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Comment by throwawaylinux | Hacker News Reader

Scaling the weight linearly down to a soup can (1/63) gives you about 5 horsepower. The Model-T had 20 horsepower though, as does your typical riding lawn mower. So you'd have to soup it up a bit more than that.

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You can already get a soup-can sized 3.5kw motor on Amazon for ~$100, so that exists already. Their tech might be optimized for the size motor, but if not, there might be room for improvement at the smaller size.

https://www.amazon.com/Flipsky-Hardened-Skateboard-Brushless...

(The idea that an e-skateboard can have four of those motors blows my mind. Suicide!)

The point is that's not enough for a car. You need around 20x that power as a minimum starting point for a small car.

Motor torque capability generally scales in proportion to volume, but shaft speed capability typically increases as the motor shrinks, so the motor power will usually fall at a smaller rate. A motor with 1/64 volume will probably output between 1/30th and 1/16th of its power. So 20 horsepower might barely be achievable. It all really depends on the driver though.

Why don't you see a lot of very long thin, high revving motors for higher power with smaller size than large motors? This one has completely the opposite scale of dimensions even.

Heat dissipation mostly. Square-cubed laws are brutal for getting the heat out.

Long and thin - volume and area increase proportionally with length. Think of 10 small motors put together end to end.

So, mainly two reasons that I am aware of. First, with a long spindly motor running at high speeds, eventually shaft whip becomes an issue; the distance between the bearings ends up being really far. Second, designers usually want low-speed torque more than super-high-speed power, and at high diameters you get a bit more torque than small long motors of equal volume, because torque scales more like the volume of a somewhat hollow cylinder rather than a fully solid cylinder.

For most brushless motors--though not axial flux motors like the one under consideration here--it's much easier to get heat out by passing air axially across the stator. This means that while the surface area is increasing proportionally with length, your ability to flow air across that surface area is not keeping up. For that same motor volume, a shorter and fatter motor will have better airflow and cooling than a long skinny motor.

Moving coils at faster surface speeds require faster switching at less efficiency. Extra diameter for same volume buys extra performance for free.

I have a motor the size of a film canister that produces 0.5HP. It was built in 2003. I'm sure with better magnets and scaling to the size of a soup can would get it well above 5HP. The main caveat is that it spins at 20k RPM, so it would need a gearbox.

Would love to hear more about that baby

It's a Mega ACn 16/15/4. Permanent magnet inrunner with 2200kV. Made in Czechia.

Officially specced for 25A, commonly run from 4S LiPo = 14V, but the current can go higher with good cooling or in bursts.

Wow, thanks. I am really behind the times. $80 gets you some oomph:

https://www.hyperflight.co.uk/products.asp?code=MEGA-16-15-4...

Here's a guy using a soup-can sized motor in an older BMW. (25:00 to see the car moving).

https://www.youtube.com/watch?v=HcoVYZ20PwM

5HP (3.7kW) is not enough for a two ton car, but it is plenty for a small city vehicle. 1kW is enough to reach about 40km/h on a bicycle.

> 5HP (3.7kW) is not enough for a two ton car, but it is plenty for a small city vehicle.

It's not enough for a car. I don't want to get pedantic about it, sure it could power a clown car or two scooters and an umbrella duct taped together.

The (BTW I have seen one in person and it is actually cute IMHO) bare-bare minimum is the Citroen Ami (which is not a car, but rather a quadricycle) :

https://www.auto-data.net/en/citroen-ami-electric-5.5-kwh-8h...

5.5 kW to get 45 kmh speed (and 70 km range)

Since the Ami is a car that actually makes a little sense for cities, it probably counts as "clown car" for people who are used to the huge hunks of steel that you usually see on streets.

Yep, though "clown car" seems dismissive, maybe we could call it "sort of legal for the road golf cart", as it does makes lot of sense for some EU city centres or for local deliveries in places where streets and parking spaces are tiny.

The UK site for it envisions a possible use for the "cargo" version for "last mile deliveries": https://www.citroen.co.uk/models/future-models/ami.html

That's not too far off if you go by volume instead? Perhaps other bits of the car can be more efficient than the model T to make up for it.

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