# Help calculating voltage needed to reaching desired top speed



## MatthewBowker (Mar 6, 2016)

Before I begin I want to just clarify that I am new to the EV conversion scene. In fact, I am pretty new to car specifications in general whether it’s regular combustion or electric engines so please give me some slack when it comes to terms and all. I am a nooby.

The other week I was looking at the new 2016 Ford Mustang GT and stumbled across a car called the Zombie 222 (if you haven't heard of it here is a link: https://www.youtube.com/watch?v=ZAwIsKC7ROQ) and since that moment have been super intrigued by this. I have been doing some reading into EVs and have learnt some basic fundamentals like the batteries are what makes an EV fast, not the motor and such but I have been diving into compiling some components to use to give me a bit of an idea of how much it would cost to do it myself.

Anyway the last post I read was actually this one: http://www.diyelectriccar.com/forums/showthread.php?t=11709

I felt like mattW did a great job of explaining the process of determining what size battery pack is needed for the users’ needs and the fundamentals of how voltage and amperage effect different variables in a EVs performance. The thing that I am lost on is calculating the voltage needed to reach 100mph. In the post mattW used a DC converted motorbike for example needing 72V to approximately reach 60mph but he doesn’t mention what bike he is basing this off so I have no indication of weight, aerodynamics and rolling resistance which he mentions play a part in efficiency. Is there some sort of formula or rule of thumb that helps people determine the voltage needed to achieve the desired top speed?

I would appreciate some help and thanks in advance.


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## major (Apr 4, 2008)

Speed takes power. That's Watts (= Volts * Amps).

Range takes energy. That's Watt hours (= Volts * Amp hours).

Acceleration requires torque.

There used to be on-line EV calculators. Maybe someone will chime in with a link.


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## minispeed (Jun 30, 2013)

If you're converting an ICE and trying to keep the same top speed and that car is drag limited then you'd should need the same amount of HP that that ICE turns out at the time the car is drag limited. You also need to have the gearing right so you don't over rev the motor before you reach that speed.

In the video you linked they said that set up would cost $125000. The cost to go fast is just like aerodynamics, not linear at all.


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## minispeed (Jun 30, 2013)

http://ecomodder.com/forum/tool-aero-rolling-resistance.php

I used leaf numbers and .01 for rolling resistance. For a car that will do 100 you prob won't be using LRR so you might want to make it higher.

http://ecomodder.com/wiki/index.php/Vehicle_Coefficient_of_Drag_List

Use this to get an idea of the CD and frontal area of other cars. Ignore the fuel stuff it only changes the mpg. In the real world there will be other factors that require more power but this is a good way to guess your minimum needed. According to leaf numbers you need 42kW to go 100 mph.


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## miscrms (Sep 25, 2013)

Although some of the specific info is a bit dated now, I always thought this book had a pretty good approach to the design calculations:
http://ev-bg.com/wordpress1/wp-content/uploads/2011/08/build_your_own_electric_vehicle.pdf

Also a bit dated, but this site's calculator is based on the same basic approach as I recall and can still be useful to play around with the effects of different design trades. It really only works with DC motors though, and was designed for lead acid battery calculations.
http://www.evconvert.com/tools/evcalc/

These methods can tell you a lot about range, efficiency and top speed based on various vehicle and drive system choices. They can give you an idea of acceleration based on the 5% incline = ~1 mph/s rule of thumb.

For more detailed acceleration modeling, I've been using this excel sheet:
http://www.offroadvw.net/exceldyno/

I can't vouch for its accuracy, but when I put in the vehicle and motor data for a stock Nissan Leaf its predictions were very close to real world reports on 0-60 and 1/4 mile. You can put in AC motor specs pretty directly, as they usually give you torque and hp vs. rpm. DC motors are trickier, best bet is if you can find cases where someone has dyno'd a particular controller/motor combination. Here's some of the motor data that I came up with:









If you are using a single speed gear ratio it does seem to gack on it occasionally, I've founding switching back to a standard transmission and then re-editting to single speed seems to clear it up. Also using a 2 speed with nearly the same ratios seems to keep it happy.

Rob


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## jetpax (Oct 28, 2013)

You also might like to try this Google Sheet.

You can play with the values or make your own copy and modify as you like.

I've loaded it with some well-known vehicles and projects that you can compare your design to, see the Notes sheet for more info.


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## ga2500ev (Apr 20, 2008)

You can work your way backwards from the wheels. The basic idea is that in order to achieve 100 MPH, then the wheels have to be turning at a specific RPM. That RPM is determined by the circumference of the tire.

Take your 2016 Mustang GT. Stock tire size is 235/50R18. according to this calculator:

http://www.csgnetwork.com/tiresizescalc.html

The circumference is 85.615 inches. 100 miles is translated to inches:

100 miles * (5280 ft/mi) * (12 in/ft) = 6336000 inches. To get RPM figure out how many inches per minute needs to be achieved to get that distance in an hour:

6336000 inches/hr / 60 min/hr = 105600 inches/min

Divide by the circumference to get the RPM:

105600 inches/min / 85.65 inches/rev = 1232.92 RPM

Now the gearing of the transmission and the differential determine that RPM. According to this:

http://www.edmunds.com/ford/mustang...-mustang-gt-short-gearing-is-fun-gearing.html

The 2015 Mustang had final drive gearing in the differentials of 1:3.73 1:3.55 and 1:3.13 for performance, intermediate, and standard gearing. So for each revolution of the wheels, the driveshaft turns 3.73, 3.55, or 3.13 times. Using the 3.73 performance number:

1232.92 * 3.73 = 4598.79 RPM. Now according to this post:

http://www.mustang6g.com/forums/showthread.php?t=10323

The 6 speed manual transmission has the following gearing from 1st to 6th respectively:
3.657,2.430,1.686,1.315,1.00,0.651

These ratios are the gearing from the engine RPM to the driveshaft RPM. So if the car is in 6th gear, the engine/motor needs to spin at:

4598.79 * 0.651 = 2993.81 RPM 

in 6th gear.

So now we've finally gotten back to the motor, which needs to spin at 3000 RPM. Motor RPM is determined by voltage. For each motor there are specifications for maximum RPM and a linear measure of RPM/Volt that determines the voltage necessary to gain the necessary RPM at a specific torque. For example let's look at a WarP-9 motor:

http://www.evsource.com/tls_warp9.php

According to the FAQ: http://www.go-ev.com/PDFs/005_008_SU_NGT_FAQ.pdf

The RPM/voltage ratio is nearly linear. The page above has a 72V performance chart. So all that needs to be figured is the required torque.

Hope this helps,

ga2500ev


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## kennybobby (Aug 10, 2012)

And if you know the weight, frontal surface area, coeff of drag, coeff of rolling resistance of the tires, then you can calculate the resisting loads that must be overcome in order to travel at the desired speed, and from that you can determine the torque required. With rpm and torque you can determine the motor hp required also.


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