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Offset Math Help

Joined
23 May 2003
Messages
395
Location
El Dorado Hills CA
I just put my wheels on my car and it looks like the fronts stick out a bit more than the rears. In order to figure out the difference in stock wheels, I took the stock offsets (55mm front on 7" wheels) and (60mm rear on 8.5" wheels) to figure out the stock distace from the hub to the end of the wheel. If my calculations are correct, the fronts are 33.9mm or 1.3" and the rears are 48mm or 1.9" from the hub to the end of the wheel.

Does this make sense? Why is this?

If it does, should i try to maintain this ratio? That is, should I shift both the fronts and the rears equally?

Thanks for your help.

Ed
 
Actually changing the offset is good way to make your wheel "fill" the fender well - even a same size wheel will look better if the offset brings it out to the edge of the fender. So it's not critical to preserve the same ratio of offset - in fact it may be better to tune exactly.

If you want to customize, first take diameter of old & desired new and divide the difference between old & new by 2 & convert to mm. If your offset was the same on your new wheels as original, then this would move the outside rim of the new wheel out towards the edge of the car by this much.
i.e. on the rear - go from 8" to 10" - difference is 2"; divide by 2 = 1" = 25.4mm. So by retaining the same offset as original 8" wheel (60mm) the new 10" would move out towards the edge of the fender by 25.4mm.
OK, so all well & good - so why not stick with 60mm offset? After all, some guys are fitting 25mm spacers, which do the same thing? Because then lip comes into it - by decreasing the offset, you will move the centre hub further in still, relative to the edge of teh rim and this will give you a bigger lip - which tends to be very desirable re appearance. This will also move the wheel further out still, so you can't go crazy with wider wheel and too small offset or you'll end up being outside the fender. You also have to recognize that when going wider wheel, with no offset change, your new wheel inside rim edge is getting closer to the shock, so you may need to adjust for less offset to accomodate that too.

You can just use a rule & measure up how much gap you have now, where you want the edge of rim to end up, what width wheel you will use, how much lip you want and then calculate all this into the equation.

But much simpler - typically you can go by what others have used as a guide for given width of wheel & check out their pics of finished article.
For example, you can see mine here - the fronts are 38mm on 8" wide and the rears are 45mm on 10" wheels.

See the little explanatory diagrams in the Wheel Tech FAQ to get good understanding of how offset works.

edit:
p.s.
emdoller said:
....In order to figure out the difference in stock wheels, I took the stock offsets (55mm front on 7" wheels) and (60mm rear on 8.5" wheels) to figure out the stock distace from the hub to the end of the wheel. If my calculations are correct, the fronts are 33.9mm or 1.3" and the rears are 48mm or 1.9" from the hub to the end of the wheel.....
Your math is correct - now see notes above to figure out how increasing width of wheel will change the final new outside rim position wrt to the hub.
Remember for same offset, just add half the width increase to your calculated number, then adjust again for any change in new wheel offset vs stock.
e.g. your new front wheel is 8" with offset of 40mm -
then the new outside rim edge distance from the hub = 34 mm (as you calculated for OEM wheel) + 12.7mm (wheel width increase) + 15mm (offset change) = 61.7mm, or a change of 27.7mm closer to the outside of fender well.
 
Last edited:
Thanks. So here's what I have:

Front: 7.5" wheels, 37mm offset
Rear: 9" wheels, 47mm offset

If I do the math, relative to the stock wheels heres what I get:

Fronts: 24.35mm closer to the fender over stock
Rear: 19.35mm closer to the fender over stock

So if I wanted to be consistent, relative to stock, I'm about 0.2" off on the back.

If I do the math on your wheels (which look awesome by the way) I get:

Fronts: 29.70mm closer to the fender over stock
Rear: 34.05mm closer to the fender over stock

So if I wanted to use the same ratio as you, your rears are "pushed out" 4.35 mm more than your fronts, I'd have to move my rears out to (24.35 + 4.35 = 28.70) or another 28.70-19.35 = 9.35mm or if I was happy with the stock ratio, 5mm (24.35-19.35) to get them the same.

Does this make sense? I think this is why mine don't look quite right.
 
Front tires: 215/40/17
Rear tires: 255/35/18

I just put went out and added washers to push it out 5mm and it's amazing how different 5mm makes.

I think I'm going to order 5mm spacers.

Comments and thoughts always appreciated. I can post pictures if that helps.

