# High level feasibility



## sergiu tofanel (Jan 13, 2014)

Most people use the transmission because it's there (especially for FWD cars). In theory, the motor you listed has plenty of torque to move a small car without a transmission. However, I don't think that the mechanical headaches created by the removal of the transmission are worth it mainly because it's just so easy to slap an adapter plate and be done with it.


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## Jeffe101 (Feb 28, 2014)

That's good to hear, I was hoping that there wasn't going to be some weird gearing thing. I figured that it would be more complicated to mount, but I like the fact that it will be cleaner and no weight of the transmission and drive shaft. It has to be a bit more efficient without all that drivetrain as well I would think.


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## RIPPERTON (Jan 26, 2010)

Take the no gearbox concept to the fullest of its theory before you rule it out.
First find out what the standard diff ratio is and go from there.
You might need 4 or 5 to 1.
That AC-51 is relatively small diameter so might fit in the tunnel.


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## Jeffe101 (Feb 28, 2014)

I believe the differential is 4.3 to 1. I think once the drive shaft, exhaust system, gas tank etc. are removed, there may be enough room. If I have to chop and cut, so be it.


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## dougingraham (Jul 26, 2011)

The usual reasons to keep the transmission is insufficient torque and bad match of the motor torque curve to the final drive. I made up a spreadsheet a couple of years ago that calculates this for you. Make a copy of it and then punch in the numbers for your car. Without the transmission the numbers for the direct drive gear will be correct. Usually this would be 4th gear on a 5 speed transmission.

Link to my vehicle speed calculation.

On my RX-7 which is a little lighter than your car with a WarP9 which has quite a lot more torque than the AC-51 I would not be happy only having the direct drive into the transmission. My transmission only weighs 72 lbs and if you add in the flywheel I used with clutch and pressure plate it comes out to be about 120 lbs that you would save. I think you would need a final drive ratio closer to 6 to make this work out.

Hope this helps you out.


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## Duncan (Dec 8, 2008)

My car is like that
http://www.diyelectriccar.com/forum...-dubious-device-44370p2.html?highlight=duncan

Its a bit lighter than a Miata - 
and I could do with a bit more torque (need a more powerful controller)

I don't think you are going to be able to do that with an AC motor - unless you are rich

DC may be old tech but its cheap and powerful
AC is lovely and the way of the future
But here and now its either expensive or wimpy or both

Something like my 11 inch forklift motor, a Soliton Controller and a 144+v pack would do very nicely in your Miata

The only issue would be a limited top speed - about 150Kph

A 9 inch motor would need more current (the Soliton should still be OK) but would have a higher maximum speed (200Kph??) (That would need more volts - 250v??)


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## Jeffe101 (Feb 28, 2014)

@dougingraham - Great excel graph! I put in my numbers as best as I could, I attached a pic of the results. On the motor rpm and torque part, I was just looking at the power graphs at HPEVS to get the numbers. Was I right in putting the torque and RPM that is at the HP peak?


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## Ivansgarage (Sep 3, 2011)

Hi Jeff here is a interesting thread about doing direct drive.

http://ivanbennett.com/forum/index.php?topic=6.0


_Ivan_


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## RIPPERTON (Jan 26, 2010)

Single speed is ok for a commuter.
My Mira is currently 136v and 5.5:1 diff ratio, 6200rpm 105kmh no gearbox but would be better on 175v and 6.5:1 135kmh, 8000rpm. still single speed.

Seeing as your Miata is a "sports car" you might want more speed than that so your looking at a 2 speed plus its very hard to find a diff with a ratio of 6:1 that can also handle huge torque. The bigger the diff the taller the ratio.


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## dougingraham (Jul 26, 2011)

Jeffe101 said:


> @dougingraham - Great excel graph! I put in my numbers as best as I could, I attached a pic of the results. On the motor rpm and torque part, I was just looking at the power graphs at HPEVS to get the numbers. Was I right in putting the torque and RPM that is at the HP peak?


Thanks!

The torque and RPM at HP peak is what I have done. I think that motor will go up to 8 krpm but as you see when you look at the dyno graphs the torque drops off in a fairly linear fashion up to the 8k limit. But it doesn't just quit when you get to 6000. So if you want to do a single speed (no transmission) setup your top speed is excessive (read that as unrealistic) and your torque is going to be a little low so sluggish acceleration. Look to see if there are other rear end ratios you could use. Or keep the transmission and just use 3rd gear most of the time. I did this spreadsheet so I could easily play the what if game.


