VT Mitsu

iTrader: (3)

LOng title, I know. It's a marathon But I believe it raises a Question that has never been fully explored/asked...Optimizing the balance between Load Index and Section Width as they relate to a given Rim Width and the resulting Sidewall profile/shape when the rim width is not ideal (we can't all afford nice Expensive dedicated track wheels Some of us are stuck with the stockers)

Question is: how to balance the values/parameters to get crisp steering response? Not as concerned with Maximum Lateral Load at a million MPH as I am with Immediate Turn-In

Aspect ratio is just that, a ratio between height and Width of tread. With Evo's usually looking at 40 versus 45 Aspect ratio. Some widths are not standard, ie Dunlop Starspec's 255 run wider than Nitto NT05 in absolute numbers. More rubber "usually" more grip, BUT ...

Too wide a tire on 8 inch stock rim will "balloon" over rim edge, taking away steering response as car weight must first take up "slack" in sidewall before contact patch initiates car turning...BUT...

Also take into account Sidewall Load Index. I believe this is a "real" Standard, as in same values/scale used across all tire manufacturers (Unlike testing numbers for treadwear, but that is a differnt discussion). Again for most of us, The numbers ar usually in the 90's. Higher = stiffer sidewall.

The ideal is easy to figure out, but what happens when that doesn't exist?

My quandary is this: Need new tires (who doesn't..) I am Sticking With stock Evo wheels, = 8 inch rim width.

Narrowed down to NT05 vs. Dunlop Starspec. BUT... Which size? I've not seen anyone discuss Load Index with these two tires and it is disparate/different between sizes of the same brand and between brands.

NT05 255/40/17 98(load index) will balloon on 8" rim, but very stiff sidewall (Question is... will the stiffer sidewall counteract the shift in the sidewall that occurs as you initiate the turn?

...OR...

NT05 235/40/17 90(load index) Stock width = Straight sidewalls/no ballooning of sidewalls = less sidewall deflection = contact patch "turns" quicker BUT...Lower Load Index =softer Sidewall,

ADD to the mix...

Dunlop Starspec 235/40/17 90(load index) Lower aspect Ratio helps but low load index.

Dunlop Starspec 235/45/17 93(load index) Stock sidewall and higher Load Index = good.. but higher Aspect Ratio not so good for turn-in

Dunlop Starspec 245/40/17 91(load index) Slight ballooning? and Sidewall Shift at turn initiation; a bit more rubber (245)... BUT lower Load Index = Softer Sidewall.

Dunlop Starspec 245/45/17 95(load index) Slight ballooning? and Sidewall Shift at turn initiation; a bit more rubber (245)... higher Load Index = harder Sidewall BUT 45 Aspect Ratio = taller tire = not as good for turn-in


Dunlop Starspec 255/40/17 94(Load Index). .. Also will balloon on 8" rim (More Rubber than NT05 so will definitely balloon on rim) sidewall not as Stiff as NT05 (94 load index on Dunlop versus 98 on Nitto)

I am leaning towards one of the Nitto's, based on reviews I have read on-site and off. Other reviews have stated less slip angle per degree of steering lock versus the Starspec, which I like the idea of, and their propensity to behave more like an R-Comp tire with less tread howl than the Starspecs at adhesion limit. Just not sure which one. If the 235's had a load index Equal to the 255's, I probably wouldn't even hesitate, but it is Quite a bit lower, but... better sidewall profile, etc. Ideally an NT05 in 245/40/17 98W would suit me perfectly, but doesn't exist :]

If anyone has read this far...Thanks for any input you may have :]

Tom T.

AlwaysinBoost

iTrader: (17)

I'm not sure what you're asking here. you seem to have alot of info, but if your looking for someone who's run all those different sized tires on a single 8" rim I think your outta luck.

I've run 255/40/17 Z1's on my EVO w/ MR rims and didn't notice much if any slowed response @ turn-in. the evo rack is very quick to begin with, with a 2.1 lock to lock ratio. sometimes slowing the response is a good thing if you have a habit of turning early (not me, I'm usually way late).

nightwalker

You have your information mixed up. A higher load index doesn't mean a stiffer sidewall. You would have to check on the tire manufacturers website to find out if that particular tire has a beefier sidewall construction or not.

load index is usually determined by how much air the tire can hold.

VT Mitsu

iTrader: (3)

Certainly not looking for someone who as run them all . That would be ridiculous of me. Looking for a way to look at diminishing returns for te variables mentioned.

My question can be somewhat summed up as : At what point does a stiffer sidewall compensate for a tire that may be a bit too wide to perfectly fit a rim and overcome the ballooning effect (and resultant less-than-ideal sidewall) of being on a rim that is a bit too narrow for it.

Were the MR rims 8 or 8.5 inches? years ago Warrtalon posted a bunch of times about GSR vs. MR vs SE rim measurements, but the numbers escape me right now.

I do love the rack on this car. That is a definite bonus in discussiing the idea of quick turn-in/steering response.

LV///R

iTrader: (36)

All Evo VIII and IX wheels are 8" wide (MR/GSR/RS/SE)

VT Mitsu

iTrader: (3)

Sidewall construction is certainly a part of my question, but that can be different between manufacturers (obviously, right? Sheer genious,Tom) and so cannot be quantified/compared between brands. ie. there are a number of ways you can make 400 horsepower. Also a number of different cars that can go 0-60 in x.xx seconds. Those results we can compare directly to one another because a standard is set. How they go about accomplishing 400 hp and 0-60 in x.xx seconds (NA, turbo,supercharger, better heads, lighter weight car); that differs from car to car. There are a number of ways to make a sidewall stiffer and every manufacturer probably feels their way is best, but without a standard measuring tool (ie 0-60), we have no way to quantify it. How that is measured is something else. ie: A run-flat tire has a stiff sidewall by definition, but you wouldn't want to run it on a track

My inferences about Load Index and how that reflects Sidewall Stiffness are taken from a conversation I had with Neal at TireRack a while back on this site.
I hope he stumbles on this Thread to shed some light on the Question of where sidewall stiffness can make up for a tire that is a bit pinched on a rim., (smallish255 (Nitto narrower than Starspec) on an 8 inch rim.)

