Subject:General Wheel/Tire Fitment and Suspension Guide.


Since these questions arise quite often, we have decided to put together just a general overview of the basic suspension and wheel upgrades owners of 78-88 G-bodies commonly make. We have tried to outline the most common wheels sizes, and as such, anything over 17" diameter and 9" width will not be discussed.

Disclaimer: No guarantee of fitment is made or implied. Each car is different, and you should take the time to carefully measure YOUR car to make sure the chosen wheel will fit without interference. As the title says, this is only a general fitment guide. Also, the owner, administration, staff, membership nor anyone else affiliated with www.montecarloss.com assume no legal liablity implied or otherwise for the content in this guide. Use at your own risk!

Wheels & Tires

The stock 4th Generation Monte Carlo SS wheel is 15 x 7", a 5 on 4 3/4" bolt pattern, and 3.75" backspacing. The SS was outfitted with
215/65-R15 tires from the factory. The following are recommended backspacings for a variety of wheel sizes, followed by the recommended range of tire sizes according to www.tirerack.com in italics, overall height of the tire in parentheses. Realize that the overall height of the tire will have an effect on speedometer and mileage readings. See The Tire Size Calculator to determine the percentage error of your speedometer with non-stock sized tires.

Follow this link for John Bzdel's excellent diagram explaining the various terms commonly used in tire/wheel discussion.

Click here for a great tire math website.

-Thank you to Larry (whitess) (Added 2/18/04)

Please note: Rubbing or other interference problems may occur with non-stock wheels/tires, especially extremely wide tires. This problem can usually be remedied with the use of a steering box with shorter stops, either the XH F-body box or other aftermarket boxes.

<blockquote>

For 15" wheels:

<blockquote>
15 x 7": 3.5 - 4.0" B.S.

• 215/65-R15 (26.0")
• 225/60-R15 (25.6")
• 235/60-R15 (26.1")
• 245/60-R15 (26.6")

15 x 8": 3.5 - 4.0" B.S.

• 225/60-R15 (25.6")
• 235/60-R15 (26.1")
• 245/60-R15 (26.6")
• 255/60-R15 (27.1")
• 265/50-R15 (25.5")
• 275/50-R15 (25.9")

15 x 9": 4.0 - 4.5" B.S.

• 255/60-R15 (27.1")
• 265/50-R15 (25.5")
• 275/50-R15 (25.9")

</blockquote>

For 16" wheels:
<blockquote>
16 x 8": 3.75 - 4.0" B.S.

• 225/60-R15 (24.9")
• 245/50-R15 (25.7")
• 255/50-R15 (26.1")
• 265/50-R15 (26.5")

16 x 9": 4.0 - 4.5" B.S.

• 255/50-R15 (26.1")
• 265/50-R15 (26.5")

</blockquote>

For 17" wheels:
<blockquote>

17 x 8": 3.75 - 4.0" B.S.

• 235/40-R17 (25.3")
• 245/40-R17 (24.7")
• 245/45-R17 (25.6")
• 255/45-R17 (26.1")

17 x 9": ~4.5" B.S.

• 255/45-R17 (26.1")
• 265/40-R17 (25.3")
• 275/40-R17 (25.7")

</blockquote>

</blockquote>

Springs and Shocks

This is only a partial listing of available springs and shocks available for the G-body. We have tried to gather part numbers for the most
commonly used spring/shock combos. Most part numbers taken from www.jegs.com . Part numbers may vary by vendor. Check before you order to ensure that you have ordered the correct spring or shock for your application.
<blockquote>
Springs:

• Eibach - 363-3803-140</font>
• Hotchkis - 515-1902F & 515-1902K (1" drop)</font></font>
• Edelbrock - 350-5230
• Moog - See Jeff Davidson's Moog Spring Chart for part
  
  numbers as well as exhaustive specs.

Shocks:

• Bilstein - B36-0949 (F) & B46-0929 (R)</font>
• Edelbrock IAS - 350-3377 (F) & 350-3477 (R)</font></font>
• Belltech Nitro Drop - 146-7012 (F) & 146-7517 (R)
• KYB - KG4513 (F) & KG5548 (F) - Thanks to Zack (ZMonte85)
• Monroe Sensa Trac - MNR 5840 (F) & MNR 5802 (R)
• Competition Engineering - ???

</blockquote>

Brakes

By far, the most popular major suspension upgrade is the "Big Brake Swap." This swap consists of either Global West, Hotchkis, or UB Machine upper control arms, spindles and calipers from a '77-'96 B-body, 1LE or comparable rotors, B-body master/booster or G-body dual diaphragm booster, and assorted hardware. For an excellent write up on the 12" Brake Swap, reference one of the following links:
<blockquote>

• J. Matthew Daugherty's write-up .
• Steve Parker's write-up .
• Anthony Kiska's write-up with pictures.
• Car Craft's article here . - Thanks to L. Garcia (82caballero)
• C5 Calipers/Rotors . - Thanks to L. Garcia (82caballero)
• Another C5 conversion . - Thanks to L. Garcia (82 caballero)

</blockquote>

Frame Braces

As you may well know, the stock G frame isn't very rigid. Simple, but extremely effective, handling mods is the addition of frame bracing. Some have chosen to fabricate their own, and various braces can be pulled from Pontiac Grand Prixs. Marcus, an authorized vendor on this site, makes very nice custom bars for the G chassis. Contact him for more information.

