Functional will it fit thread
11-02-2013 | 08:53 PM
#1
Thread Starter
Unregistered User
Joined: Apr 2006
Posts: 824
Likes: 0
Functional will it fit thread
I wanted to ask what's the widest rear tire & wheel combo you can fit in our wells without sacrificing function. I currently have 18x9.5 with 285 @ the rear with around 40 offset. I don't want to go any more than 8-9 in the front for drivability at high speeds. I am getting brake bias valve, softer front sways, and stiff rear sways to adjust for the difference. I haven't sorted out the spring rates yet, but that's next. Can anyone chime in?
Last edited by stickmantijuana; 11-02-2013 at 09:29 PM.
11-02-2013 | 09:39 PM
#4
11-03-2013 | 07:09 PM
#5
It depends on context. For autocross, going as wide as possible is usually faster, since the speeds are lower and you're cornering more of the time. At higher speeds on track, the extra width can start slowing you down on the strights more than you can make up for with extra cornering speed, but again, it's dependant on the course. See the quote below:
But most would agree that an RX8 works best with the same size tire front and rear, that way there's no need to take grip away from the rear to make the car not understeer like crazy.
Ok, i think i've been convinced on the 255's but i'm still not sure about the offset/width. I emailed Brian Goodwin at Goodwin Racing and he said he is faster on long courses with 255's but faster on tighter courses with 275's. I think my home course is in between fast and tight so i think a 255 will do just fine. He did, however, say the 17x9.5 +38 was probably the better option. I doubt he was thinking about wear and tear like some of you are but he mentioned the turn in response would be much better with the 9.5's with the same tire since the sidewall would be stiffer when mounted on a wider wheel. Makes me lean toward the 17x9.5's.. 
11-05-2013 | 08:16 AM
#11
Is this a contest to see who can make the most ridiculous assertion?
Maybe he should have said that he was only interested in feedback from people who have 450+ whp in an RX8 chassis, then none of you would be in a position to show how you can regurgitate stuff you read somewhere in your lifetime.
.
Maybe he should have said that he was only interested in feedback from people who have 450+ whp in an RX8 chassis, then none of you would be in a position to show how you can regurgitate stuff you read somewhere in your lifetime.
.
11-05-2013 | 06:40 PM
#13
Well, right now I have 17x9" +45 RPF1's and usually run 275/40R17 Nitto NT01's on them in all four corners no problem with a slight fender roll. I have also run 275/40R17 Hoosier R6's and these BARELY fit. When you stack them up next to the NT01's of the "same size" they are considerably wider, and I believe they measure out more in the 285mm to 295mm range.
I'm going to be getting a set of 17x10" +38 RPF1's to try and run 275/35R17 Hoosier R6's, might need some more rolling and slight pull for these, and then keep 275/40R17 NT01's on the 17x9" set.
I'm going to be getting a set of 17x10" +38 RPF1's to try and run 275/35R17 Hoosier R6's, might need some more rolling and slight pull for these, and then keep 275/40R17 NT01's on the 17x9" set.
11-05-2013 | 06:46 PM
#14
Bam.
Teamrx8 says:
A 10" wide wheel is the max width you can use that sticks close to the factory offset and takes advantage of all the inboard wheel space @ 50 - 52mm offset. With a fat tire (285 or 295) it will fill out the fender and also look mean as hell with all that rubber under there. You also aren't going to experience handling and driveability issues that result from wheel offsets lower than factory.
If you want to run wider than 10" then you have no choice but to start lowering the offset because there is no more inboard space available without extreme mods like tubbing and major suspension relocation/redesign. With every width increment above a 10" width @ +50-52mm offset you'd have to move the offset out by half that amount.
So for 1/2" increase above 10" width (10.5") half that amount is 1/4" = 6.3mm or 7mm for practical purposes. At each 1.5" increase you could subtract 1mm to account for the fractions adding up to a whole number, but there aren't too many RX8s using 11.5" wide wheels let alone 13" wide, lol. At anything past 10.5" wide using this basis the fender will need pulling or flaring for an equally wide tire not to rub because it will be sticking too far outboard of the factory fender lip to clear. So the resulting size/offsets would be:
10.0" @ +50 to +52mm
10.5" @ +43 to +45mm
11.0" @ +36 to +38mm
11.5" @ +30 to +32mm
12.0" @ +23 to +25mm
Otherwise running undersize tires to accomplish this fails on several accounts. To make the tire sidewall lean for fender clearance means that it has to lean both on the inside and the outside. A ridiculously narrow tire is required the further outboard the wheel lip sits outside the fender lip. Way back when tire sidewalls lacked good support but todays performance tires are designed such that a wheel wider than the tread width/contact patch width accomplishes no performance benefit. So a wide wheel with a high positive offset just to put a skiny tire on it is pointless and silly. Some wheel width/tire size reference points are:
9" wheel @ 245 to 255 tire
10" wheel @ 275/285 tire
11" wheel @ 305/315 tire
12" wheel @ 335 tire
Some change in offset from the original +50mm (S1 models) is not going to have much impact, but as you continue to push it out the resulting scrub radius moment arm is creating a force that makes the tires want to turn out, putting the steering components in constant tension while driving in a straight line. As such a setup encounters uneven pavement, bumps, water puddles etc. that moment arm is going to try and rip the steering wheel out of your hand position. Even more so for these conditions under braking because this same scrub radius moment arm is going to magnify the braking deceleration forces against the steering wheel as well.
