crai6m4ck

I am looking at a set of rims for my 94 RX7. These are staggered 18 X 8.5 in the front and 18 X 9.5 in the rear. I am planning on lowering my vehicle with coilovers and do not know just how low I will go. What I need help with is finding out what offset I should use in the front and rear? I've been told I can't really use the same offset on both the front and rear because offset changes with differences in width. I want to have the rims look even without one sticking farther out than the other one and I want to be able to lower the car and have the tires tuck. I was told that I would need a 44mm in the front and 31mm in the rear and then another person said that I would need a 44 in the rear and a 31 in the front. Someone else told me a 45 in the front and a 44 rear. Please help me with this one and only reply if you truly know what your talking about or have experience. I've had too many different answers and I'm fed up with being told things that aren't true by people that THINK they are... such as a few Civic owners that tell me that I have a 5 X 100 lug pattern!!!

dclin

iTrader: (4)

Amen - it's so easy for people to post 'whatever'. Alot of people are just rehashing what they heard, rather then go out and experiment. It's expensive to experiment, but at least you'll know.

Anyways, 18X8.5 +45 and 18X9.5 +44 are good set of numbers. My previous wheels were SSRs 18X8.5 +40 and 18X9.5 +43. Current wheels are now 18X9 +42 all four.

You have a bit of room to work with as far as offset goes, with narrower widths. I've seen people with anywhere from +38 to +45 on the 9.5 rears. I've been underneath my car plenty of times to check clearance - lots of room with that range.

Having said all that, another crucial part of the equation is your tire size choices. A mistake many people make (and a misconception) is that they mount larger then stock diameter tires when going to a larger diameter wheel.

Stock = 24.86in dia, and you want to keep that diameter or smaller (especially if you lower your car).

Here is some real world experience I can share with you. I had 235/40/18s (25.4in dia) mounted on the 18X8.5 +40s. I had lip contact when entering a inclined driveway at faster then normal sppeds, yet when I switched to a wider wheels that sat further out (18X9 +42) and a 255/35/18 (25.02 dia) combo, the 'crunch' I was use to went away!

A note on the 235/40/18s - they were Kumhos, and were sized larger then normal, more like a 235/45/18 . Other tire brands may be OK in that size.

So, if you lower your car, make sure you go with smaller diameter to compensate, such as 225/35/18 (24.02 dia), 225/40/18 (25.09in dia) or 235/35/18 ( 24.48in dia) . I have had great experience with 255/35/18s stock suspension, but YMMV.

Hope that helps!

HedgeHog

Hi Daniel,

Wondering if you can figure this out for me. I was hoping to run 255/40-17 at all corners (easier to dial in for autox). 17x9 should fit the tires. But any idea what the offsets will be? I was thinking a large offset to move the wheels in to clear the fenders. Like 48 or 50. Any thoughts?

TIA.

crai6m4ck

I plan to tackle the tire problem later after I first get the rims... I will get a tire size that when mounted on the rim will be perfectly straight flush, up and down. (no stretching the tire to fit the rim or having the tire mushroom because it is too wide) So if I get a 8.5 with a +45 in the front and a 9.5 with a +44 in the rear both rims will be even without one sticking out farther than the other and I will be able to lower it without problems?

dclin

iTrader: (4)

You're gonna hate this answer, but it depends....

Here's why. Your fitment depends on quite a few factors, including:

1. wheel width
2. wheel offset
3. tire size (sectional height and width)
4. tire manufacturer and model. A 225/40 Pirelli can be VERY different in actual size then a Kumho 225/40, and tires from the same manufacturer can be different - see this thread for and example: https://www.rx7club.com/vforums/show...threadid=15526
5. How much you lower it
6. Your camber settings

I think that's it - my mind is a bit numb from a stupid Shakespeare play at an outdoor theater I just came back from...things you do for women.

Given that we have a set of wheels with a given set of specs, we have to consider 3., 4. and 6. to determine 5. The wheel specs given above are pretty safe, and not quite the extreme.

For 3., tire size, I definitely can tell you that the shorter in diameter, the better when it comes to lowering. Stock diameter (225/50/16) equals a theoretical 24.86in dia. I suggest the sizes above in my first post, for diameter reasons. That does not mean you can't use a different size - just keep it short as you are willing to deal with.

I'm a little less concerned with width, based on my experience and from looking at other's setups. 245 or less is a safe number. FYI, 245/35/18 equals a theoretical 24.75in dia (ok) and 245/40/18 equals 25.72in dia (not ok). However, see the next paragraph.

For 4., tire manufacturer, I suggest making use of the tire specs available at www.tirerack.com to give you a guide as to how a manufacturer sizes a particular model. Choose a brand/model of tire, then look for diameter, sectional width , and what wheel width was used to measure the specs.

Remember that if you were to put a given tire on a different width then what it was designed on, the dimensions will change. A little, but possibly enough to cause rubbing etc. It takes only one extra mm to either rub or not, if you're that close.