Here are some words from Larry (whitess) concerning some of the braces on his car. (Added 2/18/04)

For other information not contained in this guide, see the Suspension Technical Section on this site. There are many excellent links detailing specific chassis components and modifications for the G-body.


Q & A:
<blockquote>
It has been measured to be about 3/8" per side. It is slightly noticable, but will not cause problems unless your current wheels are extremely tight-fitting.

If you measure from inside lip to inside lip (where the tire comes in contact with the rim), it should be 7", assuming they are stock

4.5" backspacing should be just fine.
</blockquote>
General Comments:
<blockquote>
This is a great idea, Scott. We will work on this list soon. Anybody with anything to contribute to this idea, please do so
</blockquote>

Okay guys, in an effort to put together an exhaustive FAQ for the basic suspension/wheel/tire questions often asked here, we have started this "guide." Please realize it is in its infancy, and anything and everything any of you can contribute will be added. Please keep replies on topic and only to contribute to the guide.

Also, we realize that 275 is not the widest tire that has ever been put on a G-body. I know several people here are running 295s, and I'm sure somebody out there has a 3-- tire in the stock wheel wells. But that is severly streching the limitations of the fenders, and we do not recommend it.

If you disagree with anything posted here, by all means voice your opinions and it will be discussed.

Again, post any and all part numbers or other useful links you have and we'll add it.

Thanks again,
Brandon

Alright, errors have been corrected and it's been updated, I think Sorry for the long delay in updating this thing.

We've given credit to those who have contributed part numbers and links. (Thank you )

Also, I added a "Q&A" and "General Comments" section because I felt bad for just deleting everybody's posts.

Any comments/questions/corrections you have, let us hear 'em.

Also, Phil (phil240) or anyone, do you have the C.E. shock part numbers?

Thanks again,

Brandon

Under the KYB shocks, you have an F for the rear shocks, the second number. Just thought I'd point it out so it doesn't confuse anyone.

This was definetly a good idea!

"the recommended range of tire sizes according to [url= http://www.tirerack.com"]http://www.tirerack.com"[/url]

Are the sizes listed recomendations from tirerack or are they tried and true sizes known to work on our Montes? Also, It would be nice to differentiat between the front and the rear. In other words...Front, range of tire sizes,BS,rim size etc and the same for the rear.

on the stock aluminum wheels for 86-88 ss's is there
supposed to be more torque used to keep these wheels
on these cars.in other words should i use a air impact gun on these wheels or can i just use my four
way lug wrench.thanks for any help.

^^^ just make sure that you tighten them equally. ive never torqued my wheels, but i always triple check to make sure theyre equally tight.

never use an impact, and always torque them. They call for 85, but I run 100 in the rears. They can be sensitive and warp rotors if over torqued.

Are you sure about this?

Why would a 235/40 be taller than a 245/40?
It is my understanding that the 40 means that the profile of the tire is 40% of the 235mm or 245mm respectively. Is this incorrect?

To figure a tires diameter, here is the method.

Lets use 275/40/17 for an example.
275 is the metric cross section width of the tire.
40 is the heighth of the tire from the rim to the OD. This is a percentage of the tire width.
17, of course is the rim diameter.

So for this tire we take 275/25.4 to get the english width of 10.8". 40% of that is 4.3". So the diameter of this tire is 17" + 4.3" + 4.3" = 25.6".

I get so tired of hearing people say that they have 50 series tires, "they are wide!", wider than my 40 series tires. Not necessarily, depending on what the first number is. My 275/40/17 & 245/45/17 tires are about the same diameter.

Scot

I think thats a typo it would seem to me 40% pf 245 would be more than 40% of 235...and you can use an impact if you have the special torque sockets that are meant for bolting up wheels

good, thats exactly how I understood it. thanks


my thoughts exactly. The only reason I ask is because I am looking into a set of 17X8's or 17X9's and I wanted a taller profile in the rear, which SHOULD result in a taller tire.

Also... will a 17X9 fit in the rear without modification? I will already be modifying the inner fenders on the front because I want to lower it more than they will allow

So how bout them 17X9's?

Will they fit without notching the frame rails or any other mods?

I currently have 17X9.5" Centerlines in the back with 275/40/17 tires. It is close but they clear.

Scot

what backspacing are you running?

Just wanted to let everyone know that I managed to shove a set of 295/50/15's on the rear of my car with an 8" wide rim and they fit PERFECT. The were about 1/4" from the frame rails and right under the fender lip. Didnt stick out at all. And the car has belltech rear springs, so its lower than stock. I can get the back spacing off of them for anyone that is interested.

Would f-body spindles(70-78) with a 2" drop still be fine for the big brake swap? I was looking into the possibilities and found these and a coil over setup that is designed to replace stock GM parts which would allow you to adjust ride height. Would that be too much?

on my 87ss im looking for 2in drop in front and rear..have 5660 springs, wheels looking to use
16x8 or 17x8 will i still have to use drop spindles???/

i learned today that 275/60/r15s will fit on an 8" rim.

these are 255/70r15s on my stock rims(rear only)they are not very wide.i have lift springs in the rear and i mounted the spring seats higher than stock when we built the ford 9 inch.the only close spot i had was near my 3 inch exhaust but it has never hit even with a full carload of people.im very happy with this setup.ive had these on for a few months now and i do all kinds of driving.

Lower it.

never!

