>Weight has nothing to do with top speed. The only factors that affect top
>speed are rolling resistance, air resistance, horsepower, and of course, speed
>limit signs.
Weight does affect top speed through Rolling Resistence.
RollingResistenceForce = Cr * CarMass * AccelerationDueToGravity
AerodynamicForce = 1/2 Cd * DensityOfAir * area * speed^2
Cr and Cd are the coeficient of RollingResistence, and coeficient of drag
respectively, usually in the neighborhood of .013, and .35 respectively.
top speed is governed by the force equation...
AerodynamicForce + RollingResistenceForce == power / speed
In my car, RollingResistenceForce = AerodynamicForce at 42 mph, so
Aerodynamics is relatively small at 30 mph.
> I have read in a number of places that it is saves gas to run the
>air conditioner than to open the windows. I have a problem with this. At low
>speeds, aerodynamics has little impact on your fuel economy, so rollong the
>windows down has to save gas over running the AC. However, at some speed,
>there is a break even point that makes the AC the way to go. This point is
>usually claimed to be less than highway speeds. However, what kind of car was
>used? For a very sleek car I could see window open drag kicking in at low
>speeds. However, for something like a van, (or a mini :-) the drag is so
>high,
>that an open window might be a small component of the total drag. The last
>portion is with my non-lbc. It has a sun roof that can either flip up at the
>back, or slide completely into the roof for a large hole.
The trade-off is not directly related to how sleek the car is initially.
It is governed only by the change in drag coeficient.
If putting down the windows/sunroof/convertible-top changes the drag coeficient
by DeltaCd the power required to overcome this change is
1/2 DeltaCd * DensityOfAir * area * speed^3
The trade-off favors windows down when this power is less than the power
of the AirConditioner. The speed at which they are equal is
speed = (AirConditionerPower/(1/2 DeltaCd * DensityOfAir * area))^(1/3)
Assuming: AirConditionerPower = 3 HorsePower, DeltaCd = 0.1, area= 2.2
Meter^2, DensityOfAir = 1.1 Kilogram/ Meter^3 We get
>>>>>> speed = 60 mph. <<<<<<<
Note: this speed has a very weak dependence on the assumed numerical values,
since they all enter into a cube-root. Consequently I think you save gas by
putting the top down at all but the highest speeds.
Your Mileage May Vary.
-Ted
weverka@drip.colorado.edu
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