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## Jeffe101 (Feb 28, 2014)

So when you say sluggish, what do you think 0-60, 12secs, 15secs, Prius?  At this point I don't really care whether it's a rocket ship, I just figure I'll tweak it over time, find new gears, different controller etc. I'm also going to be trying to make it as light as I can, new lighter wheels (less rotational mass), remove most of the stereo system I have put in over the years LOL! and probably get lighter seats as well.


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## Duncan (Dec 8, 2008)

Jeffe101 said:


> So when you say sluggish, what do you think 0-60, 12secs, 15secs, Prius?  At this point I don't really care whether it's a rocket ship, I just figure I'll tweak it over time, find new gears, different controller etc. I'm also going to be trying to make it as light as I can, new lighter wheels (less rotational mass), remove most of the stereo system I have put in over the years LOL! and probably get lighter seats as well.


I intend going to the max (with road tires)
So with a 50/50 weight distribution and a low center of mass (low weight shift)
and an 800Kg car
400Kg on rear wheels
80% (road tires) = 320Kg force
Tires are almost exactly 1ft radius - 320kg = 704lbs
So I need 704 Ftlbs
My diff is 4.1:1 = 704/4.1 = 171 Ftlbs

With my motor that would be about 700amps - 
I only have 500 amps just now

A 1000 amp controller (Soliton) would be able to give 50% MORE torque than that

Not that the extra torque would actually be any use

Saying that the "Device" is not slow now it feels quite sprightly - on the track it was a little bit slower than a 230SLK

I have an 11inch motor because it was available for $100 - if they had had a 9 inch I would have gone for that

A 9 inch would need ~ 25% more current for the same torque - but it would be safe for a higher Rpm

My 11 inch is OK at 4000rpm (108Kph) - probably OK at 5000rpm (145Kph)
A 9 inch should be OK to 7000rpm (190Kph)

A 9 inch would be 40Kg lighter as well!

You will need a short prop shaft from the motor shaft to the diff - mine is about 300mm 
I was able to use a drive spider that came with the motor with a simple disc adapter to couple to the UJ on the prop shaft


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## dougingraham (Jul 26, 2011)

Jeffe101 said:


> So when you say sluggish, what do you think 0-60, 12secs, 15secs, Prius?  At this point I don't really care whether it's a rocket ship, I just figure I'll tweak it over time, find new gears, different controller etc. I'm also going to be trying to make it as light as I can, new lighter wheels (less rotational mass), remove most of the stereo system I have put in over the years LOL! and probably get lighter seats as well.


I don't have a good feel for this but from the numbers about 1/5 g acceleration up to 80 mph in your car. My 4th gear performance is about 0.45 g up to 70mph and it probably takes 15 seconds to get there. I really need to time some of these things. While my 2nd gear performance takes your breath away because it is just under 1 g. If I push the pedal too hard the tires will break loose briefly. 1st gear isn't much quicker than 2nd because it just spins the tires unless you are careful. In my car 3rd gear typically runs away from most traffic but you don't win street races in 3rd. When I am driving in traffic I just leave it in 3rd. Third gear is about 0.6 g.

My numbers are quite a lot higher than your numbers will be. I am running 1000 amps and my torque band is quite a lot narrower than yours. You should probably play with a lot more numbers from more of the HPEVS motors and also see if there are any other ring and pinion sets available for the Miata diff. Also run the high voltage and the high current Curtis controller numbers. I am thinking that to feel good you probably want at least 0.5 g. One of the HPEVS setups might get you close. I am sure people have opinions about this.

Good luck!


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## Jeffe101 (Feb 28, 2014)

dougingraham said:


> I don't have a good feel for this but from the numbers about 1/5 g acceleration up to 80 mph in your car. My 4th gear performance is about 0.45 g up to 70mph and it probably takes 15 seconds to get there. I really need to time some of these things. While my 2nd gear performance takes your breath away because it is just under 1 g. If I push the pedal too hard the tires will break loose briefly. 1st gear isn't much quicker than 2nd because it just spins the tires unless you are careful. In my car 3rd gear typically runs away from most traffic but you don't win street races in 3rd. When I am driving in traffic I just leave it in 3rd. Third gear is about 0.6 g.
> 
> My numbers are quite a lot higher than your numbers will be. I am running 1000 amps and my torque band is quite a lot narrower than yours. You should probably play with a lot more numbers from more of the HPEVS motors and also see if there are any other ring and pinion sets available for the Miata diff. Also run the high voltage and the high current Curtis controller numbers. I am thinking that to feel good you probably want at least 0.5 g. One of the HPEVS setups might get you close. I am sure people have opinions about this.
> 
> Good luck!


Thank you, that's good info. I have my doubts about finding any different gear sets for the Miata, but I will look. You think I should look for something around 5 to 1 or higher?


