Getting axles weighed at weighbridge

Discussion in 'Tech/Mech General' started by stevec, Jan 29, 2016.

  1. stevec

    stevec Funster

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    My local weighbridge has a ramp on each side. I'd read that for accurate readings the weighbridge should have level approaches. I've been doing some experiments with a wooden box (actually one of my bookshelf speakers!) and some kitchen scales. I'm finding the differences between level and angled readings (eight degrees) to be less than 2 percent. This was over several readings averaged. I'm sure that with the rear wheels at the end of the ramp (just before the weigh plate) the MH would be at less than eight degrees. Can someone offer any mathematical explanation of why it is so bad to use a weighbridge with an angled approach. I can appreciate it depends on the angle. Maybe I'll check the inclination next time I use that particular weighbridge.
     
  2. funflair

    funflair Funster Life Member

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    Quite simply the weight transfers to the lowest wheels relative to the inclination, so if you just put the front axle on and left the rear down the slope and then did the same with the rear you could be out a bit, if you weight the whole thing and then take one axle just off the plate you can't be far off at all.

    Martin
     
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  3. John Laidler

    John Laidler Funster

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    Don't worry, a small lift will move the centre of gravity slightly further away from the end which has been raised but unless the weighbridge is on a massive raised plinth I would guess the error would be less than one percent. I could do the sums but it isn't worth it for such a small discrepancy.

    To give you an idea imagine a long ladder lying on the ground. Let's say the c of g will be in the middle of the ladder. Now raise one end of the ladder six inches. Because the ladder is pivoting around the other end the c of g will move back a bit but only a fraction and the end of the ladder will still feel the same. Even at an angle of 45 degrees the weight will have only reduced by about 30%.

    If you can, weigh each axle then the whole MH and from the latter you will be able to see if anything was lost, but I suspect the margin of error from the scales will be greater!
     
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  4. John Laidler

    John Laidler Funster

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    Just done a few sums. If the MH has a wheelbase of 5m and you raise one axle by 100mm then the c of g will move back by about 9mm. (had to dust off my sines and cosines for that one!)

    A rough and ready look at the impact on axle weights suggests a reduction on the raised axle of 0.3% which isn't worth worrying about. The angle incidentally of the MH I calculated as 1.15 degrees.
     
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  5. stevec

    stevec Funster

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    Thanks for the replies. That means I don't have to look out for another (flat) weighbridge!
     
  6. gus-lopez

    gus-lopez Funster

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    Are you bored ? :) Snowed in ?:D
     
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  7. Bertie Bassett

    Bertie Bassett Read Only Funster

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    Despite what Gus says:D thanks a lot DBK Our local weighbridge does have a small incline and each time I've done the weigh I've flapped about the accuracy. For putting my mind at rest 12 beers of varying types should we meet somewhere in Espana.(y)
     
  8. Wyaye wires

    Wyaye wires

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    Just take the bloody axles off, weigh them, replace job done!!!





    Not very helpful, I suppose...
     
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  9. Bertie Bassett

    Bertie Bassett Read Only Funster

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    You're daft:LOL:
     
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  10. DavidG58

    DavidG58 Funster

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    I weighed mine on a weighbridge at a customers site close to us, they have a ramp nearly 1m high to accommodate the gubbins at floor level underneath the plate

    it more or less confirms all above, we had a total weight of roughly 10% more than the sum of both axles which would have been offset by the effect of the slope (y)

    I cant find my notes now, but did put a thread on at the time, will go off on a search for that now :) cant do the maths but can search :)
     
  11. DavidG58

    DavidG58 Funster

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  12. Wyaye wires

    Wyaye wires

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    I defer to superior knowledge concerning my grading of daftness...

    But I could not disagree with my grading...
     
  13. funflair

    funflair Funster Life Member

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    Not wishing to be pedantic but don't you need to know the vertical position of the centre of gravity to do the calculation as the higher it is from the base line the greater the effect will be, either way I don't dispute it will make little difference, just thought the little grey cells might want more exercise!

    Martin
     
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  14. Wyaye wires

    Wyaye wires

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    Go have a drink old friend...
     
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  15. funflair

    funflair Funster Life Member

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    I have.

    I think that's the problem.

    I'm spoiling for a fight now:LOL:

    Martin
     
  16. John Laidler

    John Laidler Funster

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    You're right of course, I just took it as halfway between the axles and at ground level, which it isn't but good enough for government work as the saying goes.
     
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  17. Munchie

    Munchie Funster Life Member

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    Listen you lot. One gram either way might be ok for you furriners! In Yarkshire it as t be "cock on" d'int tha nose!
     
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