) So, thought I'd post here on 311S.org and if it might help just one person, then it was worth it. If there's anything inaccurate or unclear, feel free to PM me.============================================================
Not sure if this has ever been covered here PICTORIALLY. But if not, well, this might help some with the concept of WHEEL OFFSET.
Most of you on this forum can probably skip this; as WHEEL OFFSET is something that most know and understand. But I had these NEW wheels to use as a clean whiteboard so thought I might give it a shot to help anyone not quite clear on the concept (because offset is often a confusing subject being somewhat counter intuitive.)
For the benefit of anyone who DOESN'T understand wheel offset, this pictorial might help you get a better understanding for when you go to choose a set of wheels for your Z. This pertains mostly to aftermarket wheels but applies equally to purchasing a set of OEM wheels - as is the example here - to fit the way you might want them.
First thing is the explanation of what wheel offset is:
Offset is the distance that the MOUNTING FACE of the wheel is away from the center line of the wheel.
Terms you'll see/hear bandied about when discussing or fitting wheels to your car:
"Positive offset" means the wheel's mounting face is OUTBOARD of the center line, e.g., +30mm
"Negative offset" means the wheel's mounting face is INBOARD of the center line, e.g., -10mm
"Zero-offset" means the mounting face is directly at the wheel center line, e.g., +0 offset
It should be noted that most contemporary cars GENERALLY use a positive offset.
Something else you might need to know about how offset influences the car's look and behavior:
Decreasing the amount of offset, say, from +30mm down to +15mm, will move the wheel and tire assembly away from the car and closer to the fender lip. (Example pics - before/after - shown below.) Keep this in mind when purchasing wheels as clearance with the fender lip may be compromised and require fender lip "rolling", the practice of bending the inner lip up and away from the tire for clearance.
Offset is engineered into the overall suspension design by the manufacturer to account for the different type and physical geometry of the suspension. It is designed in for purposes of steering response (how quickly and how smoothly the car reacts to steering input) as well as for comfort and, most importantly, how the car handles and "feels" when delivered to the end user/customer.
Factory Z33 wheels are "mostly" +30mm (and +33mm rears) which means the mounting face sits 30mm outboard of the centerline. This amount of offset is engineered into the wheel to provide smooth running as well as giving your Z moderately crisp steering response - given the Z's stock suspension. The picture here shows these wheels carry an offset of approximately +50mm as they are 370Z specific wheels, whose front wheels use a larger offset than the 350Z. (More on this below.)
Deviating too far from this "could" ultimately cause problems with clearance, increased wheel bearing wear, and sluggish steering response; but for the most part, a modest deviation (+/- 15mm) will be fine and actually can improve your Z's handling - up to a point of diminishing returns.
Those effects won't be covered here. Suffice to say that you CAN go too far positive OR negative and any gains in performance you got by going for a modest deviation from stock will start to fall off drastically, hence "diminishing returns".
So let's say you bought a car with a set of aftermarket wheels on it or you want to pick up a set from someone on craigslist or the classified section here. (Generally speaking, when selecting NEW wheels, the specifications are noted and should be strongly heeded.)
How can you be sure of what you're getting and/or whether they will fit properly; and if so, how WELL do they fit? Best way is to actually take a physical measurement (barring casting marks indicating offset on the inside of the wheel - which is actually pretty rare and not a given for most wheels).
Start with a bare wheel, off the car. Get a ruler or tape measure (ruler is better as it's stiffer and will give a more accurate measurement). And find the centerline of the wheel. Mark it with a Sharpie.
In this pic, while the wheel is advertised as a 9" wide wheel, the actual width - including the inner and outer lips - is closer to 10" - give or take a few mm, dependent upon where you take your measurements from. This isn't always a precision measurement and actual measurements will be within a few mm, give/take. Just be consistent when measuring, e.g. outer lip to outer lip or inside bead to inside bead. (I personally find it best to measure outside to outside to find the most "precise" center line location.)
Note that I marked the center line. (I'll get to the second line in a second.) By the way, please note that there's a popular misconception that the center of gravity and the rotational line is or should be at the wheel center line. Nope, pretty easy to see that the center of gravity is going to be somewhere near or in the spoke line of direction, outboard of the center line (and sometimes inboard). Don't concern yourself with this for measurement and fit.
Next, drop a stiff straight edge down along the mounting face and mark the wheel with a second line where the mounting face theoretically intersects the barrel of the wheel.
The distance between the two lines is the wheel offset.
You'll note that in this example, the offset measures about 50mm - which is pretty close to accurate as this is a 370Z specific wheel whose claimed offset is 47mm. Close enough for general work.
Note: if you are bothered by this 6.4% difference, you shouldn't be. In practical application, it's almost negligible. But if you are, you can take the claimed measurement and start the center line measurement from the point inside the wheel lip on each side or by the difference between the claimed width and actual width/2 and start from that distance inwards from the outer and inner lips.