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## Jeffe101 (Feb 28, 2014)

Duncan said:


> You will need a short prop shaft from the motor shaft to the diff - mine is about 300mm
> I was able to use a drive spider that came with the motor with a simple disc adapter to couple to the UJ on the prop shaft


I wanted to ask where I might find something like this, is it custom from a shop? All I've seen online is a keyed one for a Warp motor. Also, the AC50/51 motors have like 6 different shaft options and the write ups aren't great on them. Some have splines, some are keyed, which one would be best for a direct hook up? 
Thanks!
Jeff


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## dougingraham (Jul 26, 2011)

Jeffe101 said:


> Thank you, that's good info. I have my doubts about finding any different gear sets for the Miata, but I will look. You think I should look for something around 5 to 1 or higher?


Yes. I found what looks like a 4.44:1 ratio available for $680. I think that would fit. They talk about some model using the same ring and pinion set as the 82-85 RX-7 and I know there was a 5.25:1 for that but its cost is prohibitive.

If you really want to do this pick a top speed you want for the car and figure out what you need to change such that the motor is turning at redline (8000 rpm with the AC-51) at that speed. For example 80 mph. You get that with your current ratio at 5000 RPM. It looks like a 6.91 ratio would give you 80 mph at 8000 RPM. The redline of the motor would be the fastest you could possibly go. I doubt you would ever get to 80 but probably make somewhere between 70 and 75. This would give you a torque at the wheel of 718 ft-lb.

An AC-35 x2 would give you 1221 ft-lb on the 144v controlllers with the 6.91 final drive which is close to the 1/2 G level I think you need to feel comfortable.


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## Plymouth60 (Sep 4, 2012)

I have a 1993 Miata with a 650Amp AC50. This has 150Nm of torque vs the AC51 with 135Nm. I ususally leave it in 3rd gear around the city, but if I need to get away from an intersection quickly, I need to drop it into 2nd gear. 4th gear is 1:1 through the gearbox, and I can drive it this way but it is too slow to be acceptable. 
It might be OK to direct drive with an AC34 dual at 275Nm torque.
There is an after market diff with a higher ratio, I think it was 4.95:1. This would improve performance.
Cheers
John


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## dougingraham (Jul 26, 2011)

Plymouth60 said:


> I have a 1993 Miata with a 650Amp AC50. This has 150Nm of torque vs the AC51 with 135Nm.


This didn't seem correct so I looked at the HPEVS dyno charts again. The AC-51 has 25% more torque than the AC50 at the same current but the Torque band is 25% smaller. This makes sense since the only difference is in the way the motors are wound.


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## Jeffe101 (Feb 28, 2014)

dougingraham said:


> An AC-35 x2 would give you 1221 ft-lb on the 144v controlllers with the 6.91 final drive which is close to the 1/2 G level I think you need to feel comfortable.


Have you seen what they want for an AC-35x2? A bit too rich for me, I think I could buy 2 AC-51's with the controller and still be under what one of those cost !
Part of what I was thinking when wanting to do it this way, was that it has to be more efficient without the motor having to turn the transmission and drive shaft and that I could gain a bit more torque back without them. Does that make any sense?


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## Jeffe101 (Feb 28, 2014)

Hey on that thought, maybe I _should_ get 2 AC-51's and put them in the rear back to back, one to each wheel!  The controller says it will do 2 motors.


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## mizlplix (May 1, 2011)

YMMV: My 1,900 Lb car when using an AC50, had 6.14 gears and 32" tall tires. It had a ridiculous 118 MPH at the motor's advertised 8,ooo RPMs.

It needs a lower drive gear. 6.14 is the lowest I can find. So, I plan to use a common Saginaw 3 speed in second gear at 1.58 giving a 7.43 ratio and a more usable 78 mph. BTW, I almost never get over 7,000 RPMs where the real torque drops off.

If picky, one could remove gears and replace with spacers, lowering the drag.

It will then drive in one gear without shifting at all.-no linkage or holes in the floor.

if one wanted to shift, get q T5 with a dog ring kit installed. easy cutchless shifting even under power.

Mix


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## dougingraham (Jul 26, 2011)

Jeffe101 said:


> Have you seen what they want for an AC-35x2? A bit too rich for me, I think I could buy 2 AC-51's with the controller and still be under what one of those cost !


Yes, I have. You need two controllers to make it work. One is configured as a master and the other is a slave. The cost of the AC-35x2 and two controllers is a little less than two AC-35 motors and two controllers.



Jeffe101 said:


> Part of what I was thinking when wanting to do it this way, was that it has to be more efficient without the motor having to turn the transmission and drive shaft and that I could gain a bit more torque back without them. Does that make any sense?


