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Are there any automotive engineers out there who can indicate or point me toward determining optimal tire sizes for a given car?

The Solstice will be equipped with P245s on each corner. I just read a post by slowandlow stating he might go to 255s on the rear for more grip and I wonder if it would even be of any benefit.

How did the Solstice engineers determine the optimal width of tire for the car? I don't know how to determine that quantitatively. For all I know it is a complicated balancing act. You want to optimize sports car performance and maximize launch from a standing stop, maximize grip on the skidpad and mimimize braking distance. For all I know, one tire width may not satisfy all 3 conditions.

But wider is not always better. Wider tires have several disadvantages and rolling resistance is the primary enemy. Not only do they increase unsprung weigth, they have less grip on wet pavement, have more inertia to overcome accelerating and at steady state speeds, hurt fuel economy, and possibly increase wheelspin upon launch from a standing start due to less pressure on the rubber surface due to the same weight acting over a greater contact area.

How do they determine optimum tire widths for race cars? Is there a forumula or something accounting for weight, wheelbase, front and rear weight distribution, track, and wheel diameter that they use to determine the best width for racing tires? Not everybody races with 700 HP machines. And I'm sure your SCCA Miatas don't wear the same width rubber as Nascar machines.

So how do race teams determine the best width of tire and how did the GM engineers determine to go with P245s for the Solstice? I'm chomping at the bit to know.
 

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cdb said:
look through the links to the test mule articles....i think the opinion is that the solstice has plenty of tire... a bit too much for some.
In other words, they tried to lay tracks, especially with LSD and couldn't "light 'em up" :lol:
 

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I'm really interested in an answer for jimbo's questions too. Because I'd like to get a set of 17" rims with semi slicks for track days, and I was thinking of getting wider rear tires though. Dropping down to a 17" would allow for going wider without going over the weight of the 18" thinner tire do to a smaller diameter. However I really don't know what negative effects having a wider tire would do to the cars handling because I've never really looked into it before. I had been interested in either working with someone to make a wide body rear panel kit, or wait for someone else to make one, and then just use spacers for the 18" wheels when the wider 17" wheels aren't on it.
 

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jimbo said:
Are there any automotive engineers out there who can indicate or point me toward determining optimal tire sizes for a given car?.
Not an engineer with a degree, but have some experience and will try to help.

jimbo said:
The Solstice will be equipped with P245s on each corner. I just read a post by slowandlow stating he might go to 255s on the rear for more grip and I wonder if it would even be of any benefit.
If you go with wider tires in the rear you may get an increase in understeer. It is unlikely that the car will be oversteery out of the box so wider tires in the rear may do more harm than good if you leave the fronts as is.

jimbo said:
How did the Solstice engineers determine the optimal width of tire for the car? I don't know how to determine that quantitatively. For all I know it is a complicated balancing act. You want to optimize sports car performance and maximize launch from a standing stop, maximize grip on the skidpad and mimimize braking distance. For all I know, one tire width may not satisfy all 3 conditions.
Optimal width? Probably measured how much space was avaialable in the wheel wells, calculated the maximum size they COULD fit, then balanced the co$t and added weight of wider rims and tires against the benefit of improved lateral grip.
One width, 3 jobs. To be sure, its a blancing act. At some point, wider or narrower will give up too much in one area to be of net benefit. The balance would be more for lateral and longitudinal grip though (turning and accelerating) as braking is more a funcition of tire compound, softer yeilding shorter stopping distance.

jimbo said:
But wider is not always better. Wider tires have several disadvantages and rolling resistance is the primary enemy. Not only do they increase unsprung weigth, they have less grip on wet pavement, have more inertia to overcome accelerating and at steady state speeds, hurt fuel economy, and possibly increase wheelspin upon launch from a standing start due to less pressure on the rubber surface due to the same weight acting over a greater contact area.
Quite correct right up until the last sentence. Doesn't matter what the tire dimensions are, the contact patch is the same size, so long as the sprung weight of the vehicle pushing down on the tires remains constant.

jimbo said:
How do they determine optimum tire widths for race cars? Is there a forumula or something accounting for weight, wheelbase, front and rear weight distribution, track, and wheel diameter that they use to determine the best width for racing tires? Not everybody races with 700 HP machines. And I'm sure your SCCA Miatas don't wear the same width rubber as Nascar machines.

So how do race teams determine the best width of tire and how did the GM engineers determine to go with P245s for the Solstice? I'm chomping at the bit to know.
In racing, the best tire selection changes with each track. But mostly, its the sanctioning body rules that govern tire choice. Teams will have a finite number of options and they test as many as possible and use empirical data, i.e. lap times, to determine the best choice. Next time they go to a given track, they have the benefit of prior experience. My guess is that GM engineers work in a similar fashion but with the cost to the consumer as an added factor.

