Backspaceing and offset claification for woldson

[woldson]

Gollum was kind enough to give me some great insight to wheel sizing.

I thought that it was worth shareing this with others, and with Gollums permission here it is: (Thanks again Gollum)

 

 

I just remembered I'd said I was going to PM about this topic, and explain offset to you. Note that the heavy use of capitolization is for accenting words that you need to remember, I'm not sitting behind the computer angry or anything.

 

Ok, so the most important thing to understand is that for the most part you care about BACKSPACING and that's what really matters, but most rim manufactures go by OFFSET.

 

Both terms refer to the MOUNT surface of the rim, basically where the rim will seat against the rotor/hub assembly. The OFFSET is the distance the mouting surfce is to the outside or inside lip of the rim.

 

If it's closer to the OUTSIDE then it's a POSITIVE offset.

 

If it's closer to the INSIDE then it's a NEGATIVE offset.

 

Generally negative offset rims will have more lip, but it depends on how the offset is achieved. If a manufacture just adds more meat behind the mounting seface, basically making a cast in spacer, then no lip distance is gained.

 

 

BACKSPACING is the measured DISTANCE between the mounting surface and the INNER LIP.

 

So here's some examples (all in millimeters since that's what offset is normally measured in, but rims are normally in inches so you'll have to convert figures once you start looking at rims online)

 

 

So say a rim is 200mm wide with a +40mm offset. That means it has a 140mm backspacing. Here's why

 

200 divided by 2 (since a ZERO offset is in the MIDDLE, splitted the rim width in half)

 

200 / 2 = 100

 

Now ADD the offset (since moving the mouting surface outward puts the rim farther inward)

 

100 + 40 = 140

 

Now, if we have a 200mm wide rim with a -40mm offset the backspacing will be 60mm. The calculation will look like this:

 

200 / 2 = 100 - 40 = 60

 

Easy right? So lets do an online example.

 

http://www.racinglab.com/rota-rb-113x8-4100-4.html

 

Ok, so it's a 8 inch rim wich comes out to 203.2mm. So if it's a ZERO offset we'd have half of that width in backspacing.

 

203.2 / 2 = 101.6

 

And it's a +4mm offset

 

101.6 + 4 = 105.6

 

105.6mm = 4.157 inches.

 

 

So for clarification, why is offset so important? Say you buy a rim that is 9 inches wide but it's offset is only giving you 3.5 inches of backspacing then in a S30 odds are it's going to rub the fender pretty bad and you can't do anything about it, since adding spacers will make it WORSE.

 

Then there's the opposite. Say you get a 9 inch rim with 8 inches of backspacing (very common for FWD vehicles) well now you're goint to need a good 3-4 inch spacer to keep it from hitting your suspension. Though this works it's a bit dangerous. Either you'll need to install longer studs which will be weaker, or you'll need an adapter with new studs and you'll be running the risk of not only the rim loosening over time, but the adapter as well, and you can't get to the adapter while the rim is on the car. Then also factor in that it's adding weight to the car (and it's rotating weight, so it's not just static, fast you're going the more resistance it'll be causing)

 

So it's best to have a little EXTRA backspacing since you CAN use small adapters with some reliability. Generally a S30 Z can fit about 5 inches of backspacing stock, maybe 5.25. With coilovers 5.5 is easy to fit on a S30. So you can go with a rim about that range and then run small 5-15mm adapters as needed to get the rim where it needs to be.

 

 

Check out this graph to get an idea of offset/backspacing sizes, and you can use it as a guide in the future.

 

Also remember that tire width is also factor in all of this, as it's normally the tire that rubs anything, not the rim. The rim gives you the window of what tires will fit, but tire size is everything. So make sure you can find tires easily enough that will clear everything as well. So don't assume because you have 6 inches of clearence somewhere that you can fit 6 inches of backspacing, because the tire will add a bit of backspacing in itself.

 

Finding tires can be everything. It's the whole reason rudypoochris is putting so much effort into a group buy on a 17 inch rim, because it's so hard to find street tires in a 275 wide tire for a 16 inch rim.

 

I probably didn't help to simplify the whole concept at all, but with any luck you understood everything I said and it will make finding the "right" rim easier for you.

 

 

 

Then I inquired more about rudypoochris, and this was his reply:

 

The reason he wants 275 is because they' one of the most common tires around that width. Obviously he wants wide tires for traction (he'll be running 255 up front) and that's just a good size to go with.

 

In case you didn't know, tire width measurements increase 10mm at a time, and always end in 5. Examples:

 

195 / 205 / 215 / 225 / 235 / 245 / 255 / 265 / 275

 

Now, selecting a rim is VERY important because say you want to do some racing with your car. Well you want the most economical setup with the most options you can get. 235 for example is a very uncommon tire size and as such you might not have very many options.

