Considering widening my rear wheels, would like to verify my thoughts on fitment

[2000xp8]

So I have a handful of saleen speedline wheels. I've probably run these since the 90's.
All 18x8.5
Some are 10mm offset, some are 30mm.
For years i've run the 10mm in the front and the 30mm in the rear. To be clear, the 10mm stick out further than the 30mm.

Tires these days are a real issue for 18x8.5 wheels. The selection is terrible. Should be like a 255 or so to fit and look right and those choices suck and are a little narrow.
I currently use a 275/35/18s but honestly they are too wide for an 8.5. Though in this size there is a huge selection. (I will NOT be using a tire with over 200 treadwear, i don't care how long they last or how they run in the rain).
In my case the tires bulge more than i would like but clearance everywhere is ok.
My thinking is to take my spare 10mm wheels and run them on the back. That would push the wheels out rough 3/4 of an inch. (back spacing on them is roughly 5.25)
I was planning on having them widened 3/4 to 1 inch. I've seen pics from weldcraft and it can be done with this particular set.
The fenders are rolled and i do run a panhard bar and torque arm, which if i remember correctly MM warns you about cutting it too close because of the way the rear articulates. The rear can't move side to side, so it's more like it pivots on the torque arm.

I'm not really interested in more tire width. If it was a sure thing? Absolutely, but I'd prefer not ruin these particular wheels. They don't exactly grow on trees.

If i'm doing the math correctly (and i may not be) If i added 3/4 to the wheel, i'd be at a 6 inch backspace which I was under the impression was the acceptable spacing on a 9 inch wheel. That should keep the inside of the wheel where it sits currently and push the outside out 3/4 (where there is room, especially without the tire bulging over the lip of the rim.
My guess is that i'd also have an extra 1/4 on the inside to just goto a full inch widening.


I don't seek second opinions often, but in this case where the mod is permanent, i'm open to opinions (i may not listen, but at least i was warned).
I'm not very tolerant of the tires rubbing and just can't have that happen.

I'm having the chrome stripped as we speak on this particular set (chemically, it does not remove any of the important speedline markings) and i will mount one up without a tire to see how it works. I'd say i mount a tire and check, but that would be with it at 8.5 and not sure if that helps.
I do have other wheels this offset, but they are NIB and would prefer not to nick or scratch them, so i can wait until the weekend.

A few wild cards may affect the situation.
I run 275/35/18 BFG drag radials, they tend to run wide (measured about 11 inches with a tape measure). Not sure how the next set of tires will be.
Then there is the fact that i'm messing with multiple things at once. A wider rim, moving it outward and tires that should fit correctly. I'll suspect an 8.5 with this tire and a 10mm offset would rub on the fender. But widening it should in theory move some of that tire inward and it won't overhang the wheel nearly as far.

Thoughts? Am i on point here?
I've went over this in my head for a few months and just decided to have the wheels stripped (i'm not a chrome type of guy and they were in mediocre shape).
I need tires, so it's time to make a final decision.

Joe

 

[AeroCoupe]

Just so you know I run an MM panhard bar and torque arm on the Coupe. I am currently running 17x9 wheels with a +45mm offset. This requires a 1/4" spacer to keep the 275/40R17 Nitto 555R2's from rubbing on the inner fender in hard cornering. Fender lips are not rolled but are trimmed at the top and faded to the sides and I still have about a 1/4" of lip at the top. I do not and have never run quad shocks so I have no idea if they would clear or not.

With all of that I would only run a 9" wide wheel on a Fox with a panhard bar and torque arm due to the geometry change in how the body articulates around the rear end. Standard four link you can get away with a 10" wheel but you will have to use a BFH and persuade the inner fenders to move and I am still not sure that will be enough as most that run a 10" wide wheel with a four link have to do this.

 

[2000xp8]

Thanks for the reply. Obviously in this case there is no worry about quad shocks, i don't even think you can run them with the torque arm.

At a minimum if I send them out, i'd have 3/4 added, which would be a 9.25. That number should put the wheel exactly where it is on the inside as it currently sits with no rubbing. Another quarter may not sound like much, but i struggle to believe with how much wider my tires are than my rims that .5 inches is enough to accommodate a 275 properly. I know calculators suggest 9.0 is the minimum, but my eyes are telling me different.

For my liking, i don't think 10s would help anyway. There aren't any good 285 tires, so that eliminates that size, then at 295 30 series becomes too short and 35 becomes too tall (for what i prefer).

After a little math (again that could be off), it would appear if i made the wheels 3/4 inch wider, it would still have 10-15mm more clearance on the inside than your set. That would be in the vicinity of 1/2 inch.
It sounds like your wheels would be slightly more inboard than mine even after mine are widened.
At a full inch widening that should still be 5-10mm more space inboard.

