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  1. #1

    Default "drawbar"hp?

    Attn: TBN "Combined-smarts" dept., [img]/w3tcompact/icons/wink.gif[/img]

    Several recent posts have mentioned "drawbar" horsepower. Is there many meaningful way to estimate the "drawbar" hp of a given tractor, when the gross and pto hp's are known, or is some sort of "tractor-pull" measurement the only way to tell? [img]/w3tcompact/icons/shocked.gif[/img]

    Seems that the traction of the particular tractor/tire combo, and whether-or-not it was mfwd, etc., would be large factors.(?)



  2. #2
    Veteran Member
    Join Date
    Dec 2000
    Southern VT, Southern ME
    John Deere 4100 HST /410 FEL, R4s

    Default Re: "drawbar"hp?

    On my web search I saw this simple definition of Drawbar HP.
    "Total hp less the amount of hp it takes to move itself, the balance to being available to pull the load."

    This article What's the story with tractor horsepower?
    asks some of those same questions.

    The independent tests run by The Nebraska Tractor Test Laboratory are explained here. Nebraska Tractor Tests.

    The article on Tractor Tire and Ballast Management had alot to say about traction for sure.
    <font color=blue>Maximum tractive efficiency results from a compromise between minimizing rolling resistance and minimizing wheel slip.</font color=blue>


  3. #3

    Default Re: "drawbar"hp?

    Thanks DFB,
    I've seen those sites, but maybe it's time to visit them again [img]/w3tcompact/icons/smile.gif[/img] .

    "Total hp less the amount of hp it takes to move itself, the balance to being available to pull the load." (almost hafta add: "or spin the tires!".

  4. #4

    Join Date
    Apr 2000
    Central Maryland
    Kubota BX 2200

    Default Re: "drawbar"hp?

    <font color=blue>"Total hp less the amount of hp it takes to move itself, the balance to being available to pull the load." </font color=blue> I know exactly what they mean, but I think they consulted IRS publications on how to state it[img]/w3tcompact/icons/smile.gif[/img].


  5. #5
    Veteran Member gerard's Avatar
    Join Date
    Apr 2000
    Syracuse NY
    Kubota L2500DT w/FEL

    Default Re: "drawbar"hp?

    Just an uninformed opinion but I think the easiest way is to think of it as gross engine HP less mechanical/friction/transmission loss due to inherent energy cost in running an engine and transmitting power. (That's why you'll get more "drawbar" HP from a gear transmission vs a hydrostatic - there's more "loss" in transmitting the power to the wheels through a fluid coupling than a direct mechanical linkage). Other than comparing stats I don't see it as overly useful since most engines are going to have similar "losses". (The same theory applies to "PTO HP", same principal ie how much HP is available at the PTO after transmission of power loss, usually a few HP less than gross engine HP)

  6. #6

    Default Re: "drawbar"hp?

    {from the web}

    Tractor Drawbar HP = FS/375 where F is the force in pounds required to pull the implement and S is the ground speed in mph.

    It would appear that doubling S would simply double the DBHP required, a direct relationship. However the amount of force, F, required to pull an implement through the soil increases with speed. In a clay soil, it takes 1 1/2 times as much force to pull a tillage tool at 6 mph as at 3 mph. Thus, both F and S increase, and more energy is required per unit of work done at higher speeds.

  7. #7

    Default Re: "drawbar"hp?

    On the contrary, gerard, it is the most useful value available for comparison if your application is to pull implements through soil. This value must come from real world measurements to be useful, and thus take into account overall weight, traction, ballast location, wheel slip, etc. Real farmers even care about how specific tires and ballasting work on their own soil profiles.

    Granted that if your application is to mow the lawn you don't give a hoot about the drawbar pull of a tractor.

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