I saw this post in another thread, and it got me thinking: would 285/40-17 Kumho MX be a good choice for 17x9.5/17x11 afs zr1 combo? I like 315s on the back, but the tire selection sucks (especially considering the price). So would it be possible to put the 285/40/17s on both the front 9.5s and rear 11s (pics anyone?) and actually keep it balanced and safe to use in track situations? what's the point of the wider wheel then?

It's a strange, un-happy middle for me, as I would have the weight of the big wheel, but the grip of the smaller wheel. So I either go to 17x9.5s everywhere, or bite the bullet and pay a grand for a set of KD's and be done?

Please enlight me.
Dazed and Confused,
Marcin

The person I was addressing in that thread had 17x11 rims front and rear, and wanted street tires.

A tire that's too wide for the rim is effectively a little softer, and you can't use the full width of the tire, at least not laterally.

The 285 on 9.5 rims in front won't be quite as crisp as the same tire on the back on 11 inch rims. It will certainly work, but might feel (and look) a little weird, because out cars actually like a softer setup in the rear. It may understeer a little.

For street use, I think 275s on 4 9.5 rims would be a better combo, or 4 285 on 11" rims, and would allow rotating and reduce understeer. With 9.5 front and 11 rear (no rotating) it might work well to use 285 in the rear and 275/40 or even 255/45 in the front, though it would probably still push a little. For reference, the Z06 uses a 265/40-17 on 17x9.5 rims on the front, but there aren't many choices in that size, and the OEM tire actually runs a little on the narrow side.

I'm considering trying 4 255/45-17 Ecsta MXs on my 17x9 rims for street use. 275 is a little too wide for a 9" rim, though it does protect the rims a little better. BTW, the 255 is a LOT lighter. 255f/275r might be interesting on my rims, but then I couldn't rotate the tires.

OK, how about 275 front, 285 rear then?

In general though, is there a rule of thumb/formula/whatever to tell us whether we're gonna get under/oversteer based on tire sizes, assuming the rest is constant? I would guess that the under/oversteer characteristics would depend on the slip curves of fronts vs rears. The closer the curves would be, the more neutral the car would be. Weight distribution (dynamic), would probably have something to do with it too, as I've seen cars that can show under or oversteer depending on how hard you're changing speed (aka large weight transfers, affecting weight distribution, thus forces on individual wheels, thus cornering force, thus slip angle).

Am I even on the right page here?

Yes, a properly set up, neutral car can exhibit both severe understeer and severe oversteer depending on driver input, though a smooth driver won't get too far from neutral (some manipulation of the balnce is desireable, as in trail braking).

One rule of thumb - the rim width should ideally be no less than about 90% of the section width.

Another rule of thumb - the ideal rim size for a given tire is usually somewhere between the middle and top of it's rated rim size range.

For example, a 275/40-17 is rated for a 9-11 rim. A 10" rim is probably about right, but they aren't very common in our bolt pattern.

The 315 is a decent match to the 11" rim, but 315 isn't a great size for street driving (neither is 275, really).

FWIW, even with stiff race tires, a 265/45-16 on a 16x8 rim doesn't really provide any more grip than a 245/45 tire on the same rim. The main difference is that the 265 will be a little softer, so it may work better at the rear for putting power down in a straight line, while the crisper 245 at the front will probably turn in a little better.

In general, wider tires at the rear will cause understeer. However, as tires wear and harden they lose grip. When you can't rotate the tires you can't keep the aging balanced either, so the handling balance will tend to shift over time.

Choosing tires is not an exact science

So what do i pick for zr1's to keep it balanced?

I feel your pain. I have the zr1 setup I got for my z06. On the z06 I run 275/315's which is a pretty neutral combo for the z06. Not so for the z28, which will understeer. I am going to go with 17x11's all around with 315's. Need 2 more 11 inch wheels. SO I can try to find someone who wants to get rid of their 11 inchers for 9.5's or buy 2 new ones for $550 or Buy a new set of 4 and sell the 17x9.5 set.

I am no expert, but if I have to keep the 9.5 11 combo, I would just run 275/315 and let it understeer, as I think it would still be better than 4 275's but I may be wrong. Power on corner exit is great with the big tires on the back

Well, the answer is that it's going to be hard to get it balanced with that rim combination. The best choice would be to buy a second set, and use all 4 11" rims for 315 race tires and all 4 9.5" rims for street tires.

If you do go with 275/315, a bigger rear swaybar will help, but watch out in the rain. Big tires and a big rear bar will make it treacherous.

315's all around would be great, Tim but depending upon how much money you want to spend you could pick up a bigger rear bar pretty cheap and that might be enough to make you happy with what you already have for wheels & tires. You've got those big meats in the back--put them to work and let them take some weight off the outside front.

I already have 32.5/21.5 LG Motorsports sways, alongside with the full strano koni/gc kit, so I'm fine in this department.

So do i go with Kumho MX 275/285 or just go big with KD's 275/315? Any other good combos I should be aware of?

It's not just a matter of having "good" swaybars. Different tire combinations may need different swaybar combinations to tune the balance of the car. Even if it's balanced now, with (for example) 275 all around on SS rims, when you change to 275/315 you probably need a different rear bar to get it balanced again. BTW, never sell your swaybars. Since they are a tuning device, it's handy to have an inventory of different sizes on hand (especially for the rear) for the next time you change the tires or suspension.

Ouch, those KD's are expensive. The price difference between 4 275 MXs and a mixed set of KDs would almost pay for a second set of 9.5/11 rims from Ebay, and leave you a set of 4 11s for track days.

Of course that still leaves you with a set of 4 315 race tires to buy, but street tires don't make good track tires, and track tires don't make good street tires.

Another possibility might be Nitto 275 555R2 in the front, and Nitto 315 555R in the rear. That would be a lot cheaper than the KDs, and would be fairly well suited to track use with a suitable rear bar. At least in 275, the Nittos are moderately streetable, and even tolerable in the rain, though like I said 315s and a big rear bar may not be fun in the rain.