armt350z

iTrader: (6)

Gents who have raced and run multiple tire sizes, I am wondering the following:

What is your decision process for what tire size to fit on what wheel. Up until last season I have always been of the thought process that to fit the widest tire on a wheel as possible while maintaining a vertical sidewall. The SMiata guys swear to running a 225 on a 9 inch wide rim. That puts a tire with a tread width that is smaller than rim width on the wheel. Yes I know they have to use a spec tire etc.. but the theory of using sidewall tension to increase peak grip by limiting contact patch squirm is solidly backed by physics.

All things being equal I would think that by limiting movement of the carcass and tread squirm you will increase peak grip but also increase probability of snap loss of traction.

Last year I ran on a 9.5/10.5 wide setup with 275/295 width tires. this year i'm moving to a 10.5 width square setup. I'm thinking of moving down to a 285 width tire all around to increase sidewall tension.

What does the Z cohort think?

dkmura

iTrader: (64)

In the States, wheel and tire combos are usually limited by the rules set of the sanctioning body. For example, when the 350Z was first classified in SCCA Touring 2 class, the contact patch was clearly used to equalize competition and we used 285-30-18 R-compound tires on 18 X 9 (F) and 18 X 10 (R) rims- the largest allowed. After the move to T3, the comp board added weight and lowered the tire size to 275-35-18 (with the same size wheels) for the Z.

Of course, I could argue that this only partially offset class inequities (V8s and turbo power), but once you decide to run a particular class, those are the rules you must run with. It seems you are asking for the "best" combo to run on your Z, and based on my experience, the 285-30-18 on a square setup (9.5" rim) should be an effective combination.

03threefiftyz

iTrader: (25)

275 and 285 work well on 10.5's. Anything more than a 285 and I would be looking 11's.

armt350z

iTrader: (6)

I'm not so much looking for a whats best setup since I 100% agree that class requirements will usually dictate what tire size you can play with. For me I have no restrictions as long as I can fit it under the fenders. I was more interested in your thoughts on the use of tire size relative to rim size to alter grip or handling characteristics.

Of course anyone who has been around a tire knows the primary way to adjust the load sensitivity/response curve has been tire pressures. But it stands that since the sidewall is also a load bearing part of the wheel, you could tune handling by adjusting the sidewall angle.

dkmura

iTrader: (64)

Actually, the primary way(s) to adjust load sensitivity/response is via suspension tuning. Camber, caster and toe are the static adjusters, followed by swaybars and shock (bounce and rebound) have a much greater effect than tire pressures. I utilize tire pressure to do fine tuning of the tires. Certainly, sidewall profile is another factor, but sidewall angle is not a huge factor, and certainly not as big as a degree or two of static camber in the overall handling of the Z.

eye-5

Stretched tires do offer a stiffer sidewall. If you can fit a wider wheel to get a little stretch, there will be an advantage over the same size tire on a narrower wheel.

armt350z

iTrader: (6)

Kind of outside of the scope I was hoping the input or discussion would delve into, but yes there is no debating that chassis, suspension and alignment changes would net much larger gains for the car. However response curves for the tire, all other things being equal, could be changed on the end user side by primarily tire pressures and rim width.

Take the R888 in a 295 width for example, Toyo recommends a 10.5 width rim on it. The tread width on the particular tire is listed as 11.2 inches wide. Factually A 295 width r888 on a 10.5 width wheel leaves the sidewall in a not vertical state at rest. I would argue that sidewall angle is a factor. Maybe not huge, but it could easily be felt by an average club level driver. The tire would be more prone to rolling over at this point with a sidewall that is past vertical.

my line of thinking along is along the lines of what 03threefiftyz is saying in regards to 285 being about the limit on a 10.5 in that the tread width is more closely matches the rim width.

JMII

Interesting discussion. There must some optimal tire width / contact patch / rim size ratio because clearly too wide = no good (squirm) while too narrow = no good (stretch).

The factory sizes (for my 03) are 225 (8.85") front and 245 (9.64") rear yet the rims are both 8". In '05 the rear rims got .5" bigger but the factory tires back there were the same size. Why the change? Did the Nissan engineers determine there was too much squirm?

I personally hate the stretched look, even if it offered faster times I wouldn't do it.

BryanH

The JMII? Awesome. 'CudaMan' from the old GT forums back in the day...