Confused? Yup, meant to be.
For general application, just measure outside to outside.Now take note here as this is why this whole thing came about in the first place. The +47mm offset of this wheel is unsuitable for the Z33 platform (including G35) despite this being a Nissan OEM product - built by Rays, as are all of the Z "sport wheels" - as the mounting face sits too far outwards of the wheel center line (too much positive offset) and because of this, steering response will be sluggish and take away from the driving experience that is the Z33. (It works fine on the Z34 because it was engineered to work that way with different suspension geometry, sizing, etc.)
To make it work for the Z33, a spacer is "required" to bring the offset back into an "acceptable range".. Let me clarify that... The wheel clears the brakes and suspension without a spacer but again, steering response suffers unless you "restore" the offset to closer to stock or below. (This is due to an alteration in the scrub radius, which I won't cover here either to keep this more general in nature; but suffice that a "too large/positive" positive offset is just as detrimental as a "too negative" offset without some pretty serious suspension modifications to compensate for the change in overall suspension geometry, of which the wheel is a big part of).
Note in the following pic, I have installed a 20mm spacer on the wheel (for illustration only; as the spacer mounts to the hub (or sits freely between the wheel and hub in the case of slip-on spacers) then the wheel attaches to the spacer). Yeah yeah, some might think, "Mic, that's pretty obvious!" Uhhh, no..... it's not. Trust me, best to write to a lowest common denominator and take the ridicule for sounding so "obvious."
Note: I do NOT like spacers. But contradicting that, I do have probably 5 sets of varying widths in my tool box and I DO use them where warranted. (Depends on which of my wheel sets I'm using.) But, I do dislike them so much so that even though my rears could use them and I have another brand new set of 15mm spacers on hand, I chose to NOT use them since the rear wheels already have a "stockish" 30mm offset.
BUT, to use the fronts, it almost isn't an option due to the "huge" 47mm offset and I had to use them to restore them to something closer to stock wheel offset in order to maintain proper wheel-to-suspension geometry.
Now I take a mounting face measurement from the back of the new spacers and transfer that to the barrel of the wheel as I did the original back plane of the wheel. Note the NEW offset is 20mm smaller than the original. The new ~30mm offset from the spacer is what's called "effective offset". Artificial, yes, optimum for performance, no. BUT.... it's pretty close and more neutral vis a vis the stock offset wheels.
And that's a tangental, but related point on how and why spacers do have a place in the modder's tool chest. It might not be the best solution (buying the proper offset and hub centric diameters is always the best) but they work as long as you get quality spacers and install them with as much diligence as you do your wheels. (Proper torque upon installation and periodic re-checks.)
So, with that all said, let's take a look at how offset affects the placement of the tire and wheel in the wheel well and in relation to the suspension and fenders:
Here's a before and after set of pics:
Without spacer:
Note how "sunken into the fender well" the wheel sits. This not only looks funky, it hurts performance as mentioned above.
With spacer:
Subtle difference to the naked eye, I know, but if you were standing directly in back (or front in this case) looking down the center line of the tire, you'd see that the "subtle difference" relative to the fender lip is not so subtle. 20mm is fairly substantial when it comes to tire placement and how it affects the steering response, handling, and overall "feel" of the car.
Anyhow, hope this helps take some of the confusion out of wheel offset. This just touches on the subject but should answer the most basic questions about what and how it helps you set up your tire/wheel combo the way you want it.
Bottom line: the most popular offsets for performance application range from about +15 to +25 or so (Z33 ONLY, all others may vary). There are obviously exceptions to this for those seeking the "flush" or "cambered" look but my advice for best all around performance, one should keep the offset in this range unless you're tracking, stancing, drifting, etc. - in which case, best to consult with others who have or use wheels that deviate from this for "the hot tip". Again, this is just for general knowledge, not recommendation.
As a post note, it is highly recommended that any time you get new wheels/tires - particularly if you've deviated from OE sizing - get an alignment! All four. Nothing hurts a car's performance worse than a sizing change that screws up your alignment. And it will. You'll be sitting there thinking: "Aww hell, I just dropped a grand on new rolling stock and the car doesn't handle like it did! This sucks!"
On OE suspension, you can only set toe, which is OK. But if you are serious about a genuine improvement in your Z's handling, you'll consider adjustable alignment componentry that allows for suspension adjustment in three dimensions: camber, caster, toe. At the least, do so when changing suspension to performance gear. ESPECIALLY lowering it. Again, don't be that person that ends up disappointed with the new suspension when it's actually a simple alignment you need. Many other suspension/tire threads out there describing all of this, but it bears mentioning here.
Cheers,
Mic
Both of those sub-systems rely on wheel speed sensors looking for a F-R delta in wheel speed, too much at one end indicates possible loss of traction/tire spin; taking out or adding too much height on one or both ends can give a false-trigger on either.
