# What's the minimum torque for safe driving in city and hwy?



## bjfreeman (Dec 7, 2011)




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## madderscience (Jun 28, 2008)

Depends on a lot of factors other than just car weight.

Gear ratio? Tire size? Expected top speed? Hilly or flat Terrain? Driver preference? Some people are happy driving a 40hp beetle and some people "need" a 300 horsepower corvette.

I can throw out one data point. My car weights 3400lbs, and has the 500 amp curtis controller (1231C) and an advanced DC 9" motor.

This means I have maximum motor torque of about 100 foot pounds of torque available. My car would be considered "low performance" but it is still fine to go on local freeways (55-60mph) and it can hit 70mph or more if I want to push it. I use 2nd gear (8:1 ratio with 23" diameter tires) for all driving up to about 45mph and I can climb a 15% hill in that gear. I use 3rd or 4th on the freeway, and 1st only in those rare cases I have a very low battery and a hill to climb.

Hope this helps.


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## sideburn (Dec 29, 2011)

@bjfreeman: Thanks for the graph! I am not sure how it would help me figure out the minimum torque.

Let's use WarP9 as an example. According to their FAQ it states:


> VOLTs=RPMs in an almost linear manner. If you double the voltage you will double the RPMs of
> the motor. Usually, RPMs increase just slightly more than double as most losses are fixed. You will
> notice that the performance graphs for our motors are all at 72 Volts. If you plan on running at 144
> volts you can simply multiply the RPMs by 2. AMPs=Torque. Torque will remain constant if the
> ...


WarP9 motor performance graph:








According to the graph, I assume at 100% duty cycle the minimum RPM is around 2100 and the maximum RPM is 5000. If input volt is 72 then, at 5000 RPM the Amp is around 80 and Torque is around 8. Now if I reduce the input voltage until RPM is down at minimum continuous RPM which is 2100 then the voltage would be 30.24 and Amp still be around 80 and Torque still be around 8. So at continuous 2100 RPM the lowest Torque I can setup is around 8 Ft/Lbs. I bet it is energy saver for sure but I have a feeling it is not safe to drive in city/hwy. My question was what's the minimum torque that is safe? I bet it depends on weight of the car. If so, what about maximum weight of car per torque if that makes sense.


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## sideburn (Dec 29, 2011)

@madderscience:
Thanks for your car specs!
I think I got top speed at given RPM, gear ratio and tire size figured out. I have the formula in my spreadsheet. However, I haven't figured out the minimum torque/acceleration/climb for flat and hilly terrian. At least let's start with 40hp beetle performance or just enough that the police won't frown at you lol.


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## lithiumlogic (Aug 24, 2011)

Does torque matter? Is it easier to talk in terms of power-to-weight ratio?

In terms of what constitutes the minimum acceptable, comparisons with low power ICE vehicles may be misleading. Extracting max power from a petrol engine makes for a lot of drama - wide open throttle, hitting the redline, jerky, rushed gear shifting, jerky clutch-slip starts. Pedestrians will stare, passengers will grip their seats and complain, you'll arrive at your destination with a sore head.

In an EV, no-one will notice you revving out to 4000 rpm or that the accelerator pedal is mashed to the floorboards. So long as your cooling is good enough to cope with frequent use of max power, no problems.

When when moving house, i've driven a few hire vans with 3500kg gross weight and 85hp diesel engine. Very noisy progress, but no problems keeping up with the flow.. including 70mph motorway sections. Then again, people give those hire vans a wide berth, i know i sure as hell do.

I suppose there's gear ratios to consider. AC motors hit peak power at similar rpms to diesel engines, which is about the highest most people ever go. DC motors hit peak lower down, but still higher than you'd go with your mother-in-law in the car.


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## MalcolmB (Jun 10, 2008)

I've thought about this question a bit for my own direct drive application. From what you say I'm not sure if you realise that electric motors develop maximum torque from zero rpm, so there is no lower rpm limit.

Because power = torque x rpm, it makes sense to look at torque when you're considering an electric car's ability to pull away at an intersection, pull away on a hill or climb a curb. The minimum acceleration for modern cars seems to be about one third g, so I would aim for similar acceleration in an electric car to be on the safe side. That means you need a thrust of one third the weight of the car at the driving wheels. For my car, weighing roughly 1500 lbs, that means a minimum thrust of 500 lbs at the wheels. The wheel radius is 10" (5/6 of a foot), so the minimum required torque at the wheel is 500 x 5/6 = 416 ft lbs. I need to choose a gear ratio that will multiply the maximum torque from the motor to at least 416 ft lbs.

