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Discussion Starter · #1 · (Edited)
Old sportscars cars can be bought cheap if they have drivetrain problems, and sometimes you can buy an excellent car and sell its drivetrain for the car's purchase price. With careful buying, this means a complete working sportscar such as a 1980's Corvette or Nissan 350Z/370Z without a working drive train can be sourced as cheaply as a Civic, at maybe $2K including parts to set the rest of the car to functional. Replacement parts cost should not be an issue; because it won't drive many miles nor very fast.

Build it to perform safely and predictably on an autocross course with as much performance as the average good driver can handle; and no more. It doesn't need to win a class. Autocross is about having fun and bettering one's driving skills. Forget new car specifications, on an autocross track, average drivers will never use better than 0-60 in five seconds. This specification could be a great second car for a lot of people.

How cheap can a sportscar be electrified with the following specifications?
  • quick acceleration only needed from 20mph to 60mph, bettering stock performance in only this event
  • a charge only needs to last for three runs of about two minutes plus waiting in line, so call it ten minutes and ten miles total low speed driving
  • standard brakes, no regeneration
  • one motor driving a limited slip rear differential for predictable behavior
  • EDIT: 0-60 in 10 seconds would be fine, because it doesn't need to launch.
    Must be quicker than stock at speeds found in the race - 20-50mph.
  • EDIT - a battery trailer

Would this approach work, and what design would you use?

I'm not asking the best approach for a sportscar, but the cheapest way to build an electric car for the autocross event only. Autocross is the cheapest and safest way to get involved in competitive driving as a hobby, with car insurance available through the racing organizations. You can drive anything. So why not drive a modern full sized go-kart?
 

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I built a purpose designed autocross car. It was great. Torque off the line is what you are looking for and it can be easily achieved with an electric conversion. You'll be wanting more than 1000amps of current and somewhere around 300v. I have 172v and 1000amps in a 500kg car (620kg with driver). It was awesome off the line and would hit 45 in great time. But acceleration drops off quickly as revs rise. Using higher gearing didn't help because it would require even more torque to overcome the gearing. I believe the answer is higher voltage to overcome the back EMF and extend the acceleration higher into the rev range. For your purpose I recommend a DC driveline.
 

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I just realised I didn't address the key question of cost. The key here is weight and expectations. If you go with a big heavy car (370z) your cost will increase. If you are targeting a super competitive car your costs will increase. If you just want to have fun in a light sports car then you can reduce your budget. Actual cost depends Your willingness to search and wait for the components at the right price. Here (Australia) we don't have too many choices and prices are more expensive so there isn't much point me quoting prices. If you are wanting to be super competitive just keep in mind how competitive a Model 3 already is in Autocross. Are you really going to be able to build something better? If so it is probably going to be a similar cost.
 

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Discussion Starter · #4 ·
Thank you for replying.
I love your car. It sounds like a blast to drive. Do your autocross events start off the line? That's almost a different event, and hopefully my specs are quite a bit less demanding on the build. The best racing opportunity for me is SCCA SOLO II autocross. Any car will do. They encourage daily drivers and penalize stripped cars. A better car will not help a beginner, but I'm not into competing. That's the cheapest way to play other than tearing up public roads, which is part of why they started these events. My interest in using a luxury sportscar body for a cheap build is a bit of a lark. A running joke of sorts. At races and EV shows, people should look under the hood and get a laugh. The build should look appropriate to a tiny economy car. This car should win the frugal build awards, not its racing bracket. My goal is to use minimum specifications to be just faster than stock in a given car for one type of event. Wouldn't it be fun to spank the modified Corvettes with a cheap electric build? Why not use one of their over-glorified bodies to make a green car that cannot exceed 80mph? That's my thinking.

SOLO II starts with a rolling start for at least 20' that triggers the timer. This eliminates peeling out, and eliminates value of torque off the line. Peak acceleration is only needed from 20-50mph, with the ability to achieve 80mph so it can drive to the track. It need only barely get off the line, so gearing should make this range easy. Even within that narrow range, it only compete with modified cars of the same model. It must be RWD with a limited slip differential - only because that's the most fun to drive. Range for racing would be ~10 miles and 10 minutes. A 50 mile range would be enough to use it for most local driving, and that can be achieved with a bank of lead acid batteries that are removed for racing.

I've seen luxury sportscars sold for the value of the engine, or include expensive new parts with receipts. I can buy the car and sell the parts. Likewise such a car with a problem. Assume a 3200lbs/1450kg curb weight on the donor car, minus the sportscar's heavy drivetrain. Assume 200k miles/300km. It could be a Corvette or 370z, but I'd 'waste' another 300lbs and use a luxury car like a G37 (fat 370z) and have everything work. For cost, this car would stay stock. The car with no electric conversion should be nearly free in good condition, especially in my local market. Consider $3K for the finished ready donor using my free labor.

