Thick maths/physics question

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  • Thick maths/physics question
  • mildred
    Member

    Ok, this is driving me nuts – We’re new to caravanning & currently loading it up (how I miss the motorhome – chuck it all in & drive).

    I’ve been trying to sort the nose weight out – my yeti has a tow-ball weight limit of 85kg. When I measured the van’s nose weight it was 110kg and bent the base of the scales..! This didn’t seem right given there was nothing particularly heavy at the front (even the gas bottles are in the centre over the axle), so I’ve been wading through the many caravan forums looking for clues. I’ve had enough.

    I did happen upon some advice saying that the nose weight should be measured with the hitch at the same heights as the tow-ball. My scales were a good 12cm lower so I chocked these up to the correct height and measured again – lo and behold it dropped 10kg by magic. WHAT IS GOING ON?

    Thinking I’ve been losing the plot I used my father in law’s scales and measured again – same result it loses weight by lifting the nose. In other words, the point at which the hitch makes contact is now higher and results in a lower weight. Why is this? Also, nothing moves in the van – its all static & well secured.

    when going downhill on your bike, if you acted as a “static load” would you have more weight through the front wheel then when on flat gorund?

    something to do with Fulcrums

    trail_rat
    Member
    crikey
    Member

    Think of a seesaw. It’s easier to lift when it’s horizontal.

    Premier Icon matt_outandabout
    Subscriber

    Move a gas bottle to the rear bunk?

    Premier Icon aracer
    Member

    Just to simplify things, imagine there is a weight directly above the axle. With the hitch at the correct height and the caravan level that will have no effect on nose weight. If you lower the hitch, then the whole caravan rotates around the axle and that weight is now in front of the axle rather than directly above it. Therefore it now contributes to nose weight.

    What you’re seeing is a slightly more complex version of that – also weight not directly over the axle will move forwards when you lower the nose, increasing the nose weight.

    Nose weight is a bit misleading term – your measuring the moment weight. If you’re hitch was too low then you’re shortening the moment arm length as it’s not horizontal and therefore increasing the force.

    mogrim
    Member

    Basically weight has two “components”, a horizontal one and a vertical one. When the bar is 100 horizontal all the weight is pushing down, when it’s on a slant some of the weight is pushing forwards.

    You can try it out with some bathroom scales – weigh yourself, then weigh yourself as you lean forward with your hands supporting yourself against a wall. In the second case you’ll feel some of your weight in your arms, and the measured weight on the scales will drop.

    Premier Icon maccruiskeen
    Subscriber

    Also, nothing moves in the van – its all static & well secured.

    Its not static though – remember that the whole thing has weight – not just the things you put in it. And it is all moving – when you raise or lower the hitch the whole weight of the caravan is pivoting around the caravan axel – everything pivoting past the axel as you lift is weight moving from one side of the axel to the other.

    Premier Icon maccruiskeen
    Subscriber

    Supplimentary question as you’ve attracted all the clever people to the thread.

    I sometimes need to weight items that are much bigger than I can put of a set of scales…..

    Say you have a irregularly shaped object that touches the ground at three points but its as big as a car. If I wanted to weigh it with a set of bathroom scale would I be right to thing that if I put the scale under each ‘foot’ in turn and added the three readings that would give the total weight of the object?

    Premier Icon edhornby
    Subscriber

    As crikey and Mattoab say, move the mass about in the van , if the mass is behind the rear axle it will lift the rear car axle and put the weight onto the nose.

    mogrim
    Member

    Say you have a irregularly shaped object that touches the ground at three points but its as big as a car. If I wanted to weigh it with a set of bathroom scale would I be right to thing that if I put the scale under each ‘foot’ in turn and added the three readings that would give the total weight of the object?

    How far apart are they? By pivoting up the leg (to get the scales underneath) you’ll be lowering the measured weight.

    But yeah, think that should work (given the above proviso). Easy to test anyway, just weigh yourself doing a press up (left arm, right arm, both feet on the scales).

    mogrim
    Member

    if the mass is behind the rear axle it will lift the rear car axle and put the weight onto the nose.

    Other way round, surely?

    mildred
    Member

    That’s brilliant – you’re all the best..!

    I instinctively knew there was an explanation but just couldn’t work it out – my inner head voice kept saying “its a seesaw innit” but that’s where my brain stopped working.

    mildred
    Member

    I can’t move stuff further back – apparently we all die in a lay-wasting mushroom cloud if you counter it by putting stuff at the back – van starts to pendulum if you get a weave on.

