| Hi, I'm preparing to buy new wheels, so i've spent many hours researching wheel sizing issues, including scouring hundreds of posts here. Fortunately, there's a great deal of information and specific data to be found. Unfortunately, it's scattered throughout hundreds of posts, the FAQ, and other websites, and in some cases there's contradictory or incorrect information that was never explicitly resolved or corrected. One reason for this situation is that there's usually no simple answer to the question, "What will fit?". Even though it's difficult to generalize, it should be possible to at least determine reasonable limits for offset for a given wheel size, with the understanding that when approaching the limits of offset, the answer may be affected by unusual spoke designs, tire sizes, suspension setups, etc. I'd like to summarize my findings in a way that (with any corrections, comments, or additions from others here who are surely more knowledgable than i) could serve as a definitive guideline for future reference (as well as helping focus my own wheel search).
First, i will define "offset" as the horizontal distance from the centerline of the rim to the hub mating surface, with positive offset indicating that the rim extends farther inward, and negative offset (unheard of for our cars) indicating that the rim extends farther outward.
Next, backspacing is defined as the horizontal distance from the hub mating surface to the inner edge of the rim. Backspacing is related to offset by the formula:
bsp = width/2 + offset
However, width is conventionally given in inches and offset in mm, so we can use:
bsp (inches) = width/2 + offset/25.4 Finally, i'm not sure if this is a common term, but i'll define "frontspacing" as the horizontal distance from the hub mating surface to the outer edge of the rim. Frontspacing is related to offset by the formula:
fsp = width/2 - offset,
or
fsp (inches) = width/2 - offset/25.4 So naturally:
bsp + fsp = (width/2 + offset) + (width/2 - offset) = width
While wheels are described in terms of diameter, width, and offset, the critical dimensions for ensuring that a wheel will fit a particular vehicle are backspacing and frontspacing.
Backspacing is limited by the potential for the wheel or tire to contact the brake caliper, suspension or steering linkage, or inner fender. The maximum possible backspacing varies depending on the wheel diameter - for instance, it may be possible to use slightly greater backspacing on a larger diameter wheel, because the larger wheel diameter may help clear suspension parts.
Frontspacing is limited by the protrusion of the wheel and tire from the fender, and is not affected by wheel diameter (assuming proper tire size, of course!). With too much frontspacing, the tire may contact the fender lip when the suspension is compressed, or the front or rear of the fender well at the steering limits.
So, i've set out to find the maximum backspacing and frontspacing possible on the 300ZX TT, then use those limits to determine the possible range of offsets for a given wheel size.
By scouring this forum and some manufacturer and retailer fitment guides, I've compiled a fairly large set of data into a spreadsheet containing wheel and tire sizes (and models) that are reported to work on the Z32TT (without fender modifications), and those that have been reported to cause problems.
Based on these data, i've found that the maximum backspacing that has consistently worked with 17" wheels is 6.0" on the front and 6.42" on the rear. For 18" wheels, the maxima are 6.0" in front and 6.77" rear. I've seen one report of a 17x10 +41 rear wheel (6.61" backspacing) with no clearance issues, and another report where the exact same wheel dimensions result in "rubbing the HICAS sometimes", but 6.42" is the largest backspacing dimension i've seen reported with no problems for a 17" rear wheel.
Based on these data, i've found that the maximum frontspacing that has consistently worked is about 3.12" (front) and 3.41" (rear). Larger values (3.27" front, 3.58" rear) have been reported to "protrude slightly", though not specifically to contact the fender. On the other hand, some have warned that offsets that would (by my calculation) result in frontspacing of 3.28" in the rear would cause fender contact. Clearly there is a grey area at the limits, and based on the available data, i'd personally stay within 3.0" at the front, and 3.2" at the rear.
These maximum backspacing and frontspacing values limit the range of feasible offsets front and rear for 17" and 18" wheels as follows: (See caveats and other considerations below before reading too much into this!)
MINIMUM offsets (limited by maximum frontspacing of 3.12" F, 3.41" R):
- front:
-- 7.5" width -> +16
-- 8.0" width -> +23
-- 8.5" width -> +29
-- 9.0" width -> +35
-- 9.5" width -> +42 * see below for 17"
- rear:
-- 8.5" width -> +22
-- 9.0" width -> +28
-- 9.5" width -> +34
-- 10.0" width -> +39 * see below for 17"
MAXIMUM offsets (limited by maximum backspacing of 6.0" F, 6.42" R with 17" wheels, and 6.0" F, 6.77" R with 18" wheels):
- front:
-- 7.5" width -> +57
-- 8.0" width -> +50
-- 8.5" width -> +44
-- 9.0" width -> +38
-- 9.5" width -> +31 (note that MAX < MIN for this width; i have not seen any reported successful use of this size in front!)
- rear:
-- 17x8.5 -> +55
-- 17x9.0 -> +48
-- 17x9.5 -> +42
-- 17x10.0 -> +36 (note that MAX < MIN for this width, though i have seen one report of successful use of 17x10 +41 with no clearance issues) -- 18x8.5 -> +64
-- 18x9.0 -> +57
-- 18x9.5 -> +51
-- 18x10.0 -> +45
For convenience, i'll summarize the same information in terms of offset limits for a given wheel size that should avoid clearance issues:
- front:
-- 7.5" width -> +16 - +57
-- 8.0" width -> +23 - +50
-- 8.5" width -> +29 - +44
-- 9.0" width -> +35 - +38
-- 9.5" width -> not generally possible
- rear:
-- 17x8.5 -> +22 - +55
-- 17x9.0 -> +28 - +48
-- 17x9.5 -> +34 - +42
-- 17x10.0 -> not generally possible, though +41 has been done -- 18x8.5 -> +22 - +64
-- 18x9.0 -> +28 - +57
-- 18x9.5 -> +34 - +51
-- 18x10.0 -> +39 - +45
Caveats and other considerations:
- Regardless of the maximum and minimum offset that will fit without clearance issues, it's desirable to keep the offset close to stock values to reduce "tramlining". Stock wheel sizes are:
-- 16x7.5 +40 front (backspacing = 5.32"; frontspacing = 2.18")
-- 16x8.5 +35 rear (backspacing = 5.63"; frontspacing = 2.87")
For reference, the SMZ used the following wheel sizes:
-- 17x9.0 +38 front (backspacing = 6.00"; frontspacing = 3.00")
-- 18x9.5 +40 rear (backspacing = 6.32"; frontspacing = 3.18")
- When offset is near the maximum limit (i.e. the wheel rim is as far "in" as possible), spoke design affects brake clearance, and camber affects fender and suspension clearance. Using a tire bigger than normal for the wheel will of course also reduce clearance!
- For very close fits, calipers and HICAS linkage can be slightly shaved. - Spacers can be used to move entire wheel outward, similar to the effect of smaller positive offset, except that the spokes also move outward (not just rim), which could help with caliper clearance. However, spacers have their drawbacks as well. - Remember - this discussion pertians to TTs. N/As have no HICAS, so could potentially run more backspacing in the rear. Someone else will have to collect the relevant data for N/As!
I would be very interested in reading any thoughts, corrections, additions, and exceptions - e.g. if your wheels fall outside the limits listed here with no problems, or if they fall inside these limits but you've still had problems. It would be great if once this information is refined with wisdom and data from the more knowledgable members, it could serve as a reference for others, so nobody else would ever have to ask the question, "What will fit?"
...dave
|
definitive discussion of wheel sizes and offsets? - 