For those that are also young and stupid......
295/50/15's will fit on a stock 87-88 15x7 wheel just fine with stock suspension and a 5/16" spacer, and it is safe as I have just went on a trip to Texas and back with absolutely no trouble at all, which was 1400 miles round trip.

I'm not so young, but maybe stupid. Do you think a set of 275/60/r15's will fit under the car? I'm gonna try since I just bought a new set of four for $190. I got a suspicion I'll only be using two of them if any.

^ 275's work fine on the rear. I have them on stock 86' Rims(I know dangerous, I didnt put them on)

John B has 275's up front!

Hi Folks,

Here's an approximate rule of thumb I use for calculating rim width to keep the sidewall neutrally biased, and not bowed inward nor outward at the rim, relative to the tread area:

Wr = Wt*[1-0.42*(AR/100)]+8.4mm

(This should get you within about a half-inch of the correct value.)

Where:
Wr = rim width in mm (measured inside-to-inside)
AR = tire Aspect Ratio (40,50,60,etc.)
Wt = advertised tire section width (245mm, etc.)

Remember that 1" = 25.4mm (exactly.)

Example: I have a 265-50 R15 tire. What rim width should I buy?

Wr = 265mm*[1-0.42*(50/100)]+8.4mm
= 265mm*0.79 +8.4mm
= 209.4mm +8.4mm
= 217.8mm
= 217.8mm*(1"/25.4mm)
= 8.575" > ans.: a rim width of 8.5" or 9" should be viable.

Best,
MAP

On edit: I've changed the AR factor from 0.44 to 0.42, to give better agreement with the data I have. As a sanity check, I also compared the recommended tire and rim sizes posted at the beginning of this thread, and they agree well with predictions from this formula.

On further edit: The prediction you get from this formula probably won't yield a value which corresponds exactly to a stock rim width (as in the example cited here: you can't buy a rim that's 8.575" wide off-the-shelf.) This being the case, do you go wider or narrower than the indicated value? I believe that from a handling perspective, it's generally better to go wider. Here's why: as the car rolls in a turn, the wheel top generally tilts away from the center of the car's direction of turn. As the tire top tilts outward, the centrifugal force simultaneously acting on the wheel causes the rim to be pushed outboard with respect to the center of the tread area in contact with the ground. This lateral displacement between rim and tread causes a distortion of the sidewalls. If the rim is narrower than the tread area, then this lateral displacement causes the outboard sidewall to "want" to assume a more planar shape, while the inner sidewall "wants" to do the opposite and assume a more conical shape. In so doing, the outer sidewall "pushes down" on the contact patch, while the inner sidewall "pulls up" on the contact patch. The net result is a distortion of the tread area where the contact pressure is concentrated on the outboard side of the contact patch: the tire almost literally wants to "roll over" itself in a lateral sense. Such pressure concentration reduces total lateral traction.

If the rim, on the other hand, is wider than the tread area, then the lateral displacement causes the inner sidewall to get more planar, and the outer more conical. In so doing, the inner sidewall "pushes down" in the tread area, while the outer sidewall "pulls up" on the tread area. This is the exact opposite, of course, of the narrow-rim scenario. The net result is that the tread area in contact with the ground is distorted in a way that tends to oppose that created by the outward tilting of the wheel overall. This counterbalancing effect tends to maintain uniform pressure across the contact patch, and so lateral traction is maximized.

Hopefully the crude sketch below conveys what I'm trying to say (shapes, angles, and distortions have been simplified and exaggerated to increase clarity...)

With further consideration given to how much each wheel tilts outward as a function of body roll, one can generally conclude that a solid live axle should probably have a neutrally-biased sidewall, while the front end should be biased toward a wider rim in relation to the tread area, with the difference in width: 1.) growing as the front roll center height decreases (relating to the matter of camber gain,) and 2.) diminishing as the car's roll stiffness increases.



Comments:

1.) Despite the optical illusion effect, both tread widths (and presumed advertised section widths) are equal.
2.) In the accelerated case, we have the angle of wheel roll, theta (which looks like a zero with a horizontal line through its middle,) which is a function of: the magnitude of centripetal acceleration a sub c; the car's roll stiffness; and the camber gain at that particular wheel as a function of suspension deflection at that wheel.
3.) Note that in the static case, the tread centerline is coincident with the rim's centerline.
4.) Note that in the accelerated case, because of centrifugal force, the rim centerline is displaced outboard of the tread centerline by a distance delta (which vaguely resembles a "six."). It is this displacement which fundamentally causes the kind of sidewall distortion shown here (again, simplified for the sake of clarity. Sidewall vertical compliance and its resulting deflection aren't even suggested in this sketch, for instance.)
5.) The displacement delta is roughly proportional to the centripetal acceleration, a sub c.

Very nice explanation MAP!

That explains how you came up with that tire width vs. rim width formula... I was at a loss at first as to how you did that.

Hi SSM,

Thanks! But truth be told, I never did explain how I got that formula, but rather the reasons why one would, or wouldn't, want to go with a higher rim width than what the formula predicts. The formula is only an approximation for the rim width that yields a neutrally-biased sidewall, i.e., one that ignoring the overall convex bulge of the sidewall viewed in cross-section, generally conforms to a plane, rather than to a frustum (frustum = section of a cone, from the Latin.)

If you or anyone else would be interested, I could post the derivation of the formula.

Thanks,
MAP

Yes, I would be interested in seeing that. I thought you might have worked backwards from the geometry above to optimize a ratio between tire and rim width.

