# Miata NA EV direct drive rear diff



## EVPolpy (Feb 17, 2015)

Hello,

I am toying with different conversion options at the moment. I'd like to convert a Miata NA and I would like it to be zippy.

I want to run the motor directly into the rear differential. The diff ratio is 4.3.

Required top speed is 120 km/h = 33.3 m/s (= 75 mph).

some (easy) math:

Considering 185/60 R14H (=stock) tires:


radius(tire) = 289 mm
circumference(tire) = 1815 mm = 1.815 m
N(wheels) = speed[m/s] / circumference [m] 18.34 /s = 1100 /min

So, considering a rear diff ratio of 4.3 I need to rev the motor at:

1100/min * 4.3 = 4730 /min, let's say 4800 /min.

Weight goal for the car:

1200 kg (about 2600 lbs)

Oh, and I'd like the car to feel torquey. Better then a stock miata for sure is the aim.

Is there a electric motor that fits my criteria? I am sure there is... Somewhere  Just a matter of finding it.


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## Tomdb (Jan 28, 2013)

How about a HPEVS system?

something like a AC51 http://hpevs.com/catalog-ac-50.htm

or the dual ac30's http://hpevs.com/catalog-ac-35x2.htm

Have you looked at the miata conversions done before? http://evalbum.com/type/MAZD

Where are you based?


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## Duncan (Dec 8, 2008)

AC systems are OK but expensive
How about a nice 11 inch forklift motor with a 1000 amp controller like the Soliton


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## tylerwatts (Feb 9, 2012)

Hi EVPolpy
Check out the Latvian Mazda rx8 project thread for a perfect example of doing direct drive on your Miata. You will need a front motor mount like there but basically should end up with 3 mounts, the original diff mounts and a front motor mount. And the miata sub frame needs to be properly braced as it will flex otherwise. 

Check out Jozzer's build also, very smart car. The auzies have done some great direct drives also using industrial AC motors but I would recommend speaking to Ruckus about a Scott drive setup! Might struggle fitting it in the tunnel in front of the diff but you could modify the output of the stock transmission so you bolt on only the output shaft end and slot the stock drive shaft in. You can then also bolt the torsion frame (forgot Mazda's name) to that and make a 2 point mount for the motor up front. Then the battery pack can live in the engine bay and original fuel tank areas. Good weight distribution then also. 

I look forward to your progress.


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## dougingraham (Jul 26, 2011)

With a Warp9 and 1000 amps on your stock tires with a 4.3 differential ratio I compute your 75mph will be reached at 4767 rpm. The torque at the face of the tire will be fairly constant up to an rpm of nearly 4000 (63mph) with 170 volts applied to the motor. The torque at the face of the tire will be 1207 lbs minus the losses in the differential of perhaps as little as 5%. This would be 1147 lbs or an acceleration of 0.44 g if the car weighs 2600 lbs. Your peak input power to the motor would be 228 HP of which you might get 70% to the tires.

It won't feel quite as quick as a Stock Miata off the line but at the time the Miata reaches 3rd gear you are ahead torque wise in the conversion. It is a very subjective thing and until you have driven a car with no shifting you cant really get a feel for it. If you keep the gearbox you will have neck snapping torque that makes expletives come out of your passengers mouths. Without the gearbox you have a freaky quiet and smooth acceleration. With just you in the car you could expect around a 7 to 8 sec 0-60 time. Not heroic but not bad either.

If you want more I would switch to a Warp11HV and put in a battery that maxes out at the Soliton Limit of 340 volts. I think the WarP11HV torque is about 20% more than the WarP9 so it will feel faster off the line. Your peak input power to the motor will be 386 HP with this setup of which you might get 70% to the tires.

You wont get anything close to this with an AC setup for the money. You would need an AC-35x2 setup for around twice the money to get close.

