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Discussion Starter #1 (Edited)
The magazines have all said that the 245/45/18" tire size is a sure thing. My question is, isn't the 18" rim mostly just for PR and looks? I've decided on getting replacment rims and was looking at the advantages of a 245/50/17". I keep thinking of the 50 series tires on the last Camaros and Firebirds. I realize less sidewall height, generally, equals better handling, but when do you exceed a point of diminishing return? Would it significantly detract from handling?

Assuming that the brake rotors are out of the GM parts bin and they would allow downsizing the rims, what are the advantages:

First off, less unsprung weight. Because most of the weight of a radial tire is around the circumferance, the two tires weigh nearly the same, but the smaller diameter rims are at least a couple of pounds lighter. This rolling, unsprung weight reduction would improve both acceleration and braking acting as, essentially, a lighter flywheel. It would also allow the car to handle better on uneven road surfaces. Would this be enough to counteract whatever additional sidewall flex from that extra half inch of sidewall height? Is it possible that a lighter 50 series would actually get better times around a course than a 45?

Second, less likelyhood of a bent rim. Although the roads around here aren't terrible, they're good enough to lull you into not watching out for pot holes. That extra half inch of sidewall might make a difference.

Third, ride. I know it's a sports car where handling is paramount, but if you could improve the ride on the straight roads without hurting the performance when you eventually get to the fun stuff, that's an improvement. I keep thinking of all the suspension mods I made to a '75 1/2 280Z when I lived in LA and would run through the hills. When I moved to Houston (where you have to drive interstate 3 hours just to get to a "fun" road) sacrificeing ride for handling was no longer as such a good an idea.

Last, cost. Not wanting to compare apples to oranges, the only tire the TireRack sells that's available in both sizes is a Michelin Pilot Sport. The 245/45/18 goes for $246 and the 245/50/17 for $205. Of all brands, the cheapest 18" tire is $198 but BF Goodrich has that one with the colored stripes for just $109 in 17"(niether apple or orange, but more banana). I know it's not nearly as good as a Z rated, but if your putting as many highway miles on a car as I plan on doing with my Solstice, that $400 a set is a factor, as well as the longer wear rating.

So is a 45 series all that better than a 50? Would I notice the difference?
OK, let me have it! :seeya
 

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I tend aggree with you on those points. I'm hoping for wheel and tire options. I raised this issue here before, and some believe that bigger wheels with less rubber is better. That may be true on a race track, but IMO, not so much in the real driving world. I also believe the car could use a smaller overall diameter wheel/tire than is spected now. I agree with you, alot of the wheel and tire choice is based on fashion and or trend. I just hope they can restrain themselves from making them chrome plated as standard.

The best would be to offer a more "city" wheel and tire and a "Performance" wheel and tire. People that are nuts about wheels and tires are going to go aftermarket anyhow. I think it will end up this way.
 

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I think the difference in ride would be noticable given the same tire with those different profiles. The lowest profile I've run was a 40 series which was the stock size of the rear tires on the 1999 BMW M3. The suspension on that car was tuned so that the ride was fine for me though some may have found it a little too harsh.

The vehicle I have now has 45 series tires all around and a worse ride than the M3 due to other aspects of the suspension. I guess my point is that other factors may have as much or more impact on the ride as the tire profile.

My main concern with 18" wheels would be the weight and the durability if they weren't forged wheels.
 

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In any case, the best way to work out wheel size is:

Estimate vehicle weight (using overall track and wheelbase), add the weight of two passengers, plus a little bit...

THEN

Figure out how much brake you need to get the right amount of output to decelerate that weight by around 1.2 g's, then add 10% to the output, and look at the size of the rotor. If you have enough to avoid significant fade (thermal mass and swept area), then you have enough brake. This should get you a decent rotor size.

THEN

Get the smallest wheel that fits around your brake package. In this case, progbably a 17" will be almost optimum. Minimizes wheel size (and weight). 18" is almost surely a styling thing, 'cause there are many out there who think bigger is better with no limit.

Even the show vehicle had 19" front and 20" rear. Once again, styling decisions sometimes override the things that might make sense for performance.
 

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I agree with Solsticeman. Seems like you can most likely fit a 12.5" dia. rotor under a 17" wheel with plenty of room to spare. That will probably be plenty. I know, for instance that my GTP came stock with 10.9-inch rotors under 16" wheels. I upgraded to 11.9" rotors and they still fit under the stock wheels with stock calipers on different GM brackets with about 1/4" of room between the caliper and the rim. Add to that the fact that FWD cars have more offset in the wheels typically than RWD cars, and we may be looking at fitting 13"+ front rotors inside 17" wheels.