Wanting it makes sense. You might gain a percent of efficiency from not having the transmission in the way. Heck, I just figured out that I would gain at about 2 miles of range if I got rid of 100 lbs which is about what I would save getting rid of the transmission, flywheel, pressure plate and clutch. This should be considered a gain in efficiency. But without greater torque from the motor and a much higher rear end ratio it is not practical to go without a transmission. One has only to look at the OEM offerings and see that they all do it this way. Only they have motors that operate at above 300 volts and have rpm limits of 10-12k rpm with gobs of torque and around 8:1 final drive ratios.

So to do this is possible. The OEM's do it. What is not practical is using a single HPEVS motor with the stock final drive. Can you make it work? Well, yes the car will move and eventually even get up to highway speeds. But it isn't going to be fun to drive and probably not really safe.

If you were local to me I would turn down my motor controller to give the torque of the motor you were looking for adjusted for tire size and final drive ratio and let you drive it to see if you could live with it. I played this game once to see if I would have liked a Soliton Jr instead of the Soliton 1 and I eventually came to the conclusion that it would have been ok but once having driven the higher power I would not have been happy.


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## Duncan (Dec 8, 2008)

Jeffe101 said:


> I wanted to ask where I might find something like this, is it custom from a shop? All I've seen online is a keyed one for a Warp motor. Also, the AC50/51 motors have like 6 different shaft options and the write ups aren't great on them. Some have splines, some are keyed, which one would be best for a direct hook up?
> Thanks!
> Jeff


My motor had a splined drive shaft - both com end and drive end with the same splines on both ends

The motor had a brake on the comm end - emergency brake or handbrake for the forklift

So I simply took the brake apart and I found a "spider" that drove the brake drum 
It had four studs - the prop shaft also had four bolts
They were not the same PCD - or it would have been totally trivial 
but a simple adapter worked 

From an engineering perspective I have never seen anything being driven through a keyway
Normally you either have a male/female spline - where there is some movement
Or you clamp the two together and drive through friction - with (of course) no movement
Keyways are normally a backup to friction drive - and if you drive through the key it fails in a short time

Saying that a lot of people in the EV world seem to use keys/keyways
I wouldn't - but that's just me

Reading the comments about AC motors - it would appear that you can't buy a diff that will suit their rpm range - another reason to go DC

You also asked about using two motors - one for each rear wheel
The issue is that torque is roughly proportional to motor weight
if you don't use the gearing in the diff you would need two motors EACH twice the weight of the single motor
Or else you gear each motor in which case - twice the weight of gears


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## Jeffe101 (Feb 28, 2014)

Duncan said:


> You also asked about using two motors - one for each rear wheel. The issue is that torque is roughly proportional to motor weight
> if you don't use the gearing in the diff you would need two motors EACH twice the weight of the single motor
> Or else you gear each motor in which case - twice the weight of gears


I was sort of joking about this, I think Tesla uses a twin motor setup, but I know it's all custom motors, gearing etc. 
You know, I played around with high performance RC cars for years back in the 90's and adapting the tech to a car seems way more difficult than it should be.  
I appreciate all the feedback.....it's not exactly the rainbows and unicorns that I was looking for, but I'd rather explore everything now before I spend a bunch of money on the wrong equipment.


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## Jeffe101 (Feb 28, 2014)

dougingraham said:


> An AC-35 x2 would give you 1221 ft-lb on the 144v controlllers with the 6.91 final drive which is close to the 1/2 G level I think you need to feel comfortable.


So to what you said here, if I were to actually explore this option, it seems that I would still be stuck trying to get that final drive ratio? All I could find was mention of a 5.125:1 ring/pinion and no mention that it was actually for sale anywhere. 
I found this here, and the price is not as bad as I originally thought, still bad, but given the fact that it does come with the controllers.


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## Plymouth60 (Sep 4, 2012)

A MX5 / Miata was converted by Zeva in Australia some time ago with direct drive. Here is a link to their build.
http://zeva.com.au/Projects/MX5/


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## dougingraham (Jul 26, 2011)

Jeffe101 said:


> So to what you said here, if I were to actually explore this option, it seems that I would still be stuck trying to get that final drive ratio? All I could find was mention of a 5.125:1 ring/pinion and no mention that it was actually for sale anywhere.
> I found this here, and the price is not as bad as I originally thought, still bad, but given the fact that it does come with the controllers.


If your model year is compatible with FB RX-7's which it might be then there was a 5.25:1 ratio but it was something like $2500 so no way would it have been worth it. Madzatrix used to list it on their web site.

