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Thread: Weight Question

  1. #1
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    Weight Question

    On another web site, it is claimed that when you put a towball weight on your car (does not matter what the weight is) weight comes off the front of the car and as such more than the tow ball weight is added to the rear axle load. The quote is much like the fulcum on a see saw. What has been quoted is that a ball weight of 350kg means about 500kg is loaded onto the rear axle. I failed physics so I am unsure if this is correct. If this is the case then I question if any car could tow a 3.5T van with a notional ball weight of 350kg. Can any of you gurus here answer my question in simple terms. I dont know how you can accurately guess the tare weight on a rear axle unless you drive over a weigh bridge, so just halving the Tare weight would not give an accurate figure, or so i would deduce as I would think more weight would be biased to the front due to the engine and gearbox being more located to the front of the vehicle.Thanks David
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  2. #2
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    David, I don’t believe it to be a massive “see saw” as such’ but yes’ if you put downforce on the rear, I believe it’ll relieve some weight off the front per se.
    Dont know if there’s a way of working it out via percentages as such, hopefully someone will be along shortly and fill us in. I think it’s easy to see on older sprung cars where you gain “power steering”.
    Personally, I would think that the only real answer would be with a set of scales under each axle/wheel and see where it takes you. Drive through roadside truck scales would probably be ok for this, just a bit of faffing around to disconnect/reconnect.

  3. #3
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    I would have thought that roughly the weight that comes off the front would be proportionate to the distances between the rear axle and the tow ball vs the distance from the rear axle to the front axle.
    Lets say the distance to the towball is 1 metre from the rear axle, and the distance from the rear axle to the front axle or more correctly the mid point of the tyre ground contact is 4 metres then the uplift on the front would be 1/4 of the towball weight.
    So say the towball weight is 200Kg then the uplift would be 1/4 of that or 50Kg. The drop or lift would depend on the spring rates , so say for example the front spring rates are 100Kg per 25MM X2 , then the uplift would be 6MM roughly. The rear spring rates would also have something to contribute. They are usually harder than the front but the drop would be greater as the distance from the load is less. Lets say rate is 150Kg per 25MM so the drop at the back should be about 25- 30MM.
    Regards PhilipA

  4. #4
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    you are correct...

    the math is simple for static work...

    the total weight of the ball is added directly to the rear axle and then you add in a really complicated formula that adds extra weight as a function of the total leverage required to balance that weight including a funky bit that cancels some of it out as the math for the reverse is calculated in to account for the overhang in front of the front axle.... its not fun.

    heres the simple way, and remember if you do just this you'll have an answer, it wont be right but it will be pretty close.

    add the downforce of the ball to the back axle.

    calculate the distance from the hitch to the center of rear axle (set if you have a 6x6) multiply this by the downforce on the hitch to get a number...
    calculate the distance from the center of the rear axle (set) to the center of the front axle (use the same units as before) divide the previous number by this measurement.

    the number that comes out is how much you need to add onto the back axle and take from the front.

    so.... perfect world...

    I put 200kg on the hitch
    the hitch is 2m from the rear axle
    200x2=400.

    the front axle is 8m from the rear
    400/8=50

    the rear axle now has to support the original 200kg + an extra 50Kg of transferred weight. and the front axle is 50kg lighter.

    assuming you started out unladen, this is now as simple as plugging those numbers into the unladen axle weight of the landy (which once upon a time were included in the datasheet).

    if you have those and extra load the distribution math is pretty easy. each item (and we'll work purely on point load numbers for a single load point about the COG/center of foot print for each item) and in 2d so we dont have to worry about l/right loading)
    measure the distance between the 2 axles and then measure out as a percentage where on the distance the center of the load will be, invert the percentage and that is how much of the weight the axle bears...

    for example a load placed directly over the rear axle is 100% of the distance away from the front axle so the front axle bares 0% and the rear 100%.

    if the load overhangs an axle go through the maths for the towball again using the distance from the center of the load to the axle as the first number.

    Add all of those together and you will eventually come up with something thats within a few % of your axle loads.


    and then we get into dynamics.................

    which is a much longer and requires far more coffee than I've had today.
    Dave

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  5. #5
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    What are you actually trying to find out?

    The D4 handling dynamics have been designed with the ability to have 350kg on the ball.
    Over 250kg you need to reduce internal payload by 100kg to avoid exceeding rear axle weight.

    On lesser vehicles you would fit load levellers to compensate for the poor design.

  6. #6
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    [QUOTE=Tombie;2935724]What are you actually trying to find out?

