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Conventional wisdom is that, for performance, 17" is the largest you want to go. Above that, the wheels get too heavy, you lose sidewall flex (a little is good for maintaining traction), the ride gets harsher, and the car becomes slower.

Given that the brakes on the Solstice hardly fill the wheels (the one & only styling mistake that I can find), is anyone else thinking that it might be a good idea to go for function over form and swap the stock 18" wheels for 17s? I can't imagine that even aftermarket brakes for this car would require 18s. I have 13" Baer brakes for the front of my '66 Malibu and they fit with 17s.

All comments welcome, all opinions valid. I'm interested in performance moreso than looks (but not exclusively).
 

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Yep. Thinking of it. Depends on if they fit. I'm mainly concerned with contact of the wheel to control arms in compression and droop, and wheel contact to the tie rod end at full steering lock.

I'm thinking P245/40R17 on a 17X8 rim. Should be several pounds lighter as a whole than the P245/45R18 on 18X8 rim.
 

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If performance is enhanced with the 17" rims why did Pontiac go with the 18". Is it merely the bling factor? Also, how will the performance increase if the rims are changed... better handling, acceleration, top speed, etc?

Also, wouldn't changing the rim/tire size cause you to get worse gas milage, more wear on the engine, and have your miles count up faster than reality? Are all those things worth the increase in performance?
 

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I changes the 14s on my 2000 Miata to 16s and the overall handling increased. I was certain to keep the tire diameter as close as possible to the factory overall tire size so my speedometer wasn't changed, nor was my odometer. The idea of changing to 17's is interesting to me since I intend to get aftermarket rims anyway. My original rims will be used to mount my snow tires.
 

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No desire to change the wheels. Just want to drive, not race, the car. Should be fine for a daily driver for me. If anyone does change to 17"s; it would be nice to hear how the performance was changed and if the cost is effective.
 

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mikefred1952 said:
If anyone does change to 17"s; it would be nice to hear how the performance was changed and if the cost is effective.
:agree: If somebody is planning on doing this, how about doing some road handling and speed tests before and after and let us know the results. I'm mostly curious about something like a slalom to see what kind of difference it would make in cornering.
 

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I posted my results of research into tire size and will recap here. Identical tire diameters in same Eagle RS/A as the 245/45R18 is 225/60R16. First off you would not upset the ground clearance or speedo calibration. Second, for better or worse you would gain quicker steering input, greater tire flex, possibly better wet traction due to higher ground pressure(less hydroplaning). You would probably give up lateral g's as the narrower tire would have less tread on the pavement and would have less grip. Off the line might be faster as you prolly could "light up" the narrower tires easier.

I did a search for compatable wheels on tirerack and found that 18's will weigh about 5# more each than 16's ,27 to 22,and that the posted tire weight by Goodyear is exactly the same at 28# each. You would gain anet reduction in tire/wheel if using the same wheels of 20#. Will this offset the loss in grip? I don't know.
 

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I am thinking of buying lightweight 17-inch rims with 50-series tires. I haven't decided whether to stay with P245s or drop down to P225s with 55 aspect ratio. It is likely I'll stay with a 245 width but I am just not convinced the car needs it.

Wheels and tires still appear to me to be more art than science, despite some members here trying to enlighten me.
 

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solsticeman said:
I'm thinking P245/40R17 on a 17X8 rim. Should be several pounds lighter as a whole than the P245/45R18 on 18X8 rim.
S-man,

Why do you want to reduce tire diameter and rollout so much? For autocross performance? How will this affect tire service life? Are you planning to change the rear-end ratios to restore speedometer accuracy?
 

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solsticeman said:
Yep. Thinking of it. Depends on if they fit. I'm mainly concerned with contact of the wheel to control arms in compression and droop, and wheel contact to the tie rod end at full steering lock.

I'm thinking P245/40R17 on a 17X8 rim. Should be several pounds lighter as a whole than the P245/45R18 on 18X8 rim.
I'm wonderig what will fit, too. I'm hoping the bolt pattern and offset is similar to recent Corvette's and that C5 or C6 wheels will fit without spacers. The genuine GM Corvette wheels are pretty light even when complared to aftermarket wheels.

I have C5 front wheels on all four corners of my Camaro, now, 245/45R17, on a 17x8 rim. I actually have a couple of sets of compatible wheels, and it would be nice to re-use them.
 