Hi SSM,

Here's the derivation of the equation:

Wr = Wt*[1-0.42*(AR/100)]+8.4mm



Thanks,
MAP

MAP's formula probably works well in a lot of cases. However,folks should know that tire and wheel manufacturers design to the standards of the Tire & Rim Association. TRA produces their guidelines with updates every year. So when you look up a tire size on a manufacturer's website, normally they show you the approved rim widths for a given tire. Set of formulas they use for rim width hasn't been published to the public that I have seen, though I do know that there is a series of "if/and" statements and is based largely on aspect ratio.

Hi FM,

I found their website here:

http://www.us-tra.org/traHome.htm

But for non-members, there's very little to see, and I admit that the price of admission is higher than my cost of curiosity.
Anyone have an inside scoop?

Thanks,
MAP

Here is a link I found helpful
Wheel Technical Information
Hope its useful to others as well

I have 275/60/15's on the rear of my 87, no problems with rubbing at all.

Did you see the shiny area on your frame? If not, you will eventually...

Hi Folks,

Since I posted the formula for rim width (see a few posts back with drawing) about one year ago, I thought I'd offer an update.

I've used this formula and compared results with known data over the past year quite extensively; in fact, to a far greater extent than all of the times previous to the posting of said formula.

What I've learned is that the formula is considerably more accurate and general than I first suspected. This surprised me, since some of the presumed values of constants (in particular, that of "k",) were based on eyeball estimates of averages for many tires and many rims.

For the record once again, the equation is:

Wr = Wt*[1-0.42*(AR/100)]+8.4mm

Where:
Wr = rim width for neutral sidewall bias, in mm. (1in = 25.4mm exactly.)
Wt = advertised tire section width, in mm. (e.g., 215,225,235,245,...)
AR = advertised tire aspect ratio. (e.g., 40,45,50,60,65,...)

Best,
MAP

i have a 275/50/15 on the rear of my 85' with no probs.the car is lowered and without tailpipes.im liking this guide and look forward to using it when the time comes.

Never, been that way for over two years now.

Has anyone posted the Z-28/Trans Am/GTA wheel fitment to the Monte? I'm interested in the GTA 16" wheels that look like the GNX wheels and heard you need spacers to fit the rear wheels. also heard you can run front wheels on the rear and rear wheels on the front without any problem? anyone know for sure what needs to be done?
Thanks,
Dave

when they say cross section arent they actuallt saying at the widest part of the bulge and not actually the tread width?
i have 255r70 15s on the rear and 245r60 15s on the front of my 86 cl.the bulge in the 255s is wide but the actual tread width is kinda skinny

Holy mega bump...

But why is it not safe on an 86 wheel but is on an 87-88?

Fronts only on our cars, is what I've heard....Ask SSNinja (Dave)....thats what he's running I believe

Hi Folks,

A thread from 2/09 titled, "Your rim opinion," speaks of rim options for a 275mm-wide tire on an MCSS. Since this tire is about as large as can fitted on these cars, I thought the following post in that thread merits inclusion here:

"Hi Mike D,

About what rims I recommend - use the equation in the sticky. You'll just about always be right within a half-inch of the correct value.

Also, agree about going at least 17" with the rim to maximize performance options with tires. In the first decade of the 21st century, 17" is what 16" used to be in the '90's, and 15" in the 80's.

Next - with a 275 and an 8-1/2" to 9" wide rim, you'll be pushing the envelope to the very edge in terms of what's likely to fit on the car without rubbing. Rim backspacing will need to be exactly on the money, or you'll have problems. You'll also want to make sure the car body is centered on the frame.

If I were you, I would remove all four springs from the car and disconnect both sway bars. Make a tire/rim template that mounts to each hub just the way the actual rim would. The template will have a puck that represents the rim/tire radial cross-section. This puck gets mounted parallel to the hub's axis of rotation. The puck mounts in an adjustable way to the hub-mounting feature of the template, so you can mock-up and adjust backspacing as well as rim diameter. Obviously, you swing the template a full 360 degrees on the axle to sweep-out the full displacement of a real tire/rim. Making such a template is a snap - I made mine in about an hour with some 1/2" plywood and a block or two of simple wood. A C-clamp is used to adjust the puck position relative to the rest of the template, to explore different rim diameter and backspacing possibilties.

Once you get to this point, check for interference over the full range of suspension travel at each corner of the car. In addition, at the front, repeat over the full range of steering input.

Don't be surprised if you find that a 275 doesn't fit no matter what you do with the rim diameter and backspacing - you'll probably find at least one point of interference. At this point, you'll then have to judge the probability and severity of this occurence (I'm talking a DFMEA for you engineering types here,) and determine whether you could tolerate this. If the answer is "no," then your only easy option is a smaller tire - sorry. Or, if you're really, really determined: some serious surgery on the car."

Best,
MAP

I want to run a 24x12 wheel on a 365/15/24 tire in the back what are some good mods to do without hacking up the body.

Mini Tubbing?

Only the car craft 12"brake conversion write up link is functional.

Hi Folks,

After watching questions on this forum about tires and rims for several years, it seems that all conform to just two simple cases.

CASE 1: I've got a RIM of a certain width (Wr) and a certain diameter (Phir). Question: What's the best size of tire to get?

Answer: You'll need at least one additional piece of information before you can proceed.
Scenario A: I'm set on a particular tire aspect ratio (AR).