Now you know the gear ratio you can work out the theoretical top speed of your car at maximum motor revs. I say theoretical because your actual top speed will depend on how much power you can deliver to the motor at its maximum rpm. That in turn depends on the pack voltage and the maximum current your controller/battery can deliver at that voltage.


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## major (Apr 4, 2008)

sideburn said:


> Let's use WarP9 as an example......
> WarP9 motor performance graph:According to the graph, I assume at 100% duty cycle the minimum RPM is around 2100 and the maximum RPM is 5000.


This graph is a "snapshot" representing what the motor will do over a range of load with a specific voltage applied to the motor terminals. In this case, 72 volts. There is no implication of a minimum or maximum RPM. You can change the voltage applied to the motor (using the controller) and essentially get any RPM you desire, certainly any RPM below that curve. 

Please search and find some of my previous posts about how to properly read the motor speed torque characteristic graph. You obviously don't understand it, along with many others (so please don't take offense, none meant ). I'll try to find my tutorial post or rewrite it later, but don't have time now.

Regards,

major

Here is an example: http://www.diyelectriccar.com/forum...ng-motor-specs-and-vehicle-gearing-27320.html And I know there have been others. I used the forum search for "characteristic".


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## sideburn (Dec 29, 2011)

@major:

Thanks for the link...I thought the graph is *rated* at 72V? 'Rated' means all the curves (rpm, amp, etc) shown on the graph can perform continuosly for long period of time. So that's why the graph shows *rated* rpm curve cuts off at 2100 RPM and at 5000 RPM. If you're talking about peak power, yeah you can do any RPM for limited amount of time. From my previous post I said reduce volt from 72V to 30.24V which shifts RPM curve down therefore 5000 RPM @ 8 Ft/lbs is shifted down to 2100 RPM @ 8 ft/lbs. What part did I didn't understand?


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## sideburn (Dec 29, 2011)

@MalcolmB

Thanks for your insight! That was helpful!!!


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## major (Apr 4, 2008)

Hi side,



sideburn said:


> ...I thought the graph is *rated* at 72V? 'Rated' means all the curves (rpm, amp, etc) shown on the graph can perform continuosly for long period of time.


This is incorrect. There is no time scale on the graph. 



> So that's why the graph shows *rated* rpm curve cuts off at 2100 RPM and at 5000 RPM.


It likely does not go above 5000 RPM because they don't recommend operation higher than that. It doesn't go below 2100 RPM because they elected not to extend it out to heavy loads. BUT, you can certainly operate the motor at lower speed.....everybody does.



> If you're talking about peak power, yeah you can do any RPM for limited amount of time.


I wasn't talking about peak and no, you can't do any RPM for limited time. There is a speed at which mechanical damage will occur, so an upper limit. 



> From my previous post I said reduce volt from 72V to 30.24V which shifts RPM curve down therefore 5000 RPM @ 8 Ft/lbs is shifted down to 2100 RPM @ 8 ft/lbs. What part did I didn't understand?


With statements like these:



> Now if I reduce the input voltage until RPM is down at minimum continuous RPM which is 2100





> So at continuous 2100 RPM the lowest Torque I can setup is around 8 Ft/Lbs.


I'd say there is a lot you don't understand. "Minimum continuous RPM" doesn't make any sense. You can run the motor at just about any RPM (less than the maximum) continuously at some load. 

What does "lowest Torque I can setup is around 8 Ft/Lbs" mean? Torque is the load. You can have a load condition less than 8 lb.ft. and at 2100 RPM or any other speed (less than max).

Again, I am trying to help, not belittle you. I see many, many folks come on this board confused as hell about electric motor curves. A lot of them try to relate directly to gas engine curves which confounds the matter. I have numerous posts trying to help and is why I suggested you read some of those.

Regards,

major

edit: 
Here are some threads about motor characteristics which may help you understand better.

http://www.diyelectriccar.com/forums/showthread.php/dc-motor-graph-39649.html

http://www.diyelectriccar.com/forums/showthread.php?p=267280&highlight=torque#post267280 

http://www.diyelectriccar.com/forums/showthread.php?p=249229&highlight=torque#post249229 

http://www.diyelectriccar.com/forums/showthread.php?p=226956&highlight=torque#post226956 

http://www.diyelectriccar.com/forums/showthread.php/electric-motor-output-wrt-voltage-32492.html 

http://www.diyelectriccar.com/forums/showthread.php?p=149088&highlight=characteristic#post149088 

http://www.diyelectriccar.com/forums/showthread.php?p=210983&highlight=torque#post210983 

http://www.diyelectriccar.com/forums/showthread.php?p=209585&highlight=torque#post209585 

http://www.diyelectriccar.com/forums/showthread.php?p=191407&highlight=torque#post191407 

http://www.diyelectriccar.com/forums/showthread.php?p=114852&highlight=torque#post114852 

http://www.diyelectriccar.com/forums/showthread.php/understanding-motor-specs-and-vehicle-gearing-27320.html?highlight=torque 

http://www.diyelectriccar.com/forums/showthread.php?p=82123&highlight=torque#post82123


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## sideburn (Dec 29, 2011)

@major:
Thanks for all of those links!

