acprokeith

Been reading for a long time but since I always find the answer I'm after (thanks to you fine folks), I never post. Appreciate all the tech in here and all who contribute! I can't count the number of times it's helped while putting together this car.

Before you say "go search" I've read every thread on here and many on other sites about fitment of 9.5-10.5" wide 18" wheels on these cars. At least 12 hours of it. I also have an 87 that I ran front 18" (x8"?) IROC wheels on all the way around years ago, and hit the rear fender lip on bumps (275/40/18). Trying to avoid that but I dont want it to look tucked or like it skipped leg day either. Trying to find the happy medium so to say and go out as far as I can without hitting the outer fender lips.

I know every car is different, but only to a small extent. I'm about order wheels (Forgestar, Forgeline, or CCW) and am stuck on what BS to order. Seems +13mm aka 6" of BS (6.5" on a 10.5" wide) up front is the key to not hitting the strut and also not hitting the fender lip. The rear seems to be 6.5" BS aka 27mm, OR just the same as the front. Cant seem to find a solid answer and I dont want to order custom wheels twice. I could measure and do all this and that, but thatll never tell me if it'll hit when the suspension cycles. Most threads with pictures and solid info are for cars with 10.5" wide wheels. I am planning on 295/35/18s so a 10" wide is what I understand I need.

Car details that pertain to fitment: 1989 Iroc, yellow Konis, Eibach sportlines, Global west DelAlum bushings up front and their control arms out back, BMR panhard, Detroit speed strut towers, old Baer 13" brake kit up front (soon to have their 12" rear kit as well) and a stock 3rd gen 9 bolt axle. Car will be used for carving country highways and a track day from time to time. I don't mind using the BFH inside the fenders, but have no desire to roll the fender lips.

Seems many think the limits of backspacing is the mid to high 5s, but then I see folks doing 6 and 6.5". Some posts by @//<86TA>\ have been real real helpful but the more I read the more it seems I question my numbers. Ready to buy just need some clarification. Thanks!

PS, I'm a long time Master AC technician and am more than willing to trade wisdom/ info/ parts with anyone who takes the time to help me. Also willing to pay a shop to make sure these numbers are right, but don't know of any that do that. I'm in NE TX

ShiftyCapone

iTrader: (7)

Nice car. What rear end is in the car? Running 10.5 up front is going to be hard. Have you watched McLovin's videos on youtube? He goes by 3rd gen Camaro guy and he has a video where he shows what he did to make 18x10's work up front. I still don't know how folks do it. I am running a 9.5" wide up front with a 265 tire and rub at full lock. Ride height and alignment certainly come into play, but unless you are tracking the car, you don't need to go that wide up front. The pain doesn't offset the gains IMHO.

acprokeith

Thanks! I have a few hundred hours in it already. It has the stock 9 bolt rear axle. I do not plan to run 10.5s, just 10's. Goal is 295/35/18 all the way around. I will go watch his videos today, I appreciate the heads up! A little rub at full lock is fine, and we will be doing some leisurely track stuff from time to time. Otherwise it'll be a sunday cruiser and backroad killer.

RoadRace

Hi
I run 18x10’s all around on my 85, I started with 295/30/18’s square. I currently run 315’s and 275’s. But might go back to 295’s up front - but like was said in the earlier post - I only track the car. I started with the same alignment I had with the 16’s and that didn’t work. I went from 3 deg neg camber to 1 deg neg with the big tires. I run on a shorter track (1.3 mi, 9 turn, 2 are RH) and go with zero toe…not ideal for a street car. To be honest, if the track conditions dictate, I will toe-out the DS and leave the PS at Zero.

I use the same offset rim front and rear (25mm) and use a 12mm spacer in the rear on top of the 3mm spacer the car came with from the factory. I use a 17mm spacer up front.
ARP studs (2.5”) all around.
they did rub - I rolled the inner wheel lips all around and “clearanced” the front of the rear inner wheel well.
with the 315’s on the same 10” rim, they didn’t rub until i hit the track and hard right handers caused rubbing again…I’m working on that lol.

acprokeith

Appreciate the input! So if the calc is right that's about 7" backspacing in the rear and 7.125" front?

With all our different setups its really hard to decide exactly whats what and what will fit. Curious how much your front brake setup moved out your track width and how much wider your rear axle is than a 9 bolt. Guessing your car came with drums and theyve been converted over to disks or its been 4th gen axle swapped.

Deposit has been paid for some Forgeline CF1 open lugs, I just have to nail down the BS.