If you want even more you could go to a Zilla 2k and run up to 2000 amps on either of these motors. This would more than double the torque which means you might be able to spin the stock tires at will with the Warp9 and almost certainly with the WarP 11HV. I have no idea how this will affect the life of the brushes. I have 7100 miles on my WarP9 at this point and I punch it almost every time I drive hitting 1000 amps. I will probably inspect the brushes at 10k miles.

Good luck on your build!


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## tylerwatts (Feb 9, 2012)

Doug, EVPolpy
If you look at the Latvian rx8, it has the same torque from it's PMAC motor and similar performance with alot less current and better efficiency. And I do believe that kit is easily competitive on price with a Warp. Depends if you are willing to buy from China but my opinion is the quality is getting good and worth the gamble.


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## Duncan (Dec 8, 2008)

tylerwatts said:


> Doug, EVPolpy
> If you look at the Latvian rx8, it has the same torque from it's PMAC motor and similar performance with alot less current and better efficiency. And I do believe that kit is easily competitive on price with a Warp. Depends if you are willing to buy from China but my opinion is the quality is getting good and worth the gamble.


Is it just me?
The idea that some "new" technology will get "_similar performance with alot less current and better efficiency_" activates my "scam detector"

With an old school DC motor achieving efficiencies in the high 80's there isn't a lot of room for a "great leap forwards"

If it takes X volts and Y amps to produce a certain torque at a certain rpm 
then I am suspicious of a claim to produce 
_similar performance with alot less current_


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## tylerwatts (Feb 9, 2012)

Detect away. The difference is that because the DC motor chucks so much current it makes torque to 4000rpm. The PMAC won't hold it for as long, but runs a higher voltage so field current is similar and the PMAC is only powering the stator to generate the torque. The net power in/output of the PMAC system won't be the same so you lose out at top end but the torque will give a similar seat of the pants performance so is in my opinion comparable and worthy of consideration. 

I have no desire or agenda to scam anybody... And the decision lies with the builder. So only treat my posts as my views, not the EV gospel...

PS, nothing new about PMAC being more power dense and higher torque than DC or AC drives.


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## Duncan (Dec 8, 2008)

tylerwatts said:


> Detect away. The difference is that because the DC motor chucks so much current it makes torque to 4000rpm. The PMAC won't hold it for as long, but runs a higher voltage so field current is similar and the PMAC is only powering the stator to generate the torque. The net power in/output of the PMAC system won't be the same so you lose out at top end but the torque will give a similar seat of the pants performance so is in my opinion comparable and worthy of consideration.
> 
> I have no desire or agenda to scam anybody... And the decision lies with the builder. So only treat my posts as my views, not the EV gospel...
> 
> PS, nothing new about PMAC being more power dense and higher torque than DC or AC drives.


Ok so I'm only an engineer but I just don't understand you here

_The difference is that because the DC motor chucks so much current it makes torque to 4000rpm

The net power in/output of the PMAC system won't be the same so you lose out at top end but the torque will give a similar seat of the pants performance so is in my opinion comparable and worthy of consideration. _

Can you explain that again please


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## tylerwatts (Feb 9, 2012)

Duncan 
I'm only an engineer myself, but my opinion is that because the PMAC delivers the same torque to 2000 odd rpm, you will experience similar drive to that speed. Then as the nature of AC drives, you will achieve a constant power onwards and so the differences in acceleration will be due to resistance. The DC will keep making torque therefore power will increase and acceleration will be less impeded by resistance and the DC will give greater acceleration at higher speeds. 

I'm not saying one is better. My opinion is that the PMAC gives good performance, comparable at lower speeds, but offers greater efficiency (10% or so) and better range and will be cost effective as it is a Chinese product. Also, for a net range less batteries are required, so less weight IE improving performance, and less cost. And remember a miata has less space for batteries without structural modifications so these all add up to a well compromised solution for direct drive in a miata, IN MY OPINION. 

That's all, just my opinion. I don't know what major travesty I have befallen you, but I apologise. 

And apologies to EVPolpy and others reading for taking the thread off track.


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## tylerwatts (Feb 9, 2012)

Have a look at the BMW 330Ci conversion also, great idea there for you EVPolpy.


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