StanceWorks » Style » Wheels and Brakes- Sponsored by Rotiform

[DrieStone]

Offsets aren't very useful without a frame of reference, and in a lot of ways they're about as confusing as using "ratio" as the measurement of a tire's sidewall height.

The two important pieces of information you should be able to gleen from an offset is the front-spacing and the back-spacing. The offset itself is a useless number on its own, but you can calculate both front and back-spacing which are very important.

The only reason you should really care about back-spacing is to make sure it doesn't interfere with the mechanicals of your car. The beauty is that you should be able to figure out "how much more" can my car handle by putting a tape measure on the back of your existing wheels and figuring out what you're going to impact.

For instance if my current car has an 8" wheel with a +20ET and it looks like I can run another 1" of wheel on the inside I know the most back-spacing I'm going to want would be 147mm (8*25.4/2)+20ET+(1*25.4). I don't know what offset that is yet because you need to know the width of the wheel I want to run to calculate offset.

As a note, you multiply by 25.4 to convert inches to mm.

Calaculating front-spacing is important for two reasons. Interference and making sure you get the right "look". For the most part you want to make sure the front and rear wheels look parallel. It would suck for your rear wheels to be a narrower than your fronts.

A good piece of information about your car is the stagger of your hubs on your car. You want to know if your rear wheels are inset. My car is pretty extreme with a positive offset on the front and a negative offset on the rear. The OEM wheels were +23ET on a 7" wheel and -20ET on an 8" wheel. Using this information I can calculate that my rear mounting location is 55.7mm shallower than the front (or at least if I wanted to match the look of what the manufacturer originally decided).

I figured this out by calculating the front-spacing of the OEM wheels and subtracting so I know the difference. When I build/buy new wheels I want the same difference so that the outside edge of my wheels are somewhat parallel (I'm not trying to match the OEM position, just making sure the wheels are parallel).

If I had a set of 8" wheels with a +20ET for the front and I want a 10" wheel on my rear, I can calculate that my target offset would actually be -10ET. I'll either need to build a wheel to match, or I'll need to buy spacers to make it work.

The math:

(8*25.4/2)-20 = [my front-spacing of my front wheels] = 81.6

(10*25.4/2) - (81.6 + 55.7) = [my target offset] = -10.3

NOTE: the 81.6 is my front-spacing of the front wheels and the 55.7 is the difference between the front and rear mounting locations.

I can't figure out the size of the barrel or the lip I'll need though, because that depends on the difference between the mounting pad and the mounting face of the center section. If you had that number you could also figure out your barrel and lip sizes for 3 piece wheels.

[SeanDub]

K

[Dudermagee]

Tl;dr

[Average_Jerk]

Originally Posted by Dudermagee

Tl;dr

your head would explode. its about wheel math.

[Kielan]

Originally Posted by Average_Jerk

your head would explode. its about wheel math.

Oh dudeman, and his inability to grasp the concept.

[Dudermagee]

No pictures no care

[got splits?]

Originally Posted by Average_Jerk

your head would explode. its about wheel math.

omfg hahahahahahahahahah

[SeanDub]

Originally Posted by Average_Jerk

your head would explode. its about wheel math.

Hahahahahahaha

[Average_Jerk]

Originally Posted by SeanDub

Hahahahahahaha

the rest of us already knew all that shit thought lol

[SeanDub]

Originally Posted by Average_Jerk

the rest of us already knew all that shit thought lol

On this site ya either know this shit, or you copy someone's shit. Not much in between.

[andybob]

Originally Posted by Average_Jerk

your head would explode. its about wheel math.

[CJay]

Originally Posted by SeanDub

On this site ya either know this shit, or you copy someone's shit. Not much in between.

I didn't know this, still learning about fitment. Neither, did I copy someone elses. I guess that makes me the in between.

[SeanDub]

Originally Posted by CJay

I didn't know this, still learning about fitment. Neither, did I copy someone elses. I guess that makes me the in between.

You'll notice I said "not much in between" not "zero in between."

[CJay]

Originally Posted by SeanDub

You'll notice I said "not much in between" not "zero in between."

I apologize for over-reacting.

[P78]

ha!

[Rebounder]

wait, there's no question

[DrieStone]

Originally Posted by Rebounder

wait, there's no question

Yeah I was going to post this in response to another comment, but it quickly went off-topic so I thought I'd just create a new post.

My point was just that a lot of people treat offset as some magic number, but it's really just part of a bigger equation. I see posts about calculating your offset, but not very many describing what to do with it.

I know the info is out there, and most of us know what to do with it. It's not new information by any means.

[MommysLittleMonster]

[SeanDub]

You're absolutely right, but most times we speak of offset it is in relation to a car. Which ultimately does make it a magic number.

Nevertheless, you posted a shit ton of information that I read none of, but I'm assuming it's accurate, I'm sure it will be helpful to at least a couple people.

[Dani]

Originally Posted by SeanDub

You're absolutely right, but most times we speak of offset it is in relation to a car. Which ultimately does make it a magic number.

