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Thread: Draw bar extensions

  1. #1
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    Draw bar extensions

    This should be of particular interest to our members with older vans,,,

    I know we have some very clever people as members,,
    I would welcome any comment...


    How does the 350 mm A-frame extension affect the dynamic forces on the ball coupling? As explained, the dynamic forces exerted by the ball on the coupling are much greater than the static weight force on the ball, and are as a result of large vertical accelerations of the ball, when the tow vehicle drives over bumps. I can’t reliably calculate the actual dynamic forces, because that depends on the size of the bump, and the moment of inertia of the van, both unknown quantities. However, we can calculate the percentage change in the dynamic forces, as a result of increasing drawbar length, which is what we want to know.

    Need some background theory. Moment of inertia (I) can be thought of as ‘rotational mass’. A flywheel has a large moment of inertia because it is heavy, and because the mass is deliberately placed around the circumference, at the maximum radius. A caravan has a moment of inertia about it’s axle, and for a given overall weight, the moment of inertia is greatest if the weight is concentrated at the ends, meaning at the greatest radius. For many reasons, we want the moment of inertia of the van to be as small as possible.

    When the ball on the tow vehicle rapidly accelerates upward over a bump, then this linear upward motion is converted to rotational motion of the van about it’s axle. For a given upward acceleration at the ball, the dynamic upward force exerted on the coupling is proportional to the moment of inertia of the van about it’s axle. The following formula relates the linear upward acceleration at the coupling, to the rotational acceleration of the van :-

    A = a/R (Equation 1) where

    A is the rotational acceleration, in radian/s/s (degree/s/s of you prefer)
    a is the linear upward acceleration of the ball, in m/s/s
    R is the distance from coupling to van axle in meters

    This is not rocket science. All this formula is saying, is that the rotational acceleration of the van is proportional to the linear upward acceleration of the ball, and inversely proportional to the length of the lever arm about which the van rotates. Intuitive common sense.

    Many would be familiar with the formula F=ma. There is a directly analogous rotation form of this formula, also very intuitive :-

    T = IA (Equation 2) where

    T is the Torque, in Nm
    I is the moment of inertia, in kg-m^2
    A is the angular acceleration, in radians/s/s


    The final equation, that should be second nature by now, that ties it all together is :-

    T = FR (Equation 3) where

    T is the torque applied to the coupling, in Nm
    F is the upward force exerted on the coupling by the ball (N)
    R is the lever arm length, from coupling to axle (m)


    Combining these three equations gives us the equation we want :-

    F = Ia / (L^2) (Equation 4)

    Thus we see that the dynamic force exerted upward on the coupling by the ball is proportional to the linear upward acceleration of the ball and the moment of inertia of the van – no surprises there.

    The interesting part though, is that for a given size of bump (given value of a), and a given van having a particular moment of inertia, the dynamic force exerted on the coupling by the ball is inversely proportional to the square of the distance from coupling to axle.

    How very interesting! And how very desirable. The percentage increase in total drawbar length is an increase of 350mm in an original length of 4840m, so the percent increase is 350/4840 = 7.2% The L-squared term in Equation 4 means that if we increase total drawbar length (from coupling to axle) by 7.2%, then the dynamic forces exerted on the coupling decrease by 14.4%

    This is a beautiful result in every way. It means that there is no structural problem with the increased-length A-frame, because although the static bending moment is increased, the much-more-important dynamic loading is very substantially reduced, though I need to fully tie up the dynamic bending moment in the next posting.

    Even more importantly, it means that in general, the dynamic forces exerted by the van coupling onto the tow vehicle, are reduced by twice as much as you might think, on account of the L-squared term in equation 4, namely a 14% decrease rather than the 7% decrease you might expect.

    In a similar way, having excessive weight at the ends of the van has a bigger dynamic effect that you might expect, because the formula for moment-of-inertia is mR^2, so if the mass that you are packing is placed at double the radius (distance from axle), then it’s moment of inertia and thus it’s dynamic influence, is increased by a factor of four. I can hear Collyn Rivers nodding his head.