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achieftain said:
I posted my results of research into tire size and will recap here. Identical tire diameters in same Eagle RS/A as the 245/45R18 is 225/60R16. First off you would not upset the ground clearance or speedo calibration. Second, for better or worse you would gain quicker steering input, greater tire flex, possibly better wet traction due to higher ground pressure(less hydroplaning). You would probably give up lateral g's as the narrower tire would have less tread on the pavement and would have less grip. Off the line might be faster as you prolly could "light up" the narrower tires easier.

I did a search for compatable wheels on tirerack and found that 18's will weigh about 5# more each than 16's ,27 to 22,and that the posted tire weight by Goodyear is exactly the same at 28# each. You would gain anet reduction in tire/wheel if using the same wheels of 20#. Will this offset the loss in grip? I don't know.
How would you get quicker steering input with MORE tire flex? Handling would suffer, both from the narrower tire and taller sidewall. Hydroplaning wouln't change at all, since hydroplaning is a function of tire pressure and tread depth.

The only improvement I can see is the lighter unsprung weight will help acceleration, but not by much. The moment of inertia will reduce by 5 lbs at 9 inches from the center, so that's 9/12 * 20 lbs = 15 ft/lbs, doubled for rotational inertia, effectively lightens the car by 30 lbs. Rule of thumb is lightening by 100 lbs gives you a tenth in the quarter, so you will improve your quarter mile time by about 3 hundredths of a second or so; maybe 5 hundredths since the car is so light to begin with.

If you're that concerned with handling and acceleration, you might think about a set of racing rims and tires. Hoosier DOT legal road racing tires weigh 21 lbs in 245/40. The stock wheels are actually pretty light already - my 17" 5 spoke aluminum impala wheels weigh 44 lbs apiece. Of course, they're designed for a 4500 lb car.
 

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why bother changing all the wheels and tires for only 3-5 hundredths of a second?

Improving your launch, 60ft times, playing with tire pressures and MANY other performance mods (re: engine tuning mods) will yield greater results imo.
 

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psycho-squirrel said:
why bother changing all the wheels and tires for only 3-5 hundredths of a second?

Improving your launch, 60ft times, playing with tire pressures and MANY other performance mods (re: engine tuning mods) will yield greater results imo.
Hmmm.... I always thought reducing unsprung weight was valued for improving ride and handling, not acceleration.

Light wheels cycle more quickly and stay planted on road undulations better. Relatively light wheels transfer less shock into the car in the form of bumps and stay planted on rough roads better.

Maybe one of our resident technical experts can give us a better explanation of the role of increasing or decreasing unsprung weight. As for me, I never considered the usefulness toward lightening rotational mass for accelerating and breaking, though some benefit would result. I want lighter wheels for the benefits of ride and handling.

Or am I nuts? :skep:
 

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I'd consider it as long as they fit, but only if I decided to drive it year round. Then I would mount some snows to 17's, and go a little skinnier in tire width to get a better bite into the white stuff!

Otherwise, I'll probably stay stock. The car is being engineered for the tire and wheel package it is coming with, so I will assume it will ride and drive to my satisfaction. I really wish it had 17's standard to begin with, but those are the breaks. The 18's do look good!
 

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jimbo said:
Hmmm.... I always thought reducing unsprung weight was valued for improving ride and handling, not acceleration.

Light wheels cycle more quickly and stay planted on road undulations better. Relatively light wheels transfer less shock into the car in the form of bumps and stay planted on rough roads better.

Maybe one of our resident technical experts can give us a better explanation of the role of increasing or decreasing unsprung weight. As for me, I never considered the usefulness toward lightening rotational mass for accelerating and breaking, though some benefit would result. I want lighter wheels for the benefits of ride and handling.

Or am I nuts? :skep:
Well, I hadn't thought about bumpy roads. Good point. And with the roads around here, that's something to consider.
 

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18's are a good performance size. I don't know why you would go 17's but thats just my opinion.
 

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I would also consider P245/40R18's as well. Most likely, if I decide to race mine, that's the size I'd end up using.

SolsticeDesire: Bling factor is a big thing. Designers (basically "artists" that have little understanding of engineering or race/performance aspects - with a few exceptions - Kip Wasenko comes to mind as one of the rare exceptions) ALWAYS think bigger is better. The Concept solstice had 19" front and 20" rear to start with. The Chevy SS? Try 24s!!! The Caddy Sixteen had 24s too, IIRC.

