How do you calculate torque at the wheels or axles?
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    mike01's Avatar
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    How do you calculate torque at the wheels or axles?

    Hi all,
    Does anyone know a way to calculate how much torque a tractor is producing at the wheels or axles? I'm curious how much all of the reduction amplifies the engine's output.
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    On most tractors,
    the maximum torque at the rear axle is meaningless because you can not get enough traction to use all the torque available,,,

    So, if your tractor can pull a maximum of 3,000 pounds,,
    and
    the distance from the dirt to the center of the axle is 2 feet,,,

    your tractor is producing (2X3000=6000) 6,000 foot-pounds of torque.

    If the tires are spinning and the load is not moving, the torque could be anything,,,
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    Quote Originally Posted by mike01 View Post
    Hi all,
    Does anyone know a way to calculate how much torque a tractor is producing at the wheels or axles? I'm curious how much all of the reduction amplifies the engine's output.
    Probably easier to take it somewhere and put it on a Dyno. :-)
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    I can't help with that but I can show you how to calculate PTO torque, you might find interesting.....especially if you ever get the idea to step close to a spinning oneClick image for larger version. 

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    I know how to calculate torque coming out of the engine. But tractors amplify torque. Unlike HP, torque can be amplified almost infinitely by slowing things down with gearing, which is what tractors do. So a tractor that puts out 50 ft lbs of torque from its engine at the rated RPM might amplify that to some obscene number at the wheels. I'm really curious how much torque these things are actually putting to the ground.
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    Quote Originally Posted by mike01 View Post
    I know how to calculate torque coming out of the engine. But tractors amplify torque. Unlike HP, torque can be amplified almost infinitely by slowing things down with gearing, which is what tractors do. So a tractor that puts out 50 ft lbs of torque from its engine at the rated RPM might amplify that to some obscene number at the wheels. I'm really curious how much torque these things are actually putting to the ground.
    IIRC, the theoretical value is simply the torque at the engine multiplied by the drivetrain gear ratio. So if the torque at the engine is 100 lbs/ft and the drivetrain in 1st gear is 32:1 then the torque at the axle is 3200 lbs/ft.

    Of course, transmissions have losses. Unless you know that loss, you can't determine the actual value.

    Al
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    Quote Originally Posted by AlKozak View Post
    IIRC, the theoretical value is simply the torque at the engine multiplied by the drivetrain gear ratio. So if the torque at the engine is 100 lbs/ft and the drivetrain in 1st gear is 32:1 then the torque at the axle is 3200 lbs/ft.

    Of course, transmissions have losses. Unless you know that loss, you can't determine the actual value.

    Al
    Is the drivetrain ratio included in tractor specs somewhere? I've never seen it. And does it include the rear tires into the equation?
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    Quote Originally Posted by mike01 View Post
    Is the drivetrain ratio included in tractor specs somewhere? I've never seen it. And does it include the rear tires into the equation?
    a. I don't know. I'm sure the manufacturer knows it, but whether they publish it...?
    b. The wheel diameter doesn't affect the torque. It does determine the tractive effort.

    Al
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    Torque also has to do with rotating mass. Case in point is the old flywheel engines. 6 hp engines weighed several hundred pounds and with those flywheels turning a robust 600rpm or less, they could move the world.
    A lil Farmall Cub was like what..9 hp.I think a good judge of torque would be the drawbar test. If you look at actual tractor tests you will see hp, pto hp, drawbar hp and even belt hp.

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    Quote Originally Posted by mike01 View Post
    Is the drivetrain ratio included in tractor specs somewhere? I've never seen it. And does it include the rear tires into the equation?
    This only works for a true geared transmission, hydros are another story altogether, to figure out the gear ration of the transmission as a whole you simply divide engine RPM by rear tire RPM. That will give you the ratio.

    So say you have an engine at 1800 rpm and a rear tire at 600rpm that would be 1800/600= 3. So you would multiply your rated engine output torque at 1800 rpm by 3 to figure out wheel torque.
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