As I said, i could still be off, it was making my head spin just doing the math on mine, that adding yours to the mix didn't help.
Maybe i should start writing stuff down. lol

Now all of this assumes it will clear on the outside. Which i think it will without the tire sticking off the rim a full inch per side.

 

[AeroCoupe]

Ever thought about buying a wheel firmament tool?

 

[nicholase]

I've built a few chassis from scratch. Figuring this out is an important step. Especially when you're having an axle housing narrowed and axle lengths custom made and figuring out wheels and backspacing so it all fits and looks right. Some times having a deep dish wheel fits the build, sometimes not so much. So figuring this out is a big part of a scratch build frame. I can share what I've learned. It does require drawing it out if you really want to do it right and do it once. Here's how.

It's important to note that wheel width is measured from the inboard BEAD SEAT to the outboard BEAD SEST. Backspacing is measured from the wheel lip to the mounting flange. Here's an image.

So remember when you measure backspacing from the wheel lip to the mounting flange you are including the thickness of the wheel lip, which isn't measured in the wheel width. Wheel lip can measure anywhere from 1/4" - 1/2".

To figure out actual wheel and tire width go to a place like tirerack.com (https://www.tirerack.com/tires/tires.). Search by size. Click specs tab. This will give you the a chart like this:

Different brands and makes have different measurements. You'll be looking for the "section width" measurement. Thats how wide the tire is at its widest point at the sidewall. This example is for a Goodyear F1 in one of the sizes you listed. It's measured at 10.9". This is on a 9.5" wheel.

Heres a quick scribble. We'll be using a 9.5" wheel with 6" backspacing for all of them.

So in this example you calculate total wheel width which equals the advertised width (9.5") plus the lips on both sides. This varies each lip could be 1/4"-1/2". Already having the wheels in your case makes this very easy. Just measure them. Let's say they are 1/4" each side. Total wheel width = 10.0". (Or 9.5 + .25 + .25 =10 )

Total tire width (section width) = 10.9"

10.9" - 10.0" = .9

The tire is .9 wider than the wheel. Divide this in half will tell you how much buldge sticks out past the wheel on each side. In this case .45

So let's add that to our drawing and see how it effects backspacing

So here you can figure the distance from the mounting flange to the widest point of the tire sidewall.

To figure out the distance to tue outboard side you just use the same math. In this example the measurement from the mountong flange to the outside of the tire sidewall is 4.45".

So if you add the 6.45 + 4.45 it equals the 10.9 section width.

 

[2000xp8]

Thanks Nick, I've done a lot of what you are showing. Just without the fancy drawings, lol.
I usually use tirerack for the tires and https://www.1010tires.com/Tools/Wheel-Offset-Calculator and https://www.1010tires.com/Tools/Tire-Size-Calculato
I don't have a wheel tool, just didn't think i would need it in this case because i have the wheels themselves. But if i have to buy/borrow one, i'm not opposed to it.
When i get them back today i'll measure to the from the existing rim to the tub and then install the 10mm offset and see how they compare.
If the calculations are right, the new wheel should be 3/4 more outboard.
The F1 is a good example, considering I may want to run the F1 3R.
Thanks for the drawings.
The bulge is i believe is what i was having the most issue with, mainly because i have a lack of faith in width specs.
Right now the tire bulge is 13/16 (.81) per side (measured with a straight edge and tape to lip). So that should give a little more room.
With a number that high you can see why i'd like to widen the wheels.

 

[2000xp8]

So I swapped out the wheel today.
At it's closest spot (for the rim) the new wheels have about 2 inches on either side of the car before accounting for a widened wheel.
In my head, measuring was going to be super easy, really wasn't with tires on the ones on the car. Regardless, the new wheel does indeed add about 3/4 space.
Going to 9.25 is most likely a sure thing.
Going to 9.5 and getting roughly 1/3 inch back from less tire bulge could work.
Might be slightly risky.

Oddly enough with the 10mm wheels on the car, the rim is still inside the outer wheel well. Again, with less tire bulge that shouldn't be an issue on the outside.
I probably should have never used the 30mm wheels in the first place.
But that is a mistake i made 30 years ago.

Most of the good tires are also available in 265/35/18. It's .3 shorter, but split in two, that shouldn't really be a factor.
It may just be the wisest choice is to have them widened 3/4 then use a 265 tire.

 

[AeroCoupe]

You need to jack one side of the car up and watch how the body articulates over the wheel. If you spend all that money and they rub you are gonna be pissed. Better yet call MM and talk to them about it.

 

[2000xp8]

I had spoken with them a couple months ago via email.
They actually suggested that I should run 275s on all 4 corners.
Sounded like a great idea, but i just can't wrap my head around how I would fit 275s in the front.