    It has been quoted to me that when you put 350Kg on a tow ball, because of the see saw effect you are really putting nearly 500 kgs extra on the rear axle. As such you cannot load the car up to 350kg below GVM as the rear axle load has been exceeded. It was said that the New RAM cannot tow 3.5T because of that and neither can a D4. If we want to tow 3.5T then we need to use a DOG trailer. As I cannot find what the rear axle load weight is before a trailer is hooked on, I am just wondering what weight can we put on the ball before the axle load is exceeded. Whilst i read what LR says, that 100Kg seems to be a very "loose" type of figure. How do you remove 100Kg if you have nothing in the back? How much can i put in and still not be over? Then i can fire back with some authority. Physics was never my strong point at school
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  7. #7
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    [QUOTE=dirvine;2935726]
    Quote Originally Posted by Tombie View Post
    What are you actually trying to find out?

    It has been quoted to me that when you put 350Kg on a tow ball, because of the see saw effect you are really putting nearly 500 kgs extra on the rear axle. As such you cannot load the car up to 350kg below GVM as the rear axle load has been exceeded. It was said that the New RAM cannot tow 3.5T because of that and neither can a D4. If we want to tow 3.5T then we need to use a DOG trailer. As I cannot find what the rear axle load weight is before a trailer is hooked on, I am just wondering what weight can we put on the ball before the axle load is exceeded. Whilst i read what LR says, that 100Kg seems to be a very "loose" type of figure. How do you remove 100Kg if you have nothing in the back? How much can i put in and still not be over? Then i can fire back with some authority. Physics was never my strong point at school
    Just get your book out - drop it in front of the other persons face and point Weight Question

    Let me hazard a guess - they drive a Toyota? That’s where this generally comes from.


    In short they’re wrong, the answer is in the black and white. And you can’t exceed axle weight if the vehicle is empty Weight Question

  8. #8
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    No sure what they drive. I have had a look at my 16.5 D4 handbook and cannot see any reduction mentioned, But it did tell me what to add up when making sure carrying capacity was not exceeded.
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  9. #9
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    weights

    We have a registered weighbridge and have weighed many vans and vehicles. It is easy to calculate the weight on the steer with a van attached. We just place the front wheels on the bridge , take the weight, then take the van off. The difference will give you the weight on or off the front wheels. You can then attach the load distributing bars on and then take a weight reading. You can do this with the rear as well. What we have found is that the majority of vans exceed the stated tare weight of the van and also the towball weight when loaded. I have had people having to trade their Prado in on something heavier to tow, upgrade van suspensions, and postpone trips till they redistribute things like water tanks. Had one bloke who had to relocate water tanks from the front of the van to the back to reduce towball weight as with full water and gas the van far exceeded the towball limit. This van could only carry an extra 90 Kgs of bedding, clothes, food or crockery before it exceeded its A.T.M. and this was a new van.
    You would be surprised at how many vehicles are overloaded with the tojo/ nissan weights pretty frightening when you weigh individual axles.
    The other thing that needs to be considered is how far the towball protrudes out from the rear of the car or the rear axle as the further you go out the more weight you place on that rear axle due to the cantilever effect even though you towball weight hasn't increased.
    If anyone wants to play with weights pm me.
    Lindsay

  10. #10
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    Quote Originally Posted by oldyella 76 View Post
    We have a registered weighbridge and have weighed many vans and vehicles. It is easy to calculate the weight on the steer with a van attached. We just place the front wheels on the bridge , take the weight, then take the van off. The difference will give you the weight on or off the front wheels. You can then attach the load distributing bars on and then take a weight reading. You can do this with the rear as well. What we have found is that the majority of vans exceed the stated tare weight of the van and also the towball weight when loaded. I have had people having to trade their Prado in on something heavier to tow, upgrade van suspensions, and postpone trips till they redistribute things like water tanks. Had one bloke who had to relocate water tanks from the front of the van to the back to reduce towball weight as with full water and gas the van far exceeded the towball limit. This van could only carry an extra 90 Kgs of bedding, clothes, food or crockery before it exceeded its A.T.M. and this was a new van.
    You would be surprised at how many vehicles are overloaded with the tojo/ nissan weights pretty frightening when you weigh individual axles.
    The other thing that needs to be considered is how far the towball protrudes out from the rear of the car or the rear axle as the further you go out the more weight you place on that rear axle due to the cantilever effect even though you towball weight hasn't increased.
    If anyone wants to play with weights pm me.
    Lindsay
    Thanks for that tip Lindsay,, sometimes the most practical, easiest of methods eludes us,,, we’ll, me anyway

    As I’ve pointed out before about van weights etc. I think people forget that every item weighs something. It’s a pretty big burly bloke who can carry the weeks grocery shopping on his own,, let alone all the other combined items which are only bought say once a month or so. Pots and pans are weighty too, a few books/magazines nick nacks etc,, boots jackets handbags, etc. It all adds up.

    We’ve were informed our van can carry 600kgs, (legally written on the plate) which of course includes almost 200lts of water. It was easy to overload the forward fold camper, and I’m thinking it will be just as easy to overload the caravan. If your only using it for weekend jaunts etc, it’s probably much easier to keep in check. We’ll be living in ours, so some serious consideration, checking and experimenting of weights etc will be undertaken over the coming months.

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