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Electric motor into rear subframe of a Miata NA?

6131 Views 15 Replies 4 Participants Last post by  JDubya, 
J
Has anybody fit an electric motor into the rear subframe of a Miata NA? Can it be done? Can it be modified to fit a decent motor? It would be pretty nifty to free up the tranny tunnel to pack some batteries low and central.
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MattsAwesomeStuff
Almost everything I've seen has re-used the transmission.

Of course it can be done, with some constraints. You have space, you need a motor, you put a motor in the space. What/how/where is up to what you want to do.

Starting the same way you start with any build, what were you hoping for in terms of budget, effort, and performance?
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B
It's not just the motor - there's also the reduction gearing and differential. My guess would be that there isn't enough width between the suspension arms of an NA/NB - even with subframe modification - to fit any available drive unit (motor with transaxle). It does work in some larger (wider) vehicles, and with some combinations of specific drive units and simpler suspensions (e.g. Tesla Model S with semi-transparent arms). I have even seen a Model S unit with Corvette C5 double A-arm suspension, but on a wider vehicle.

The closest that I've seen in a Mazda is a motor coupled directly to the input shaft of the stock final drive (diff); that was an RX-8 which is mechanically like a NC, but would also work in an NA/NB if the motor didn't run into the seat space.

You could look at turning the final drive around to face rearward and coupling the motor to it behind the axle.

I don't think there's enough space in a NA tunnel to effectively place battery modules, but a motor in the transmission tunnel (driving the final drive without a transmission or with just a small reduction box such a an ev-Torquebox) would at least leave the entire engine compartment available.
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J
brian_ said:
It's not just the motor - there's also the reduction gearing and differential. My guess would be that there isn't enough width between the suspension arms of an NA/NB - even with subframe modification - to fit any available drive unit (motor with transaxle). It does work in some larger (wider) vehicles, and with some combinations of specific drive units and simpler suspensions (e.g. Tesla Model S with semi-transparent arms). I have even seen a Model S unit with Corvette C5 double A-arm suspension, but on a wider vehicle.

The closest that I've seen in a Mazda is a motor coupled directly to the input shaft of the stock final drive (diff); that was an RX-8 which is mechanically like a NC, but would also work in an NA/NB if the motor didn't run into the seat space.

You could look at turning the final drive around to face rearward and coupling the motor to it behind the axle.

I don't think there's enough space in a NA tunnel to effectively place battery modules, but a motor in the transmission tunnel (driving the final drive without a transmission or with just a small reduction box such a an ev-Torquebox) would at least leave the entire engine compartment available.
Click to expand...
Fitting batteries into the transmission tunnel would be dependent on the form factor of the battery cells/packs, it may not be worth it if the fabrication and customization at the rear subframe is onerous. It may not afford much capacity. Zero EV placed a motor in the transmission tunnel, that appears to be a good solution. They make their own transmission/reduction gear unit (not sure what to call it). Broad brush conceptual what-iffy, it would be nice to remove the weight of the tranny and the diff and replace with purpose built units such as the Chevy Bolt drive unit or the small Tesla unit. But of course all the custom work may not be worth it.

I was also considering an NC, so that is interesting, thanks.
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JDubya said:
I was also considering an NC, so that is interesting, thanks.
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The NC (and ND) have a very different rear suspension from the NA/NB, and the NC is the widest of the generations. If a drive unit will fit within the suspension of any Miata, I would guess it would be the NC. Also, if you put the motor immediately in front of the diff, the NC has more space (it's longer from driver's seat to rear axle) than any other Miata generation.

The NC rear suspension and subframe:
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Duncan
121804


I would be looking at putting the entire Nissan Leaf front subframe into the back of the Miata and using most of Nissan suspension

BUT - I am quite happy about cutting huge pieces out of the chassis and welding in a completely new structure
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Duncan said:
View attachment 121804

I would be looking at putting the entire Nissan Leaf front subframe into the back of the Miata and using most of Nissan suspension

BUT - I am quite happy about cutting huge pieces out of the chassis and welding in a completely new structure
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Sure... but the Leaf's McPherson struts would be a downgrade from the stock suspension in any Miata, and especially an NC or ND.

It would take some structural fabrication to bridge the low Leaf subframe to the high body structure in the Miata, but that should be manageable. The Leaf strut upper mounts would likely be both higher and more widely spaced than the Miata spring and shock mounts, especially with the NA/NB; that should be manageable but may require significant structural work.

