locks no longer fit!
A good source of advice on wheel fitments is www.wheelenhancement.com. They offer a full range of wheels and spacers as well as telephone advice. They know what works.
An excellent presentation of wheels that can be made to fit is at www.gmpperformance.com.
All wheels aren’t equal. Just because they look alike doesn’t mean they have the same weight, durability or strength. Ask around on PPBB for people who have had experience with the wheel you are craving and be sure and tell them the model year and any suspension modifications or options your car has.
Daniel Flour (who provided much of this information suggested that if you are considering buying a wheel where there is any question about fit, (and of course the seller 'guarantees' the fit), that you get the seller to ship one wheel. Take off the old wheel/tire and mount the new wheel without even a tire tire on the hub. This will allow you to see a lot. Certainly you'll see the inside wheel/strut clearance. You'll also see how far the new wheel will stick out. Consider it insurance.
The arguments for bigger wheels I hear are:
1. Looks better. Fills the wheel well up.
2. Allows a lower profile tire with less sidewall flex which gives more precise handling.
3. Gives opportunity for using a wider wheel and tire to alter/improve handling.
The arguments against I hear:
1. Cost of wheels to upgrade (partially offset by selling your old ones (on eBay?)
2. Cost of larger radius tires is more than the smaller radius ones.
3. Ride is harsher as there is less give in the tire.
4. Danger of damage to tire and wheel increases as radius of tire increases.
I'd try to drive a car equipped the way you plan yours to be to see how you like the setup. With the cost of wheels and tires for our car, you don't want to make a purchase you'll later regret.
You make the call. It is ultimately your car and your money.
The suspension was strengthened on the '98 and later cars to allow for the additional strain that the larger wheels and their wider tires with additional grip imposed on the suspension parts. In theory, you aren't supposed to use other-than-stock 16" or 17" wheels on the '97 model. But the truth is that failures came under competition conditions so my feeling is if you just want the look and aren't going to track or Auto-X the car, you'll be fine. Many have done this. But read below.
As you go up in wheel size, you use a tire with less crush space. So, when your tire hits a bump, pothole or whatever, the space the tire can crush to absorb the blow before the expensive wheel gets involved is less the larger the wheel you use. And if the tire runs out of crush room, it gets pinched between the hole/bump and the rim and the tire gets cut. So if you are going to use these wheels on your Boxster, you'd better be from an area that has awfully smooth roads or you'll be making repeated painful $ acquaintances with one of the wheel repair places at beyond $150 an incident (if it can even be fixed) and with the tire shops.
from a "mee" post on ppbb
For the front, you don't need anything to fit 996 wheels on a 986. The front wheels are generally interchangeable. The front 996 wheels are typically 1/2" wider but not always. The front of the 986 is nearly identical to the 996. Ironically, the more "manly" 996 front wheels are needed more on the Boxster which has proportionally much more front weight bias.
47mm to 48mm is the ideal offset for a 10" rear on a 986. 65mm offset from the wheel minus 17mm from the spacer = 48mm
Wheels that look like or were originally Porsche marketed models are available at less than new at the dealer prices from:
http://shop.wheelsrims.net/search.html?make=porsche&model=boxster&year=2003&part_type=
and
http://www.hartmannwheels.com/1-425-895-0000/wheels/Porsche/3
and
http://www.wheelenhancement.com/index.php?t=Wheels
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