Then: Tire width = Wt = (Wr-8.4mm)/[1-0.42*(AR/100)]
Tire OD = Phit = Phir + Wt*2*AR/100
Tire ID = Rim OD = Phir

Scenario B: I'm set on a particular tire OD (Phit,) probably because I want to keep speedometer calibration.

Then: AR = [100/(2*Wt)]*(Phit - Phir)
Wt = (Wr-8.4mm)/[1-0.42*(AR/100)]

CASE 2: I've got a TIRE of a certain width (Wt,) a certain aspect ratio (AR,) and a certain ID (Phir.) Question: What's the best size of rim to get?

Answer: Wr = Wt*[1-0.42*(AR/100)]+8.4mm
Tire OD = Phit = Phir + Wt*2*AR/100
Rim OD = Tire ID = Phir

A caution about rims: rim OD (Phir) and width (Wr) are measured at the mounting surface of the tire, and do not equate with the extreme width and diameter you would measure on the rim. A typical 17" x 8" rim would probably have an extreme OD of about 18.5" and an extreme width of about 9.2", for example. As always, use the conversion factor of 1" = 25.4mm (exactly,) since industry standards are persisting to keep units jumbled. You've got to keep units consistent within any given equation to get valid answers. And, the answers you get will rarely correspond exactly to something you can buy off the shelf. For instance, let's try a Case 2 example:

I've got a 245/45/17 tire. What's the best size of rim to get?

Plugging-in:
Wr = 245mm*[1-0.42*(45/100)]+8.4mm = 207.095mm. Round to 207mm. Units conversion: 207mm*(1"/25.4mm) = 8.15"
Tire OD = 17"+(245mm*1"/25.4mm)*2*45/100 = 25.68"
Rim OD = Tire ID = 17"

Interpretation: the math tells us we need a 17" x 8.15" rim. Obviously, unless this is a custom rim, you can't get that width. Plus, the width equations aren't exact, but will almost certainly get you to the nearest half-inch. So what to do? 8" would work, but 8-1/2" would probably work even better, since it's usually better (see discussion previous posts) to go a bit wider than a bit narrower than what the equation would tell you for neutral sidewall bias. This is especially true for the front of the car.

Let's try another Case 2 example, and go back two posts to the person with the 365/15/24 tire. Is his desire to run a 24" x 12" rim a good idea?

Wr = 365mm*[1-0.42(15/100)]+8.4mm = 350mm = 13.8". Conclusion: a 12" rim is way, way off. Btw, even with the right rim, this is far beyond anything that mini-tubbing could afford unless you just let the wheels "hang out" and hope you never hit any big bumps.
Tire OD = 24"+(365mm*1"/25.4mm)*2*15/100 = 28.31".
Rim OD = Tire ID = 24"

Now, let's turn this into a Case 1-a question: I've got a 24" x 12" rim, and want to run a tire with a 15 aspect ratio. What tire do I buy?

Wt = (12"*25.4mm/1"-8.4mm)/[1-0.42*(15/100)] = 316.3mm. Get a 315mm tire.
Tire OD = 24" + (315mm*1"/25.4mm)*2*15/100 = 27.72"
Tire ID = Rim OD = 24"

Answer: I buy a 315/15/24 tire.

Best,
MAP

Hi Folks,

Some further clarification. In case 1-b the equations seem to be largely reciprocal since they involve similar unknowns. They way to approach this is as follows:

1.) Use a first-guess for tire width (Wt) to get aspect ratio.
2.) Plug the resulting aspect ratio (AR) into the second equation to see if the resulting Wt matches the first guess. If yes, then stop. If no, then adjust Wt accordingly and plug back into the first equation. Iterate as many times as necessary to get convergence.

About having the rim to be wider than these equations would indicate, go back to roughly the middle of the thread to the post which shows a picture of how the tire gets distorted in a turn in the case of a rim that's too wide versus one that's too narrow for a full explanation.

Best,
MAP

Hi Folks,

As time has passed and I've tested more and more tires against the equation I first posted, I've noticed that an improvement and refinement are probably called for.

The issue is the presumed nature of convexity in the sidewall bulge. The factor of 0.42 comes from the presumption that the transverse bulge of the sidewall is, on average, about 21% of the exposed sidewall height (see derivation a number of posts back.) But more recent tests have revealed that this fraction declines as we make the sidewall shorter.

That is, instead of k = constant = 0.21, it should probably refined according to:

k = a1 + a2*H, where "a1" and "a2" are two, new constants.

The refinement amounts to presuming that the sidewall bulge is not a linear function of H, but quadratic, and where a2 is negative. Once I have a more comprehensive study finished about this, I'll share.

Best,
MAP

Just picked up a set of 1987 aluminum 15 inch SS rims to put on my 1986 G body last night. Bolt pattern is correct, but this morning I noticed (1) that the SS rim mounting holes are much larger than on the stock steel wheels on my car, and (2) the mounting holes are not chamfered.

Is there a special nut available / required to mount the aluminum rims? Couldn't find this in the FAQs so ...........

Hello Everyone,

I am looking to purchase 2009+ Corvette wheels. A couple of options I am looking into are:

Front
18x8.5in either +40 or +56mm offset
18x9.5in +56mm offset
19x9.5in +

Rear
19x10 either +40, +56 or +79mm offset
19x11 either +63mm offset or +73mm offset
19x12 +59mm offset

I'd like to fit as wide a tire as I can, but I would like to know is any of this possible? Would I need to do any mods? Mini tubs? If possible, what tires should I use? I searched and I could not find the answers I was looking for so any insight is greatly appreciated. This would be for an '87 SS BTW.