What are the minimum specifications that will meet this definition?

What's the cheapest implementation in today's market, with new and used parts?
 

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Hi
I do Autocross here in NZ
The events are a bit different but great fun
I would suggest getting a "Locost" type car

 

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I technically don't do anything anymore as they banned EVs from racing. But back in the day, I was doing Khanacross, which is similar to autocross. I just use the word Autocross because only locals will know what khanacross is. I also did motorkhana and hill climbs but its all off the table for now.
 

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Discussion Starter · #7 ·
Banned from each competition type? That sucks. Gotta let the ICE vehicles play with themselves, I guess.
That's why I figure a cheap system can compete, but I don't know the math. The SCCA will let it run, but may put it in an unlimited category. I don't need to win a trophy, just have fun. I want to beat a Corvette with a mildly upgraded engine with a laughable system, which sacrifices off-the-line and high speed to perform well at one event. It may as well double as an around town scooter. The joke is funnier (and more repeatable) the cheaper it can be built.

Using a pretentious body is all the more fun. Designing around 3400lbs would make the setup pretty universal. Can anyone offer up the math on minimum specifications to meet the idea?
 

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I don't need to win a trophy, just have fun. I want to beat a Corvette with a mildly upgraded engine with a laughable system, which sacrifices off-the-line and high speed to perform well at one event. It may as well double as an around town scooter. The joke is funnier (and more repeatable) the cheaper it can be built.
Many years ago I ran slaloms (parking lot style, so Solo II in SCCA terms) in a sports car club that ran with other clubs, including two for Corvettes. I would beat some of the Corvettes (which were C3's, due to the era) - depending mostly on the Corvette driver's experience - with my dead stock Toyota Tercel; it was hilarious. They didn't feel too bad, because we were in different classes (although their class was supposed to be much faster).

Have fun! :)
 

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Banned from each competition type? That sucks. Gotta let the ICE vehicles play with themselves, I guess.
I think it's more a matter of being unable (given a lack of expertise and available effort in amateur motorsports, and simply a lack of will) to form a reasonable set of rules. The easy way out is to just put EVs in their own class, but even then there are issues of classification within EVs (clearly they're not all the same and it is unfair to expect them to have similar performance) and with safety (organizers are, with good reason, not confident that they will not have issues with batteries or high voltage in crashes or just failures).

The SCCA will let it run, but may put it in an unlimited category.
I think that's relatively likely. With an engine it's easy to assign consequences in classification to engine modifications, but with an EV it's not at all clear what to measure. If you swap the engine for a completely unrelated engine that is not a production fit for anything, you'll probably end up in an unlimited class; and EV conversion is similar.
 

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A 50 mile range would be enough to use it for most local driving, and that can be achieved with a bank of lead acid batteries that are removed for racing.
That would be really awkward to do. If you can stand to tow it, a battery trailer would be more practical.
 

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Banned from each competition type? That sucks. Gotta let the ICE vehicles play with themselves, I guess.
That's why I figure a cheap system can compete, but I don't know the math. The SCCA will let it run, but may put it in an unlimited category. I don't need to win a trophy, just have fun. I want to beat a Corvette with a mildly upgraded engine with a laughable system, which sacrifices off-the-line and high speed to perform well at one event. It may as well double as an around town scooter. The joke is funnier (and more repeatable) the cheaper it can be built.

Using a pretentious body is all the more fun. Designing around 3400lbs would make the setup pretty universal. Can anyone offer up the math on minimum specifications to meet the idea?
Our main governing body here have a published set of rules. I built my car to comply with those rules. Then 4 years later they tell me I should never have been competing. Apparently they have an unpublished set of rules against EVs. It is technically possible to pass those rules but it is extremely expensive as the competitor is responsible to pay for a team of trained/professional emergency staff. So there goes all my time and development into this car.

Nobody can give you a definitive figure. There are too many variables. You need to choose the variables that suit your needs and constraints.
But here are some of the things you'll need to balance:
Higher Volts = higher revs = higher speeds = more battery cells in series.
Higher current = higher torque = better acceleration = more cells in parrallel.
But if you try and achieve both high voltage and higher current it gets expensive and results in a heavier battery.
Both Duncan and I have light weight vehicles. I have taken the low voltage (172v) moderate current approach. While Duncan has taken the higher voltage approach. Neither is right or wrong. We are just working without our constraints. But for a heavier vehicle you will need some reasonable current in order to get good acceleration. I am hesitant to put a precise figure on it as there are too many variables.
Another constraint is the components. From what I can see there are various motor and controller options that run all the way from low voltage and small increments up to around the 170v mark but then there seems to be a gap between systems at 170v and around 300v. Similarly there seems to be a gap between systems running 1000amps and 2000amps.

