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I wonder if one of our esteemed engineering types may have a formula they can plunk weight, drag, power, gear ratios, etc into and come up with a good guesstimate.

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http://www.solsticeforum.com/forum/showthread.php?t=2253

Here's an equation to calculate the power required for a particular speed in MPH.

Horsepower=((frontalareasqfeet*DragCoefficient*.00 256*(MPH^3))/375)+15

.00256 is the english unit average air density. Frontal area can be simply estimated by [(Overall Height X Overall Width)*0.80] - the 80% takes into account the rounded corners. Drag coefficient is generally 0.4-0.5 for short tailed things with large negative pressure zones (like a roadster), 0.33-0.38 for something like a sedan or a cavalier, and a bit lower for a well-managed aerodynamic car.

The +15 is a ballpark estimate for chassis parasitic loads (like tire rolling resistance).

Then, don't forget to divide your answer by 0.85 - to account for the power loss through the transmission and drive train, and get your answer in engine bhp.

1.3*1.8*0.80 translated to square feet is about 20.2. Using a Cd of 0.40, it works out to:

134 MPH is about max speed for 177 Hp, for a car of this size and drag. Keep in mind the Cd is probably conservative - the solstice has an even shorter tail than most, and it's drag COULD be higher.

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Assuming a 26 inch diameter tire:

Tire revolutions per mile: 775.33

Rear gear ratio: 3.91

Fourth gear ratio: 1.00:1

Peak hp: 177 hp @ 6600 rpm

6600 rpm/3.91 = 1688 tire revs per minute

775.3 tire revs per minute = 60 mph

1688 tire revs per minute = X mph

1688/775.3 = X/60

(1688/775.3)*60 = X

X = 131 mph at peak hp.

Looks like Solsticeman has a PhD in ******* Engineering...

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I looked at it and came up with closer to around 785 revs/mile, but not bad. The only thing we really don't know is the Cd. If the Cd is up as high as 0.45, the top speed drops down closer to 125 ish.stonebreaker said:

Assuming a 26 inch diameter tire:

Tire revolutions per mile: 775.33

Rear gear ratio: 3.91

Fourth gear ratio: 1.00:1

Peak hp: 177 hp @ 6600 rpm

6600 rpm/3.91 = 1688 tire revs per minute

775.3 tire revs per minute = 60 mph

1688 tire revs per minute = X mph

1688/775.3 = X/60

(1688/775.3)*60 = X

X = 131 mph at peak hp.

Looks like Solsticeman has a PhD in ******* Engineering...

PS: Got my PhD in RE a while ago. Of course, you know what PhD is short for... . That's exactly my garage: "Piled higher and Deeper". :lol:

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Cartest estimates 128mph, using a CD of 0.40.

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just thought of this, we really don't know the EXACT frontal area either, but I'm sure we're not too far off.

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BTW, how did you come up with 785 revs per mile?

((26*pi)/12) *5280 = 775.

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18.2" (true wheel size for an 18" standard rim) X .5 * 25.4 = 231.14 mm.stonebreaker said:

BTW, how did you come up with 785 revs per mile?

((26*pi)/12) *5280 = 775.

245mm X 0.45 aspect ratio = 110.25mm. Total unloaded radius= 341.39mm.

Typical deflection to get true rolling radius is approximately 3/4" or around 18mm, so rolling radius is in the 324mm range. Or about 492 revs per km. or around 785 ish revs/mile (787.4 to be a bit closer).

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One thing the impala club found out: aspect ratios on tires are only approximate. Tires generally come in 24", 26", and 28" diameters, regardless of what the aspect ratio says it should work out to.

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That's very interesting. Maybe I can go to a tire store and measure the cirumferences of various tires as a check.stonebreaker said:One thing the impala club found out: aspect ratios on tires are only approximate. Tires generally come in 24", 26", and 28" diameters, regardless of what the aspect ratio says it should work out to.

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How about off a cliff? :lol: (although way too cruel to do that to the Sol!) :leaving:PAS22 said:Down hill, with a back wind; the skies the limit! :lol:

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Fformula88 said:I knew S-man or someone else would have the formula down to give us a good estimate!

How about off a cliff? :lol: (although way too cruel to do that to the Sol!) :leaving:

Maybe the s-man can give us an equation with gravity figured in! :lol:

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9.8 (m/s)/s until you reach terminal velocity, or the ground.PAS22 said:Maybe the s-man can give us an equation with gravity figured in! :lol:

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Be that as it may, the "effective rolling radius" is usually 2%-6% less than the measured circumference of the tire. It is much like a tank tread, but not exactly like a tank tread. The steel belts do not run circumferentially around the tire, they run at an angle within the belt package - separated by and encased in rubber. These angles are generally more than 15 degrees and usually less than 30 degrees. And the upper and lower belts are at different angles to each other. There is deflection and longitudinal compliance in the tread (if you look at a tire tread on a glass traction plate when it is under torque, you can actually see there is longitudinal slip).stonebreaker said:

One thing the impala club found out: aspect ratios on tires are only approximate. Tires generally come in 24", 26", and 28" diameters, regardless of what the aspect ratio says it should work out to.

It is this longitudinal slip that makes the tire rotate more times than the calculated circumference of the tread.

The speed rating is "V", which means 149 mph for the tire, unless they use a tire placard pressure less than standard. IIRC std pressure is 32 or 35, can't remember exactly. I think the lowest they could placard this tire at is 29 PSI, (depends on what the Tire and Rim Association manual says). Therefore, even at this lower pressure, the tire should still be rated to >135MPH or so. Conclusion: Solstice is NOT tire limited.mcronec5 said:With RSA's it is going to be limited at about 126mph because of the tire rating. For the price that this vehicle is going to be at I can't see it going faster than that. However with 170hp it wouldn't go much over that anyway. I drove a 1995 Mustang GT with a 215hp V8 to 137mph top speed, of course it was a brick. I know the mustang is heavier, and aerodynamically challanged, but I don't see how with 170hp that this car will break 130. We will find out soon enough.

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