A 10" wide wheel is the max width you can use that sticks close to the factory offset and takes advantage of all the inboard wheel space @ 50 - 52mm offset. With a fat tire (285 or 295) it will fill out the fender and also look mean as hell with all that rubber under there. You also aren't going to experience handling and driveability issues that result from wheel offsets lower than factory.
If you want to run wider than 10" then you have no choice but to start lowering the offset because there is no more inboard space available without extreme mods like tubbing and major suspension relocation/redesign. With every width increment above a 10" width @ +50-52mm offset you'd have to move the offset out by half that amount.
So for 1/2" increase above 10" width (10.5") half that amount is 1/4" = 6.3mm or 7mm for practical purposes. At each 1.5" increase you could subtract 1mm to account for the fractions adding up to a whole number, but there aren't too many RX8s using 11.5" wide wheels let alone 13" wide, lol. At anything past 10.5" wide using this basis the fender will need pulling or flaring for an equally wide tire not to rub because it will be sticking too far outboard of the factory fender lip to clear. So the resulting size/offsets would be:
10.0" @ +50 to +52mm
10.5" @ +43 to +45mm
11.0" @ +36 to +38mm
11.5" @ +30 to +32mm
12.0" @ +23 to +25mm
Otherwise running undersize tires to accomplish this fails on several accounts. To make the tire sidewall lean for fender clearance means that it has to lean both on the inside and the outside. A ridiculously narrow tire is required the further outboard the wheel lip sits outside the fender lip. Way back when tire sidewalls lacked good support but todays performance tires are designed such that a wheel wider than the tread width/contact patch width accomplishes no performance benefit. So a wide wheel with a high positive offset just to put a skiny tire on it is pointless and silly. Some wheel width/tire size reference points are:
9" wheel @ 245 to 255 tire
10" wheel @ 275/285 tire
11" wheel @ 305/315 tire
12" wheel @ 335 tire
Some change in offset from the original +50mm (S1 models) is not going to have much impact, but as you continue to push it out the resulting scrub radius moment arm is creating a force that makes the tires want to turn out, putting the steering components in constant tension while driving in a straight line. As such a setup encounters uneven pavement, bumps, water puddles etc. that moment arm is going to try and rip the steering wheel out of your hand position. Even more so for these conditions under braking because this same scrub radius moment arm is going to magnify the braking deceleration forces against the steering wheel as well.
11-05-2013 | 06:57 PM
#16
245 is ok for 8, but definitely not enough friction for a well tuned 13b-rew or 20b.
Personally i would go for 255 on stock 8, others have good success on 265, 17x9.5.
Just bored at work.
Personally i would go for 255 on stock 8, others have good success on 265, 17x9.5.
Just bored at work.
11-06-2013 | 01:21 AM
#17
Registered
Joined: Sep 2005
Posts: 2,372
Likes: 51
From: Hayward, CA
I ran a few track days on stock wheels with 245/45/18 RE-01R's which were great tires.
Currently running on my street setup 245/40/19's front and 255/40/19's rear on 19x8 front and 19x9 rears both +42 offset.
Track set up is still 245/45/18 RE-11's on all 4 with 18x8.5 +40
Currently running on my street setup 245/40/19's front and 255/40/19's rear on 19x8 front and 19x9 rears both +42 offset.
Track set up is still 245/45/18 RE-11's on all 4 with 18x8.5 +40
11-06-2013 | 01:31 AM
#18
I ran a few track days on stock wheels with 245/45/18 RE-01R's which were great tires.
Currently running on my street setup 245/40/19's front and 255/40/19's rear on 19x8 front and 19x9 rears both +42 offset.
Track set up is still 245/45/18 RE-11's on all 4 with 18x8.5 +40
Currently running on my street setup 245/40/19's front and 255/40/19's rear on 19x8 front and 19x9 rears both +42 offset.
Track set up is still 245/45/18 RE-11's on all 4 with 18x8.5 +40
11-06-2013 | 01:32 AM
#19
Registered
Joined: Sep 2005
Posts: 2,372
Likes: 51
From: Hayward, CA
11-06-2013 | 11:43 AM
#20
Well, right now I have 17x9" +45 RPF1's and usually run 275/40R17 Nitto NT01's on them in all four corners no problem with a slight fender roll. I have also run 275/40R17 Hoosier R6's and these BARELY fit. When you stack them up next to the NT01's of the "same size" they are considerably wider, and I believe they measure out more in the 285mm to 295mm range.
I'm going to be getting a set of 17x10" +38 RPF1's to try and run 275/35R17 Hoosier R6's, might need some more rolling and slight pull for these, and then keep 275/40R17 NT01's on the 17x9" set.
I'm going to be getting a set of 17x10" +38 RPF1's to try and run 275/35R17 Hoosier R6's, might need some more rolling and slight pull for these, and then keep 275/40R17 NT01's on the 17x9" set.
I've been running 275/40/17 BFG R1-S (Which are wider than the 275 Hoosiers) on a 17x9 +28 and they fit with just a fender roll and some camber..
11-06-2013 | 12:13 PM
#21
The rears were fine but the fronts with -2.7 degrees camber, if you loaded up the suspension with a jack there was maybe 1/16" between the tire and the fender lip. A +28 with that same tire definitely wouldn't have fit.on my car, not sure what could be different.
Thread
Thread Starter
Forum
Replies
Last Post
JimmyBlack
Series I Major Horsepower Upgrades
273
02-10-2020 11:23 PM
yankeepicker
Series I Trouble Shooting
3
09-11-2015 06:42 PM
projectr13b
Series I Do It Yourself Forum
1
09-06-2015 02:04 PM
