Wheel backspacing, what to look for, what to avoid?

**Zapper2003** · 04-17-2007, 04:07 PM

I think we need a sticky on this subject. Regardless, onto my question.

I've got a 97 Formula, and I'm going to a swapmeet this weekend. I'm in need of knowledge here. I realize that the 5-4.75 boltpattern is required, and I know minimum size is 16" diameter. However, I want to know what I will need for backspacing with my following wheel sizes. I am going to be looking for a set of 17" wheels, either 8" or 9" wide in front, and either 9", 10", 10.5", 11" wide in the back. I'm looking to keep the front tires in the same relative 'stock' location inside of the fender well, i don't want them sticking out any, as with the rears, i do not want them to come outside of the body any, but i DO want them to remain inside of the rear quarters without touching the lip on the inside. The car is of stock ride height as well. I'm shooting for the widest I can find in the back. So what will be the minimum/maximum required backspacing for these size wheels? Also, i would like a bit of a lip in the back, if at all possible.

**Eugenio_SS** · 04-18-2007, 07:55 AM

if you stay close to +50mm offset you can fit all the above mentioned sizes for the rear.
Stock wheels are 17x9, +50mm offset.
For example, going w/ a 17x11 +50mm offset, the wheel will stick in an extra 1" and an extra 1" will stick out... based from the centerline of the wheel, you're still 50mm away from the mounting point on the hub.

**Zapper2003** · 04-18-2007, 05:58 PM

Alright, so what does 50mm convert to in american?

Mine has 16" wheels on it now, assuming those same dimensions, I will be looking for a 17x9 in front, 17x11 rear with +50mm offset, do the fronts have 50mm offset too?

**Eugenio_SS** · 04-19-2007, 11:01 AM

stock wheels 17x9 are +50mm offset.
offset is calculated in mm (conventional)

an image is worth 1000 words:
offset = http://www.tirerack.com/wheels/tech/...jsp?techid=101

**Zapper2003** · 04-19-2007, 03:45 PM

strange. According to that image, what you're telling me is that from the center of a 9" wheel, the 50 mm offset it towards the outside of the wheel and not towards the hub on the car, meaning there is less lip than say, negative 50 mm offset? And with negative 50mm offset, the wheels would be sticking WAY out past the quarterpanel, correct?

**Eugenio_SS** · 04-20-2007, 07:47 AM

when offset is positive, it's towards the outside... when offset is negative, it's towards the inside.
the point where the wheel gets in contact w/ the hub on our stock wheels are 50mm (~2") towards the outside of the wheel from the center line.

you are correct about what you are saying... by decreasing the offset, the wheel will stick out more. by increasing the offset, the wheel will tuck in more.

**Zapper2003** · 04-20-2007, 01:23 PM

Okay. So If i find a set of wheels, the absolute baseline i should be looking for is at least +50mm offset in any size, 8-11"? What else would be acceptable?

**hamburger68** · 04-21-2007, 12:38 PM

i just put 18x8 front and 18x9 rear on my Z, both 35mm offset. works fine, but i wouldn't recommend any lower offset, especially for the front wheels.
http://www.ls1.com/forums/showthread.php?t=48385

**Zapper2003** · 04-21-2007, 02:19 PM

Well, I picked up a set of these wheels today at Carlisle at the swapmeet. All 4 brand new for $550. Not sure if it was a good deal or not, but they were along the lines of what I was looking for and to me the price was right. 17x9, 35mm offset. Only problem I have so far is that standard lugnuts wont fit, they're too fat around. I assume I need some sort of thin walled acorn style lugnut, but where would I get a set? Also, I'm not sure what size tires i should run, i would like to have wider in the rear if at all possible and narrower in the front, but not so narrow that it leaves the edge of the rim unprotected.

http://www.sportmaxwheel.com/sportmax_962.shtml#

**Eugenio_SS** · 04-22-2007, 01:05 PM

Originally Posted by Zapper2003

Okay. So If i find a set of wheels, the absolute baseline i should be looking for is at least +50mm offset in any size, 8-11"? What else would be acceptable?

the wider the wheel, the less play you have both on the inside and the outside.
let's take a look @ a well known setup... 17x11 w/ +50mm offset.
that is very close to the fender... some will roll the outer fender for safety... and also requires some BFH massage therapy on the inner fender... so you're pretty much close to the limit on both sides of the wheel.

