# hp vs mph graph



## brinkhamer (Jul 8, 2016)

just as a simple exercise I put a page together to graph mph vs hp given CDA and weight. Eventually would like to add motor torque and ratio/wheel diameter graph, but don't have a simple way to do that yet (getting the motor data in easily and intuitively is the hard part).

https://davebrink.github.io/hpvmph.html


----------



## BWA (Mar 14, 2015)

Nice.

Going in my file of useful EV stuff...


----------



## galderdi (Nov 17, 2015)

Yes agreed. Quite useful. I wish I knew my CD.
Regardless it proves that something is fundamentally wrong with my car. Conservatively I should be seeing 90mph but I am topping out at 55. I am hopeful it was my stuck brushes.


----------



## Duncan (Dec 8, 2008)

Hi Galderdi

remind me about motor, pack voltage and rpms - please

My car was topped out at 59mph - I simply didn't have enough volts to drive a high current at high rpm (130v)

It is going back together with 325v


----------



## galderdi (Nov 17, 2015)

Duncan said:


> Hi Galderdi
> 
> remind me about motor, pack voltage and rpms - please
> 
> ...


Motor - ADC 9 inch
Voltage - 170v
RPM - Estimated 5000rpm in 1st and 2nd gear but not getting to max in 3rd (not even getting to 4th or 5th)
Car weighs 2600 lbs 

I am hopeful its the brushes.


----------



## PStechPaul (May 1, 2012)

Here is my EV calculator where you can plug in various parameters to get the data points you want. I don't guarantee that the default values or calculations are accurate, but they seem reasonable when compared to actual performance as reported by others:

http://enginuitysystems.com/EVCalculator.htm

According to my calculator, a 1000 pound car at 60 MPH requires about 8 HP, while your graph shows about 1.6. For 10 MPH, I estimate 0.58 HP, while your graph shows less than 0.1 HP. I can get closer to your figures at 10 MPH if I greatly reduce the rolling resistance coefficient, but at 60 MPH the aerodynamic drag predominates.


----------



## Electric Avenue (Jul 11, 2016)

Wind drag is a very important detail to figure in. Topography is another one to figure in. You need less power going down hill than up.


----------



## brinkhamer (Jul 8, 2016)

PStechPaul said:


> Here is my EV calculator where you can plug in various parameters


Hi Paul, what CD*A did you use (in sq ft)? I can match those numbers at a CDA of 4.4 sqft.

It was inspired by this, and corresponds in calculation:
http://www.wallaceracing.com/Calculate HP For Speed.php

so it would be good to know if there is a problem.


----------



## PStechPaul (May 1, 2012)

I used a CD of 0.25. That is on the low side of the examples shown in your web link.

My default frontal area is 1.25 m^2, which is close to 10 ft^2.

Using that calculator, with CD=0.25, Area=10, Weight=1000 lb, and speed 60 MPH, it gives 3.69 HP for just drag. The rolling resistance of 2.08 makes for 5.77 HP.

My calculator shows 8.05 HP.

You can look at the HTML and JavaScript of my web page to see how I did the calculations. There may be errors, but the results seem reasonable.

http://enginuitysystems.com/EVCalculator.txt


----------



## brinkhamer (Jul 8, 2016)

PStechPaul said:


> Using that calculator, with CD=0.25, Area=10, Weight=1000 lb, and speed 60 MPH, it gives 3.69 HP for just drag. The rolling resistance of 2.08 makes for 5.77 HP.


yup, same as the interactive graph I posted, 5.77 hp.



PStechPaul said:


> My calculator shows 8.05 HP.


When I open your app, and use 455kg (1001 lbs), and 97kph, and change nothing else, it comes back with 24.345 HP...

when I use all defaults it says 49.288hp?!? 

(chrome on windows 10 fyi)

you are of course welcome to look at my code in crome dev console or github, but the meat of it is trivial and follows the original calculator precisely:

y is hp, x is mph:
var y = (Math.pow(x / 52.715, 3) * cda) + (0.000034667 * x * weight);


----------



## brinkhamer (Jul 8, 2016)

fyi https://www.rbracing-rsr.com/aerohpcalc.html (waay at the bottom)

with .25 10 1000 60 comes back with:

Horsepower Needed: 6.235294117647059
Horsepower Lost to Drag: 3.6

right click, inspect, sources tab, javascript/bonnevil2.js has all their conversions.


----------