In the case of the Solstice, I'm surprised they opted for 18" rims really. Aluminum is much more dense than rubber. Normally, more rim and less tire is a good deal heavier than its converse. It won't surprise me in the least to find that a 17" wheel and tire package is quicker.
 

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Just quickly popping in, so I don't have time for a detailed answer.
[some discussion on this thread:
http://www.solsticeforum.com/forum/showthread.php?t=127]
and here:
http://www.solsticeforum.com/forum/showthread.php?t=226

I'll post something more detailed when I get a chance, but here's a few pointers:

1) First, every tire has a load rating. This load rating in relation to the actual operating load of the tire is key in determining a decent tire size. Generally, the less of the max loading you use, the better the traction, with the obvious trade off of:

2) Larger sizes = more weight. Not just unsprung weight, which can be hard to manage, but also rotational inertia which can magnify the amount of weight you add (10 lbs on the tires can be the equivalent of 50 lbs added to the body). This goes for both width and diameter, but diameter (adding those dubs) is worse for magnifying weight through increased inertia.

3) It depends on what you are trying to do, but "GENERALLY" the wider you go the more lateral grip you get, and the less steering feedback for the same suspension settings. The taller the sidewall, the "sloppier" the steering will feel, and you'll get a ride benefit as well as pothole/wheel damage protection. (sidewall height is not just aspect ratio, but aspect ratio times the width).

Just some initial food for thought - I'll elaborate with some more tire sizing tidbits... just remember, these are all just guidelines, and there are exceptions for just about every guideline.

That being said, you can basically start with this: Start with the size brakes you think you need. Add about 15%. Then find the smallest diameter wheel that you can put around those brakes without contacting the tie rod (when lock turning) or brake calipers/suspension/etc.

Then, get the widest wheel you can get in that diameter. This width will limit your lower bound for tire width (generally, >235 tire width, you'll need a minimum of 7 1/2 or 8" wheel). Then, if you can afford it, bump the rear tire width (for a near 50/50 weight distribution car) by 10 or 20 mm, so if you have a 235 front, look for a 245 rear to start with. This gives you a bit of cushion in agressive/transient handling (i.e. gives you a bit more understeer).

Then set the aspect ratio so you get about 95 or more for the sidewall height, unless you are fearless and don't care about damaging wheels.

Then check the relation of your tire load rating (that number before the speed rating which corresponds to a max load) to the loading of your car - you want to make sure you are at least as good as, say, the ratio of other cars (like the Honda S2000, I think their front and rear load ratings are 89 or 90).

There are exceptions - for example, you may end up with a BUNCH of extra load rating, which is where the Solstice appears to be. In this case, you may have so much tire that it's OK to have similar sizes front and rear without agressive handling concerns.

OK, now I've gone on too long and I've got to run, but can elaborate with examples if you want...

:cheers:
 

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Good thing that wasn't a detailed answer, otherwise I'd have to print it up to take home to read over night with me. :D
 

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I'll just go out on a short limb here. Since these 245/45-18 are same size as Caddy CTS which has 14" 4 piston Brembro brakes I would think first off there would be room on the Solstice for that kind of aircraft carrier landing tailhook brakes. All kinds of upgrade possible for the track.
 

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It started brief... then I walked away from the computer... then I came back and added a bit, then had to run again... then when I got back I just said WTF and finished was I was thinking I was going to say:).

Regarding rolling resistance, Jimbo, it's actually more of a function of a few things:

1) how much you load your tire vs. what the load rating of the tire is,

2) the nature of the construction and tread compound (the stickier or more performance, the more rolling resistance), and

3) the operating tire pressure. Tire size is a factor, but it turns out to be a minor influence (not insignificant, just minor when compared to the other three).

Which brings me to a really rough segue... tire pressure. Once you've established a tire size, you have a lot of leeway with pressure - generally you want to optimize the tire footprint by selecting the appropriate tire pressure for the size tire you've picked when operating at your load. So, even if you end up picking a really oversized tire, you can improve your grip by lowering the tire pressure (because it's likely if you have a car that would normally use a 90 load rating and you chose a 98 load rating tire, at normal tire pressures your tire would likely be overinflated).

Ran into this when autocrossing - in a Calais, it was necessary to drop the rear tire pressures down to about 18 psi when running competition - because the weight distribution was about 64% front, the rear tires at normal pressures had too much for the load placed on them (about half what was on the front tires).

I hope this is helping...
 

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And another thing to remember when spec'ing different tire front and rear is the affect on or inability to rotate the tires to even out wear. With these oem tires selling for $193 at tirerack as opposed to the 225/60-16 on my GTP which are only $105 you want to watch excessive wear like a hawk.
 