I'm thinking about having some (Stockton) ralley wheels built for my SS 17x8 4.5 & 17x9.5 5 backspacing. Will be able to run them year-round........ Please help with feed back.

Hi Folks,

I have a small, but significant, refinement to report for calculating rim and tire widths. I was able to conform the refinement to something simpler than a quadratic fit, so it would remain easy to use on a calculator.

CASE 1: I've got a RIM of a certain width (Wr) and a certain diameter (Phir). Question: What's the best size of tire to get?

Answer: You'll need at least one additional piece of information before you can proceed.
Scenario A: I'm set on a particular tire aspect ratio (AR).

Then: Tire width = Wt = (Wr-8.4mm)/[1-0.42*((AR-5)/100)]
Tire OD = Phit = Phir + Wt*2*AR/100
Tire ID = Rim OD = Phir
Scenario B: I'm set on a particular tire OD (Phit,) probably because I want to keep speedometer calibration.

Then: AR = [100/(2*Wt)]*(Phit - Phir)
Wt = (Wr-8.4mm)/[1-0.42*((AR-5)/100)]

CASE 2: I've got a TIRE of a certain width (Wt,) a certain aspect ratio (AR,) and a certain ID (Phir.) Question: What's the best size of rim to get?

Answer: Wr = Wt*[1-0.42*((AR-5)/100)]+8.4mm
Tire OD = Phit = Phir + Wt*2*AR/100
Rim OD = Tire ID = Phir

A caution about rims: rim OD (Phir) and width (Wr) are measured at the mounting surface of the tire, and do not equate with the extreme width and diameter you would measure on the rim. A typical 17" x 8" rim would probably have an extreme OD of about 18.5" and an extreme width of about 9.2", for example. As always, use the conversion factor of 1" = 25.4mm (exactly,) since industry standards are persisting to keep units jumbled. You've got to keep units consistent within any given equation to get valid answers. And, the answers you get will rarely correspond exactly to something you can buy off the shelf. For instance, let's try a Case 2 example:

I've got a 245-45/17 tire. What's the best size of rim to get?

Plugging-in:
Wr = 245mm*[1-0.42*((40-5)/100)]+8.4mm = 217.39mm. Round to 217mm. Units conversion: 217mm*(1"/25.4mm) = 8.54"
Tire OD = 17"+(245mm*1"/25.4mm)*2*45/100 = 25.68"
Rim OD = Tire ID = 17"

Interpretation: the math tells us we need a 17" x 8.54" rim. Obviously, unless this is a custom rim, you can't get this exact width. Plus, the width equations aren't infinitely precise, but will almost certainly get you to the nearest half-inch. So what to do? 8-1/2" would probably work well. In cases of doubt, and especially when body roll plants the outer tire with positive camber, as is likely to happen mostly at the front of the car, go wider than the equation dictates. 9" at the front of the car should work quite well.

Let's try another Case 2 example, and go back several posts to the person with the 365/15/24 tire. Is his desire to run a 24" x 12" rim a good idea?

Wr = 365mm*[1-0.42((15-5)/100)]+8.4mm = 358mm = 14.1". Conclusion: a 12" rim is way, way off. Btw, even with the right rim, this is far beyond anything that mini-tubbing could afford unless you just let the wheels "hang out" and hope you never hit any big bumps.

Tire OD = 24"+(365mm*1"/25.4mm)*2*15/100 = 28.31".
Rim OD = Tire ID = 24"

Now, let's turn this into a Case 1-a question: I've got a 24" x 12" rim, and want to run a tire with a 15 aspect ratio. What tire do I buy?

Wt = (12"*25.4mm/1"-8.4mm)/[1-0.42*((15-5)/100)] = 309.39mm. Get a 305mm tire.
Tire OD = 24" + (305mm*1"/25.4mm)*2*15/100 = 27.60"
Tire ID = Rim OD = 24"

Answer: I buy a 305/15/24 tire.


Best,
MAP

PS: It would make things a whole lot easier if I could paste a spreadsheet in here. Does anyone know if this can be done, or do I need to post a link external to the forum?

Hi Folks,

In case the eye-strain factor is overwhelming in the post above, just remember this one equation:

Wr = Wt*[1-0.42*((AR-5)/100]+8.4mm

The refinement compared to the previous expression, is that we now deduct 5 points from the aspect ratio when making the sidewall convexity correction. Predicted rim widths increase modestly as a result, and more so for the lower aspect ratios.

Best,
MAP

285/40/17 the ultimate rear 17' tire for g-body. has anyone made them fit w/ stock rear springs no lip rolling ?

Running 17x9.5 5" bs, Nitto 555 Extreme in 285/40/17, with a 5/16 spacer on both sides.
A 4.75" bs would be best, as they are VERY close to the frame.
I would not recommend for everybody, rear UCA/LCA bushings need to be in good shape, otherwise you will have light contact when you drive it hard.
Front are 17x8 4.5" bs w/ Nitto 555 in 255/45/17, still running stock coils,
new coils going on this summer along w front adjustable SPC LCA.

