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Old 12-11-2014, 10:35 PM   #1
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Default Mechanical engineers: help me calculate stuff.

Such as how much HP does it take to move an NA down the highway at a constant 50 MPH on level ground, assuming ordinary all-season tires in the ~195 width range.

With the top up / down?

With the headlights up / down?

What about 65 MPH?

Etc.
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Old 12-11-2014, 11:09 PM   #2
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Is this what you are looking for ?
Calculate HP For Speed
and
Part 6: Speed and Horsepower
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Old 12-11-2014, 11:15 PM   #3
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It all depends on the coefficient of drag on the car, which can be then converted into a force, which can then be converted into power/torque

so unless someone has the experimental C_d values for what your looking for from full scale model testing, then your guess is as good as mine. Even then its going to differ from real-world results

I mean, how accurate of an answer are you looking for?
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Old 12-11-2014, 11:18 PM   #4
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Doesn't MS have a HP readout given the required sensor inputs? I feel like dataloging that once its calibrated would give the best answer for your brake drag, bearing condition, headlight gaps, etc etc
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Old 12-11-2014, 11:22 PM   #5
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Whoever wrote Virtual Dyno assembled all that. His documentation and possibly the program has it in it.
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Old 12-11-2014, 11:28 PM   #6
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this calculator has the basic necessities included on an accurate estimation

Aerodynamic & rolling resistance, power & MPG calculator - EcoModder.com
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Old 12-11-2014, 11:42 PM   #7
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Quote:
Originally Posted by curly View Post
Doesn't MS have a HP readout given the required sensor inputs? I feel like dataloging that once its calibrated would give the best answer for your brake drag, bearing condition, headlight gaps, etc etc
The problem with this is that yes you theoretically should be able to estimate the power the car MAKES given the sensor inputs/outputs, but this doesnt tell you the power the car NEEDS to go that speed.

Drag Force = .5 * Drag Coefficient * Velocity^2 * Frontal Area * Air Density

^this can be converted to power fairly easily if all the variables are known

Estimated Engine HP based off MS = some complex function of the sensors given calibration is accurate. The uncertainty in this I'd imagine would be pretty high, but then again it should give you a ballpark
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Old 12-11-2014, 11:53 PM   #8
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Quote:
Originally Posted by Joe Perez View Post
Such as how much HP does it take to move an NA down the highway at a constant 50 MPH on level ground, assuming ordinary all-season tires in the ~195 width range.

With the top up / down?

With the headlights up / down?

What about 65 MPH?

Etc.
Vehicle Coefficient of Drag List - EcoModder

Calculate HP For Speed

Top down drag coefficient is .46, which means that 50mph takes 10.9 hp
Top up drag coefficient of .38 means 50mph takes 9.69 hp
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Old 12-12-2014, 12:09 AM   #9
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Quote:
Originally Posted by DNMakinson View Post
Whoever wrote Virtual Dyno assembled all that. His documentation and possibly the program has it in it.
That algorithm relies on acceleration. You can get the drag force empirically by doing a cost down test from 80 to 60mph and then get the rolling friction force by doing a coast down test from below 40 to like 10 mph. And thats just some math. And once you have that you can do your calculations on the horsepower required.
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Old 12-12-2014, 01:38 AM   #10
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I love this thread already.
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Old 12-12-2014, 02:28 AM   #11
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Quote:
Originally Posted by Leafy View Post
That algorithm relies on acceleration. You can get the drag force empirically by doing a cost down test from 80 to 60mph and then get the rolling friction force by doing a coast down test from below 40 to like 10 mph. And thats just some math. And once you have that you can do your calculations on the horsepower required.
lol.


EDIT: For the record, I did not write the paragraph seen there using the word "ration"
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Old 12-12-2014, 09:10 AM   #12
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the mpgs in my WRX drop significantly when cruising above 60mph compared to cruising at 55mph behind a big truck.

hope that helps.

“( ͡ ͜ʖ ͡)”
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Old 12-12-2014, 10:15 AM   #13
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Note Frontal Area and Cd. VD includes drag.

Anyway, there is the info for a NB. I don't know where to find lights up / lights down. CC gave you top up and top down.
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Mechanical engineers: help me calculate stuff.-vd_input.png  
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Old 12-12-2014, 05:29 PM   #14
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It would be FAR easier and better to do some logs then hire a dyno for an hour.

Dann
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Old 12-17-2014, 01:23 PM   #15
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I remember that class!

Quote:
Originally Posted by Schuyler View Post
lol.


EDIT: For the record, I did not write the paragraph seen there using the word "ration"
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Old 12-17-2014, 04:14 PM   #16
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Quote:
Originally Posted by Leafy View Post
That algorithm relies on acceleration. You can get the drag force empirically by doing a cost down test from 80 to 60mph and then get the rolling friction force by doing a coast down test from below 40 to like 10 mph. And thats just some math. And once you have that you can do your calculations on the horsepower required.
^This. What Leafy describes is basically a SAE test to determine the rolling resistance as well as the drag force, but with a slight deviation.

The TL;DR process is in red.

For the rolling resistance, hook a force gauge up to a strap that is connected to the front tie down hooks. Pull the car from a stop using the force gauge, as close to parallel to the ground as possible. This has to be done on a smooth surface that is level. At a steady 1mph or so, record the steady state force reading, just make sure that you are not accelerating or slowing down, this will greatly affect the readings. This is your rolling resistance in Lbs.

Perform a coast down test at a relatively high speed, such as 70-40mph. Knowing the weight of the car as well as the frontal area and other parameters, subtract the rolling resistance from the resistance calculated from the high speed tests, this will tell you the aero drag force, and from this you can find the coefficient of drag.

Using the information above with the calculations you guys posted, you can find out approximately how much HP is needed to go a certain speed.

Keep in mind, when you find this HP rating, this is the raw HP needed to accomplish this speed, it does not take any driveline or other losses into account. So if you are doing some project like trying to do a EV conversion and want to know how powerful the motor needs to be to achieve a certain speed, take a conservative estimate of the losses and go from there.

Long winded, but hope this helps.
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Old 12-17-2014, 05:23 PM   #17
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why can't we just get a REALLY LONG inclined road of known angle and let the car coast to terminal velocity?
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Old 12-17-2014, 05:27 PM   #18
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Quote:
Originally Posted by y8s View Post
why can't we just get a REALLY LONG inclined road of known angle and let the car coast to terminal velocity?
with frictionless rollers on it in a perfect vacuum at absolute zero?
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Old 12-17-2014, 05:43 PM   #19
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Quote:
Originally Posted by Leafy View Post
with frictionless rollers on it in a perfect vacuum at absolute zero?
Simple. Build a road at Mt. Everest Peak down to the bottom with a run off road and all in a sealed tube to pull a vacuum.

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Old 12-17-2014, 05:59 PM   #20
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Going back to why dont youy just do a log and then use a dyno?
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