Wheel and tire options

[SolsticeMan]

Originally Posted by padgett Can almost promise the base car will not come with 18s & even 17s are unlikely. ... Base automatic Solstice will probably get 16s, and maybe manual trans the 17s (General is now charging extra for M/T (see GTO) so hope we get a few extras thrown in)... Those who have seen my maunderings before may remember that I said that a 245 was a silly size for a non-race tire, a 2800 lb 50/50 Solstice would not be able to load sufficiently for peak cornering though looks great standing still. ...

Sorry Padgett, gotta big time disagree with you. I'll bet you a lunch that 18" are standard. They showed P245/45R18 tires at the Detroit Autoshow, and stated over and over that this is the car you can buy for "around" $20,000. I asked about wheels and tires and they said: "This is the production car, that's the size wheels that will be on it.."

Granted, that is just the showperson, but there's article after article out there emphasizing the 18's as a feature. It will have 18 inch P245 wheels and tires standard - they would be crazy not to after all this harping on it.

As far as peak cornering, I respectfully think your statement is oversimplification. There's plenty of data that shows a 245 wide tire is or can be a very appropriate size for great cornering. The new honda s2000 rear tire is exactly a 245 wide tire. Z4 is running with a 225/255 split (at curb a little heavier than Solstice). The extension of your statement is that heavier loaded tires corner better - this is obviously not the case. If it was, than the corvette would still be running around on 185 tires (max load for the width, right?)

[TIRE LESSON FOR THOSE WHO CAN'T SLEEP:]

Peak cornering is highly dependent on distribution of footprint pressure, and the wider the tire the lower the footprint pressure.

Also, the lower the load on the tire, the lower the footprint pressure.

Both of these will generally increase peak lateral traction.

What does tend to happen is the "edge" or peak lateral force gets "peakier", meaning that if the tire doens't have enough sidewall give to allow deflection and redistribution of normal force through the tirepatch, the car gets less controllable at the peak lateral traction. This is all carefully balanced (hopefully) when the tire is "tuned" by the guys who develop ride and handling.

Also playing into the equation, probably as big if not bigger a factor, is the tread compound used (assuming you have a decent hi-perf tread pattern). A good all 'round compound will have a different mix of characteristics, grippier compounds tend to generate heat which (on road racing applications) increased pressure, which increases footprint pressure (if you started at optimal pressure) which leads to cornering fade as the race goes on.... you get the picture.

This isn't as much a factor on autox (heat generation), and one easy way to get around non-optimal tire grip is to work with a tire pressure and camber setting that generates the right amount of grip. Problem is you only get four runs to tune this in, so it helps if you've figured out a decent setting to start with...

[/TIRE LESSON FOR THOSE WHO CAN'T SLEEP:]

You can have too much tire, sometimes, but at 2800 lbs or even 2900 lbs (curb) I don't think the Solsice is in danger of being there.

Max lat will tell some of the picture, but haven't seen anywhere that information on the solstice exists (at least not yet).

18's are a little big (and heavy) for this car, but looks sell cars, so what can you do?

-solsticeman

[padgett]

Well yes and no. If you plot lateral vs downforce for a given tire you get a skewed bell curve. Exceed the max and you fall off a cliff (slide) but the backside is just as important. At the top you have a plateau of max lateral for downforce but beyond the plateau lateral drops off faster than downforce.

If lateral acceleration is important, you want to size the tire/rim/pressure to the expected loading. This is especially important when using street tires that do not have the choice of compounds that racing tires do. Racing tires are also constructed for extreme use, something a DOT approved tire is (usually) not. (I have seen some DOT approved from Kumho and others that have interesting characteristics if you do not mind a 10,000 mile useful life).

Footprint is very important, it determines whether you can reach the max sideforce capability available. This is why I like a rim that is wider than the tread, it reduces the distortion while cornering and you need less of a kludge (increased static camber) to compensate.

Everthing combines to determine what your performance on an autocross circuit is going to be (and some closely approximate racing conditions, have been well over the ton several times, in fact speeds exceeding what is possible with a B/P Corvette at Waterford Hills).

Never said that a Corvette would be faster with 185s (well maybe with studs in ice racing) just that the tire size needs to be matched to the car and that a 245 street tire is too big for something the size/weight of a Solstice to load properly. A 215 would be a better match with an 8.5" rim.

You mentioned cars with different sized front and rear tires. Yes you can go larger on the rear of a RWD car, it is not as critical as the front, is not as involved in the "turn in" at the start of a curve, and can produce both driving and lateral forces. With an exceptionally powerful car like the Corvette it can also help match tire life. Also looks good.

Personally I prefer to use the same sized tires front and rear. Put new ones on the back and rotate them to the front when run in enough to really grip. Of course I also prefer slight oversteer in an Autocross car.

Keep in mind that the above is specific for maximum performance with street tires, my daily drivers have 225x60s on 7" rims which works very well.

As to the last bit I have no doubt that 18s will be available on some Solstice versions, just doubt that they will be on the base model.

BTW does anyone have rotational inertia figures for 18s vs say 16s ?

[SolsticeMan]

I know we're probably a thou miles apart, but I betcha a dinner the base will have 18's.

Or a $30 Amazon gift card... to be resolved next year?

As for 245 street tire too big... again depends on how well the 'street' tire is tuned. We'll have to see.

