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Calculate
Aerodynamic Drag and Rolling Resistance with the Auto Dyno Coast Down
Calculator
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Auto Dyno Coast Down Calculator
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Simply record a data run starting with your speed just above
60mph and let your vehicle coast down to 30mph. Auto Dyno uses this
data to calculate your Aerodynamic Drag Cd and your total drag Cda.
This data is also used to calculate your Rolling Resistance. |
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The Cda is calculated from empirical data |
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The Cd is calculated using Cda and the vehicle Frontal Area
you entered into the Aerodynamic Information box under the Options / Test
Run Information drop down menu. |
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The Coast Down technique measures your Drag Coefficient empirically. This is more accurate than the vehicle factory
specification which commonly removes rear view mirrors, etc before
completing this test. It also accounts for any aerodynamic changes
that may have been made to your vehicle including:
- Lowering
- Larger Wheels / Tires
- Body / Aero Kits
- Wings / Spoilers
- Convertible Top Down Vs Up
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Your Rolling Coefficient needs to be measured as well to
insure accurate HP/TQ data. |
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After you have calculated your Aerodynamic Drag you can use
this data to increase the accuracy of the HP/TQ data by allowing Auto Dyno
to calculate the HP/TQ required to overcome air resistance and rolling
resistance. |
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You can also Graph empirical Drag Losses which include
Aerodynamic Drag and Rolling Resistance by selecting Custom in the Graph
window, and selecting Coast Down Aero+Rolling Drag. This drag is calculated in
real time, and thus it is the sum of Cda [Cda = Cd + Frontal Area] and Rolling Drag. This graph
is only accurate if the vehicle is in neutral and no brakes are applied.
In the following graph:
The Blue
line is the calculated Aero+Rolling Resistance in HP
The Green
line is the Coast-Down Aero+Rolling Drag in HP measured by the deceleration of the
vehicle
The Orange line is the
vehicle Speed.
Note how close the Blue and Green values are. [Click
for larger Graph] |
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Math:
A vehicles HP is the force accelerating your vehicle F = M*A, where
the mass is the weight of your vehicle, and the acceleration is your
change in speed.
This formula nets the HP required to accelerate your vehicle, but it
does not take into account the HP required to overcome rolling
resistance or to overcome aerodynamic drag.
To measure the force or HP required to overcome rolling resistance
and aerodynamic drag you will need the Frontal Area of your vehicle, its
Cd [Coefficient of Drag], and Weight
Rolling Resistance (LBS)=Weight * (Rolling Coefficient + (Speed
Effect Coefficient*3.24*(Speed/100)^2.5)
Aero Drag = 0.5 * Air Density[kgm3*kgm3_to_slugs[slugs]*Frontal Area*DragCoeff*(speed*1.467)^2
Rolling Resistance [LBS] + Aero Drag [LBS] = Total Drag in LBS
Convert Total Drag [LBS] to HP
HP = Speed * 1.467 * Total Drag / 550
The math is actually far more complicated, this is just a brief
explanation of the mechanics.
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| How does AutoDyno calculate
Cd? |
| Auto Dyno calculates Cda and
uses the Frontal Area to extract Cd from Cda |
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| How does AutoDyno calculate
Coast-Down Cda Drag? |
| Auto Dyno calculates Coast-Down
Cda from deceleration HP. The only reason the vehicle
decelerates is because of Aerodynamic Drag and Rolling
Resistance. Thus you can easily calculate the amount of HP that
is exerted to generate the measured deceleration. |
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| I am confused, why are there
3 Cda's? |
| 1. Options menu in the
Aerodynamic Information box |
| A: The Drag Coefficient
here is entered by the user and is the factory Cd for your vehicle
make, model, and year. This value is probably close enough, but
might not be accurate. |
| 2. In the Data View in
the Calculated Drag section |
| A: This is actually the
measured Cda from a coast-down run 60-30mph. Cda is the sum of Cd and
Frontal Area. It is calculated
by performing a coast down from 60-30 mph. It calculates Total
drag and separates it based on the fact that from 60-50 most of the
deceleration is because of Aero Drag and from 40-30 most of the
deceleration is because of Rolling Drag. |
| 3. In the Data View /
Test Run Info: Section [Drag Coefficient = Cd] |
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A: This is the Cd that was in the Options box when the run was
recorded. The same is true for all settings in the Test run Info
section.
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| More
Information on AutoDyno |
| View full product details on
AutoDyno. |
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| Purchase AutoDyno |
| Purchase AutoDyno Online. |
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| Purchase AutoDyno |
| Purchase AutoDyno Online. |
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