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Premium Member
5,206 Posts
Discussion Starter #1
[science lesson alert!!!]


(re-post from skyroadster and another forum that came from a discussion of how to tell if top speed is possible)

As for top speed, power and drag dominate the capability over about 115 MPH (maybe slower). Roadsters, especially with short tails, are notoriously bad for drag coefficient, generally ABOVE 0.40. The Solstice is around 1.3 meters tall, and 1.85 or so meters wide - that's a huge frontal area.

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.

To go 142, you'd need 210 Hp. That's assuming you can find a gear at exactly the correct speed to get the max power.

That's how to figure out if the information is accurate. And 142 for 177 hp for a car like the Solstice is about about 30 horsepower (maybe more) shy.

Likewise, power and torque can be used to estimate 0-60 times very simply, and you can even take a stab at the 1/4 mile time and speed. You can get easily within a half second, and probably better if you know the real weight, drag, etc.

To estimate 0-60 times, take the curb weight in lbs, divided by peak horsepower, then multiply by 45%.

SKY = 2860 lbs, 177 hp (assuming same engine as Solstice). 21860/177 = 16.16 curb lbs/hp. Multiplied by 0.45 = 7.27. Solstice advertises 7.2 seconds 0-60 - so it's got to be close enough.

Equation breaks down for things more powerful than about 11 curb lbs/hp.

[/science lesson alert]

[further discussion alert]
By size I meant frontal area, not weight.

Weight only really affects top speed as a function of the parasitic loss (rolling resistance, bearing resistance...) but those can be thought of as pretty much a constant (about 15 HP at wheels).

As far as the weight and inertial effects - these are tiny when compared to the aero-drag. That's because f=ma, or a=f/m, purely a linear part of the function. A change from 2600 to 2800 lbs means a tiny change in the inertial resistance, and it's the same to go from 30mph to 40mph in 5 seconds as it is to go from 130-140 MPH in 5 seconds.

Mathematically, weight really doesn't have any appreciable effect in how fast you get to top speed (if you're close to 'terminal' top speed) because 90% of the power near terminal velocity is absorbed by aerodynamic drag.

At ungoverned top speed or 'terminal velocity', it's literally how much force is pusing the car forward (speed of the engine, gearing, wheel, velocity of the car, and ultimately power) EXACTLY BALANCED with paratisitc drag (10%, semi-constant) plus the aerodynamic drag (90%).

Since you are at terminal velocity, you speed is not changing, therefore zero acceleration, so any inertial forces are also zero.

Additionally, as you approach terminal velocity, the acceleration of the car is very low - to get up to that last 10 MPH, you're creeping along at a very low acceleration - sometimes it takes a good 10-15 seconds to gain that extra velocity. This acceleration is so low, that the power required to overcome inertia (the mass of the vehicle) during this acceleration is also very low (just a couple, if that, horsepower). It's only the amount of horsepower required to propel a car 0-60 in 600-700 seconds (not counting aero drag or parasitic losses) - a miniscule amount of power. Even if you double the vehicle weight, it's still 'miniscule' multiplied by two.
[/further indepth discussion]

1,248 Posts
slowandlow said:
would a wing or aero package help with this? a nice front spoler
Probably not. They would increase down force for cornering, but they would increase drag and increase the CD. What would help would be a tail cone and a sloped panel in the back instead of a flat glass vertical window.

By way of example, the GM EV1 acheived >170 MPH with 135 HP because of low drag. To get above 170, they added front wheel farings (covers from the body down) and a tail cone. The basic shape was already pretty slippery CD wise, and presto, a really fast electric car. Could you drive it that way on the street? Well, it's a bit like driving a long stake bed truck because of the tail cone, and the front farings would prevent turning more than about 5-10 degrees, so your turning radius would be measured in fractions of a mile, rather than in feet, but, SURE, you could try...

In all seriousness, though, the roadster is not a form factor for speeding down long straight roads at 190 MPH; roadsters are for roads with turns in them. That's why they call my favorite type of racing "road racing". It's a track, yes, but it's got a lot of turns! ;)

Premium Member
8,901 Posts
Yeah usually front and rear spoilers are designed to create downward force to keep your car from coming up off the road at high speed. Which as Crimson Avenger said, it also increased the drag factor usually.

You would have to redesign the entire front facia of the car, plus the rear to help decrease drag by mkaing it smaller and more aerodynamic.

Premium Member
5,206 Posts
Discussion Starter #5
A word on downforce or lift...

Downforce or lift are both usually generated at the expense of drag.

Simply stated, to create downforce you deflect airflow (the mass of the air, anyway) upwards using a flat or bernoulli surface. This creates a negative pressure zone on the back side of the panel/wing. The plan view "shadow" and the pressure determine the downforce. However, there is also a frontal view "shadow" of this same negative pressure - meaning there is a corresponding force rearward.

It is possible to reduce the drag effect using wing shape. A flat panel is the worst. Depending on the speed you want to generate your downforce, a curved bernoulli surface is usually the best (flatter on the topside, curved on the bottom side - the reverse of an airplane wing). It is NOT possible to completely eliminate the drag.

Since you mostly need the downforce in situations where you need stability when the rear has reduced force, like high speed braking, and you don't really need downforce for straight line driving at maximum speed, you can get the best of both worlds by making your wing self adjustable.

The way to do this is start with a relatively flat panel. Spring load it on a pivot so at rest it is at maximum downforce angle. It is better if you use an "over-center" mechanism, with variable effort that will sort of "pop" it from maximum downforce to pretty much no downforce (and also minimum drag) once a certain angle is reached. Then you spend some time adjusting the spring so that the panel is pretty much zero angle of attack at or near max speed of your now developing race car.

Ta-DA! Max speed down the straightaways with minimum drag. Slam on the brakes, when you drop below your threshold speed, your wing pops up, you get great stability, you can use more of your rear brakes, AND you have the equivalent of an added air brake. Cars like the old Chapparals could generate in excess of 2.5g's of braking using such a system. How's THAT for seemingly violating the laws of physics? :thumbs:

I started this thread to help people understand the difference between pure top speed (balance of drag and engine power) versus acceleration (balance of weight and engine power). Indy cars have such incredible top speeds because of the engine power - they actually have pretty crappy coefficient of drag. So much so that just letting off the throttle at 195 mph, the drag alone generates over 1 g of braking - without touching the brakes.
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