I'll read *ALL* of your recommended links slowly one at a time ...I'll be back when I'm done with homework.


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## iti_uk (Oct 24, 2011)

I agree with MalcolmB, I think it would be more useful to specify (or enquire about) a minimum acceleration, as this takes into account the mass of the vehicle, the gearing and wheel sizes used AND the motor torque (a = F/m).

1/3g acceleration is roughly 3.27 ms^-2, which if assumed constant, means a 0-60mph time of 8.2 seconds which is very adequate for a commuter car.

From there, as he mentioned, calculate your required motive force by multiplying your a by your car's m (F = m.a), and from that you can work out the torque at the wheel required for the acceleration (M = F.r where r is the radius of the wheel), then divide through whatever gear ratios you have (say you have a gearbox ratio of 1.2:1, and a final drive ratio of 3.6:1, the calculation would be M / (1.2*3.6) ), this will end up with a required maximum torque at the motor shaft so that you can then specify a motor suitable for your build.

As major said, this calculated maximum is just the limit of your vehicle's operational envelope.

(assuming a vehicle of mass = 800kg and wheel radius of 25cm, the above calculations and values given in my example above would require a motor with maximum available torque at the motor shaft of 380Nm (280lb.ft). This also hilights the importance of gearing choice - a gearbox ratio of 1.2:1 is equivalent of a fairly standard 4th gear in a 5 speed 'box. Doing the same calculation with a typical 1st gear (e.g. 3.6:1) would result in a motor torque requirement of 127Nm (94lb.ft)

Chris


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## sideburn (Dec 29, 2011)

@Chris:

Wahooo!!! That's exactly what I was looking for how to solve for N.m if you have the desired acceleration!!! Thank you so much for the formulas!!!! I think there is small error when you came up with 380N.m. It looks like you used a = 8.2 sec but it should be a = 3.27 ms^2 which comes out to 151N.m?


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## iti_uk (Oct 24, 2011)

sideburn said:


> @Chris:
> 
> Wahooo!!! That's exactly what I was looking for how to solve for N.m if you have the desired acceleration!!! Thank you so much for the formulas!!!! I think there is small error when you came up with 380N.m. It looks like you used a = 8.2 sec but it should be a = 3.27 ms^2 which comes out to 151N.m?


No probs.

Lol you are correct - picked the wrong cell in my knocked-together excel sheet. :-D

As a further correction, this would mean that the assumed first gear (3.6:1) motor torque would be 50.46Nm.

Also bear in mind that the formula doesn't include any efficiency calculation (drivetrain losses). To be on the safe side, assume that you lose 20% between the motor and the wheels. It is better to over-specify than to under-specify, and in this case an increase of this magnitude probably won't even effect motor choice given that you will choose a motor with a rating of at least your maximum required torque.

If eff = 0.8, going back to our 3.6:1 gearbox ratio,

Motor torque = M / 0.8
= 151/0.8
= 189Nm

Chris


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## sideburn (Dec 29, 2011)

@Chris

And one more thing (I hope lol)...once you solve for motor torque, which do you apply it to:
initial velocity (0 MPH at 0 RPM)
or
final velocity (60 MPH at let's say 5000 RPM)


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## sideburn (Dec 29, 2011)

Or does 189Nm represents as average Torque between 0MPH and 60MPH (0 to 5000RPM for example)?


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## major (Apr 4, 2008)

sideburn said:


> Or does 189Nm represents as average Torque between 0MPH and 60MPH (0 to 5000RPM for example)?


The way I read this....


iti_uk said:


> ....1/3g acceleration is roughly 3.27 ms^-2, which if assumed constant, means a 0-60mph time of 8.2 seconds which is very adequate for a commuter car. ....


....is that Chris would need a constant torque from 0 to 60mph in a frictionless vacuum to get his assumed constant acceleration.

If you design your system appropriately you can get constant motor torque from 0 to 60mph. But that will not get you constant acceleration due to aero and friction loads on the vehicle which are speed dependent. So he was giving you an approximation to demonstrate the principle.


