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      06-06-2023, 01:40 PM   #1
dre99gsx
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315 size tires in the rear - let's have fun

I know this is going to cause a stir, but here goes...

In my quest to get more corner-exit rear end grip on the turns, I wanted to run 315s rear / 275 front.

If you look at this image, you'll see that I am able to fit a 315/30-18 tire with 11" wide rim (et44 with 15mm spacer). To do this, I un-clipped the rear bumper and sideskirt, loosened the fender bolts and pulled the fender out about 1".

This causes the fender to no longer align in a few areas as you can see. I attempted to create a custom bracket to reattach the rear bumper. I haven't worked on the bracket for the sideskirt yet. You'll see I filled in the gap to the rear tail light with some plastic. There is a gap between the fender and the trunk.

Other than that, all of this is 100% reversible.

I'm actually not sure this will give me any improvement in rear end traction. My current track setup is 295/30 rear and 255/35 front, and they've been pretty sticky for my limited driving capabilities so far.

But, here are some things that I've been reading
1. Wider tire = same contact patch for the same weight, so I'm not really getting more tire on the road
2. Wider tire/wheel will be 3lbs weight extra. This is still LESS than an oem wheel/tire combo fyi.
3. Benefit or more tire = heat management: I can probably run a few more laps before the tires starts to get overheated or greasy
4. I could run slightly lower air pressure, giving me MORE contact patch
5. Break-away may be more abrupt, this may be a bad thing
6. Costs go up, it's another $150/tire for the rears
7. Longer to get tire up to temp. I noticed my rears are slower to get up to hot pressure than the fronts already with 295s.

Upside:
1. Allows me to run taller front 275/35-18 tire for more front end grip. If I run 275/35 with 295/30 rear, front tire is 16mm taller (or reduces my rake by 8mm making the car have higher ground clearance in the front than the rear)

So, I'm finding more negatives than positives to go through with this. I have no idea if I'll get more grip. My goal is to be on 100% throttle much earlier before the apex. But, so far, I have been getting on it earlier and earlier and these 295s haven't let me down yet.

I need convincing that this is not worth it. I'm not too thrilled about mis-aligned panels. I haven't tried this on the passenger side yet to see if the filler cap area prevents me from doing this... Do you guys feel I will see a benefit from this upgrade?
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      06-09-2023, 06:24 AM   #2
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If it’s more grip you seek, I would first try a stickier tire. Hoosier R7 or Hankook Z214.
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      06-09-2023, 08:01 AM   #3
dre99gsx
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It's a dilemma since:

a) I'm trying to near lap times of actual spec race cars, and am close, but haven't had these wheels slip out from under me yet (remember I had massive mid-corner oversteer? Apparently replacing 24 HC old rear RRs fixed that issue). So, I'm not sure I'm even at the limit of these tires.
b) Are Hoosiers/true slicks really going to give me more grip over Toyo RRs? I don't know the answer and from searching, no one really seems to know for sure. Perhaps I can get 1 sec on a 1:30 lap time, no idea
c) Back to (a), going wider may give me more margin of error, but I just don't know.

I've decided to bite the bullet and try. I got new tires on order and am going to do a back to back on my local track. My main issue is corner-exit confidence. I am really waiting until almost perfectly straight before going full throttle. I'm hoping wider rears give me some more room.

SIDE NOTE: The cool thing about this modification is for those of you who want more room in the rear for wider wheels, you may be able to loosen the rear fenders and just shift them out about 3-5mm more without much of any other customization.
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      06-09-2023, 12:42 PM   #4
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Quote:
Originally Posted by dre99gsx View Post
b) Are Hoosiers/true slicks really going to give me more grip over Toyo RRs? I don't know the answer and from searching, no one really seems to know for sure. Perhaps I can get 1 sec on a 1:30 lap time, no idea
Yes. Of course they have more grip. Why else would people race on them and not RR’s? RR’s are known for having a hard compound that wear well. Not known for having great grip. SpecE46 uses them, but it’s a spec series so everyone has to use them. Where tire choice is an option, most pick Hoosier or maybe Hankook.
A friend did a back to back in his IP racecar of the RR and the Z214 at Roebling Road (a relatively short lap at @1:18). Z214’s were 2 seconds faster.
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      06-18-2023, 08:32 PM   #5
dre99gsx
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Got them to fit. I did have to fab some custom spacers but once you loosen the fender mounting bolts, it's fairly easy to pull them out almost an inch without bending anything permanently.

I had to run 9mm spacers to clear the inner body work. The entire wheel can tuck in as far as it needs with no rubbing.

I gained 2lbs total each side with wheel/tire/spacer/hub extender.
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      06-19-2023, 12:31 PM   #6
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How did you reattach the rear bumper and side skirts? Got any pictures of those areas? Thanks
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      06-19-2023, 09:23 PM   #7
dre99gsx
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No pictures, but I duplicated the plastic clip brackets out of thin aluminum angles from hardware store but notched the holes at the proper locations so the bumper re-attaches but slightly shifted.