If you run it low, you will need to roll the rear fender lips, the lines on the tire are the tire
design, not slices.

wondering if a 8.5 or 9.0 rim width would suck the sidewall in to help this or does the 285 need a 9.5 min. rim width. when I put 275/50/15 on a 8' rim it looked bubbled on sidewall and probablly lost some tire contact patch as it needed a min 9.0' rim . A fine line...... a fine line

How about 215-70R15 on the front and 255-70R15 on the back? I think they'd fit.

Fit perfect, even lowered 3", with the right backspaced wheels. Ran this setup on my old '88 SS.

Front: 15x6 with 3.5" backspace. 215/70-15
Rear: 15x8 with 4.5" backspace. 255/70-15

Eric that looks amazing...it has the perfect stance and look. I like the 3 star center caps on the Welds. That is the first I seen anything like that done. By the way love the first name.

Eric

It does look great!! Thanks for the response.

Thanks! You are welcome.


Thanks, Eric. Yeah, the 3 bar knockoffs was just something to be different. I miss that old car sometimes, but wouldn't take 100 on trade for my '86.

Here is a short video of it: http://www.youtube.com/watch?v=1mSb-coZHyQ

You can see that tire fitment a little better. You know, there was a lot of great men named Eric. Still is...... LOL! Great to meet you!

ERIC

Man Eric, your old 88 was awesome. That thing looked and sounded great. I may just need to copy that stance on it. I will send you a link to the way my Monte sounds with a 406 in it.

Thanks, bud. My '86 has the exact same stance. A 3" drop on these cars seems to be perfect in my opinion. I think I have already heard that big 406 which sounds great! But, you can send me that link anyway.

A few members have inquired so I thought I'd share some info-as seen in my sig., I run Impala SS wheels (funny story about those too) which measure out as a 17x9 wheel (5" BS) and I have 255/45/17 up front and 285/40/17 out back. Overall installed height is 28" in front and 28.1" in the rear-running Firestone Firehawk SZ50 EP series tires which have a stiff sidewall but a fairly soft tread compound-imo the best road racing style tire for the street. Full Hotchkis suspension allows me to run as tight as 1/8" at the rear frame and not have any rubbing issues. Car also sits 3" lower up front and 2.5" in rear-all custom parts-not many "catalog" parts on this car-mostly all GM components but from a variety of cars-at least 5 different cars that IO remember....lol. This car does everything extremely well-accelerates hard, stops quick and corners amazingly.

here is a 295/50/15 on an 8" rim with 4" back spacing

Would some c5 rims off a 99 vette work for a stock suspension monte? I do plan on lowering it eventually though.

The wheels are :

17x8 1/2 up front with 245/45/17 tires
and
18x9 1/2 in the back with 275/40/18 tires

I just bought an new set of 4, ZQ8 Chevrolet 16x8 with 5 x 4 3/4" bolt pattern with a 4 1/4" backspace. Looking for the widest tire I can get on the front and rear without modifying anything.

"84 ZZ4 posted pics earlier in this post of his tires: Front are 17x8 4.5" bs w/ Nitto 555 in 255/45/17 and 17x9.5 5" bs, Nitto 555 Extreme in 285/40/17, with a 5/16 spacer on both sides."

I like the height of 84 ZZ4 tires, so Im looking for a similar look of tire's for my 16's. 84 ZZ4 setup is just slightly to wide in the rear, because it would require lip rolling if I ever lowered the car 2in or 3in in the future(which I will). So, I need a tire that will not be as wide. I also want a slightly taller tire in the rear by 1 inch to 2 inches than the front. Any help or suggestions is greatly appriciated?
Thanks

So, on a 9" wheel at the rear, what will be the right offset? This is know in each wheel as the ET and on must of the brands measuere this in mm. So we know the Backspacing is 4-4.5" but what we really need to know is ths ET on the wheel for the rear and the ET for the front on 8" wide.

This is what I´m talking about

A stock 1985 SS wheel is a 15 * 7 et 10 mm

With a 18 * 9 the et -15 mm

The Inner clearence is the same and the outer Position is extended by 50mm

Check it out in this web page

http://www.1010tires.com/WheelOffsetCalculator.asp

Offset is the distance from the center of the wheel to the wheel flange (positive if the wheel flange is outboard of the centerline, negative if it is inboard.)

Backspacing is the distance from the wheel flange to the most inboard point of the rim, including the lip for the tire bead.

Width is measured from inside of the tire bead flanges, so an 8" wide rim typically measures 8.5" at it's widest point.

To convert from backspacing (BS, in inches) to offset (OS, in mm), knowing rim width (W, in inches):

OS(mm) = [(BS-0.25)-(W/2)]*25.4

conversely:

BS(in) = (OS/25.4)+(W/2)+0.25

25.4 is just the conversion from mm to in.

looking at getting some new wheels and was wondering what is the most backspacing you can run without usuing spacers.stock suspension.looking at 16x7 for the front and 16x8 on the rear.

I had posted the “chart” below in another thread but thought it might be useful here. The measurements were taken utilizing stock upper and lower controls arms with drop spindles. The spindles won’t affect the sub-frame and fender clearances, but by raising the mounting point of the wheel two inches in order to give the body a 2” drop, 17” or smaller wheels with 4.5” backspacing (or more) can or will give clearance problems at the tie rod. I know it was extremely close on my SS (less than 1/8”) when using a 17x8 SLP ZR-1 wheel with a 4.5” backspacing. However, 1/2" more radius provided by an 18” wheel (compared to the 17” wheel I used for measurements) should give ample clearance at the tie rod for more than 4.5” backspacing – I have no idea of how much more.



