This 944 Wheels FAQ was created to show you how
to measure and compute your own wheel specifications.
This will alleviate a lot of the confusion arising from the 14.4
Wheels section of the 944FAQ (more on this later).
Rather than throw up a bunch of numbers for you to memorize and
regurgitate as fact and gospel, I'd rather give you the ability
to generate these numbers yourself to confirm the validity of
other "experts".
Ok, let's go over the basics of wheel measurements. Refer to the wheel chart on the right for the terms discussed below.
WHEEL DIAMETER - This is the distance across the face of the wheel. A 16x7" wheel has a nominal diameter of 16-inches at the tire bead.
RIM WIDTH - Also known as the bead width. That's because this is the inner width between the rim flanges where the tire-bead sits. On a 16x7" wheel, this bead width measures 7-inches across. However, this dimension is impossible to measure with the tire on the rim. NEVER, EVER use this rim-width measurement in ANY of your calculations or you'll have errors and confusion (like in the Wheels Table of the 944FAQ).
OVERALL WIDTH - This can also be called outside width and is the distance measured across the outside edges of the rim. The overall width is ALWAYS wider than the rim-width (bead-width) by 0.5" to 1.0". This measurement can be obtained even with the tire on the rim.
Easy way to measure this is to lay two straight-edges across the diameter of the rim on the outside & inside edges (cut to clear tire). Then use a bungie-cord or giant rubber-band to hold them together and measure with a ruler through the hole in the center of the rim.
ALWAYS, always use this empirical measurement in computing the backspacing and offset of a wheel. So even if a rim is stamped 16x7", never use 7" (bead-width) in any of your computations; instead use the outside-width that you actually measured (will be 7.5" to 8.0"). That's because the computations for backspacing and offset uses the outside edge of the rim, NOT the bead-width (refer to chart).
BACKSPACING - This is the distance from the inner edge of the rim down to the hub-mounting surface. To measure this, lay a straight-edge across the backside of the rim and drop a ruler or tape-measure down to the hub-mounting surface. This measurement is useful for determining clearances with the struts and springs.
OFFSET - This is the distance between the centerline of the rim and the hub-mounting surface. An offset of ZERO would have the mounting surface exactly in the middle of the rim's width, centered between the rim edges and tire-bead. On our Porsches, there is an offset of 23 to 65mm which places the mounting surface OUTBOARD of the rim's centerline.
This measurement is useful in upgrading to wider rims. Maintaining offset close to stock measurements would split the extra width evenly between the outer and inner halves of the rim. So a 2" wider rim with the same offset as stock would have 1" more on the inside and 1" more on the outside. This preserves the steering geometry and feel as well as the wheel-bearing loads.
How to compute OFFSET (given OverallWidth and Backspacing)
Having measured the Overall Width and Backspacing, use the following
formulae:
Offset = Backspacing - 0.5*(OverallWidth)
For example, let's say we have a 16x7" Fuchs wheel with a measured Overall Width of 7.625" and Backspacing of 4.75", what is the Offset?
Offset = 4.75" - 0.5*(7.625") = 0.94" (23.8mm)
which is pretty close to the actual offset of 23.3mm stamped on the wheel.
How to compute BACKSPACING (given OverallWidth and Offset)
Backspacing = 0.5*(OverallWidth) +
Offset
So let's take that 16x7" Fuchs wheel again where we measured a 7.625" Overall Width. Also given the 23.3mm offset stamped on the rim, what is the Backspacing?
Backspacing = 0.5*(7.625") + 23.3mm = 3.81" + 0.92" = 4.73"
which matches closely with the 4.75" that we actually measured for the Backspacing.
As you can see, given any two of the three variables of OverallWidth, Offset and Backspacing, we can compute the remaining one. Going through BOTH sets of equations is a good way to double-check your measurements and calculations because it should be the same both ways. This is also a good way to confirm that others' measurements and computations are correct.
Problems and Errors with section 14.4
Wheels from the 944FAQ
Personally, I think this single piece of mis-information has caused
more confusion and headaches in regards to 944 wheels than anything
else (in particular the '86 951). For example, let's look at this
particular entry at the end of the table:
Year/Model |
Original Tire Size Front Rear |
Wheel Size |
Backspacing mm / inch |
Offset mm / inch |
|
| 944
Turbo 1987-1991 front: rear: |
205/55 VR 16 | 225/50 VR 16 |
7
J x 16 8 J x 16 |
153.9mm/6.06" 153.9mm/6.06" |
52.3mm/2.09" 52.3mm/2.09" |
| Sport/Club
Sport: front: rear: |
225/50 VR 16 | 245/45 VR 16 |
8
J x 16 9 J x 16 |
161.6mm/6.36" 174.3mm/6.86" |
60mm/2.36" 60mm/2.36" |
| M030
Option: front: rear: |
225/50 VR 16 | 245/45 VR 16 |
7.5
J x 16 9 J x 16 |
160.25mm/6.31" 174.3mm/6.86" |
65mm/2.56" 60mm/2.36" |
Without even going through the calculations, you can tell that this entry is clearly wrong (along with numerous others as well). Let's look at the first two entries and examine the progression from the 16x7" wheel to the 16x8" wheel in relation to how it affects the backspacing and offset measurements. If you maintain the same offset, increasing rim-width by 1" would place 0.5" on the outside AND 0.5" on the inside, thus increasing the backspacing by 0.5". This is clearly NOT indicated in the 944FAQ wheels section shown above.
Or conversely, if you maintain the same backspacing in going from 16x7" to 16x8", you put ALL of the extra width on the outside of the rim, thereby changing the offset. So in summary, there is NO WAY you can increase width from a 16x7" to a 16x8" wheel and maintain identical backspacing AND offsets. You can go through the caculations to confirm this, it just doesn't compute. So I would advise NOT using the 14.4 Wheels section from the 944FAQ.
Instead, learn to measure your own wheels and calculate your own specs. That's why I created this 944WheelsFAQ to hopefully clear this up.