    Premier Icon maccruiskeen
    Subscriber

    one less caravan though – so not all bad 😆

    Put the wife in the back

    Hth
    Regards

    Les Dawson

    Premier Icon aracer
    Member

    thestabiliser wrote:

    Put the wife MIL in the back
    Hth
    Regards
    Les Dawson

    Premier Icon aracer
    Member

    MacCruiskeen wrote:

    Say you have a irregularly shaped object that touches the ground at three points but its as big as a car. If I wanted to weigh it with a set of bathroom scale would I be right to thing that if I put the scale under each ‘foot’ in turn and added the three readings that would give the total weight of the object?

    For the same reasons as with the nose weight, no – if you lift the supported point you’ll decrease the weight on it. To get it accurate you need to have supports the same height as your scale to put under the other two support points (or 3 sets of scales).

    Premier Icon theotherjonv
    Subscriber

    For the same reasons as with the nose weight, no – if you lift the supported point you’ll decrease the weight on it. To get it accurate you need to have supports the same height as your scale to put under the other two support points (or 3 sets of scales).

    This. My Dad used to weigh oil rig modules and the like with arrays of load cells under them. By spacing the array you can not only tell the weight but also how it’s distributed, which is useful before you put it on a barge on the Tyne and find it’s off centre and rapidly disappearing to the bottom.

    Premier Icon Greybeard
    Subscriber

    The reason the weight reduces when you level it out is that the centre of gravity is higher than the hitch. Same as going downhill on a bike, you have to shift your weight back or you get extra weight on the front.

    The reason a caravan needs such a high noseweight is that that the wind pressure on the front, at speed, pushes it backwards, which lifts the front. If you don’t have enough noseweight, the van lifts the hitch – either taking weight of the back wheels of the car, or if the hitch isn’t well looked after, uncoupling it.

    Nudie or Textile?

    I’m a bit rusty on this sort of thing but I dot think the analogy of a weight on a slope, or a mountain biker on a steep hill is not the correct analogy in this case. A more accurate analogy is that of a see-saw. the caravan is effectively a see-saw trying to rotate around it’s axel. And like with a see-saw, if it is horizontal and you push it down with someone sat on the other end, you have to push harder and harder as you move it towards the ground. This is because the effective moment arm is reducing in length and the component is force x distance, so as distance reduces the force has to increase.

    The component forces are different in both cases – as shown i the diagram above, in the weight on a slope analogy you have a component force parallel to the slope. You don’t have this with a see-saw/caravan as the pivot is restraining the motion to an arc.

    Probably a very similar result by the time you’ve plugged in all the numbers but the model is not the same.

    Xylene
    Member

    Stay in a hotel. Job done.

    avdave2
    Member

    Forget all this caravan nonsense I think we should all be celebrating the invention of the mogrim diet!
    Instant results guaranteed.

    mogrim
    Member

    Forget all this caravan nonsense I think we should all be celebrating the invention of the mogrim diet!
    Instant results guaranteed.

    Pleased to be of use 8)

    Premier Icon Cougar
    Subscriber

    Get a broom, stand it vertically and balance it on your hand.

    Now angle it at ten degrees and try it again.

    pdw
    Member

    The slope analogy isn’t relevant here, but it’s not really about the effective length of arm changing either. It’s primarily because the centre of mass is above the point about which it is pivoting, which is the axle.

    Imagine a trailer that weights nothing, except for a big lump of lead fixed a metre above the axle. If you hold the towing arm horizontal, the weight is directly above the axle, and no force is needed to hold it there. Lower the towing arm down, and the weight on it increases as the weight moves further forward of the axle.

    The reason nose weight on a trailer is important is because it’s a measure of how far in front of the axle the centre of mass is, and having the centre of mass in front of the axle is critical to stability.

    Premier Icon matt_outandabout
    Subscriber

    If moving weight back is wrong, surely you can reduce negatives by moving weight to nearer the axle?
    Take less stuff?

    Premier Icon benp1
    Subscriber

    By lowering the towhitch, you’re actually moving more weight in front of the axle, the opposite for when you raise it. It’s because you’re pivoting more than 50% of the mass of the caravan in front of the axle (as the axle is underneath the caravan, and not through the middle of it)

    For an extreme, imagine you lowered the hitch so much is was about 10 cm in front of the axle, would weigh loads then

    It’s like the centre of mass/gravity thing

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