Torque, technically, is the rotational equivalent of linear force. Linear force is what happens when you push a door open, torque what you use to turn the handle. But torque doesn’t need movement to exist. When you pull on a wrench to attempt to loosen a stubborn bolt, whether that bolt moves or not, you are applying torque to the wrench. Practically, in a car, more torque = more looks of terror from passengers as you push the accelerator. They’re feeling linear force as they are pushed back in their seat because the wheels are enduring a bunch of torque.
“Give me a place to stand and with a lever I will move the whole world.“— Archimedes
You may know from experience that if you are having trouble getting that bolt loose, you can always apply more force on the wrench – pushing harder and risking dislocating your shoulder for example. Or you can do things the smart way and get a “cheater bar,” adding an extension to the wrench that provides more leverage even though you aren’t applying more force. Even the rustiest-crustiest nuts come loose with long enough cheater bars. Or, you break your wrench. Either way, you now understand that the further away from the fulcrum you are on a lever, the more the force you apply is multiplied.
That combination of force and its multiplication is a measurement of torque.
In the United States, and across the automotive world with internal combustion engines, torque is typically expressed per Pound-Foot (“lb ft”). This expression is a simplified way of saying “however much torque it is when you put one pound of weight at the end of a foot-long wrench parallel to the ground, that’s one Pound-Foot.” Ergo, in putting your entire body weight on the end of a one-foot-long wrench to get a bolt loose, you might be making 200 Pounds-Foot of torque!
Yes, I prefer expressing this as “Pounds-Foot” rather than “Pound-Feet.” You aren’t changing the length of the lever, after all.
Pound-force should not be confused with foot-pound, a unit of energy, or pound-foot, a unit of torque, that may be written as “lbf⋅ft”; nor should these be confused with pound-mass (symbol: lb), often simply called pound, which is a unit of mass.Wikipedia
A Pound is a measurement of weight. Weight uses Earth’s gravity to measure mass. It’s a rough approximation since Earth’s gravity is not the same everywhere. In practice, gravity can cause weight to fluctuate up to 0.5% depending on where you are standing.
So if you don’t like what the scale says, rather than eating more salads, try moving to a different spot.
In modernity, we’ve hacked together a definition of “pound” that makes it directly comparable to mass, but it’s frankly an outdated measurement that should be driven to a farm upstate. Wikipedia has separate entries for “Pound (force)” and “Pound (mass),” and this need for clarification should tell you it’s an overloaded unit of measurement that deserves to be replaced.
With EVs, it’s becoming common to express torque Newton-Meters. Why? Are EVs creating torque in a fundamentally different way that requires a fundamentally different measurement? No! So why make the change from the warm, cozy, familiar Pound-Foot to the scientific, sterile, vaguely European Newton-Meter? The two ways of measuring it are as interchangeable as Inches and Centimeters if you understand the thinking behind the measurements.
Imagine placing a large pineapple on a greased up counter and then shoving it three feet in about a second. The force you used to do that is about how much a Newton is. What seems different about a Newton a compared to a Pound is that it involves movement, but really they are measuring the same thing: force. With Pounds, the Earth is applying Gravity to Mass. With Newtons, You are applying Your Arm Muscles to A Pineapple.
Technically, it’s expressed this way:
There’s that bugaboo that the example doesn’t account for: “at the rate of one metre per second squared.” That little “squared” part is important, because once that pineapple is moving three feet a second, assuming a perfectly frictionless greased counter it doesn’t take any energy to keep it moving. The “squared” part ensures that if you are shoving the pineapple for two, three, or God forbid even four seconds what we’re measuring is a steady force that increases the acceleration of the pineapple.
Basically, you keep pushing, it keeps going faster. That’s what the “squared” in the definition is all about.
As far as the “Meters” part of Newton-Meters, that’s self-explanatory for Europeans. For all those Yanks out there, a football field is 100 Yards, and a Yard is pretty close to a Meter, which is also pretty close to three Feet.
Correct Torque Unit for EVs
So why do car manufacturers and magazines covering EVs love to express units in Newton-Meters when they’ve been using Pounds-Foot for years? They are both measures of the same thing, torque.
For one thing, the Newton-Meter has been normalized as a unit of measurement for electrical motor torque outside the domain of cars for years. It also speaks to the “new” nature of EVs, and especially how people “feel” the torque.
In internal combustion engines, there is a generally a “peaky” torque curve, and the torque advertised for any engine is peak torque. This is the point in the power band, from 0 rpm to peak rpm, that the engine is making the most torque. The engine will be making this torque for only a moment in time and your mileage may vary about how much it makes across the rest of the curve. The example given in the graph above of the 2019 BMW M5 Competition exemplifies one of the flattest torque curves in gasoline engines, and yet when compared to a Tesla Model S, it appears like a short-lived peak.
This is because ICE power units generally have an upper limit near 7,000 rpm, while EVs such as the Tesla Model S can spin their motors up to 18,000 rpm. And during that time, while most gasoline engines are building steam up to a peak in the middle or top of the torque band, electric motors are making peak torque from 0 rpm and then it gradually tapers off.