A tire is designed to be mounted on a range of wheel widths, but the 'official size' was derived from only one width size - so keep this in mind.

For 6., camber settings, the amount of camber you have will also determine (to a smaller degree) what sizes you can use.

If you look at your car directly from the front (or back), you can see the tire lean slightly inwards at the top (actually, if your camber is within spec - negative .1 degrees - you probably can't see any leaning). Racers can typically have as much as 1.5 degrees of negative camber for track applications.

The more it leans in on the top, the more negative camber you have. Adjusting camber is a means of adjusting the handling of the vehicle. The more negative camber you have (up to a given point, then its down hill), the more 'grip' you have at a given end when cornering.


Anyways, since we are looking strictly at tire/wheel fitment, you obviously have more room to run wider tires and/or lower it more if you have more negative camber. As the more negative camber you add, the more the top edge of the tire tilts in, affording more room.

If I hadn't failed geometry, I could calculate for you the amount a given point on the top edge of tire would move back into the wheel well – given the distance from the center of the hub to the point, and how many degrees camber you add. I have since burnt all of my mathematics books though, hehe. One degree, though is pretty noticeable.

Adjusting camber is meant to adjust the handling of the vehicle, so adjusting it just for the sake of being able to put wider meats underneath is not recommended.

BTW, a general rule of thumb is, for every 1in you lower a car, you add 1 degree of negative camber. Therefore, you will have to get an alignment done upon lowering your car.

My advice? Get adjustable height coilovers, and experiment! (Sorry, crappy answer I know)

I'm about to install RB .75in front springs (thanks Rishie!) on mine, with 255/35/18 on 18X9 +42 combo - wish me luck!

Hope that did not confuse you any more…..

dclin

iTrader: (4)

Taz (the guy with the VR with black/gunmetal 17in Volk SE37Ks) has 17X9 +40 all four, negative 1.5 deg camber. He reported no problems with 245/??/17, but some slight rub with 255/??/17s.

As in the post above, your tire specs will play into the equation, so you may have differnt results if you use a different tire manufacturer.

If you really need to have wide rubber, I would suggest something around +45.

I have +42 (I have a 9in wide rim, so we can compare offsets) that currently wear 255/35/18s stock height, and I'm very pleased. I am about to lower it (waiting on my shocks) .75 via RB springs, so we shall see.

+48 might be OK ( and preferable from the fender lips veiw point), as that it moves the wheel inside more (clearing the fender lip), but then we obviously need to look at clearance on the inside. I will see if I can crawl under my car with a camera to see how much room I have, then we can get a rough idea of what we have to work with. (wish I had a 'lipstick' cam, hehe)

Do you have skinnier coilovers? Actually, we can calculate it based on other members set ups. Taking Steve Cirians setup, 18X10 +50 on front (before we continue, Steve's write up was a tad confusing as that he refers to offset as 'stock'. However, you cannot compare offsets of different wheel widths. Since stock for the 16X8 is +50mm, I am assuming he is using 18X10 +50mm: http://www.scuderiaciriani.com/rx7/wheels.html#KIN )

Given that you wish to use a 9in wide wheel, and Steve has a 10in wide wheel with a +50mm offset - if you were to have a 9in wide wheel with a +50mm offset, you will have 1/2 clearance on both the inner and outer edges of the wheel compared to Steve.

Because the center line remains the same for both wheels (both are +50mm) the 1in difference betweent the 9in width wheel and the 10in width wheel is taken evenly on both sides - 1/2 in (or 12.7mm)

If we then look at a 9in +48mm offset, this wheel is moved outwards (towards the fender lip) by an extra 2mm. Therefore, you gain 2mm between the inner wheel face and the suspension bits, and loose 2mm between the outer wheel face.

In either case, both offsets will work, assuming everything else is identical to Steve's setup (suspension, etc) Keep in mind this is only if Steve's 18X10s are +50mm offsets. Also remember he has skinnier coilovers.

Hope that helps!

dclin

iTrader: (4)

See my respnse to eBoost.:p

Tires are given a certain range of widths they can be mounted on, so don't worry about overdoing it. Just follow whatever rim width recommendations a tire manufacturer says. For example, Pirelli's P-Zero Assimmetricals in 225/40/18 has an approved wheel width of 7.5in to 9in. Any one one of these widths will not result in too much stretching or balloning.

The problem is, there are less then reputable tire dealers that are willing to mount sizes to a given wheel width other then what is recommended by the manufacturer.

I lived in El Paso for a few years, and it was very popular for the low rider trucks to sport skinny tires over wide rims - the stretch effect.

BTW guys, I hope I'm not coming across a 'smarty pants', hehe. I'll be first to say I do not know everything, and what I have gained was over a looooonnnng period of time, and figuring it out the hard way. Just wanted to share with you guys what I've learned.