Thanks!
 
RP-Motorsports said:
... A 225 tire with a +37mm offset will look different on your car as a 215 tire +37mm or a 235 tire and a +37mm offset. We need to know your tire widths before we can help......
Fair comment - thought that was a given though.

emdoller said:
...I just put went out and added washers to push it out 5mm and it's amazing how different 5mm makes.

I think I'm going to order 5mm spacers. ...

5mm doesn't sound like much (~ 1/4") but make sure you have enough thread left to safely secure the lugs - you should have 6 full turns as a good rule of thumb. You can't say generally whether you need or not with after-market wheels and a 5mm spacer as a lot depends on how thick the wheel hub itself is. If you don't have 6 turns, you'll need longer wheel studs
 
D'Ecosse said:
Fair comment - thought that was a given though.



5mm doesn't sound like much (~ 1/4") but make sure you have enough thread left to safely secure the lugs - you should have 6 full turns as a good rule of thumb. You can;t say generally whether you need or not with after-market wheels and a 5mm spacer as a lot depends on how thick the wheel hub itself is. If you don't have 6 turns, you'll need longer wheel studs

When you say 6 turns, I'm assuming you mean 6 complete full turns, right?
 
RP-Motorsports said:
Rim width is NOT what you should be looking at, it is Tire width. A 225 tire with a +37mm offset will look different on your car as a 215 tire +37mm or a 235 tire and a +37mm offset. We need to know your tire widths before we can help......
I have to disagree. Width and offset of a wheel are probably the most important parameters to start with to determine how you want the car to look. The tire width of course is going to affect the resultant look with variations in bulge/stretch.
With Ed's set, he has room for a bolt-on 15mm spacer for the rear to fill in the gaps.
What you said is a given as Ken said.
Steve
 
emdoller said:
Front tires: 215/40/17
Rear tires: 255/35/18

I just put went out and added washers to push it out 5mm and it's amazing how different 5mm makes.

I think I'm going to order 5mm spacers.

Comments and thoughts always appreciated. I can post pictures if that helps.

Thanks!
Ed,
I am surprised that your final sizes are such. I thought the last time we spoke you decided on
"Front --> 7x7.5 +37 offset (215/40/17 tires)

Rear --> 18x9.5 +40 offset (255/35/18 tires)"
You have room for a bolt-on 15mm hubcentric spacer for the rear and I think you should use that instead of a 5mm spacer since you will lose the hubcentricity.
Steve
 
whiteNSXs said:
I have to disagree. Width and offset of a wheel are probably the most important parameters to start with to determine how you want the car to look. The tire width of course is going to affect the resultant look with variations in bulge/stretch.
With Ed's set, he has room for a bolt-on 15mm spacer for the rear to fill in the gaps.
What you said is a given as Ken said.
Steve

I have to agree. I'm slowly becoming an expert on somethign I never cared about. If I want to keep the push out to the fender ratio same as stock, I need to push the rears out another 5mm. From looking at the "ultimate wheel fit" on this nsx website, what I have found is that most push the rears out further than they do the fronts. Right now I have th eopposite and it looks off just a bit - I'm a perfectionist and realize there's not real perfect answer to this.

So my choices are:

Leave them alone... naaaa
Go with a simple 5mm spacer and keep the wheel studs
Try a 7.5mm spacer from dali racing and try to keep the studs
Go with 10mm or 15mm and reaplce the studs... seems like to much work.

I'd love to hear what others might do??
 
whiteNSXs said:
Ed,
I am surprised that your final sizes are such. I thought the last time we spoke you decided on
"Front --> 7x7.5 +37 offset (215/40/17 tires)

Rear --> 18x9.5 +40 offset (255/35/18 tires)"
You have room for a bolt-on 15mm hubcentric spacer for the rear and I think you should use that instead of a 5mm spacer since you will lose the hubcentricity.
Steve

Steve,

That's what I was headed torwards and my "friends" at Work came back with this combo. Oh well you live, you learn! I'm worried that 15mm might push ot out too much. Also, I think I can get 5mm and 7.5mm hubcentric spacers from Dali, no?
 