    One more short post to tie up the exact effect that increasing A-frame length has on dynamic bending moment, and I’ll be done.
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  2. #2
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    I doubt that I will use an extension on our van But getting one on a boat trailer for beach launching interests me.
    You only get one shot at life, Aim well

    2004 D2 "S" V8 auto, with a few Mods gone
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  3. #3
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    My 1976 Franklin which I just sold had a 450mm drawbar extension that I built and it was the best trailer I've ever towed. The new owner has also commented on this since they've had it.

    Doubtful I'll do the same on the Viscount although it does need some work in that area, I want to keep it close to original on the outside as I can. I may add some strengthening at some point but it isn't too bad.

    Nice find though Pedro - I did have a couple of Grey Nomads comment that the extended draw bar on the franklin would create more problems but it seems they were wrong. 👍😊
    If you need to contact me please email homestarrunnerau@gmail.com - thanks - Gav.

  4. #4
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    Gav,, did you cut the whole A off?
    all I really want to do is add a "nose" to the current A
    sort of a Y
    if you follow,,
    "How long since you've visited The Good Oil?"

    '93 V8 Rossi
    '97 to '07. sold.
    '01 V8 D2
    '06 to 10. written off.
    '03 4.6 V8 HSE D2a with Tornado ECM
    '10 to '21
    '16.5 RRS SDV8
    '21 to Infinity and Beyond!


    1988 Isuzu Bus. V10 15L NA Diesel
    Home is where you park it..

    [IMG][/IMG]

  5. #5
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    Draw bar extensions

    Yeah, I lopped the whole lot off and started from scratch. I ran the new drawbar right back to the front spring hangers as well as the original was only attached to the front 2 cross members. Took 3 days from start to finish but I was in a mates fully equipped workshop as well with all the tools I needed at hand.

    If you trust your welding skills it isn’t a massive undertaking nor that expensive.

    I may just run some new braces from near the hitch back to the spring hangers on the Viscount. Yet to finalise details on that one yet as it is in good condition already.
    If you need to contact me please email homestarrunnerau@gmail.com - thanks - Gav.

  6. #6
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    Quote Originally Posted by Pedro_The_Swift View Post
    Gav,, did you cut the whole A off?
    all I really want to do is add a "nose" to the current A
    sort of a Y
    if you follow,,
    your better lopping the lot off IMHO

    it isnt a big job, in reality, on most vans.

    is a y piece legal?
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  7. #7
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    depending on what you read you need to get an extension certified.
    if its all repainted it would be difficult to tell its a new part.
    "How long since you've visited The Good Oil?"

    '93 V8 Rossi
    '97 to '07. sold.
    '01 V8 D2
    '06 to 10. written off.
    '03 4.6 V8 HSE D2a with Tornado ECM
    '10 to '21
    '16.5 RRS SDV8
    '21 to Infinity and Beyond!


    1988 Isuzu Bus. V10 15L NA Diesel
    Home is where you park it..

    [IMG][/IMG]

  8. #8
    Homestar's Avatar
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    If you register it here in Vic, it's a free for all under 4,500Kg so no certification required. They are more worried that your lights work correctly but not sure about other States.

    And yes - is anyone really going to know what the original draw bar on a Golf looks like? 😉
    If you need to contact me please email homestarrunnerau@gmail.com - thanks - Gav.

  9. #9
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    I am more concerned about how much to add.
    35cm is a shot in the dark....
    50cm ??,,

    The post above gives such good results from adding only 35cm...

    Fully loaded the "Old Golf" (thanks Gav) may be getting close to 2T so its pretty much at the limit of its design


    anyone seen this method of chain attachment before?
    Attached Images Attached Images
    "How long since you've visited The Good Oil?"

    '93 V8 Rossi
    '97 to '07. sold.
    '01 V8 D2
    '06 to 10. written off.
    '03 4.6 V8 HSE D2a with Tornado ECM
    '10 to '21
    '16.5 RRS SDV8
    '21 to Infinity and Beyond!


    1988 Isuzu Bus. V10 15L NA Diesel
    Home is where you park it..

    [IMG][/IMG]

  10. #10
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    Quote Originally Posted by incisor View Post
    your better lopping the lot off IMHO

    it isnt a big job, in reality, on most vans.

    is a y piece legal ?
    Don't know the legalities but have seen a few and makes reversing easier as you can jack-knife the buggars.
    PaulT

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