Few questions:

Can someone throw me a link to a good page explaining what all the tire numbers mean?

I thought our rear tires were wider then our front tires? When my friend's dad read the numbers on the tire, the widths were the same, but the diameters were different (front and rear)....what's the deal?

I would like 19" rims, but I don't want to sacrifice my launches. Would really light 19's make up for this? Or should I stick with really light 18's, and just lower it?

 

theres links, but easier.

the big number, in the case of stock 350Z with 18" rims is 245, this means the traction patch is 245mm wide, width, front is 225, hence the wider rear tires. then you have 45, this is a percentage, it means 45% of the first number 245, about 110, this means 110cm of sidewall, remember this is only sidewall, and you have 2 sidewalls in a diameter, so if you took sidewall*2+ the last number, 18, which is rim size, youd have total diameter, but dont forget that rim size is inches, and the rest is metric millimeters.

light weight 18s will be lighter and cheaper than identical brand 19". pick a rim, compare 18" to 19" 19s cost more and weigh more, so if your going apples to apples, 18s are better from performance standpoint, but on the other hand, you could get the lightest 19" rim for 2500$, and compare it to a 1000$ set of 18" rims and come to a conclusion that 19" rims are lighter.

but 19s have the appearance, you can get pretty close to the same weight of 18 and 19, maybe 2-3lbs difference between identical styles. all up to you, your car, so your prioreties and your choice is your own.

 

http://www.miata.net/garage/tirecalc.html

bigger tires weigh more too!, even to the point of offsetting rim weight. 245/45-18 stock tires weigh 24 lbs each + 18lbs on the rays or 26 on the stock 18s. thats 50lbs for a stock 18" wheel/tire. My 19s are pretty light for 19s at 22 lbs, but with tires, my rears weigh 60lbs. My 17lb 17" race tires with race rubber weigh 44lbs combined. If you want light, get lighter 18s. I can tell a huge difference between my 17s and 19s!

 

Those rims in the pic are 19's right? Yeah , I care about performance more than looks...looks like i'm going lighter 18's

thanks for the info guys

 

yeah, they are 19s. 18s with the proper offset will look just as good, I think. I would go for 18x8 front +19 to +25 offset with and 18x10 rear with +27 to +33 offset. or equivalent offsets in 8.5-9" wide fronts and 9-9.5" rears.

245/40 and 275/40 18s

 

What exactly do you mean by offset?

 

how far it sticks out from the mount location, lower the number farther it sticks out I think... if you have a wider tire you have to make it stick out farther so it doesnt hit the wheel well.

idealy you want to keep it flush with fender for best appearance(IMO anyway) some very wide tires will stick out even farther, and you can roll fenders. in my state(GA) its illegal to have tires stick out beyond fenders. so you might want to keep that in mind depending how wide you go and what offset you get.

 

Picture an 8" wide wheel, with the mounting point right in the middle at 4". That is a 0 offset wheel.

If you move the mounting hub inward, away from the face of the wheel, that would be a negative offset wheel. (deep dish, like a cragar drag wheel)

If you move the mounting hub out, toward the face, that would be a positive offset wheel. (flat faced fwd wheels)

A +21 offset means the mounting hub is located 21mm from the midpoint of the wheel toward the face. Why wheels switch between metric and sae is a mystery to me.

calculating required offsets is a little above my pay grade, however. I left it to iforged.

 

Don't you mean from the face toward the midpoint? Cuz 21mm from the middle of the tire would be ridiculous..


..or maybe i'm jst not comprehending. What offset seems pretty obvious to me, but I wanted confirmation. I think what I really need is a pic

 

You can find a lot of good info here
http://www.yokohamatire.com/csunderstand.asp

Offset is discussed here (at the bottom of the page)
http://www.yokohamatire.com/utcustom.asp

2003z's explained correct, but like you said maybe you can't picture it.


Wheel offset
The wheel's offset is the distance from its hub mounting surface to the centerline of the wheel. To determine wheel offset:

* Position the wheel on a flat surface and measure its overall width.
* Subtract the backspace and divide by two.

The offset of a wheel can be one of the following three settings:

Zero offset:
The hub mounting surface is even with the centerline of the wheel.

Positive offset:
The hub mounting surface is toward the front or wheel side of the wheel. Positive offset wheels are generally found on front-wheel drive cars.

Negative offset:
The hub mounting surface is toward the back or brake side of the wheel's centerline. "Deep dish" wheels are typically negative offset.

 

Thanks guys. Yeah, The picture helped confirm what I thought offset was in the first place. Now I have a couple questions. First, read the post below.

OK, after reading that, my question is: What is the function of an offset wheel? Also, for 2003z, why would you have positive offsets?

 

Not all wheels come with 0 offset from the factory. The stock Z wheels has 30mm of offset (front and rear). 2003z's wheels have 21mm of offset (in the front)...that means that he has 9mm less offset....or think of it this way, his wheels are pusehd out 9mm towards the fender. The reason many here are getting lower offset wheels is because the Z looks kinda silly with skinny, tucked wheels under its huge, wide fenders. Only custom wheels let you choose your own desired offset.

Changing the offset can affect performance. By moving the front wheels out, you increase something called the scrub radius. This could cause the steering wheel to be a bit heavier, and the car will be more succeptable to bump steer and road wandering...where the car will steer with the angle of the road more than stock.

 

these are all positive offsets right? So when picking new rims I assume I should try to learn toward a positive offset in both the front and rear tires?

Russ said a posotive offset was typically sued in front wheel drive cars.. When exactly is a negative offset used? For drag applications maybe?

 

Just think of the offset measurement as a measurement...like wheels can come in differenct sizes. Engineers design the suspension geometry knowing what the wheel ofset is going to be. When seecting your new wheels, you want to get as close to stock offset as a) asthetics allow b) how much clearance you need....going to a wider wheel requires a higher offset. Check out the offset calculator at the top of this forum.

Where are negative offsets used? I have no idea.