You could choose a system running 90v and 500amps but I guarantee you'd be disappointed. You could choose a hacked Tesla drive unit and pump hundreds of volts and thousands of amps through it. Go as big as your budget will allow and as big as the components you can source.
 

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Discussion Starter · #12 ·
Higher Volts = higher revs = higher speeds = more battery cells in series.
Higher current = higher torque = better acceleration = more cells in parrallel.
Then this would be an extreme ratio of high current to low voltage. What would that look like? It only need accelerate from 20-50mph and make due with a top speed of 60mph on the track. No launch is allowed, so gear it to hit target speeds at relatively low rpms, go off peak torque at 50mph, and hopefully reach higher speeds just long enough not to be a pain in the arse.

If you can stand to tow it, a battery trailer would be more practical.
That fits this project perfectly!
 

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Hi
You will need about 170 volts to get decent current at 60 mph - in fact I would say that you will need more than that
 

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Higher Volts = higher revs = higher speeds = more battery cells in series.
True, because the controller's output voltage is limited by the battery voltage.

Higher current = higher torque = better acceleration = more cells in parrallel.
Not quite. Higher torque requires higher motor current, but the controller exchanges voltage for current, so the real limitations are the controller current capacity, and the battery power capacity (regardless of the voltage and current combination).

Duncan's controller has a very high current limit, and his large motor can take a huge amount of current. His plug-in hybrid battery is at nearly the stock voltage (it's all but a few cells of a full Chevrolet Volt pack, with all modules in series), and can't supply the current which the controller is rated to put out. Perhaps he can fill in some of his current data from an acceleration run...
 

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True, because the controller's output voltage is limited by the battery voltage.


Not quite. Higher torque requires higher motor current, but the controller exchanges voltage for current, so the real limitations are the controller current capacity, and the battery power capacity (regardless of the voltage and current combination).

Duncan's controller has a very high current limit, and his large motor can take a huge amount of current. His plug-in hybrid battery is at nearly the stock voltage (it's all but a few cells of a full Chevrolet Volt pack, with all modules in series), and can't supply the current which the controller is rated to put out. Perhaps he can fill in some of his current data from an acceleration run...
Thanks Brian, Great clarification.
 

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True, because the controller's output voltage is limited by the battery voltage.


Not quite. Higher torque requires higher motor current, but the controller exchanges voltage for current, so the real limitations are the controller current capacity, and the battery power capacity (regardless of the voltage and current combination).

Duncan's controller has a very high current limit, and his large motor can take a huge amount of current. His plug-in hybrid battery is at nearly the stock voltage (it's all but a few cells of a full Chevrolet Volt pack, with all modules in series), and can't supply the current which the controller is rated to put out. Perhaps he can fill in some of his current data from an acceleration run...
I'm running most of a Chevy Volt battery - 14 kWh - I run it from 340 volts (full) to 300 volts (empty)
The motor is an 11 inch Hitachi forklift motor with direct drive to a 4.1:1 Subaru LSD

My controller is set to 1200 amps

When I accelerate the voltage starts to sag - at about 100 kph my controller is maxed out (100%) and the battery "sag" stops increasing at about 20%

The overall final drive ratio is about right - and my top speed is something over 100 mph

With less current then you will need less volts to defeat the back EMF and drive the current through the motor

With half of my voltage (170 volts) I expect that a 600 amp controller would be maxing out (100%) at about the same speed
 

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I'm running most of a Chevy Volt battery - 14 kWh - I run it from 340 volts (full) to 300 volts (empty)
The motor is an 11 inch Hitachi forklift motor with direct drive to a 4.1:1 Subaru LSD

My controller is set to 1200 amps

When I accelerate the voltage starts to sag - at about 100 kph my controller is maxed out (100%) and the battery "sag" stops increasing at about 20%

The overall final drive ratio is about right - and my top speed is something over 100 mph

With less current then you will need less volts to defeat the back EMF and drive the current through the motor

With half of my voltage (170 volts) I expect that a 600 amp controller would be maxing out (100%) at about the same speed
You really want to at least match Duncan's numbers. Otherwise you'll be disappointed and end up paying twice over when you upgrade. Pay a bit more now and get it right first time.
 

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Discussion Starter · #18 ·
I'm running most of a Chevy Volt battery - 14 kWh - I run it from 340 volts (full) to 300 volts (empty)
The motor is an 11 inch Hitachi forklift motor with direct drive to a 4.1:1 Subaru LSD

My controller is set to 1200 amps
Duncan, yours sounds like the project to copy. How would that perform at 30-80kph with over 1500kg?
What changes would you suggest?
 
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