My lift has a hydraulic slide jack, i can just put one of the truck frame stands on one side and pick it up.
Though I know the inside will be good since it will be the same or better than before.
The outside i'm not sure without a tire on the new rim is going to tell me much.
I suppose i could space out the wheel with the 275 on it and see where it goes but that tire has nearly an inch of overhang on the outside and the new one won't.

After some thought, i think I'd also like to try and keep the tire on the rim close or just past square and it's looking like a 275 tire would take about a 10.25 rim to make it happen.
A 265 isn't that much better, would still take nearly a 10 inch wheel to get it square.
It's looking more and more like the 265 is the better choice, regardless of if I add 3/4 or 1 inch.
I think the 275s are just too much tire, period.

 

[AeroCoupe]

I think you are talking yourself in circles. I literally run a 275 Nitto on 9” wide wheels and they fit great. This was during the TKX swap but there isn’t much side wall bulge.

Don’t get me wrong I understand the contemplation, calculations and second guessing as it took me a few weeks to pull the trigger on the Enkei’s as they were not stock and would come from Japan. Waited months due to Covid and when they got to the house I was so relieved they fit.

 

[2000xp8]

Well, i certainly have been going in circles.
This is one of those projects that's been in the back of my mind for a long time and now it's moved on to being executed. I typically daydream about projects, then I get into them and get obsessive, then I get the results accept them and i move on.
Right now i think i'm going to just have them add the 3/4 (no sense in doing less, i know for sure that will work).
I can worry about the tires after i get them back.

Yours does look pretty good, how far does the tire bulge?

 

[Noobz347]

Are you widening your Pony wheels? Have you seen the ones on LMR?

 

[2000xp8]

Are you widening your Pony wheels? Have you seen the ones on LMR?

Just checking are you the guy on FB marketplace asking all the questions that I already answered in the ad? Lol

 

[Noobz347]

Just checking are you the guy on FB marketplace asking all the questions that I already answered in the ad? Lol

Nope

 

[gkomo]

Damn I run a 275 on an 8” wheel

 

[nicholase]

I have 275's on 9" wide pony wheels (rear). The wheel measures 10" lip to lip. I have 5/16" tire buldge.

I know most people won't probably want to spend this much time.... but I put jack stands under frame, remove the coil springs, put the control as back together, then a floor jack on pumpkin. You can articulate the axle easily by hand and see whats going on and where the closes points are. A straight edge on the rotor face and measure while moving the axle around.

I've never bought a backspacing tool (dummy wheel), but they are nice. A simple L channel bolted to one wheel stud with a metal ruler attached to the top and bottom with wing nuts in slots works real well for an DIY wheel guage. If I can find mine I'll post a picture. Its been a while since I've seen it. Takes about 10 minutes to make.

 

[FastDriver]

275s on a 9" wheel are fine. On a 10" wheel works, too. From what I gather, the more square the better the traction in a straight line and the corners. So, you're not wrong for wanting to do it, even if a bit obsessive, but hey... that's called having a hobby.

 

[2000xp8]

Yeah, i'm not sure how I got on this square tire detail, regardless, it's where i'm at.
Most of you guys know once an idea infiltrates your thoughts when it comes to a car, it's hard to dismiss it. Not to mention with performance tires being the sizes they are it just seems like the time to fix it.
Just one little piece of the puzzle to make things how i prefer they be.


Everyone has given good advice.
It all makes sense in theory.

My hesitation may all be coming from the actual size of my current tires vs their rated size.
Almost seems like the 275 wasn't accurate. Even here where i probably bought them, they suggest a 9-11in rim, which probably means a 10 is optimal.
If that is true, most of what you guys have suggested is correct and where the disconnect is that my experience is with a tire that truly does run wide.
Even with a 9, i just don't think it would be wide enough for what i have now, but the reality it, there is a good chance that is not what the next set is going to be.
It's a 275/35/18 Gforce drag radial (if it doesn't link correctly)

Discount Tire | Tires and Wheels for Sale | Online & In-Person

www.americastire.com

 

[FastDriver]

Did someone mention 6.5" backspacing is about right in a 10?

 

[AeroCoupe]

Without a panhard bar and torque arm it works with a Fox width rear end. Most have to massage the inner fender to make it work with any kind of tire side wall bulge with the OEM four link. Obviously if you have a 94-98 SN95 width rear end that changes things.

With the PHB and TA the body articulates around the rear end which will cause interference with the inner fender more severely than the OEM four link so I would not be overly optimistic that a 6.5” BS will come close to working with a Fox width rear end.

I can say this because the wheels I am running with an SN95 width rear end have a 6.75” backspace with a 1/4” spacer so an effective backspace of 6.5” and they work. The SN95 rear ends are 0.75” wider per side than the Fox width rear ends. With all of that the optimum backspacing for my car with a Fox width rear end would be 5.75” with the Nitto 555 R2’s.