 

The most common tire sizes for performance tires are:

 

225 / 245 / 255 / 275 / 315 / 345

 

Now, because of the NEW cars comming out starting this treand of larger rims, and with so many people changing their rims to larger rims the tires sizes for smaller rims are starting to dissapear. Sure there's options, but not like there was 5-10 years ago.

 

So say you want to run wide tires on your Z and you want to run the smaller ZG flares. That means you'll be able to run about a 315 max if I recall correctly. So you want to run a 315? Good luck finding some in a rim that's 15, 16, or even 17 inches. 18 and 19 inch rims have the most options for tires of that size nowdays. And in most cases the overall tire height is the same as the few 315 tires you'll find for a 17 or 16 inch rim. Just look at the stock Ford GT rims, they're 19 inches and the overal height isn't all that large, around 26-27" i think. Very low profile tires.

 

So chris went with a 275, as it is one of the most common sizes for performance tires, and that meant needing a 17 inch rim to maximize his options, thus the group buy is for 17 inch rims with nearly the perfect offset for Z cars.

 

Anyways, I think I've share most of what I know on the subject, or at least it feels that way. Let me know if you have any more questions and I'll try my best to help out.

 

 

Remember this information was PM'ed to me, not posted frist in general forum.

[rudypoochris]

Nice write up! I would like to add though... now this is how I measure back space and may not be technically correct... but I add an extra 1" to the overall width of the rim before claculating back space. Reason being the lips take up around a 1/2" on each side. Once your run the calculations for backspacing from offset you will be left with the real distance between mounting face and inside edge of tire. Just a thought.

[Globerunner513]

WOW awesome write up! I vote for sticky!

 

I havn't found any explanitions of the whole deal as neat and thourough as this! Thanks a bunch Gollum! *And woldson for posting it of course*!

[Gollum]

Chris makes a good point. A rim width measurement is taken at the mounting point for the tires, and then there's the lip which adds AT LEAST a good 1/4 inch per side, but 1/2 like chris said is perfectly acceptable. And backspacing is generally the measurement from the mout surface to the LIP, not just the measured edge.

 

Also don't forget that tires will add another variable, as they usually end up wider than the rim itself, giving you another thing to calculate when you're selecting a rim.

[rudypoochris]

When I get an ounce of free time I will go ahead and do a little write up about backspacing and such considering I had to go through all that stuff with the Rota group buy (I am sure tons of others are qualified to write something like this as well - its just there isn't one yet).

 

Here are the calculators I wrote a while ago for these calculations. They are very basic and I have seen better ones written with pictures and such, but the last calculator will calculate backspace for you based on tire width if your tire section sticks past the lip. Some might find that useful (probably not in the S30's case since it is MacPherson and the farther from the axle CL you go the more spacing you get... thus the rim is usually the issue, not the tire buldge - on a double wishbone this may different... hence the calculator).

 

http://autolounge.net/calculators/tirespacing.html

 

If one were to plug in the 17x9" -13 which is a size that fits and is being specified for the Rota group buy, the numbers are:

 

~4.5" BS and ~5.5" FS. Thats about the maximum in both directions I would recommend generally. A bit more can be squeezed out on the BS, but then T/C rod and frame interference may become an issue at full lock (also perch clearance possibly if you don't have coilovers). More FS can be achieved, but I am not really sure how much. I know that 6" looked too close for comfort for me when it came to loaded distance between front air dam and tire (275/40/17). I am sure 6" of FS could be run with a little finagling and such, but I felt 5.5" was really a better place to stop at.

 

The rears are much simpler. Give them 4.5" maybe 4.75" for stock suspension, more for coils and then the FS is super simple since it basically comes down to flare width.

[Mul-ti-pass]

Not perfect but here's the one I use: http://www.wheelsmaster.com/rt_specs.jsp

[rudypoochris]

That was the one I was refering to as very good. Takes a while to load though. It is nice to see it draw you a picture.

[woldson]

Found by member Temujin.

 

 

 

[260DET]

Those illustrations above say it all really, they clearly show that backspacing is measured fron the hub inner face to the outside edge of the rim lip.

[datman]

I run 17 by 9" -13mm on the front of my car with 235/45/17 tyres, they rub on the edge of the front air dam so I cut 10mm section away to clear it.

[rudypoochris]

I run 17 by 9" -13mm on the front of my car with 235/45/17 tyres, they rub on the edge of the front air dam so I cut 10mm section away to clear it.

 

Interesting. When I tested this I did not have any air dam clearance issues, only stock valance issues if stock valance was fitted. Hmmm. How far is your car lowered? Which air dam are you running?

[datman]

My car is lowered but not sure by how much, probably 2" I could measure the engine craddle to floor distance if that helps. The airdam is an MSA urethane. It rubs right where the flare joins the dam....like this

[rudypoochris]

Interesting! Thanks for the info.