Unless the factory 18's are pretty light, I'll probably look to the aftermarket for 17's. It's interesting to note that the factory wheels on that Grand Prix (torquestars, for those familiar) were made by Enkei, and were pretty dang light for factory alloy wheels. A world of difference compared to the alloy anchors on my '03 VUE. If we're lucky, GM will contract Enkei to make the wheels for the Solstice as well.
 

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I have to agree with mstorace.
I would not want the car to come looking like an early Miata or a Honda where the wheel wells looks too big for the tiny tires and wheels in them...
 

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From an appearance standpoint, I'm not disagreeing (necessarily) that bigger looks better. Just saying that the cost in additional unsprung weight will hurt performance more than the aspect ratio will help performance beyond a certain point. Also, when it comes time to replace the tires, 18's cost more than 17's. I'm not suggesting that the wheel/tire package as a whole be made smaller, but kept the same size overall. So the wheelwells will be filled just the same. There will be the same amount of gap even though the smaller wheels get swallowed a little more within the wells.
 

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No i dont need 18 inch rims, but i want them. Car styles have been favoring large rims recently and by haveing 18s standard will make the car appear to be more expensive than what it is. when i look at a miata i get the impression of an economy sports car, when i look at the solstice i think of a higher priced vehicle. GM needs to shed the image of a car company run by bean counters allways favoring the cheapest interiors and appointments, by having 18 inch wheels joe sixpack will be impressed, and the companies image may begin to change. After all, the solstice is a halo car designed to bring people into the dealarship, not designed to make as much money as possible.
 

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Aside from the issue of greater unsprung weight, and all other things being equal (which they never are), doesn't a larger wheel/tire also reduce acceleration compared with a smaller wheel/tire?

Or is this a non-issue because of transmission design?

While everyone likes the looks of a larger wheel, less fender gap, etc., some people will not go to a larger wheel unless they have a SC or turbo, and even then it deserves some thought.
 

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I'm not saying bigger does not look better. It does. I bet this is one sweet looking ride with dubs. BUT...

[Physics Lesson]

There is an unavoidable price you pay for the looks. Bigger wheels mean more rotating inertia because the major part of the weight of the total assembly is further from the center of the wheel - unavoidable. You can get BBS or Youngblood 18"X8" wheels that weigh a fraction of forged aluminum wheels. Heck, I bet the 'vette front wheel (C5 Z06) is probably almost that light.

But you cannot avoid the fact that the tire weight is concentrated further out from the center of rotation for the wheel. You cannot buy "superlightweight" tires. Even the best racing "street" tires still weigh almost as much as a normal tire, because street tires must meet DOT standards. And bigger tires ALWAYS weigh more, (assuming similar dimensions and street tire classfication, i.e. P245/45 R18 ALWAYS weighs more than a P245/45R17).

This doesn't seem like much, but think of this rotating inertia as a multiplier for the weight of the tire. If your tire and wheel is only 5 lbs heavier, that's 20 lbs for the car, but the rotational inertia of the assembly makes spinning these new "flywheels" up and slowing them down more difficult. This affects braking AND acceleration. So that extra 20 lbs can seem more like adding 35 lbs or more to the total car in it's effect.

[/Physics lesson]

Styling, like-it-or-not, has tremendous effect on appearance, and they are not always intuitive, nor complementary. I don't think the engineers and designers started out by saying "we need a roadster that only has 2 cubic feet of storage". They (or Lutz) said, "we need a hot-looking roadster." and when they got one, Lutz said, "this needs to be in production tomorrow, and it needs to look exactly like this concept car, but it must perform like it looks"

And this is what we have on the way, in all it's strengths and weaknesses.

The Corvette has a big butt. I've never liked it, but it holds 2 sets of golf clubs. Great.

The Solstice has short overhangs, and a funky top and not much of a usable trunk. But it seems like it will perform well, it's $20,000, and Dammit, it looks GOOD. Which is more than I can say for many other vehicles out there.

Could it perform better with 17" tires? Sure. Will it look as good? Probably not. It all boils down to which is more important to you, as a customer.

It's obvious that GM made the decision that "great" styling and "very good" or "appropriate" performance were the dominant factors. It was not "all out maximum performance" and "pretty good" looks.

As I wrap this up (have a rare moment where I can think before posting), those who put 17" on their car will probably go faster and brake better than those who put lightweight 18" wheels on their car and upsize the brakes (assuming GM has correctly sized the brakes with enough capacity in the first place). But those who put dubs on their Solstice will be the s**t, 'cause their ride will look the coolest. They'll just get there last.