Yes, the AC-35x2 would still need a different final drive to go direct drive and I suspect it would still feel slow. The drag racers were doing this but they had dual series wound motors and were probably running with around 500+ ft-lbs of torque and upper 3's final drive ratio. Double the final drive ratio and cut the torque in half and you still have decent performance with no transmission.

Keep doing your research and figure out what you want it to do. The reason so many of us kept the transmission is that it eliminates so many of the issues you are facing right now.


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## Jeffe101 (Feb 28, 2014)

dougingraham said:


> Keep doing your research and figure out what you want it to do. The reason so many of us kept the transmission is that it eliminates so many of the issues you are facing right now.


Yes, leave it to me to make it as complicated as possible. So if I did go DC direct drive, can you spin those backwards for reverse? 
And I guess I could live without regenerative braking.


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## dougingraham (Jul 26, 2011)

Jeffe101 said:


> Yes, leave it to me to make it as complicated as possible. So if I did go DC direct drive, can you spin those backwards for reverse?
> And I guess I could live without regenerative braking.


A series DC can be run backwards. It takes three contactors to reverse the rotation direction. You dont want to run high power levels in reverse because the brushes are set for forward operation and are wrong for reverse. Brush and comm damage is possible. So don't back up hills in direct drive.

There is always something!


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## Jeffe101 (Feb 28, 2014)

So I guess the 2 options for me are: a direct drive DC motor (maybe a Warp 11 after looking at that other Miata build) or keep the transmission and do an AC system (to keep the gearing and get regen.). If I do the AC, do I need to keep the clutch? I thought I read somewhere that you can connect without the clutch, but can you still shift OK if you need to?


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## dougingraham (Jul 26, 2011)

Jeffe101 said:


> If I do the AC, do I need to keep the clutch? I thought I read somewhere that you can connect without the clutch, but can you still shift OK if you need to?


You don't need to keep the clutch. You can shift without it and I suggest you practice on the car before you start the conversion to see if you like it. It is a little different with an EV because the motor tends to spin down slowly unlike an ice which coasts down somewhat faster allowing you to find the shaft speed match fairly quickly. One caveat is you can't do coast down regen without a clutch because when you put the gear selector in neutral the motor will just stop turning and the input and output shafts will never sync up. Regen would have to be activated with the brake. Shifts take longer without a clutch but it can be done.

I am somewhat of a driving enthusiast and from that point of view I would suggest you keep the clutch. You probably wont use it all that much but it makes shifts a lot faster and you can let pretty much anyone drive your car that can drive a manual if you keep it. Shifting without it takes a little practice.


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## Jeffe101 (Feb 28, 2014)

Well now I'm torn, I thought I had this all planned out, now I don't know what to do.


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## dougingraham (Jul 26, 2011)

It might help to really define your goals for the project. Who will be driving it and where? If the primary plan is to use it to commute to work and run errands then this is a very different thing than a weekend toy. I built a bit of both but the primary goal was to make a car that I enjoy driving to work and would not get tired of in just a couple of years. The fact that it ended up being a joy to drive is a huge plus.

I have a bunch of hobbies and when people say something like "Doesn't that cost a lot?" I reply, all hobbies cost the same. They cost whatever you are willing to spend on them. DIY EVs are no different. If you want an EV to save money then it is hard to beat buying a Leaf. I could buy a Leaf for less than I spent on parts not even considering the time I spent doing the conversion or the cost of the donor car. But if you don't happen to like any of the less expensive OEM offerings and you still want an EV then you get to DIY.


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## Jeffe101 (Feb 28, 2014)

dougingraham said:


> It might help to really define your goals for the project. Who will be driving it and where? If the primary plan is to use it to commute to work and run errands then this is a very different thing than a weekend toy. I built a bit of both but the primary goal was to make a car that I enjoy driving to work and would not get tired of in just a couple of years. The fact that it ended up being a joy to drive is a huge plus.
> 
> I have a bunch of hobbies and when people say something like "Doesn't that cost a lot?" I reply, all hobbies cost the same. They cost whatever you are willing to spend on them. DIY EVs are no different. If you want an EV to save money then it is hard to beat buying a Leaf. I could buy a Leaf for less than I spent on parts not even considering the time I spent doing the conversion or the cost of the donor car. But if you don't happen to like any of the less expensive OEM offerings and you still want an EV then you get to DIY.


I hear you, I had originally planned, because of the small nature of the car, limited trunk space etc. that it would end up just being a commuter and small errand car. I live very close to work, 5 minutes. But after thinking about how I used to drive the car (hard), I began to feel that I would miss that if I didn't do something with some power, at least equivalent.


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