Hacker News Reader

Comment by neya | Hacker News Reader

10,000 NM of torque. That's INSANE. I drive a car with 400NM torque and that thing scares the sh*t out of me when I accelerate. 10,000 NM? Isn't that something along the lines of a tram or a train (I'm not sure, I'm asking you guys). This is CRAZY. I'm getting one for sure.

View on HN

Your car has 400Nm at the engine output shaft.

After gearing, you'll have the same order of magnitude torque at the wheels as the Tesla.

Randomly picked example: Porsche G97/01 (997 Carrera 2 2005-08) First gear total reduction 13.45 (gear 3.91, rack&pinion 3.44)

So, ignoring efficiencies, you'd have a first-gear torque of 400Nm * 13.45 = 5380Nm at the wheels. Still "only" half of the Tesla, but not an earth-shattering difference anymore.

Thank you, now that makes perfect sense :)

Isn't the rack-and-pinion for steering, not for power transmission to wheels?

Yes rack and pinion is for steering. Parent may have meant "ring and pinion" which would refer to the differential gear ratio.

Yes, you're right of course. Thank you.

I was wondering, because I seemed to connect the expression with steering as well :-) At least I have the excuse of not being a native speaker.

I had the same thought [0]! They cite their wheel torque, not their engine torque, which is what we're all used to seeing.

[0]: https://news.ycombinator.com/item?id=15719411

In electric cars, isn't it basically the same?

Even if there is not a traditional transmission, there is still a gearbox behind the electric motor.

Tires only rotate at ~1000 RPM on the freeway where the electric motor in a Tesla might be at 10k rpm. That 10x reduction still needs to be factored in.

Honest question: did you just make up these numbers, or are they based in fact? I was expecting the motor to turn slower.

Not OP, but Tesla Model S has approximately 1:10 fixed gear reduction.

Motor RPM will go higher than 10000 RPM at really high speeds.

Right. And electric motors are more efficient at higher RPMs. At high RPM, the voltage drop due to back-EMF (i.e. actual work) is higher but your voltage drop due to coil resistance is about the same, so you lose proportionally less energy in the form of coil resistance at higher rpm.

> electric motors are more efficient at higher RPMs.

How big is the difference?

It must be pretty substantial to offset both friction losses and gearing efficiency.

Gearing and friction losses can be fairly small.

Consider that at ~0mph and ~0rpm, producing any kind of torque at all is done with ~0% efficiency, all just resistive losses as the back-EMF is effectively zero. (Note this is no different for an internal combustion engine, although the heat generated is in the clutch or similar such device...)

Friction losses don't change much as a proportion of power as you increase speed. For aerodynamic drag, however, your losses do proportionally increase with speed, so "windage" (i.e. drag from spinning parts of the motor) needs to be cleverly reduced by making the rotor aerodynamic if you're planning on operating at high rpm.

EDIT: for properly designed electric motors (i.e. thin wire windings, iron core optimized for low eddy current losses), coil resistance usually dominates even at high rpms.

It can be, even though many electric vehicles apparently use a fixed-ratio gear as well. But it's still an apples-to-oranges comparison since traditional ICE-driven cars tend to publish engine torque, not wheel torque.

Of course, they weren't trying all that hard to alleviate any confusion, and it worked even on the fairly tech-savvy HN crowd.

Ed: improved both facts and wording.

For those of us in the US, and vaguely familiar with traditional hot rods, the Chevy big block engine generally came with ~500-700 lb-ft of torque, which in a lead sled, relatively speaking, such as a 60-70s era Camaro or Chevelle would throw your head back and shake your chest like a roller coaster as the car started to slightly drift sideways.

10,000nm in comparison is 7,375lb-ft, which sounds like Harry Potter land in comparison, if all of my conversions and memories serve me.

I'm kinda speechless, TBH.

Look at my sibling post https://news.ycombinator.com/item?id=15720518

700lb-ft is apparently 950Nm. So assuming the same gearing as in the sibling post (very likely a false assumption -- I guess these cars had longer gears), you'd end up with 12.7kNm of torque at the wheels.

BTW, if such traditional hot rods had automatic transmissions (they did, didn't they?), the torque converter would have amplified the torque even beyond the numbers I just gave you.

My car has 1092nm of torque from about 1500rpm and it's mind-boggling... 10,000nm is hard to comprehend and surely more than enough to give the occupants some serious whiplash if not sat properly.

Wheels will lose traction, or the front of the car would lift, rather than whiplash.

4000-5000nm is the limit in Formula 1 tech. Just for perspective

may i ask what you do for a living

If you can afford one of these cars, you probably don't do anything for a living.

I'd suggest the exact opposite - if you can afford one of these cars, you must be working very hard at be very good at whatever you do for a living.

I work in cybersecurity. I'll have to set money aside for a couple years to make a considerable down payment, but I might be able to afford one of these.

Reply on news.ycombinator.com