Anyway, my contribution to this thread is only that going from a 225 on a 7" wheel to a 275 on a 7" wheel (yes, crazy, I know - that's Hoosiers in SCCA Stock autox for you!) resulted in more sloppiness from the tire, but also more overall grip and better tolerance to heat. Tire wear actually increased despite the much wider tire - I think all the movement and squirm of the tire on those narrow wheels didn't help.

I've no science or testing to back this up, but I think a lot of it depends on the tire construction, camber, and things like that. Generally, though, most tires will be pretty happy just a little bit on either side of "perfectly" matched to their wheel width. There is a school of thought among autocrossers, particularly in lightweight cars, that a little stretch is optimal (something like a 225 on a 9" wheel). I currently run 285s on 11s or 275 on 10.5s on my Z33, and it seems to work.

armt350z

iTrader: (6)

I'm not sure anyone is really advocating a stretched look. The tire choice that originally got me thinking about this was the hoosier 225 width that had a contact patch around the 9 inch point mounted onto a 9 inch wide tire.It pretty much squared off the tire. My perception of conventional thinking for most of the Z people local to me seemed to be to fit as wide of a tire as possible onto a rim.

Here you see people running 255 width tires on OEM 8 inch rims but they are not turning lap times near where I am. Admittedly my car cannot be accurately compared to any of them though.

I think right now i'm going to move from the 275/295 setup I ran this past year to either a 285 or 295 square setup mounted onto a 10.5 wide wheel. I'm still undecided on that since I'm having issues finding the tread width of the michelin sport cup 2's

03threefiftyz

iTrader: (25)

Remember that a 9" wheel is really 10" lip to lip from the outside, so a 9" tire is not square. A 245 is really more "square" on a 9" wide wheel.

Assuming you're running 10.5's all around 275 or 285 is what you want. RE71's are the ticket if you want a "street" tire, so you're limited to the 275 for now.

JMII

No flipping way ...the internet is a small place

And yes even in real life brother EZ is still schooling me He has a Golf R with APR Stage I+ plus some other goodies (tires / suspension ) and runs about 7 seconds quicker around Homestead (2.2 miles infield course) then my stock '03 Z.

Here is a video of him catching me:
https://www.youtube.com/watch?v=Syb_...ature=youtu.be

BryanH

Yeah, but, is it any surprise we still play with cars?

Looks like EZ is making up the vast majority of his time on you with braking. Tires and confidence will help with that. I rode around Homestead during a Chin Motorsports track day about a year ago - pretty fun place especially for a roval.

armt350z

iTrader: (6)

Would you say the same knowing that I am not bound by any treadwear restrictions? I had figured the pilot sport cup 2's would be a step above a extreme performance street tire considering how the pilot sport cup 1's were.

03threefiftyz

iTrader: (25)

Nope. 275-285 is what you want unless you are going to a real slick. At that point it would be best to contact the tire manufacturer and see what wheel size the tire was developed for.

armt350z

iTrader: (6)

Sorry, that last sentence was about the RE71. The sizing seems right.

And I can't go slicks for class rules, it has to resemble a street tire. Really its anything with a tread is allowed.

terrasmak

iTrader: (8)

Hoosier A7 , simple choice

dkmura

iTrader: (64)

Not that I want to take this thread too far OT, but tire choice isn't that simple. The A7/R7 is certainly one of the softest, stickiest of the DOT-R choices, but it really resembles a race slick. Furthermore, it's fine for autocross or a very short sprint race (if you have multiple sets), but for extended track time at high speeds, it wears too quickly. Maybe armt350z wants to post up some of his rules set and let us know what kind of track events he's participating in, we can offer better advise.

armt350z

iTrader: (6)

The hoosiers resemble a race slick too much and would put me in the same class as the guy who brings a Nismo GT3 GTR(who brings a GT3 car to a time attack event)...

The website is down at the moment. When it comes back up I will translate the tire restrictions into english.

03threefiftyz

iTrader: (25)

The A7 seems to have a lot more life than the A6 in similar conditions. It is what you want for Time Trial, though. Warm up lap, 2 hot laps and in. Anything more than 6-7 laps will require you to start managing tire temps.

Seems the very soft Yoko and Hankook compounds are probably the fastest of the more than two groove club these days, but tolerate even less heat and wear than an A7 from what I've heard. That is all very third hand info, however.