Now, going w/ a narrower wheel, assuming you are still @ +50mm, you have a bit of play on each end... allowing you to determine the offsets that will fit.

For example... a 18x11 wheel with +50mm will fit the same way as a 17x11 with +50mm offset.

If you go with a 18x10, that means you have 1" of play... or 1/2" on each side if your wheel is a +50mm offset.
1/2" ~ 12-13mm
That would imply that you could go to (+38mm offset = wheel would stick out to the same point as the 17x11 +50mm and give more room in the back) to (+62mm offset = tuck in on the inner fender as much as the 17x11 +50mm requiring the BFH-massage-therapy, but leaving more clearance on the outer fender).

remember same wheel width, smaller offset brings the wheel towards the outside of the car (tucks out)... bigger offset will tuck the wheel inside.

so... taking this analysis further.
C6 wheels are 18x10.5 with +58mm offset.
If they were 18x10.5 with +50mm offset, they would clear the inner fender and outer fender by 1/4" relative to the 17x11 +50mm.
but due to the extra +8mm, the wheel will stick in by 8mm more than if it was a +50mm offset.
1/4" clearance is ~6mm... but you tucked in 8mm... so it'll rub.
I'm putting 1/4" spacers on the 18x10.5 +58mm C6 wheels so that i have good clearances...

Hope this whole thing makes sense.

**Zapper2003** · 04-22-2007, 02:52 PM

That's a very good explanation! I was able to half-ass mount one of my wheels on the back (just 'stuck' it on there, no lugnuts to mount it with

because the douchebag that sold them to me gave me a set of standard lugnuts and not the 'tuner lugs' that these wheels require. Regardless, just putting it up on there made it look very close to the rear quarter (17x9 with +35mm) so once I get some lugnuts (and tires!) then I will be able to tell for sure.

As for lugs, which brand seems to be good quality against rust/chrome flaking?

**Zapper2003** · 04-22-2007, 02:53 PM

I also plan to run 245's on front and 275's on back.

**hamburger68** · 04-23-2007, 03:45 AM

Originally Posted by Zapper2003

I also plan to run 245's on front and 275's on back.

i run that setup with a 35mm offset, works fine. compared to stock, the wheels stick out a little more, 20mm front + 30mm rear. still well under the fender though.

http://www.ls1.com/forums/attachment...6&d=1177189481

**Eugenio_SS** · 04-23-2007, 05:01 AM

Originally Posted by Zapper2003

That's a very good explanation! I was able to half-ass mount one of my wheels on the back (just 'stuck' it on there, no lugnuts to mount it with

because the douchebag that sold them to me gave me a set of standard lugnuts and not the 'tuner lugs' that these wheels require. Regardless, just putting it up on there made it look very close to the rear quarter (17x9 with +35mm) so once I get some lugnuts (and tires!) then I will be able to tell for sure.

As for lugs, which brand seems to be good quality against rust/chrome flaking?

Those will be fine... they will stick out 15mm relative to the stock 17x9 wheel.
remember that you have a good 25mm clearance upto the outer fender.
It'll look nice... no need for spacer or anything... just plug + play.

**Eugenio_SS** · 04-23-2007, 05:03 AM

Originally Posted by Zapper2003

I also plan to run 245's on front and 275's on back.

if all wheels are 17x9... go with 275s on all corners.
it allows for rotation... and our cars like wide front tires for turning... after all our cars are nose heavy.

**Zapper2003** · 04-23-2007, 02:02 PM

I agree with going bigger on all corners for rotation reasons, but I like the look of the narrower front tire. Will cornering be THAT much affected, being as the car has the 16" stock rims on it right now with 235's?

*shrugs*

**Eugenio_SS** · 04-24-2007, 04:54 AM

since all wheels are 17x9, the 245/45-17 will be quite stretched in there... I'd put 275/40-17 on all 4 and have it look good and better mounted.
If your front wheels were narrower, like an 8"... than going with a 245 would be good.

**Zapper2003** · 04-24-2007, 02:52 PM

So the 245 will look odd on my wheels? (being that the contact patch will be narrower than the wheel by a measurable amount? and making the sidewall almost angled?)

**Eugenio_SS** · 04-24-2007, 06:29 PM

Originally Posted by Zapper2003

So the 245 will look odd on my wheels? (being that the contact patch will be narrower than the wheel by a measurable amount? and making the sidewall almost angled?)

That's one of the reasons i'd recommend going 275 on all 4.