[AeroDave]

As with everything in engineering, there must be compromise based on application. Remember the car also has to go in a straight line too. Big fat wide tires and rims add weight and drag. There is a happy medium and it seems to lie with what ever the individual wants to get out of his or her car.

This is why I like a rim that is wider than the tread, it reduces the distortion while cornering and you need less of a kludge (increased static camber) to compensate.

Padgett- I guess you dont live in a city and have to parrallel park very often! :lol

[padgett]

Said daily drivers have 225x60s on 7" rims :smile but thought the discussion was about maximum performance. I have always used a different set of tires/rims for autocrossing than for street since Baldinis are best on a dry track (mfrs usually provide shaved tires for serious competitors) and I like Michelin X-Ones (now replaced by something else but I forget what) for Florida's frequent "liquid sunshine". Entirely different functions.

Keep in mind that we are talking about the last few tenths of a second here, if you have a car that can take the class by a couple of seconds, the difference between 215s and 245s is not going to make much difference. However when talking state, divisional, or national championships it takes a lot of little things and attention to every detail to make up an "unfair advantage". This is one of them.

For a really nut theory, try this: I believe that on a stock Solstice, 215x45x16s would be faster both in the 1/4 mile and on an autocross circuit than with 245x45x18s on all four.

[SolsticeMan]

Originally Posted by padgett ...For a really nut theory, try this: I believe that on a stock Solstice, 215x45x16s would be faster both in the 1/4 mile and on an autocross circuit than with 245x45x18s on all four....

P:

That's not a really nut theory. That's real, sound, and sensible tire and wheel choice for a vehicle of this size :smile

Had a friend who (way back now) bought a new 1989 Chevy Cavalier Z24. The one with the 2.8 l v-6, automatic, cool 14" 215/something tires.

Then lined it up one day with another friend who had a 1988 Chevy Cavalier Wagon with crank windows and the V-6 manual and 13" P185 wheels and tires. And promptly had his a** handed to him. Consistently. After five drag races he took his Cavalier back and wanted a new car - that's when he got a lesson in styling "add-on" weight (rocker panels, door tupperware, sunroof, power amenities - it all added up to about 150 extra pounds).

His bright idea: make me ride with the "wagon guy" (at the time I weighed about 175-185lbs). Then got his a** handed to him again. :lol

A good chunk of this was wheel/tire weight and the rest was probably gearing.

I never said the 18" would be better than a 16" or a 17". BUT, my guess is that a 235/50 vs a 245/45 - the 245/45 will fare better, tire construction and wheel size equal.

At 50/50 distribution, 2860 lbs, plus 200lb driver means 765 lbs nominal load per front and rear tire. If I started with a car with those loads, I would have ended up with a minimum 225, no wider than 255. Maybe for autoX a little narrower but nothing narrower than 215. If the wheel size is locked in at 18", than the sensible width range is really quite limited - you want some sidewall, but not too much and ....

I guess it don't matter much 'cause you're getting 18's on the base vehicle and stock class won't allow you to downsize

Here's another: I'd say a 225/45 r16 will probably outperform a 215/45R16 on an autoX circuit, and the impact to 1/4mi will be JBD (just barely detectable). Tire construction/type equal.

On a road race course, I'd go to 17s minimum assuming I get to fill the wheel with brake.

[padgett]

Longer time ago than I want to admit (car was a V-8 Sunbird if that tells you anything), people hooted at the inflation pressures I was using: the scheme was to take the total weight at each end of the car, divide by the load rating of the tire, and multiply by the max inflation pressure. Rationale was that under hard cornering all the weight would be carried by the outside tire. Usually came out around 50 psi in the front. Totally against all recommendations and not for use in your living room but never had a tire failure.

Did lose an average of one wheel per weekend (usually the right front but checked all for cracks, several times in long or really fast course, once broke the whole center out of one) but was an SCCA Solo II F/S car that ran with the Porsches, car usually took the class by whole seconds, only losses were fault of driver.

Did break just about everything except the windshield, had to reweld shock towers and suspension mounts several times, finally retired when lower A-Arm separated entirely from the body and was obvious that was just a mass of cracks underneath.

Point I am making is that I have a lot of experience with making tires often ones that would seem inadequate, work. More is not always better (though I tend to agree "there ain't no substitute for cubic inches" and my style tends to be more dirt-track than pavement, might be why so many things break).

Once I get real specs like gear ratios and torrque curves sorted out I'll know more and should I get serious will try a lot of tire/wheel combinations looking for the "sweet spot" but will probably start with about a 215 section, it just feels right.

[fizzer]

I am waiting to see what styles of wheels will be availble in 8x19". I want a polished wheel. NO chrome.

I don't have a problem with the looks of the stock (polished) 18's so much, they look nice for a factory wheel, though could have been polished out more but I don't care for the tallish looking/ballon looking 45 series tires. To keep the overall diameters the same as factory and using a 40 series tire, I need to use 19" wheels.

I would also lower the front 1" and the rear 1.25" to close up some of the wheel/fender gap.

I'm also wondering if it will be possible to run a 275/35/19 on the rear and 245/35/19's up front, like what is a common upgrade on a 350z.

I think the 275's would look sick in the rear and offer more grip but I have a feeling it will kill the HP/0-60MPH times etc. All the extra weight and a 275 isn't needed on a 178HP car anyway. Kinda overkill. I seriously doubt I would spend all that money on 275s and the additional cost for 9.5" rims out back. I would rather just run 245/40/19's all the way around, so I can rotate them and SAVE money on tires