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achieftain said:
And another thing to remember when spec'ing different tire front and rear is the affect on or inability to rotate the tires to even out wear. With these oem tires selling for $193 at tirerack as opposed to the 225/60-16 on my GTP which are only $105 you want to watch excessive wear like a hawk.
:agree:
Hence the statement "if you can afford it, bump the rear..."
 

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Discussion Starter #13
Thanks for all the great information. I am just beginning to appreciate what goes into selecting a tire for a race car. It sounds like it is an empirical process through trial and error. Try a tire size and type, see what happens. Pick a type, chance size, evaluate. Pick a size, change type, evaluate.

Sounds like more art than science.

My main point is that you don't see 36-inch wide tires on road circuit racing cars. At some point there is a diminishing return of reduced performance. How do those who set up race cars know that they've hit that point? How do they know how wide is wide enough, and no wider. I've never understood that.

But thanks for all the detailed specific information in this thread. It was fascinating reading.
 

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Discussion Starter #14
Sman,

Wow, I've always inflated the tires on my personal vehicles to the maximum cold pressure printed on the sidewall without ever thinking if this is the best pressure. I really should check my owners manual and see what it says for recommended tire pressure. I guess I always thought that the max. = recommended. Live and learn. Thanks for the useful information.
 

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Breeze,

I guess I had always equated cars and tires as tanks and treads. If you double the area of a tanks treads it imparts half the pressure on the ground it is bearing on. But treads are flat and tires are round.

You seem to be saying that regardless the circumference and width of a tire, the tire will deflect until the same area of rubber is in contact with the pavement in either case, both equal to the tributary load that wheel is carrying (or 1/4 the car's weight if it had a perfect 50:50 rear/front weight distribution).

I'm still struggling with this concept. For any given tire, if you make it twice as wide, then the length of the contact patch would have to be 1/2 that of the original tire along the tire's circumference. That is very hard for me to visualize and accept. Not saying it isn't true, just that I don't see how it can be. I need to ponder that a bit.

Conversely, if you could take a tire and compare it to one of equal width but you double the circumference, you are saying the contact patches in each case are equal, therefore the length of contact on the pavement is equal (since width is equal), but the angle of the portion in contact with the pavement is 1/2 that for the larger tire than for the smaller tire. That is much easier for me to visualize than in the case of the wider vs. narrower tire.

Breeze, I'm still chewing on this one but absent my logic to the contrary I have to accept what you say. I just can't visualize it with respect to the wide/narrow tire.

Put another way to sum up my confusion, it sounds like you are saying, for any given tire compound and tread design, it doesn't matter if you have a 6-inch wide tire or a 12-inch wide tire, your friction and slip upon launch will be identical in each case because youre contact patch is the same in each case. :confused:
 

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I'm having issues wrapping my head around that too jimbo. Everythinng I learned from all of my statics and physics classes in college dictate that if you increase the surface area that a force is put upon, then the force should be decreased for any given area compared to the original smaller area. So you would think having a wider tire would distribute the force across a larger area, in essence putting less wear per unit of area compared to the smaller tire. I would think this in turn would permit for longer tire life since it would be putting less force on the area.

However I'm not a mechanical engineer by training, I just had to take a lot of their classes to be ready for the PE-Exam. There might be some "left hand rule" for tires I don't know about that dictates these types of things.
 

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jimbo said:
Breeze,

Put another way to sum up my confusion, it sounds like you are saying, for any given tire compound and tread design, it doesn't matter if you have a 6-inch wide tire or a 12-inch wide tire, your friction and slip upon launch will be identical in each case because youre contact patch is the same in each case. :confused:
Well, not exactly. As simply as I can describe it, a 3000 lb. car is being pulled toward the earth's surface by gravity. The only thing stopping it from reaching its objective is the equal and opposite force imparted by the four tires. The pressurized air in those tires is pushing the car away from the earth's surface. Given a fixed vehicle mass of 3000 lbs. and a fixed tire inflation pressure of 30 psi, you end up with a fixed total area of tire contact patch needed to equalize the forces. Does that part make sense?

If you add weight to the car but keep the tire pressure the same, the size of the contact patch must neccessarily increase. If you keep the weight of the car the same but deflate the tires, the size of the contact patch must increase in order to keep the forces equal. Hopefully this makes sense as well.

Thus, the area of the contact patch remains contant so long as inflation pressure and the car's weight remain constant. This is true irrespective of the width of the tire. It then just becomes a question of the contact patch's shape, and it is this which determines the car's ultimate handling characteristics.

