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Locost EV conversion log

10295 Views 86 Replies 11 Participants Last post by  Wayne Alexander
Hi everyone,

I am still in the early planning stages here, but I thought I may as well start a thread to document some of what I'm working on/planning, both for information and feedback.

The car:
-Lotus 7 replica (AKA 'Locost'), built by myself to original book dimensions.
-Currently powered by a 2004 Yamaha R1 motorcycle engine.
-Current weight is about 1200 lbs without driver and half a tank of gas
-Primary use is Autocross, though it is street legal

-Safe and well-built EV conversion
-Torque at low speeds but still possible to cruise on the freeway
-Keep reasonably close to the same weight
-Range isn't a huge concern but 30 miles or so would be nice
-Keep costs under $10k CAD?

Here's a photo from last season:

Tire Wheel Vehicle Car Sky

So the car is obviously a front engine RWD layout (I guess, technically the engine is 'mid' since no part of it is forward of the front axle). The rear axle is a narrowed Ford 7.5" out of a Ranger. From what I can tell, it looks like I can get up to a 4.56 gearset for it. It is currently geared to 3.45, which means the engine is running at 6-7000 rpm on the highway (not good) and still doesn't have as much low end torque as I would like. Also, due to the open intake and limited space for a muffler, I am constantly within a decibel or so of going over the noise limit when I get near redline at WOT. The car will do over 60 MPH in first gear, so I rarely even hit second on the courses we typically run here.

Since I will be left without a transmission after the bike engine is out, and to keep weight down, I am planning to go directly from motor to axle with a two-piece driveshaft. I thought long and hard about a DC motor, but decided against it due to a few reasons, including choice of controllers and availability of suitable motors locally.

So I am looking at either a Leaf motor or a Hyper 9. It would be nice to use a Leaf motor for the lower cost, and bonus if I can get the charger to work with whatever batteries I choose. Right now I can't seem to find much info on using the charger, and even less info on using it with a different battery voltage.

And speaking of batteries, I'm leaning towards second gen Chevy Volt. It looks like I could fit four modules in the car - two under the hood and two in the back where the gas tank is now. The Volt battery comes with three 16s modules and four 12s modules, so I can think of two possibilities:
1. Use two of each module size to get 28s2p (actually 4p since each module is internally 2p) (Good for Hyper 9)
2. Use all three 16s modules plus one 12s in series for 60s (Leaf motor would need at least this much voltage)

Since I designed all the suspension and everything except for the basic layout of the 'book chassis', I already had a CAD model to start with. Here's what it looks like with the Hyper 9 and a Spicer PTO u-joint adapter:
Wheel Tire Vehicle Automotive parking light Motor vehicle

And finally, some of my comparisons. I had to go and wrap my head around how synchronous motors work, so hopefully this is right. I would really appreciate some feedback on this! To get the Leaf numbers, I first took the commonly available graph for the 80kW motor and scaled it up to 110kW at the same RPM. I then scaled the RPM for a given torque value, based on going from 360v to 240v.
Sky Rectangle Astronomical object Slope Parallel

If I did that all correct, it looks like the Leaf motor would give me better torque up to about 40 mph and then the Hyper 9 takes over. The MPH line assumes 4.56 gears. On one hand, I like the added low end power, but on the other hand, power is only useful if you can put it to the ground. Maybe as a next step, I should make another graph that compares wheel torque instead of driveshaft torque, and play around with different combinations of motors and rear end gears.

And that's about where I am at now. Any input is appreciated. My timeline for buying parts is basically most of the summer, then once this season is over I plan to tear the car down and start the conversion as a winter project.

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Hey Mr Excel graphs,

Now do a plot of curves for tire force (assume infinite coeff friction), motor torque, and acceleration (a = F/m) vs vehicle speed. From there a second graph of distance vs speed from a standing start.

Seems geared too tall to me by about 2:1, though it is low mass...or was.

Zero to 100 Kanadian miles /hour in 6 seconds?
Hmm, maybe part of my problem is that I'm not understanding how to estimate power outputs of a DC motor and/or underestimating how much current I can draw from these Volt batteries in short bursts. Looking at the Warp 9 specs they say 100 ft lbs at 500 amps, so I guess I'd be looking for peaks around 1500 amps? And to do that, would I need 3 battery modules in parallel?

I guess the other thing I should look into is how to scale these graphs. A 16s+12s module arrangement would get me like 112v so I think that would be enough, but I don't know how to calculate how much torque I would end up with at a given RPM.

This is the one I'm looking at for reference:
View attachment 128807

Edit: Alright, I think I have my head wrapped around it now. All my reading about AC motors threw me for a bit of a loop but it looks like DC is a lot simpler. It looks like if you want more torque at a given RPM, just increase the voltage proportionately. I'm going to go play with more spreadsheets now..
Um...not really.

Voltage is proportional to speed in a DC and Current is proportional to torque.

Back EMF cuts back max current, which is why you increase voltage.

The square law you're seeing is horsepower, not torque.
Finite Element Analysis to capture the back EMF contribution, if you must calculate.

Easier to bench test.
2000A is a crazy amount of heat being produced (proportional to square of current) in both motor and controller, not to mention the weight of the pack needed to produce that much current.

You also have to be very careful about spec'ing a drag racing car vs a car running sustained levels on a track. Night and day what you can put into a motor continuously for the race period.
No - it says the wiring can support 350A without incinerating the car.
At 3.5V, I'd say it's "close enough", if they were mine - I would not futz with them.
So, buy a Sharpie, number the cells, measure each, put them away.

I don't understand why the risk and complexity of charging is needed at this point when all he plans to do is hoard a Volt battery 😉
While I am thinking of it, DO NOT use a cheap multimeter to measure cell voltages.

I used a Harbor Fright freebie meter (was handy in the toolbox, too lazy to grab the HP out of the house) and got dozens of millivolts of variation among the Model X modules I had harvested.

A couple of days later I walked out of the house with the HP and decided for some reason to remeasure...they were all almost perfectly matched. I have no explanation why the HF meter would vary so much, but it did.
You know you're ready to put it in drive when there's a good strong tea brewed by your precharge resistor 😂 is good for fabbing a small qty of boards. I use them all the time.

Fast, excellent quality, US made.
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Turn your relays 180 degrees for a nicer layout
My hand calcs on your circuit are 15.4W with the cap dead short (t0) and 8W when the cap is fully charged (open DC) assuming a 13V battery/DC-DC.

I'm wondering where the 5W came from, unless it was estimating coil power vs coil plus resistor power?
What, no Flir pic of the economizer board and contactor???? Science demands it...
Two of those boards in your car will cost you 170 feet of range due to resistor losses...
She started a thread here, but we apparently got cucked for the intellectuals over there 🤓
Suggest you route the steering shaft and position the steering rack before committing that box design....
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You can....some roadster builders use these:

They come in various offsets, so search for the right one. Those are just examples
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