Dear MOby,

When I was growing up as an early baby boomer (bless our initially materially rebellious, yet ultimately avaricious souls – which in my case I blame on motorcycles being so much fun), torque was righteously represented as foot-pounds: usually stated as “ft-lbs” in any self-respecting moto rag. Now, it’s represented as pound-feet of torque, “lb-ft”.

About when did it change, and why? And really, why should most people give a s%$t? As always, thanks in advance for the erudite answer that I’m sure you’ll provide.

Walter B.


 

Dear Walter,

I wondered the same thing, especially since I‘ve spent a fair amount of my lifetime plugging entries into spec charts. It’s easier than it used to be, when we’d actually measure the circumference of the rear tire to calculate speed in all six gears (or five) using primary, final and internal gear ratios using an abacus. But that was in the pre-internet days when there was plenty of time to putz around the office, smoke cigarettes, sexually harass coworkers, etc.

Bitter as a result of how much more “work” I’m now tasked with, I was happy to just use whatever format the bike manufacturer in question decided to use, though like you, I thought it was foot-pounds for years. Aprilia and Moto Guzzi and Harley express themselves in ft-lbs, BMW and Ducati use lb-ft… the Japanese usually don’t make horsepower or torque claims, but Yamaha says the new Star Venture makes 126 pound-feet.

As long as people know you’re describing torque, and more is better, what difference does it make? Either way, we’re describing the twisting force produced by applying one pound of pressure to a lever one foot long, no?

Have the people who make Tekton tools got it wrong too?

Have the people who make Tekton tools got it wrong too?

We used the terms interchangeably here at MO, depending on which editor was writing, because there was no mystery that we were referring to torque output no matter which way it was stated. But in the interest of consistency, we researched the topic to determine the most accurate term, determining that pound-feet, or lb-ft, is correct.

Since you asked, though, we decided to get a statement from an expert in the field, Dustin Schaller, Senior Product Manager at Dynojet Research, who tells us:

Torque is always measured in force at a radius. The correct measurement is lbf-ft or “pounds force foot”, which we shorten to lb-ft. The metric guys always have it right, when they express torque in terms of Nm, or Newton-meter. Never have I seen meter-Newtons, but somehow foot-pounds became common verbiage…

Okay, lb-ft it is then. All we have to do now is get our Dynojet operators to quit giving us readouts that look like the one below, or we’re doomed to a lifetime of using lb-ft and ft-lbs interchangeably. Next question please.

screen-shot-2017-08-07-at-7-38-59-am

 


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  • Born to Ride

    I consciously choose to say Ft-Lbs to describe torque and pound-feet to describe work. It comes from not only the gearhead upbringing I was subjected to, but also basic calculus. Torque is a vector quantity, and therefore the cross product of its component vectors is not commutative. You HAVE to multiply it r X F and if you switch the product to F X r, then you have not calculated torque. So by my logic, it makes more sense to list the units of measurement in the order that they are produced. Radius comes before Force, thus Ft-Lb is the intuitive unit for the Torque. Also work is always shown as F dot delta X, so it is intuitive by the same reasoning to conclude that Lb-Ft is the proper unit for work. Why engineers have reversed this convention over the years is a mystery to me.

    • Old MOron

      Just curious here: do you also say meter-Newtons?

      • Born to Ride

        Haha nope, you got me there. My crusade is only an imperial one.

      • Starmag

        Wise-assery with torque.

  • DickRuble

    Mathematically it is the same thing. The dimensional analysis lb*ft=ft*lb or m*N=N*m. Distance*force=force*Distance. Neither is correct nor incorrect.

    • Born to Ride

      Liar, did you forget your multivariable calculus?

      • DickRuble

        Nope.. You are correct that work and torque are different, one is measured in joules, the other in Nm. The difference is that there is an angle (in radians, and adimensional) that intervenes between the two. That you chose to multiply distance*force when you refer to work, possibly for mnemonic reasons, is your choice.

        • Born to Ride

          Well yeah, the dot product actually is commutative as you know. Work can be described accurately either way in the imperial system. The units are reversed between torque and work because it would be confusing otherwise to have the same units describe two different physical measurand. But i still contend that if the physics of angular versus linear coordinates forces us to calculate one value a specific way, why shouldn’t we carry the units through the calculation in the order they are multiplied?

          • DickRuble

            work = torque x angle. If you express torque as lb*ft, why would you flip the order when you discuss work? Without commutativity of multiplication, physics would be intractable.

          • Born to Ride

            In the angular frame, work is the time integral of the DOT product of torque and angular velocity, which resolves to torque times the angular displacement (assuming torque remains constant through the interval). But that operation is commutative, where the cross product in the Cartesian coords used to solve for torque is not commutative. Hence my argument for r X F leading to the unit ft-lb as more intuitive. I get you, but your point simply fails to subvert my assertion.

    • Wonko_T_S

      Yeah but…
      In the metric world mN would generally be read as milli-Newton. The ‘m’ has to be at the end to allow for such beautifully symmetrical units as mNm – milli-Newton-meter. A level of torque seen in the insect world.

  • Starmag

    Not being a engineer or a scientist, I say foot-pounds simply because it sounds better and rolls off the tongue like a fine whiskey. Also pounds-feet sounds like a podiatrist procedure.

    • Born to Ride

      Truth.

    • RMP52

      Agreed!

    • Pale Face

      FT LB is Correct.

    • Strat

      I am a Mechanical Design Engineer and Lb-ft units are used with brake torque for electric motors. Ft-lb is correct for wrench torque. I don’t think anyone uses meter-newton, it’s always newton-meter. If anyone does, I stand corrected.