Nevertheless, you posted a shit ton of information that I read none of, but I'm assuming it's accurate, I'm sure it will be helpful to at least a couple people.

True!

[kabel88]

Lovely post, but next time instead of writing so much, that most people are not interested in reading. Maybe just setup the formel for the calcalutation principal..

I will remeber this post, and use it next time i have to calcalulate a offset or simular.

[DrieStone]

Ok, long post... with some pictures, and more math. This may look complicated, but it's not that bad. Doing some calculations may give you a bit more flexibility for wheel choice, and not only help calculate the widest wheels you can run, but what the range of offset you'll need for those wheels.

NOTE: remember to convert inches to mm (or vice versa) so your units match. [inches] * 25.4 = [mm]



Using the wheel size and offsets from the OEM wheels you can figure out what the stagger of the hubs is (if there is any). As I mentioned earlier, my car is pretty extreme because of the inboard brakes on the rear. My stagger is 55.5mm. This piece of information is vital to making sure my front to rear position looks right on my car.

(([front wheel width]/2) - [front offset]) - (([rear wheel width]/2) - [rear offset]) = [hub stagger]

For my car:
((178/2) - 23) - ((203/2) +20) = -55.5 (note my rear ET is negative so I had to switch the signs, two negatives make a positive, instead of subtracting)

*keep in mind that we're using the OEM wheel position as a judge of stagger. If the OEM had wheels that aren't inline, the stagger number will preserve the OEM wheel difference.



Then we figure out where our interference might be. We do this so we can figure out the largest wheel we can run. The important information here is the maximum front (outside) spacing and the maximum back (inside) spacing. This is a pain in the butt because it's almost impossible to find everything that might interfere (suspension travel and wheel position being what they are). We just need a general idea here though. A tape measure off the existing wheel gives us a good idea.

([Existing wheel width/2]) + ET + [distance to inside interference] = [maximum back (inside) spacing]
([Existing wheel width/2]) - ET + [distance to outside interference] = [maximum front (outside) spacing]
[max front spacing] + [max back spacing] = [max wheel width]

The nice thing is that we don't need to know the exact total width, because when we buy wheels we exclude the bead, but the bead is still important to figuring out interference. If our OEM wheels are 7" wide, that's 7" without bead. Because we're measuring the additional distance and adding to the OEM size, the width of the bead is included.

For example, my OEM front was 7" with 23ET. I have another .75" inside and .75" outside before interference. Perhaps it's obvious that I can run an 8" wheel (because I have 8.5" of available room), but I don't have much room for error when choosing my offset. The question is... what offset should I be targeting? I really only need to solve for either front (outside) or rear spacing, if I solve for front (outside), I can use that to figure out what I should run for the front too.

(177.8/2) - 23 + 19 = 84.9mm (this is the maximum distance from the hub to the edge of the wheel before the wheel hits interference).

I calculate this backspacing by

([prospective wheel width]/2) - [front (outside) spacing]

(203.2/2) - 84.9 = 16.7mm (this is my minimum offset, any less and the wheel sticks out too far.)

I know, because I have 8.5" of space that this wheel has 1/2" of "wiggle room". If I want to know my maximum offset I can just add my 1/2" to the offset:

16.7 + 12.7 = 29.4mm (this is my maximum offset, any more and the wheel is too far in and hits things.)

So now I know I'm looking for an 8" wheel with somewhere between a 17 to 29 ET offset.

I think a front wheel with a 20ET offset is probably good. It should give me just a little clearance on the front and plenty in the back.

So now I want to figure out what my wheel on the rear should be, including my earlier calculation for hub stagger.

To do this, I'm going to calculate what my target front (outside) space on my rear wheel should be.

(203.2/2) - 20 = [front space for front wheels] = 81.6mm

Since I know my stagger is 55.5mm. I need to add that in:

81.6 + 55.5 = [target front spacing for rear wheels] = 137.1mm

I measured my rear wheels and I know my target width is 10" and my range of offsets is somewhere between -20 to -7 ET so as long as my calculations end up between those offsets I'll be happy.

To calculate my offset from my front (outside) spacing for my rear wheels:

([new wheel width]/2) - [front (outside) spacing] = [offset]

(254/2) - 137.1 = 10.1mm


So now I know, that assuming I calculated my interferences correctly that my new wheels should be:

8" with ET20 for the front
10" with ET-10 for the rear

I also know that they'll sit a little more than a 1/2 inch further out than the OEM wheels, but it will preserve the OEM difference between the wheel position (parallel or not).

Of course things aren't always that simple. For instance finding negative offsets are difficult. Chances are I'm not going to find the perfect wheel. Luckily I can get spacers.

If I can only find a 10" wheel with a ET15, then I need to find a spacer... luckily simple subtraction gets me there:

[target offset] - [wheel offset] = [spacer size]

-10 - 15 = 25mm

[stinkinrich]

If I smell burning rubber, low offset.

[Buck6687]

then adjust your coilovers for real lows and fuck up everything you just worked out.