scotts300

iTrader: (46)

Fellow autoxers,
We've seen guys running stock class with 7.5/8" wheels and 285/295 Hoosiers. I also followed first350 when he ran 285/295 on 10" wheels: https://my350z.com/forum/wheels-and-...r-rolling.html

I also recall that Z33SM2 ran 295 A6s on 9.5 +20s and 315s on 18x10 +20s, however, I have something new to ponder. Do 315s on a 10" wheel offer more grip than 285s on a 10.5" wheel?

First, here are the specs for 285 and 315 Hoosier A6 tires:

285/30/18: 24#, 11.5" section width, 10.9" tread width when measured on a 10" wheel
315/30/18: 27#, 12.5" section width, 11.8" tread width when measured on an 11" wheel

I currently have 285/30s on 18x10.5 +15 RPF1 wheels. The 10.5 really does stretch nicely, and I estimate that the extra 1/2" of wheel width creates 1/4-1/2" of additional tread width compared to the 285 on the 10" wheel above. Before this set, I had 305/30/18 V710s on the 18x10 +38 (spacers added) RPF1s.

Here are my initial thoughts: The 315 will not fit (clear) on the 10.5 +15, and the next size down is a 295. The 295 is basically the same width as the 285, just with more sidewall, so I'm not really considering those up front over the more-common 285. At the end of the day, I think the "stretched" 285 on the front 10.5 has a tread width closer to 11.2-11.3". Is that better than a 315 on a 10" wheel? If I put one of the high-offset 10s on the back with a 315, would I gain much in width given the values above? The 315 is basically 1" wider when mounted on a wheel that is also 1" wider, but what is the gain if it's mounted on a wheel that's 1/2" more narrow?

jnathan68

iTrader: (29)

From what I have always heard (both track and auto-x), it is better to have the proper sized or even slightly stretched tire than to have a tire that is too wide for the wheel.

I am sure there are exceptions to the rule (stiff side wall tires, limited wheel size based on class, etc.) that might dictate a wider than recommended tire on a wheel, however, everyone I talked to said to try to go with a little bit of a stretch (e.g. 285 on a 10.5 wheel).

Just my $.02 since I am by no means an expert on this.

Guilty1s

iTrader: (12)

Id say that the weight saved from not adding more wheel/tire would be more benificial then the minimal extra grip you will get.

scotts300

iTrader: (46)

Thanks for the input, guys. There are a few very strong data points - national champions that run huge tires (285/295) on very narrow wheels (7.5/8.5). Clearly that's faster than running a 225 on that wheel, which is why I'm up in arms.

In my proposed case, I'd actually have a smaller (lighter wheel), with the 315 than the heavier 10.5" wheel with the 285. The tire is 3# heavier, but the wheel is lighter. Weight savings / penalty are probably within the noise. . .

mhoward1

I think it really comes to the true contact patch area, tire characteristics, and patch vs weight for tire temps. On our FFRC cars the series actually got faster when they switched from 275's to 255's on the same 9" wide rims.

Some times when you go oversize tires on rims you get a narrow but wide contact patch vs a more square set up so you gain cornering but loose braking...and some top speed due to extra aero resistance.

betamotorsports

iTrader: (1)

We did some back to back testing on a DSP E36 325i. Here were the tire and wheel combinations :

All Hoosier A6s same size wheel and tire on all 4 corners.

245/40-18 on a 8" wide wheel.
245/40-18 on a 9" wide wheel.

285/30-18 on a 10" wide wheel.
285/30-18 on a 10.5" wide wheel.
315/30-18 on a 10.5" wide wheel.

The 285/30-18 on the 10.5" wide wheel had the lowest course times over two days of testing. Second was the 285/30-18 on the 10" wide wheel and third was the 315. YMMV.

scotts300

iTrader: (46)

John,
That's good data - thank you. Any chance you have some of the details of the lap times? I assume these were on open tracks / time trials and not autox? If they were for big courses, do you think horsepower came into play on the larger tire?

betamotorsports

iTrader: (1)

Autocross testing. For track use keep in mind that a wide tire on a narrow rim will generate more heat then a proper tire and rim combination. All of the racing I've done has rules or points covering rim width and tire size. In those situations your tire and rim combination is pretty much picked by the sanctioning body. You just to have to carefully read the rules and/or add up the points.

scotts300

iTrader: (46)

Excellent. I'll go with your expertise and experience, and stick with the 285s. Thanks for helping to solve the conundrum!

betamotorsports

iTrader: (1)

FYI... My testing above was on a 2,700 lb., 246whp E36 325is. YMMV.