As always with subframe swaps, the track dimension is critical.
Rear track dimensions:
  • first-generation Leaf: 1534 mm
  • second-generation Leaf: 1554 mm
  • Miata/MX-5 NA/NB: 1427 mm
  • Miata/MX-5 NC: 1505 mm

I would also check to ensure that the Leaf subframe and drive unit don't extend too far forward for rear-axle location in the Miata, especially in the NA/NB.
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Duncan
Not convinced about the McPhersons being in any way worse than the Miata twin wishbones
the main advantage the twin wishbone setup has is in packaging for the front suspension -
as far as actual performance is concerned its easier to make an excellent suspension using McPhersons - you reduce the "sensitivity" to small variations
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JDubya said:
Broad brush conceptual what-iffy, it would be nice to remove the weight of the tranny and the diff and replace with purpose built units such as the Chevy Bolt drive unit or the small Tesla unit.
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The Bolt drive unit is a nice package, because by using a hollow motor shaft it places the motor right on the axle line, instead of entirely behind (Tesla) or ahead (everyone else) of it. Space ahead of the axle line before running into the step in the floor in a Miata is tight (although better in the NC than the others), even with the fuel tank removed.

The biggest issue with the Bolt is that it doesn't have the aftermarket or hobbyist support of the Tesla and Leaf hardware, so there's no easy control system fix to use the drive unit without the rest of the Bolt electronics.
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JDubya said:
Zero EV placed a motor in the transmission tunnel, that appears to be a good solution. They make their own transmission/reduction gear unit (not sure what to call it).
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But do they? Zero EV has assembled a great selection of components, but I don't know if they make any of them. I don't see a reduction gearbox (transmission) in their online store. Perhaps they just used something like an ev-TorqueBox on a project and didn't mention where they got it.

I found some online articles about Zero EV's conversion of an NB MX-5:
I didn't watch all of the videos embedded in these articles, but none of the articles specifies the source of the gearbox. TheDrive sort of shows it and mentions the 1.8:1 ratio - in both respects this looks like an ev-TorqueBox. The build overview video does mention driving the car enough to validate their gearbox for production; perhaps they did build their own but never went into production. There are apparently at least seven earlier videos - maybe one gives more detail.
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MattsAwesomeStuff
brian_ said:
Rear track dimensions:
  • first-generation Leaf: 1534 mm
  • second-generation Leaf: 1554 mm
  • Miata/MX-5 NA/NB: 1427 mm
  • Miata/MX-5 NC: 1505 mm
Click to expand...
I'm shocked that the Miata-->Leaf is only about an inch difference with the NC. In my head the Miata is so slim they should be way farther apart than that.
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MattsAwesomeStuff
brian_ said:
I didn't watch all of the videos embedded in these articles, but none of the articles specifies the source of the gearbox.
Click to expand...
I presume it's the same one he was documenting on the OI board last year for a while. But it's mostly video he posted:

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MattsAwesomeStuff said:
I'm shocked that the Miata-->Leaf is only about an inch difference with the NC. In my head the Miata is so slim they should be way farther apart than that.
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I agree, but I'll note that the NC is wide for a Miata and the Leaf might be narrow for its class since drag is a big deal for energy consumption.
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MattsAwesomeStuff said:
I presume it's the same one he was documenting on the OI board last year for a while. But it's mostly video he posted:

Click to expand...
Thanks. Yes, all video, and in the three videos that I skimmed through without audio, the reduction gearbox is only an object (with a mass of 16 kg implied by mentions of the motor mass with and without gearbox) in the 3D model.
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J
brian_ said:
Sure... but the Leaf's McPherson struts would be a downgrade from the stock suspension in any Miata, and especially an NC or ND.

It would take some structural fabrication to bridge the low Leaf subframe to the high body structure in the Miata, but that should be manageable. The Leaf strut upper mounts would likely be both higher and more widely spaced than the Miata spring and shock mounts, especially with the NA/NB; that should be manageable but may require significant structural work.

As always with subframe swaps, the track dimension is critical.
Rear track dimensions:
  • first-generation Leaf: 1534 mm
  • second-generation Leaf: 1554 mm
  • Miata/MX-5 NA/NB: 1427 mm
  • Miata/MX-5 NC: 1505 mm

I would also check to ensure that the Leaf subframe and drive unit don't extend too far forward for rear-axle location in the Miata, especially in the NA/NB.
Click to expand...
Chevy Spark has a track of 1410 mm; 140hp and 327#ft of torque
Chevy Bolt track 1501mm, 200hp and 266#ft torque

www.ebay.com

2014-2016 Chevrolet Spark EV Electric Motor w/ Transmission Chevy Transaxle | eBay

Used electric motor and transmission (transaxle) from a 2014 Chevrolet Spark EV.
www.ebay.com
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J
JDubya said:
Chevy Spark has a track of 1410 mm; 140hp and 327#ft of torque
Chevy Bolt track 1501mm, 200hp and 266#ft torque

www.ebay.com

2014-2016 Chevrolet Spark EV Electric Motor w/ Transmission Chevy Transaxle | eBay

Used electric motor and transmission (transaxle) from a 2014 Chevrolet Spark EV.
www.ebay.com
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