  • john burns

    I admire my ability to start discussions in which I have no clue what people are discussing.

  • Jason

    The short answer is that unless you live in the USA, Myanmar, or Liberia it is Nm. Even in the USA, many industries have switched over to metric to better compete in a global market.

    • DickRuble

      Tell that to Congress. In 1999 a probe missed Mars by a wide margin because of conversion errors.
      http://www.cnn.com/TECH/space/9909/30/mars.metric.02/

      And that, because Congress felt that adopting the metric system at NASA would somehow diminish America and promote Russian superiority. For that, the space station has dual metric/imperial gauges, at far more than twice the cost.

      • Jason

        Congress passed the Metric Conversion Act in 1975 setting the metric system as the preferred unit of measure in all branches of the US government and Ford signed an executive order requiring the changeover. Reagan disbanded the effort in 1982.

        • DickRuble

          Starting with 1990 NASA had begun using the metric system. Yet a few years into the project, while everything was set and agreed upon, some congressman got wind that all systems would be metric. That led to hearings, and the decision. In 2007 , after the mars probe loss, NASA finally decided to adopt definitively the metric system, as a “goodwill gesture” to other nations with which they have to work.

  • Douglas

    No, no, no…..pounds feet is what describes a paratrooper doing the “airborne shuffle”…I know this, and had shin splints to prove it.

    • DickRuble

      It’s “pound the floor”..

      • Douglas

        What is?……

        • DickRuble

          lyrics from a running cadence.. I thought that’s what you were referring to.

          • Douglas

            Well, okay. Close enuf. It’s a kind of half-step trot, much harder and more tiring than a normal running gait. The pounding effect ….the 1st week is hades.

    • Rob Mitchell

      well why didn’t you wait for the plane to land before getting out😂😂😂

  • James Stewart

    I prefer to discuss something less controversial…like the proper units for velocity (aka speed – for the great unwashed non-engineers among us.)
    My prefered units are [furlongs] per [fortnight].
    Surely no one will drop their Whitworth socket wrench to disagree?

    • Gruf Rude

      Lordy, I still have Whitworth and British Standard open- and box-end wrenches (along with the degree wheel, adaptor and bent-wire pointer for setting ignition timing on 60s-era Triumphs) in my tool box. Relics from my days in a Triumph/Honda/Bultaco dealership.

      • James Stewart

        Gruf – you had me at “Bultaco”.

    • Starmag

      Just how many FPF’s were you over the “velocity” limit, (I just took a great bath), on your last ticket?

      • James Stewart

        Let see… 31MPH over (91 in a 60)… so…
        1[furlong]/0.125 [Mile] x 336 [Hours]/1 Fortnight x 31 [Mile]/[Hour] = 83,328 furlongs/fortnight. But I got him to write it for only 80,000 over, so I’m good, thanks.

        • Born to Ride

          Unit conversion done right, right thurr.

        • Starmag

          While it would be impressive, I don’t think the face of your FPF “velocimeter” is going to have room for all those zeroes.

          • James Stewart

            I’d like to say that I’d hacked the firmware in my Trail Tech Vapor to convert MPH to FPF but that would take an electron chasing EE and alas I’m only a lowly Mechanical… Them EEs is real fart smellers I tell you…

  • kenneth_moore

    I know it’s technically incorrect, but I like Jeremy Clarkson’s use of “torques.” He says the new Ferrari has “380 torques” and we all know what he means. It’s a heck of lot easier to say anyway.

    • Kevin Duke

      Agreed, at least until we start using Nm…

  • D. Paul League

    When I went to Engineering School in the late 1970s the reference was lb-ft. That doesn’t make it correct, just a reference to engineering.

  • Rob Mitchell

    Why are you concerned with the imperial system? America couldn’t even get it right, The US gallon isn’t the full gallon, the US ton isn’t even the full ton. The odd time I see an American trying to use metric they are using centimetres, can’t even get metric right either.

    • Born to Ride

      Yes, all the American engineers who are drilled in unit conversion and have to know two systems intuitively can’t get metric right. *rolls eyes*

      • Rob Mitchell

        Exacty, why do American engineers use cm? Even Autodesk have cm as the default metric unit in AutoCad.Just plain dumb.

        • Born to Ride

          I was trained on solidworks, so I can’t help you with the settings on your autodesk software, Sounds like user error to me.

          • MountainK1ng

            At this point we just use centimeters to piss off Europeans. Besides for average size parts in the 0.5-2.0 ft range, meters and mm are both sort of unwieldy, but really it’s just to piss off Europeans.

  • John A. Stockman

    Man, I love math! So much great info in the posts below. My grandpa had a compliment of Whitworth tools for the British bikes he worked on, I inherited them. The odd, old British bike I get to work on, is the only time they get used. A few asked if I could loan them out, NO. They have special meaning and even if I don’t use them but once a year or two years, he gave them to me and I learned about motorcycles and working on them from my grandfather. He had a small machine shop in his garage and many times would have to machine/fabricate a part for his ’39 Indian Chief. Back then, not as much aftermarket support for the old Indians.

  • Ted Oud

    Mathematically torque is a vector product and termed pound foot. Energy is a scalar product and termed foot pound. One poster refers to the scalar and vector aspects but reversed the two terms.

    If you’re an engineer it matters as it can confuse otherwise if you know the difference to begin with. For everyone else just keep saying ft lb of torque and we’ll understand…