The bumper snaps on just like the plastic clip brackets, but the brackets are bolted to the rear fenders in a discrete location. I'll try to snap some close ups. If you look closely you'll see some spacer-like brackets between the side skirt and the fender. Those are also aluminum angle brackets that were cut and trimmed to match the gap that was created. I wrapped it all in carbon fiber.

These RR's run very wide btw...
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      06-22-2023, 10:30 PM   #8
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Those tires just look insane lol.

For all your resourcefulness, you should just add fender flares!
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      06-23-2023, 09:00 AM   #9
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It is cool I don't want holes in my fenders and want this to be reversible, so win-win.

There is one downside to all of this - pulling out the fender raises the wheel arch slightly, about 3/8". So, I ended up with a larger than pleasing wheel gap in the rear.

I had to reduce my rake from about 3/8" to about 1/16" (barely none) just to get it to look Ok again. This means the rear suspension is further compressed which brings about potential issues with the CV shaft not being parallel to the ground, any additional angles created which could impact traction, what will happen when I sit in the car or under full throttle and sqaut, etc etc

Luckily I didn't have to touch the front end to fit 275s. Again, no spacers, just switched from 6" spring on KW clubsport to 5.5" spring and I have 1/16"+ clearance from these fat tires to the perch. 275RRs are wide. I tried on 275 Direzza and I had almost 1/4" gap to perch!

This experiment is probably going to make things worse on the track, but it's just an experiment. I've been super consistent at NJMP TBolt on same tires running 1:28s~1:29s on a clear track, I will be curious to see if the lap time improves. My goal here is to get on the throttle much earlier and see if it sticks.
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      06-28-2023, 04:31 PM   #10
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Quote:
Originally Posted by dre99gsx View Post
It is cool I don't want holes in my fenders and want this to be reversible, so win-win.

There is one downside to all of this - pulling out the fender raises the wheel arch slightly, about 3/8". So, I ended up with a larger than pleasing wheel gap in the rear.

I had to reduce my rake from about 3/8" to about 1/16" (barely none) just to get it to look Ok again. This means the rear suspension is further compressed which brings about potential issues with the CV shaft not being parallel to the ground, any additional angles created which could impact traction, what will happen when I sit in the car or under full throttle and sqaut, etc etc

Luckily I didn't have to touch the front end to fit 275s. Again, no spacers, just switched from 6" spring on KW clubsport to 5.5" spring and I have 1/16"+ clearance from these fat tires to the perch. 275RRs are wide. I tried on 275 Direzza and I had almost 1/4" gap to perch!

This experiment is probably going to make things worse on the track, but it's just an experiment. I've been super consistent at NJMP TBolt on same tires running 1:28s~1:29s on a clear track, I will be curious to see if the lap time improves. My goal here is to get on the throttle much earlier and see if it sticks.
How do you like the KW CS?

Do you trailer your Z? Always wondered how they ride on street? Always read they’re a bit harsh (whereas v3 are better for dual duty).

What made you choose KW over some of that other options?
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      06-28-2023, 04:40 PM   #11
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KW Clubsport was an improvement over my BC Racing. BC Racing did their job but I felt the car would shift weight over before the shock would dampen it (even at near full compression/rebound).

Once I went to KW, the car settled on transition much more controlled and I have more room for adjustment.

I chose them because they were cheaper than JRZ which was my next option.

I trailer it, but I do drive the car often around town. I run ~10-12 almost full soft comp/rebound around town, they feel compliant enough. I run very stiff springs too.

If I was driving 10/10 in competition and was always repeatable, I would venture for JRZ.
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      06-28-2023, 10:24 PM   #12
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Quote:
Originally Posted by dre99gsx View Post
KW Clubsport was an improvement over my BC Racing. BC Racing did their job but I felt the car would shift weight over before the shock would dampen it (even at near full compression/rebound).

Once I went to KW, the car settled on transition much more controlled and I have more room for adjustment.

I chose them because they were cheaper than JRZ which was my next option.

I trailer it, but I do drive the car often around town. I run ~10-12 almost full soft comp/rebound around town, they feel compliant enough. I run very stiff springs too.

If I was driving 10/10 in competition and was always repeatable, I would venture for JRZ.
With lower rates you think you could use them for daily driving?

What spring perch hardware did you source to convert to a linear setup?
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      06-29-2023, 07:42 AM   #13
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I think I would daily it at this spring rate. I have solid mount race seats in it with little cushion, all suspension 'joints' are ball (no bushings at all), and it's still a fairly compliant ride.

FYI I'm now at Z62-152-012 672lb/in front and Z60-127-14 784lb/in rear SWIFT springs. I run a heavier engine that N52 and S54 up front with the huge turbo setup.

Springs just swap right in. They are slightly wider than 61mm but no issues using these for years.
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