To be sure, other factors come into play and must be taken into account in determining which tire is best for a given task, but hopefully now the issue of contact patch size is more comprehensible. ;)
 

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Breeze said:
Well, not exactly. As simply as I can describe it, a 3000 lb. car is being pulled toward the earth's surface by gravity. The only thing stopping it from reaching its objective is the equal and opposite force imparted by the four tires. The pressurized air in those tires is pushing the car away from the earth's surface. Given a fixed vehicle mass of 3000 lbs. and a fixed tire inflation pressure of 30 psi, you end up with a fixed total area of tire contact patch needed to equalize the forces. Does that part make sense?

If you add weight to the car but keep the tire pressure the same, the size of the contact patch must neccessarily increase. If you keep the weight of the car the same but deflate the tires, the size of the contact patch must increase in order to keep the forces equal. Hopefully this makes sense as well.

Thus, the area of the contact patch remains contant so long as inflation pressure and the car's weight remain constant. This is true irrespective of the width of the tire. It then just becomes a question of the contact patch's shape, and it is this which determines the car's ultimate handling characteristics.

To be sure, other factors come into play and must be taken into account in determining which tire is best for a given task, but hopefully now the issue of contact patch size is more comprehensible. ;)
I'll challenge your description a little. Take a set of 4 inch wide tires from the fifties (18 inchers were on MG Tseries and race cars of that era) and a set of these new 245/45-18 and on two otherwise identical cars start them around a circular track (NASCAR comes to mind) gradually increase speed until each car loses traction and slides into the wall. You are saying both tires would have the same contact patch - I'm saying the car with the larger patch (wider tire)will stick to the pavement better. Another way to think about it is two people that weigh the same and are the same height. One wears size 9, the other size 13's. One has more traction than the other. What about a woman in heels as opposed to flats. Same weight, different psi applied to the ground.
 

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achieftain said:
I'll challenge your description a little. Take a set of 4 inch wide tires from the fifties (18 inchers were on MG Tseries and race cars of that era) and a set of these new 245/45-18 and on two otherwise identical cars start them around a circular track (NASCAR comes to mind) gradually increase speed until each car loses traction and slides into the wall. You are saying both tires would have the same contact patch - I'm saying the car with the larger patch (wider tire)will stick to the pavement better. Another way to think about it is two people that weigh the same and are the same height. One wears size 9, the other size 13's. One has more traction than the other. What about a woman in heels as opposed to flats. Same weight, different psi applied to the ground.
Correct, I am saying both tires have the same contact patch in total area. This is basic Newtonian Physics. Can't be helped. What you are doing is adding new variables that require different solutions.

Once the car is in motion, the shape of the contact patch plays a very important role in how the vehicle handles certain tasks, as I said in my previous post. A wider tire has a wider contact patch side to side, but shorter measured front to back. The converse is true of a narrower tire.

When the car is in motion there are (basically) two forces acting against the tire that affect handling. The first is coming from in front of the car trying to keep it from going forward. The other is from the side trying to keep the car from turning. These are vector forces. In your example, the wider tire works better at keeping the car from sliding sideways because thats the way the contact patch is oriented. Trying to keep it simple, does that make sense?

Basically, this is why dragsters use very narrow tires up front. They don't have to make the vehicle change direction and are better suited to overcoming the force acting against the car's forward motion. Lets leave the dragster's rear tires alone for this discussion, please.:)

This is also why road racing cars use wider tires. They are better suited to overcoming the forces trying to keep the car from changing direction. All because of the shape of the contact patch.

Racers fine tune the handling of their car by adjusting tire pressures, thus altering the size of the contact patch, as described in my previous post. And I won't complicate the illustration any further by introducing factors such as sidewall deflection, slip angles, coefficients of friction and tire compounds.
 

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achieftain said:
I'll challenge your description a little. Take a set of 4 inch wide tires from the fifties (18 inchers were on MG Tseries and race cars of that era) and a set of these new 245/45-18 and on two otherwise identical cars start them around a circular track (NASCAR comes to mind) gradually increase speed until each car loses traction and slides into the wall. You are saying both tires would have the same contact patch - I'm saying the car with the larger patch (wider tire)will stick to the pavement better. Another way to think about it is two people that weigh the same and are the same height. One wears size 9, the other size 13's. One has more traction than the other. What about a woman in heels as opposed to flats. Same weight, different psi applied to the ground.
Yeah you're hitting on what I was thinking. Because say I have a 7" wide tire now but replace it with a 10" wide tire. That's ~3 extra inches of tire that is making contact with the ground my previous tire didn't have at all no matter how low the psi was to make contact with. If both tires are pretty much already flat slicks then you're pretty much making contact with the entire tire already. I could understand rounded tires that need to be pressed down on to make the rounded edges meet the road, but not on something flat. You still end up with the same amount of fource as before, it's just smaller per unit area then before.
 
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