Basically it looks like clearance is tight, but good enough to run 245/45-17s and 245/40-18s on an 8” wheels with 4.5” backspacing. I know some have run a 255/45-17 and 255/40-18 tire on 8” wheels with 4.5” BS, but based on my measurements anything wider than a 9.8” section width is pushing things – don’t just go by the tire size since section widths vary between manufacturers and even lines of their tires. Some have reported rubbing with 245/40-18s with a 4.5” backspacing and others have not. There may be suspension differences at play, but I’d lean towards the differences in section widths and sidewall stiffness between brands of tires as the issue.

It seems from the consensus of those that participated in the original thread containing the chart, 8” wheels with the more commonly available 4.5” backspacing give the appearance the tire is sitting in a little more than “optimal”, especially when running wider tires and wheels out back. It makes sense if you think about it – If you’re taking the tire as close to the edge of the fender lip as you can in the rear, if you don’t do the same for the front, the fronts could look a little sunken. If you’re running the same size wheel/tire front and rear (or slightly larger for the rear) this probably won’t be an issue. Also, if you’re dropping the front more than a couple inches, you may need the tire to set in closer to the sub-frame than the fender anyway.

Looks aside, it appears an 8” wheel with a 4.25” backspacing will come close to centering the wheel in regard to sub-frame (under full lock) and fender lip clearance. As will an 8.5” wheel with a 4.5” BS, a 9” with a 4.75” BS and a 9.5” with a 5” BS. Technically, a 0.25” offset (bringing the center forward) is a negative 6.35mm (-6.35mm). The trend is to advertise offset and many advertise a -6mm and -7mm offset which is technically 0.236” and 0.276” respectively. I doubt that the few hundredths of an inch one way or the other will present an issue.

15x10 with 5.5 back spacing with 5/16 spacer 28x10.5 slick, notched to the seam and lip rolled. fits like a glove.

Will a 275/40/17 fit under all four corners without rubbing on a 9 inch wheel with 4.5 in backspacing and -12mm offset?

These wheels
http://www.summitracing.com/parts/brw-997793445
These tires
http://www.summitracing.com/parts/bfg-31629

Thought I would put this out there in case someone wanted to know

Vision 143 Wheels - 18x9.5 w/ 5.25BS / 0mm Offset

These pics are with no spacers, stock frame rails - about 1/4" of clearance to the rail (not the "LH" plastic thingy)

great thread very helpful

Adding the image back:

I had posted the chart below in another thread but thought it might be useful here. The measurements were taken utilizing stock upper and lower controls arms with drop spindles. The spindles won't affect the sub-frame and fender clearances, but by raising the mounting point of the wheel two inches in order to give the body a 2" drop, 17" or smaller wheels with 4.5" backspacing (or more) can or will give clearance problems at the tie rod. I know it was extremely close on my SS (less than 1/8") when using a 17x8 SLP ZR-1 wheel with a 4.5" backspacing. However, 1/2" more radius provided by an 18" wheel (compared to the 17" wheel I used for measurements) should give ample clearance at the tie rod for more than 4.5" backspacing but I have no idea how much more.



Basically it looks like clearance is tight, but good enough to run 245/45-17s and 245/40-18s on an 8" wheels with 4.5" backspacing. I know some have run a 255/45-17 and 255/40-18 tire on 8" wheels with 4.5" BS, but based on my measurements anything wider than a 9.8" section width is pushing things. Don"t just go by the tire size since section widths vary between manufacturers and even lines of their tires. Some have reported rubbing with 245/40-18s with a 4.5" backspacing and others have not. There may be suspension differences at play, but I'd lean towards the differences in section widths and sidewall stiffness between brands of tires as the issue.

It seems from the consensus of those that participated in the original thread containing the chart, 8" wheels with the more commonly available 4.5" backspacing give the appearance the tire is sitting in a little more than optimal, especially when running wider tires and wheels out back. It makes sense if you think about it. If you're taking the tire as close to the edge of the fender lip as you can in the rear, if you don't do the same for the front, the fronts could look a little sunken. If you're running the same size wheel/tire front and rear (or slightly larger for the rear) this probably won't be an issue. Also, if you're dropping the front more than a couple inches, you may need the tire to set in closer to the sub-frame than the fender anyway.

Looks aside, it appears an 8" wheel with a 4.25" backspacing will come close to centering the wheel in regard to sub-frame (under full lock) and fender lip clearance. As will an 8.5" wheel with a 4.5" BS, a 9" with a 4.75" BS and a 9.5" with a 5" BS. Technically, a 0.25" offset (bringing the center forward) is a negative 6.35mm (-6.35mm). The trend is to advertise offset and many advertise a -6mm and -7mm offset which is technically 0.236" and 0.276" respectively. I doubt that the few hundredths of an inch one way or the other will present an issue.

i just joined the forum since i just bought a 85 ss and i was looking to run a bigger rim and a wider tire since i plan on running a ls engine in the future the rim i was looking at was 17x8 with 4.5 bs the tires the recommended was a 235/50 i dont know how this would fit rubbing was in the front and rear if anyone could help me with this id be thankful

I ran a 245/45/17 on a 17x8 with 4.5in BS with no rubbing at stock ride height.

When I lowered it, I ran into some slight rubbing in the wheel wells and at the 12 o clock position of the wheel well.

how much did you lower it