Thanks!

RoadRace

hey - I'm not certain on the backspacing, i will pull off a wheel and check tonight - the car is off the ground right now anyway. I'm running the stock 10 bolt that has never been out of my car, its an 85 IROC and is a factory disc brake car. it came with a .120" thick steel shim wheel spacer from factory...I use the 12mm hub-centric spacer in addition to the factory spacer. I am looking to use a 4th gen width rear in the future.
Up front, the 295's rub the inner wheel well at full lock. No idea if the track width changed with the 6th gen brakes - I would like to think the design engineering into the kit was to maintain the OEM track width. Scott at BBU would be the authority on that.
As the 18's fit over the tie rods, more BS is available. with the 295's up front, I could barley squeeze my finger in between the tire and the strut, but at max cornering forces, the 295 didn't roll over and rub the strut. I'm pretty sure as it sits, my set up is the max BS you can run up front, any wider and you're going to have to go outboard.

acprokeith

You are the second person to mention that spacer. I have an 87 and this 89 but neither have a spacer. Thinking its an early model thing.

I would really appreciate it! Im going out to throw mine on the lift now and measure for the engineer. They have made quite a few sets for 3rd gens and I'd think they'd have it nailed down, but my understanding is that the final word in on me. Like most, I just want it to clear the outer fender lip without destroying the tire or fender, without sitting in like some "stanced" import with stretched tires. The furthest out I can get it is the goal. Seems like 6.5 is the go to iwith a stock rear, but from pics it looks like 6.25 or even 6 might be the magic number on rear with a 10" wheel. The front is more of a guess, as it's .400 wider per side than factory and they have .635 more BS in the rear than the front from the factory. Thinking the same BS front and rear might just work.

I want to stay away from any spacers of any kind, as I'm from a rock crawling/ racing background and have seen far too many come apart or get loose and come off. I know it's common place in this segment of motorsports, but I just dont have confidence in them over 100 mph. Hence the reasoning behind custom wheels vs whatever else and an adapter/spacer.

I really appreciate your time sir! Once this is all figured out I'll post in the thread I already started about the BS details, numbers, and what not, so maybe the next guy wont have to do countless hours of seemingly worthless digging.

RoadRace

Hey - so I took some dims for you:
my set up:
18x10 25mm offset = 6.468” back spaceFront 5.75” Back Space with .665” Alum spacer
Rear is 5.81” Back Space with .151” OEM steel shim Spacer and a .475” Alum spacer
NOTE: there is more space than I remember between the front rim and the Strut. Could go back another .250” no problem.


Rear spacing - shows both spacers

Steel shim .151” OEM SPACER and hun centric alum spacer

Front alum spacer

Angle alum extrusion used to measure - touching rim only

315/30 18’s on 10” rims.

acprokeith

I sure hope you didn't take the wheels off for me! Thanks for taking the time out of your day to measure and write all this up! Most wont take the time to reply, much less all that.

You're right about where I was thinking. Did you try fitting them with no spacers? Seems like they should fit with ~ 6".

I tore mine down and measured again yesterday, with Scott from Forgeline on the phone. As long as their engineering dept doesn't thumbs down the idea, we are going with 18x10 deep concave with 6" BS squared. The .400 added width up front per side makes up most of the difference in factory BS front vs rear. It's still .225 off so we almost put the rear at 6.250 BS, but after quite a bit of digging they assure me we can clear 6". I'd rather have it out as far as possible without fender contact anyway. It'll be on 295/35/18s most likely. Those AO52s seem to be well liked and pretty wide for a 295. How do those 315s like the 10" wheel? Seems like they would be pinched pretty well.

Thanks again!

RoadRace

It doesn't take long - so no problem, if it helps you at all, then that's what its all about! Like i said the car is off the ground right now anyway as I'm waiting for a couple parts so i can put it back together...just in time to store it for the season!

Sounds like you guys have a plan and that great - I'm looking forward to seeing how it all worked out, those Forgeline wheels are really top shelf, and by the sounds of it, so are the folks that work there - great to hear.

i didn't try to fit them alone, but I really should play with the spacers a bit and even try the front without a spacer just to see if/how it fits. With the 275's on there it fits easier, the 295's up front changed the fitment, but I didn't try anything with the spacers.
Looking at the back, the 315's rub the lower control arms during cornering and also rub the rolled inner lip during corning - with 5.81" back space. I might put the front .665" spacer on the back and roll the wheel well - or just go back to the 295's. But as mentioned earlier, the early cars might have a different width diff if they come with those .150" thick spacers.