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## major (Apr 4, 2008)

sideburn said:


> Or does 189Nm represents as average Torque between 0MPH and 60MPH (0 to 5000RPM for example)?


Hi side,

You might be interested in this thread where they are calculating acceleration. 
http://www.diyelectriccar.com/forums/showthread.php?p=280170#post280170 

major


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## sideburn (Dec 29, 2011)

@major

Thanks for those hints!!! Very much appreciated!



> But that will not get you constant acceleration due to aero and friction loads on the vehicle which are speed dependent.


I thought about this for awhile. If you grab someone's data such as 0-60MPH in x seconds doing actual real life test drive then the aero and friction loads are already accounted for into acceleration result, right?


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## iti_uk (Oct 24, 2011)

(Sorry, been absent all today...)

It is correct that the example I gave is only an approximation with no account taken of aerodynamic or mechanical friction forces.

In real life, of course all of the details are taken into account.

It is best to look at a plot of the proposed motor's torque characteristics and go from there. Calculating a closer approximation of the acceleration offered by the motor would involve knowing your drag coefficient, your frontal area, your mass and then doing some calculation involving the motor's torque curve and some integration...

Chris


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## sideburn (Dec 29, 2011)

Hello Chris! No biggie...I work 6 days a week...it is hard to have fun such as spending time learning about elec cars and reading a cool forum like this one! 

Ok, let's plug in real life data for 1996 Honda Civic EX Coupe (just randomly picked this car):

Weight = 2319 lbs ( 1052 kg)

Road test on flat terrain = 0 - 30 MPH 2.7 seconds (Cd * Frontal Area and friction load included because of actual road test) => 4.9671 m/s^2

Cd = 0.32

Frontal Area (ft^2) = 19.9

1st Gear = 3.25:1

Final Drive = 4.25:1

Tire = 185/65-14

With all those data above inserted into formula. I got:
0 RPM = 0 MPH
5850 RPM = 30 MPH as top speed for 1st gear
Torque at motor shaft including efficiency = 140N.m (103.94 ft/lb)

So, when I look at motor specs, which of the following would match closely to Honda Civic's performance stated above (0 - 30 MPH 2.7 seconds):

a) 140N.m @ 0 RPM
b) 140N.m @ 5850 RPM (30 MPH)
c) 140N.m @ 2925 RPM (halfway between 0 and 5850 RPM)

My newbie guess is (c) because that is close to average of "net" torque...don't you agree?


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## iti_uk (Oct 24, 2011)

sideburn said:


> So, when I look at motor specs, which of the following would match closely to Honda Civic's performance stated above (0 - 30 MPH 2.7 seconds):
> 
> a) 140N.m @ 0 RPM
> b) 140N.m @ 5850 RPM (30 MPH)
> ...


Well, yes and no... up to 30, because of the limited effect of aerodynamics in that range, an _average_ torque of 140Nm would be okay. The danger with taking individual values (peak torque/power as usually stated by manufacturers) is that it speaks nothing of the nature of the curve.

Consider option "a". It could provide 140Nm at 0rpm, but for all we know that may drop off quickly after, meaning a good punch off the line but gutlessness once moving. Alternatively, it may also have a flat torque curve (constant torque below a certain RPM).

Option "b" may be similar to "a", in that it could struggle off the line but become more powerful as the revs rise. It might also have dropped from a higher value to 140Nm by the time it reached 5850rpm, resulting in a potential "over-kill" on motor choice. This "over-kill" motor may be excessively heavy, where a smaller, better suited motor may give greater gains for daily driving by virtue of being lighter.

Option "c" is, I guess, the best choice out of the three, since we have chosen a median value rather than one at the extremes of the operating range. Still, a median value only tells a small part of the story.

A curve is far more helpful than a single value. For example, I have attached a plot for the AC50 motor and a plot of the values given for a Warp 9 motor at 72V (for the moment, just consider the shape of the curves rather than the absolute values - these will change due to controller setup). If we happened to choose the peak torque value for the AC50, we would happen to be fairly close to the mark since it seems to have fairly constant torque up to 3000rpm. However, if we did the same thing with the Warp9, the peak only occurs at a minimum RPM value and the drops off steeply. Taking the peak torque value may in this case be misleading.

Hope this helps.

Chris


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## sideburn (Dec 29, 2011)

@Chris:


> Hope this helps


Yes, your last post helps big time 
Thank you!


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## Weenam (Feb 13, 2012)

Thank you for this post. That's all I are able to say. You most absolutely have built this website into something special. 
You clearly know what you are working on, you've insured so many corners.


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