Jimbo: there have been many discussions on tire sizes - scan the tech section. Will try to update the crosslinks and index in a few days, seems to be a subject that draws a lot of discussion. Generally, lighter unsprung mass is better for rough road handling and ride. The mass itself is a detriment for acceleration performance, but the effect is compounded by the rotating inertia of the tire/wheel. Adding 10 lbs to all four tires has a similar effect to adding between 30-50 lbs to the trunk - the rotating inertia is resistance to acceleration and deceleration, and can impact ABS performance, especially in slippery conditions.

The other factor is load rating. a 96 load rating tire is more appropriate for cars with a curb weight between 750 to 1000 lbs heavier than the Solstice. The load rating of the Corvette tire, for example, is 88 or 91 - and that car weighs 300 more pounds than the solstice. The Honda S2000 load ratings are approximately 89 and 91 for the front and rear.

This is certainly an indication of a few things. First, it is an indication that there is wasted mass. Much higher load rating than is necessary means there is lost opportunity to reduce weight and rotating inertia. Second, it implies that these tires may not be fully utilized - this can affect steering feel, crispness, how linear the handling is, where the non-linear handling kicks in.

There are benefits to having too much tire too - you can usually get better grip and higher max lat with less agressive tread. Tire wear is usually noticeably improved. Rolling resistance is prolly a wash - so the only fuel economy effect would be in the weight/inertia.

As for the other parameters, lower sidewall height (calculated by tire width multiplied by the aspect ratio) usually means better handling and crisper steering at the expense of ride. Too low a sidewall height means increasing risk of wheel damage. The wider the better, but subject to "appropriate" load rating - so if you get too wide for your desired range of sidewall height, the load rating will guide you to the widest but most appropriate tire.

The sidewall height is a bit of judgement call, but something between 90 and 100 mm is prolly appropriate for a car like the Solstice.

The best way to do this would be to think about it from a mission standpoint first, then work out the simple engineering details second, then establish a weight for a new car based on the desired size.

This weight tells a designer and the engineers the appropriate family of tires to use - in this case prolly load ratings in the 87-93 range, 93 being the upper limit. Then, in the simple engineering details, you take what would be an appropriate size BRAKE system, add a bit (maybe 15%?) for a factor of safety, then find the MINIMUM size wheel that will fit the chosen brake rotor size. Once the wheel size is established, there will only be about 5-10 tire sizes that fall out of the mix. Maybe half or more can be eliminated based on the mission of the car - in the case of the Solstice this would be a tendency toward lower sidewall height and aspect ratio, and wider tread.

Then, the designer can design the wheel well around the most appropriate tire choice for the car - optimized for braking, ride, handling, steering, appearance, performance and weight.


This process does not seem to be the way the Solstice was done. It was prolly much simpler - Franz came up with the concept drawing, which looks like it had fork truck solid rubber tires. Bigger is better - so 20's is a great place to start. But 19's in the front gives the appearance of classic rake. Once the car was a hit, and they decided to build it, it prolly became apparent that 19/20 tires and wheels were either too big or too expensive for a "back to basics" roadster. 18" was the largest they could either get away with or maybe afford. My guess is that is as much consideration to tire and wheel choice that was done.
 

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One thing I've noticed is that Nitto, Hoosier and Mickey Thompson all make a road racing radial in 245/40 - 18. (M/T lists theirs as 26X10.00R18LT, but it's for an 8-9 inch rim.) Hoosier's dry tire is really a DOT slick, but the rain tire should do just fine for the street if you don't mind the low treadwear.

Some of my autocrosser friends say that the Nitto drag radial makes an excellent autocross tire if you put 50 psi in it. The drag radial is fairly light, which is whay I was looking at it. Unfortunately, Nitto doesn't make a drag tire in our size. They do have a road racing tire, though.
 

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The new C6 Vette comes with 18/19s but it has 230 more horsepower. I personally would go with 17's and lower the car about an inch. Maybe increase the rear wheels to an 8.5 or 9" width. The speedo/etc can easily be electronically recalibrated anyway. At say 70 miles an hour it should only be off by 2-3 MPH.

A 4 cylinder can really tell the difference if you were to save yourself 10 or 12 pounds by going to a smaller wheel and lighter tire.
 
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