The 315's tuck Ok onto the 10's. Hoosier says a 10.5" rim is min for 315's, so they pulled in a 1/4" per side - while not ideal - it isn't that bad either. I probably need more tire pressure and definitely need more wheel well, I'd like an 11" rim if I had the option. I will say that all tires are not created equal and when i was speaking to a proper race tire guru when I first put the 18's on, he told me that while the 295 Hoosier were Ok for a 10" rim, he would not run a 295 Toyo or Nitto comp tire on the 10. I will most likely run a 295 Toyo on my car on these 10" rims because I'd like to see how and why, but I'm just relaying what I was told in case Yokohama says something similar.
Also, the 295/35 is taller than the 295/30 I was running, but is real close to the 315 I have now and that little bit of extra height puts the tire closer to the forward inner wheel well as it curves toward the tire up and forward, so i would check clearance there once its mounted.

acprokeith

Glad some people still feel that way! I appreciate it and hopefully this post will help others in some way in the future. I couldn't do all that cold and misery you all have up there. Hell, its too cold for me in NE TX, lol.

It sure couldn't hurt to do so, and maybe get a WMS to WMS measurement while you're at it to see where it lies in comparison to others. I'd bet there is a small difference, and it also sounds like the fender wells or LCA mounts might be slightly different too. 5.81"BS and 315s rubbing the LCA sounds off (those 315s are wide though). Ever fit any 295s out back?

I used to run 275/40s on 8" wheels when I was younger and they pinched a good bit but I never felt anything bad from it. Looks far better than a stretched tire. I'd be curious to hear what you find, but I'd assume these guys know better than us. Lap times prove their points from what I've seen. The AO52 might prefer a 10.5" wheel, but 10" is in the allowable range, same as it is with the other option the Potenza RE71-RS that I ended up buying before a promotion was over. Seems like a well built tire and is almost as sticky as the sticky tires on my crawler. Hopefully they wear a little better.

I'll check up front for clearance, but I'm guessing that whole inner fender is going to have to be heated and formed to allow clearance, possibly removed. I'm sure it'll go up and down 50 times on the lift before we ever drive it. Hope it all fits!

RoadRace

hey - can you please decode WMS? i work in the airplane business, deal with a million acronyms everyday and sometimes i just gotta say i don't know that one!
Weather - ya, its a short season up here - 6" of snow last week, been below freezing for a while - actually when i took the pics for the backspacing was literally the last nice day. We have 5 competitive track events per year, one a month from May-Sept, and early May and late September events are typically barely above freezing in the morning, and 50/50 if its going to be wet - I typically never make all 5 events. My wife doesn't understand the money and time investment for 4 track days a year....but like they say, if you have to ask....

As far as 295's, yes - that's what I ran on the car when I first went to 18 x 10's. I had Hoosier R7 295-30-18's on front and rear.
Rear: had to cut the inner wheel lip off the rear wheel wells and 'massage' the front inner part of the rear wheel well also.
Front: rubbed at full lock, I rolled the inner lip at the front but it wasn't really a problem, rubbing on the fender/battery support bracket and inner wheel well was the only contact points. Also keep in mind that ride height will have an effect on where it rubs, I run my car pretty low - its track only, so I can get away with a bit lower.

that being said, i might put it on the street next year, just to make some noise once in a while

acprokeith

Sorry it's a term used often in off road motorsports. Stands for wheel mating surface to wheel mating surface. Basically width minus tires and wheels.

Yuck. I don't see how you all don't go crazy. Some women just don't get it, and then there's the rare few that'll get out there with you and work on them. Luckily I have the latter of the two. She's done all the weather stripping and interior work so far.

Well that's reassuring! I am all to familiar with clipping the lips on the bottom of the outer fenders, and hammering in the rear lower front fender area. That's to be expected, as well as some front inner fender adjustments. Trying to roll the rear fenders was what I wanted to avoid. May do the fronts (just in case) since I can borrow the tool and do it without damage. I'll keep my eyes peeled for contact with the fender/battery support. The car is 1.5-2" lower than stock, so it's about as low as I could tolerate on the street. Typical Eibach sportline springs. She rubs the air dam if I'm not very careful pretty often.

You really should get her out on the street. Surely there are some beautiful mountain roads you could tear up up there. I have to drive hours to find even a hint of a mountain, so I just enjoy the flat country roads here.