PDX_Racer

iTrader: (2)

I did testing on 245/35R18 vs 285/35R18 on my Track model (18x8/18x8.5) at an autocross test day.

My times were virtually identical between the two (0.1 second apart).

However, the car was significantly more of a handful in the latter part of the run on the 245s as they tended to overheat. The 285s were much more consistent throughout the entire run, and didn't require "saving" the tire at the beginning of the run to have tire at the end of the run.

I don't see much difference between the 285s and 295s except for longevity. The 295s are lasting much longer than the 285s.

KingBaby

iTrader: (3)

I'll just leave this here...

http://hpwizard.com/rotational-inertia.html

What's everyone's opinions on this setup

17x9 +22 275/40R17 (16lbs)
18x10.5 +15 285/40R18 (20lbs)

versus

18x9 +30 245/40R18 (21lbs)
19x10 +30 285/35R19 (28lbs)


thoughts

Ruff Z

iTrader: (8)

Those are two vastly different setups... sorta like the first would be for a Z and the second for a porsche cayman.

Anyway, just comparing those two setups, option 1 hands down. First, less weight, second much wider front tire and wider rear due to a wider rim, third, since this is the racing section, a square or closer to square setup is ideal (the second option would result in an understeering pig)...

KingBaby

iTrader: (3)

Thanks very much for your inputs. The second setup is my current setup which I'm tryna get away from.

Now my real question is how different in driving would option one feel? Would driveability suffer in anyway?

Either way I'm saving to purchase option 1 in some Enkeis

thanks again!

scotts300

iTrader: (46)

I still think you should go 18s all around! 9-10.5" front, 9.5-10.5" rear!

betamotorsports

iTrader: (1)

Your weights are all wrong. There's no way a 285/40-18 tire on a 18 x 10.5 wheel weighs 20 lbs. Wheel weight is only a small part of the equation because tire weight, further out from the wheel center, is a bigger factor.

KingBaby

iTrader: (3)

Thanks again for the replies...

I assumed we all know tire weights, and rim wrights. I'm really asking if the big drop in weight, will change the car feel. Since there is less weight, less power is need to move them. I have the little spirited experience, so wanted to know from "teh Experts" what to expect even with an 18" setup. That's if there is anything to expect.

Well wheel hop increase?

Will oversteering increase?

My little bump in power made a huge difference in driving the car. I just wanted a little insight. My gain of 10 horses and it's completely different car. So I'm pretty sure that more lost in weight will make it more wild.

Scott I have those as the backup specs, seeing what I want may be too retard.

More please!

scotts300

iTrader: (46)

You are going to make up a lot of the savings in the wheel weight with that huge-sidewalled tire. If it was a 285/30/18, then there'd be something to that (24# tire), but that 285/40/18 (not even sure who makes that?) isn't going to be lighter. If you saved a total of 4# on each of the rear wheels/tires, you may feel that especially if it's a shorter set-up.

KingBaby

iTrader: (3)

Roger was surfing from my phone for most of these posts I made here l...

Got a chance to do the math, Scott your right 40 sidewall gains it all back to being one pound lighter than this setup which I think I'll be going with.

.....pending at work

EDIT:

18x9.5 +15 (18.6lbs) 275/35R18 (25lbs) = 43.6lbs
19x10 +15 (21.3lbs) 285/35R19 (29lbs) = 50.3lbs

versus my current setup of

18x9 +30 (21lbs) 245/40R18 (25lbs) = 46lbs
19x10 +30 (28lbs) 285/35R19 (29lbs) = 57lbs

looking at it now isn't that much difference, but it is weight shaved off!

mgrotel

iTrader: (3)

and rotating weight at that.

on my rears, i went from 285s on 19x8.5" wheels (stock g35 coupe 19s) to 275s on 18x9.5" wheels (LMGT4s). Huge difference in handling, the rears were rolling pretty bad with the narrow wheels and 285s. granted, this is on the street and canyons and not on a track with lap times and whatever, but it was enough of a difference for me to notice it.