As Steven Wright said: "you can't have it all, 'cause where would you put it?"

and Solsticeman adds: "well, you can't put it in the Solstice trunk, but maybe you can store some of it in the extra space between the brakes and the wheels :lol "

:rant

Peace. Out. :seeya
 

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Personally, I rather have them put 16" steelies OEM, because I will never keep the wheels anyway and would rather not pay big$$ for 18" factory wheels I will never use. For me light 17's for driving and if for some reason I would "show" the car then I will get a set of 18's aftermarket.
 

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The problem with bigger wheels is that it increases the "arm" for the rotating forces. Braking will be longer with larger wheels unless the extra diameter is needed for lager brakes. The longer arm will also transmit more force to the under carriage. If the car is engineered for this then GM is adding weight to beef up parts that would be affected and once again it looks like style is more important than performance or function.
 

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The 18's do look good, but doesn't having those tires on there hurt the value of the car? The car is a bargain at $20K dollars so it can be purchased by a lot of people. However, isn't having $250 a piece tires on the vehicle going to start pricing out those people looking at this car in the first place? High performance tires do not last long, especially if you are driving the car in a spirited manner. The 18's should be an option for people who are willing to pay for them to get either the improved looks and/or performance, but there should be a cheaper 17' wheel for people who do not like spending 4 figures everytime the need tires! Especially if your not going to get long life out of them!
 

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Discussion Starter #15
As it's a mostly parts bin car, I'm hoping I can, eventially, sell the 18" factory rims and tires on E-bay to some CTS owner to defray cost. That's after I move them from the "old" 2006 Solstice to the "new" 200? S/C Solstice! :cheers

I've read several articles where a tuner will take a car, do standard proformance mods (cat back and cold air induction, etc) and take the wheels to Stage 2 (from 14" 60's to 17" 45's to maintain the same outside diameter) and add bigger, better brakes. When they compare the before and after, often the after is slower and has equal (or longer) stopping distances. But it looks a lot better!

Watched Jesse James (not exactly an engineer!) wax on about how the only right way to dub an SUV was to upgrade the brakes (usually dual piston and always larger diameter vented discs).

Myself, I put oversized wheels and extreme low profile tires in the same catagory as fuzzy dice and a 21 note air horn. They may look cool but they don't make the car run any better! But then, a large part of the appeal of the Solstice is the looks. I can appreciate the individual statement any modification makes. :thumbs
 

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dav8or said:
The problem with bigger wheels is that it increases the "arm" for the rotating forces. Braking will be longer with larger wheels unless the extra diameter is needed for lager brakes. The longer arm will also transmit more force to the under carriage. If the car is engineered for this then GM is adding weight to beef up parts that would be affected and once again it looks like style is more important than performance or function.
A 17" wheel with taller sidewall is going to create the same rotational torque (ignoring the very minor differnece in sidewall flex). It's the weight and how far from center the weight is that affects performance, but really only from an accelleration pov.
 

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skeptic said:
A 17" wheel with taller sidewall is going to create the same rotational torque (ignoring the very minor differnece in sidewall flex). It's the weight and how far from center the weight is that affects performance, but really only from an accelleration pov.
Given a 16" and 18" wheel/tire of the same mass the 18" will have a longer arm and more force will need to be aplied in both starts and stops.
 

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So is it correct to say that the primary performance advantage with a larger wheel/tire is in better braking, whereas the performance disadvantage is slower acceleration?

Again, all other things being equal (which they never are) ...
 

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robrecht said:
So is it correct to say that the primary performance advantage with a larger wheel/tire is in better braking, whereas the performance disadvantage is slower acceleration?

Again, all other things being equal (which they never are) ...
Nope, the primary performance advantage is better cornering and steering feedback due to less sidewall flex. The primary performance disadvantage is poorer acceleration (which can be thought of as both positive and negative, braking). Unless you upgrade the brakes with the larger wheels, your brake performance will suffer. Of course, when you go to larger rotors, the braking gets better, but you are adding even MORE rotating mass to the equation, possibly at the cost of less acceleration performance. Fortunately, larger rotors don't cost as much in performance as larger wheels since the center of mass of a rotor isn't so far away from the center of rotation.

Clear as mud?
 

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The only need for bigger wheels is if you need the space for bigger brakes. A car this size will not need monster brakes. Lower side wall tire can be placed on smaller wheels that 18". So big wheels are only for show.
 
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