SleepR1

You want to install 8.5/9.5 x 18s fr/rr, respectively, but want the wheel/tire face to be the same with respect to each other (fr/rr). What you want is equal front spaces at the front and rear wheel positions. John Purner, of Complete Custom Wheels, says 3 inches is the maximum front space for the front position. If you don't want the rears to stick out farther than the fronts, then you'll use 3 inches as the max front space in the rear position. You'll want to stay under 3 inches in the front positions to avoid rubbing problems with the fender lips.

Front space is the measurement between the mounting surface of the wheel hub to the fenderlip.

Front space is calculated as follows with all measurements in inches. Overall Wheel Width is the measurement between the outside surface of the wheel rim lips (usually 0.5 inches more than the bead-to-bead wheel width measurements).

Front Space = (Overall Wheel Width/2) - Offset

Knowing that you want a front space <3 inches, the equation is as follows:

2.8 = (9/2) - x

Solving for x you get 1.5 inches or 38.1 mm. To keep from rubbing you'll want an offset greater than 38 mm (to move the wheels slightly inward), ranging from 40 to 42 mm for the 8.5 x 18s up front.

The rear offsets will calculated in the same manner. Because the rear front space clearance is greater than 3 inches, you can use 3 inches as the max rear front space clearance.

3 = (10/2) - x

Solving for x yields ~51 mm. since you have more front space clearance in the rear positions, you have an offset range of 47 to 49 mm.

Abolute clearance will depend on tire size choice. With a plus 2 fitment, you'll need to maintain ~25-inch overall tire diameter. This means the tire choice for the 8.5 x 18 wheels would be 225/40-18. The tire size choice for the 9.5 x 18 wheels would be 265/35-18. The overall tire widths are 9.5 and 10.5 inches, resectively, so you should get the wheel/tire fit you're looking for (sidewalls flush with the rim lips)

Good luck

maxcooper

Hey Guys,

Be careful with what rim width you use in doing the calculations. I have a set of CCWs from John and I did some measurements on them to get a better idea of what offsets really work on the car. Mine don't rub at all with 285/30-18s on the same wheels at all four corners (with coil-overs, etc). The wheels are 18x10", but the rims are actually about 11.25" wide (and are wider than the tires). I measured the backspacing from the wheel pad to the rim edge, and it was exactly 7.5". My understanding is that you need less backspacing to run 17x10" wheels in back, but you can use the same offset for all four with 18"s.

What is the offset?

Backspace - (WheelWidth / 2) = Offset
7.5" - (11.25" / 2) = 1.875" = +47.625mm

However, my 11.25" wheel width measurement was not very precise, so don't mistake my precision here for accuracy. I am pretty sure I was within 1/10 of an inch, which is 2.54mm, so the real offset is somewhere between 45 and 50mm. I would select an offset closer to 50 than 45 if you have the option. That makes sense when compared against the offset page on the Fikse site:

http://www.fikse.com/offsets.html

Front Space:

WheelWidth - Backspace = ActualFrontspace
11.25" - 7.5" = 3.75" (actual)

(NominalWheelWidth / 2) - Offset = NominalFrontspace
(10" / 2) - 1.875" = 3.125" (nominal)

I think the guideline Manny gave for no more than 3" nominal front space is a good one. My front wheels are just about flush with the outer surface of the fender with -0.8 degrees camber, which is certainly too wide to tuck under the fender if you want to lower the car a lot. I am surprised they don't rub, but they really don't. I guess my ride height (~1/2" lower than stock) and stiff springs keep me out of trouble. A stiff front anti-roll bar would help limit travel, too, for some extra margin of safety. I would say that you want to be well under 3" front spacing to have the tire tuck up under the fender - maybe 2.5" or so for lots of clearance.

An 8.5" wide wheel has nominal front spacing of 2.478" at +45 offset and 2.675" at +40 offset. A 9" wide wheel has a nominal front space of 2.925" at +40 offset. A 9.5" wide wheel has a nominal frontspace of 3.175" at +40 and 2.978" at +45. Hmm... Volk SE37As might fit at all four corners in 18x9.5" +45 with some 255/35-18 or 285/30-18 tires, or maybe 255/35 in front and 275/35 in the back. That would not work on a "slammed" car, though.

-Max

SleepR1

Max brings up an important point. You must be aware of what wheel widths you're working with. If you're calculating offsets for the greatest clearances, you should use the OVERALL WHEEL WIDTHS NOT the nominal tire bead-to-tire bead measurements most wheel manufacturers use. Fikse's table that Max refers to is a good one. They apparently use overall wheel widths, because when I crunch the numbers using the nominal wheel widths, the results don't agree with the offsets listed in the table. When I use the overall wheel widths (assume 0.5 inches> than nominal), the resulting offsets agree with the offsets Fikse publishes for their wheels.

Yes I've read from others that John Purner's "10-inch wide" CCW wheels are much wider than the 10-inch widths he publishes.