Frankly I wish Tirerack sold the Nittos so I could get an unbiased opinion from someone who knows is stuff (Neal) and would ave te experience with both tires. One of te tings e recommended for me to look at was Load Index to get an idea of Sidewall stiffness.

Thanks reading the novel and for the input

Tom T.

VT Mitsu

iTrader: (3)

Good to know

Smike

iTrader: (8)

I will read through this today and put together a response.

Smike

iTrader: (8)

Ok, can add some here too. And for general use, you should read all the TireRack links. Good info there.

Writing this on the fly. So if I make a typo, my bad.

Contact patch will change response some also. More patch to more, more time to move it. Would have to do a footprint and shape factor all the tires listed to see their exact contact patches. Those are taken at 80% load index - 32 psi for Standard Load tires, 38 psi for Extra Load. Then you can measure via shape factoring.

Section width is section width. 255 will be 255mm, 245 - 245mm. That is read center of the sidewall to center of the sidewall. That is different than the actual tread width. Changing from what the tire calls for in rim sizing will change the section width numbers. Say a 255/40 calls for a 9in rim (and you get the 255 from measuring it on the 9in rim). But you run it on a 8in rim. You have to subtract .2in (5mm) for each 1/2in change. So for a 255 going down 1in, that is a 10mm decrease in section width making it a 245 section width. Is there more tire there, yes. It it heavier, yes. More ultimate grip, maybe.

Tire construction varies wildly between manufactures. And they are all closely guarded secrets. Only way to find how stiff the sidewall really by looking at the cut tire analysis (cut section of tire). There is a bead and unseat test that can give you some ideas also. Higher pressure needed to unseat a tire from a rim, generally stiffer sidewall.

Load index is the amount of load the tire can hold before failure. Not an accurate way to judge sidewall stiffness. SL v. XL - XL will have stiffer sidewalls. Might mean increased weight in construction.

Now, for DD - 235/45. AX 245/45 or 245/40. Track 245/45 or 245/40 or 255/40 (best on 9in).

MR White

iTrader: (3)

Subscribing
I'm in the same debate right now. Are the nto5 competitive autocrossing? I'm learning towards 255/40/17 star specs.

VT Mitsu

iTrader: (3)

First off Thanks for taking the time for a Thought out response

Thanks for the correction on Section width differing from Tread width. I ran into this info:

The Nitto site states NT05 255/40/17 section width = 10.12 in. Also is an XLoad tire)
235/40/17 section width = 9.57 in
TireRack states Starspec 255/40/17 section width = 10.4 in.
245/40/17 section width = 9.8 in.
235/40/17 section width = 9.6 in.

Ugh. So how do I reconcile this info? Better off w/ Nitto 255 because narrower than Starspec 255. on 8 in. rim? If I choose to go with 255.

But less contact patch to turn/shift in 235 Nitto, so quicker Turn-in, Yes?


I have found out NT05 255/40/17 is XL so I am wondering if that stiffer sidewall will make up for being on a Stock 8 in. rim. I realize 9 in. is ideal, but not in my budget currently. Nitto NT05 235/40/17 is not XL.

Are these assumptions correct?: AX 245 for quicker steering input? And 245/255 for Track because of much higher speed cornering requiring more contact patch?

Thanks again very much for the input/response. After looking at all the numbers for each tire, after a while I feel like I'm pole-vaulting a mouse turd...but I know little differences in measurement can make big canges in performance...

Just to re-iterate, what I am most interested in is Quick turn-in, not necessarily max-grip at 120mph on a corner

I'm going to go read all the TireRack links again...

Good to know there is someone else from Northern New England on the boards.

Smike

iTrader: (8)

As the Red-Green Show always put it, "if women don't find you handsome, they should at least find you handy."


XL tends to be heavier due to construction. I do run a XL tire in the stock size for my car. But its not an Evo. More of a momentum car, where I hold speed rather than brute force and slingshot out of turns.

Now, if you are not worried about gear changes (shorter tires will hurt gearing). Then the 245/40/17 is the best middle ground in my book. You get a little more width. Shorter sidewall for response. With the loss of some gearing.

245/45/17 was my go-to for AX because I got a little more width, helps with gearing (taller tire). But some concessions in turn-in response. And increased weight.

255/40/17 for track. Ultimate grip, sacrificing turn-in some, and increased weight of tire slows acceleration some. But on the track, turns and braking is where you win.

Once you have the size down, its time to look at compounds and weight of tire/rim package.

I know the feeling. Hope I am helping some.

MR White

iTrader: (3)

Smike, so running a 255 on 8inches would still increase grip even though the rim is not wide enough for the tire. I would sacrifice turn in and weight for more grip autocrossing. I feel like that's one of my weak areas, not enough grip.

Smike

iTrader: (8)

Tread width is still more than a true 245, but section width has been squished (technical term). AX I would rather have response (accel and turn-in) and gearing. 255/40 are still shorter.

MR White

iTrader: (3)

So if I want to maximize grip and keep accel I'm better off looking into a 17x9 rim and running 255/40/17.