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All out range & efficiency build/ we need YOUR help

4184 Views 85 Replies 11 Participants Last post by  SuperV8
Hello you awesome people,

I'm on the hunt for lots of info to build a very special car with very special needs.
Our small team wants to build a specific car to raise funds for an organization focused on children's wellbeing.

Of course we need to check the viability and cost involved, to inform our sponsors. You could consider this a charity start-up.

The team is technically, mechanically, and fabricationally (is that even a word?) skilled. Help is all around us.

The car in question is a 90's compact car, small petrol engine, manual gearbox, fwd, 1000kg driveable. It has a smaller tire size. (185cm circumference).

The Aerodynamics will be taken care of to bring it to much higher levels of efficiency.
The car will be purpose built, so no extra weight, no luxery for the driver... Basically, an empty shell to build on.

The car will be used for international long distance Road rallies. (+10h driving) The goal is to have a large range, short charging time, and high speeds. Since the end goal is the lowest time to arrival.
So charge fast, drive fast (sub 120mph). And efficient

I am a petrolhead at heart and this electric stuff is very new to me.. but basically the build has yet to start so we are open to any direction regarding parts etc.
So here are the questions I hope you can answer before we continue.

Efficiency factors:

- tire size, does it make much of a difference to change circumference / width
(To like i3, 155 wide, 220cm circumference).
I know it does with an ICE, so I'm guessing YES

- is there an efficient rev range in electric motors?

- size of motor or way of driving for longest range?

- most efficient air / water cooled Netgain?
HyPer9 is available in Europe.

- any other motors you would advise? Preferably available with controller as pack

- gearbox or direct drive? (HyPer9 diff 1:5 or 1:7)
See also 2nd question/ efficient rev range

- gearbox with straight cut gears worth the investment?

- is rwd more efficient in EV's ( enough to convert, because I could)?

- I understand Tesla units are the most efficient, but need a very expensive controller? And they are also aircooled?


- size & type of battery pack (I'm thinking Tesla all day long?)
You guys probably can advise when the weight outweighs the advantages of range.
I think the car could haul about 16 Tesla modules.
We would love to surpass 400miles range.

- is a 130v motor more/less efficient than a 400v with the same battery pack (say 16 cells, differently connected of course )

- dual packs, dual loading system? Halve the charging time.

- what would be the charging (stopped) time, and to what % would you advise loading?

- is it possible to add an extra pack when a very long run is planned? (I don't mean plug&play, since there is cooling etc...)

- solar panels on car efficient? kWh?

Thank you for answering all the previous.
I thought it was best to ask most questions at once, instead of scattered throughout the thread.
There is, however, more to come.

Can you please advise before we proceed so we can steer this build in the right direction before electrifying?
We really need the expertise on this forum to succeed...
We would thus like to eliminate most of trial&error and go straight to Kick-Ass build/ end product.

I have been reading A LOT already (most of it on this forum, love the Wiki), please excuse me for wanting to repeat the same questions (if that sort of thing annoys you). But I would rather have all the answers in one place to document.

That way we can anticipate (and stretch) the budget.
And this makes it also possible for everybody interested or involved to understand the complete build (not just the person who did al the research. aka myself) and form their opninions on all the options and considerations. Hopefully you all understand.

So hey, what would be the setup of YOUR dreams for our goals?

Thank you all very very much!
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- is there an efficient rev range in electric motors?
Yes, very much so. Normally a motor is designed to have a nominated rpm limit for the given supply voltage and to go faster than that you have to use field weakening which wastes energy and produces more heat in order to overcome this inherent limit in the motor. FW starts around 65MPH in a conventional ev because the majority of its life will be spent at legal speeds. I have heard a Tesla has to de-rate its motors considerably if driving excessively fast because they overheat. I have heard 50kW sort of power range after some time at high speed due to thermal limiting, although have never verified this. If you wish to run a standard EV drivetrain fast for extended periods then you will encounter this issue and your power and range will be somewhat less than you had anticipated.

Older DIY EVs use a gearbox to get around this speed limitation so that could be an option combined with a forklift style AC motor.
The top speed of any single-gear-ratio EV is determined directly by the maximum motor speed, the gearing, and the tire circumference. The top speed of a Leaf is not a failure of the vehicle or a limitation on another vehicle using the motor, it is the consequence of good design: choosing the lowest gear ratio (most reduction) which allows the vehicle to reach the highest required speed. There's no reason to drive a Leaf over 90 MPH, so the gearing isn't taller.

If you use an absolutely stock Leaf drive unit but want to go fast, use taller tires. You can also provide more battery voltage to extend the top of the motor's performance, within inverter limits for voltage and bearing limits for mechanical speed.
Yup, for sure. Engineer it for the application. If you can safely get 34% more speed with bigger tires then that'll work. Or a combination of more voltage and bigger tires. A 500V nominal system will likely achieve that too with a motor speed of 13400rpm, if it can survive mechanically, electrically and thermally.

OP wants to go up to 120mph in a race, that means the motor is going to be operating in FW mode for the majority of its use. Plan accordingly and know that the cooling is adequate to reject the extra load.
The transition from constant torque to power is usually defined by field weakening so yes, fw quite likely starts at 2800 or 50kph.
Rectangle Slope Plot Parallel Font

94~95% efficiency isn't brilliant for a modern motor design (unless that is the axle efficiency, then it is quite good) and that may be accounted for by fw operating. I haven't looked into it enough though. Each % efficiency loss from the motor adds ~0.8kW to the motor cooling system load, up to the point where it can't reject it, and then the power curve is thermally limited.

The case to avoid is where you are running on the thermal curve rather than the power curve. When your driver is unable to drive to the conditions because the powertrain is choosing how fast to go, it stops being a race and that is the main point I'm getting at here. Videos of Teslas running fast and stuck at 135mph will be sitting on the thermal limiter, even though the driver requests to go faster

This is the same graph somebody posted of a Tesla LDU with an openinverter board on a dyno. Constant(ish) torque changes to fw at 8000rpm or about 62mph
Light Rectangle Black Slope Plot
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Do you have the car already? Figuring out the gas consumption at various speeds in the 'race' trim will give you the best info before you cut anything up. I'm picturing a 1000 mile trip broken up into 80 mile runs and huge amounts of downtime pissing about wondering how to charge it, charging it, waiting for it to cool down, swapping batteries

This is an interesting story about why battery cooling is useful

Can you name the competition or the rules or something? Right now it looks like a non-engineer trying to outsource engineering (aka help) to a bunch of random grumpy internet dwellers, while wanting to achieve non-engineering goals that override the engineering. These never end well.
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