If it was me, I'd go with Steve's suggestion & do the 15mm with new studs - there are some tricks you can see here on 'Prime to be able to install them without removing the hub (which is a real pain on the rear!)
How many turns did you get with your washer spacer? I'm sure they might be marginal at 5mm & 7.5 probably take it over the top for sure. Do you really want your personal safety based on a less than ideally secured wheel?
15mm is only just over 1/2" (~ 5/8") remember - I'm surprised you can see much difference at only 5mm but 15mm should give you an appreciable look without being overboard. You can replicate this again with more washers as long as car is static.
 
emdoller said:
Steve,

That's what I was headed torwards and my "friends" at Work came back with this combo. Oh well you live, you learn! I'm worried that 15mm might push ot out too much. Also, I think I can get 5mm and 7.5mm hubcentric spacers from Dali, no?
15mm will not push your wheels out too much at all. You will end up to have a equivalence of a 18x9 et 32mm wheel and still has a lot of room to spare especially you are only running 255/35/18. Go with the bolt-on 15mm spacers. Do not get the Dali spacers since it will be tons and tons of work to change out the studs, the end results will not be satisfactory to you. The 15mm spacer is a 15 minutes job.
Steve
 
D'Ecosse said:
If it was me, I'd go with Steve's suggestion & do the 15mm with new studs - there are some tricks you can see here on 'Prime to be able to install them without removing the hub (which is a real pain on the rear!)
How many turns did you get with your washer spacer? I'm sure they might be marginal at 5mm & 7.5 probably take it over the top for sure. Do you really want your personal safety based on a less than ideally secured wheel?
15mm is only just over 1/2" (~ 5/8") remember - I'm surprised you can see much difference at only 5mm but 15mm should give you an appreciable look without being overboard. You can replicate this again with more washers as long as car is static.

Thanks. I'll try the washers (need a trip to home depot to get more) and let you know. I got about 7.5 turns with the 5mm washers.
 
whiteNSXs said:
15mm will not push your wheels out too much at all. You will end up to have a equivalence of a 18x9 et 32mm wheel and still has a lot of room to spare especially you are only running 255/35/18. Go with the bolt-on 15mm spacers. Do not get the Dali spacers since it will be tons and tons of work to change out the studs, the end results will not be satisfactory to you. Also, there is NO guarantee that you will receive your parts when you deal with Mark Johnson from what I have known and read on Prime. The 15mm spacer is a 15 minutes job.
Steve

Where can I get the 15mm hubcentric with built in studs? BTW. I still love the lights I purchased from you!!!
 
Here's a picture of them... the left side has no spcers and the right side uses washers to simulate 15mm spacers.

Ed

wheel1.JPG


wheel2.JPG
 
whiteNSXs said:
I have to disagree. Width and offset of a wheel are probably the most important parameters to start with to determine how you want the car to look. The tire width of course is going to affect the resultant look with variations in bulge/stretch.
With Ed's set, he has room for a bolt-on 15mm spacer for the rear to fill in the gaps.
What you said is a given as Ken said.
Steve

If it was a "given", he would not be asking our help?????

I mentioned tire widths as missing info. We need ALL info before we can help. I was a bit brief and to the point, because of past issues with emdoller trying to figure out offsets. (I have helped many times already, and yet he still ordered wheels with offsets, and tire sizes that I did not agree with (not from me)....and now he is trying to "fix" the "look")

With a 255 rear tire, I usually reccomend a +33mm, and you can actually mathamatically go down to a +25mm.

With a 215 front, we usally reccomend a +35mm at the lowest.
 
Those are the ones we're all familiar with Chris - only the 25mm's come with the built in adpater style with new studs.
Steve was suggesting per the picture on the referenced thread that someone makes a 15mm with built-in studs & hubcentric ring, similar to the H & R 25mm's.
 
RP-Motorsports said:
If it was a "given", he would not be asking our help?????

I mentioned tire widths as missing info. We need ALL info before we can help. I was a bit brief and to the point, because of past issues with emdoller trying to figure out offsets. (I have helped many times already, and yet he still ordered wheels with offsets, and tire sizes that I did not agree with (not from me)....and now he is trying to "fix" the "look")

With a 255 rear tire, I usually reccomend a +33mm, and you can actually mathamatically go down to a +25mm.

With a 215 front, we usally reccomend a +35mm at the lowest.

"with a 255 tire, you recommend a +33mmm offset"??? You are missing Steve's point. The key variables are wheel widths and the offsets. The tires are the least of the variables. It's the wheels and the offsets that really matter. Jeez! Regarding my size choices, this is what Work themselves told me was the best choice. Obviously they were wrong.
 
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