Trailer calculatiom formula

  1. amron Member

    Messages:
    131
    Hi I would like to build a trailer and I would like to know if there is some sort of formula to calculate the center of the axle/wheel and the length of the tow bar. I would like to make a flat utility trailer about 2m long by 1.3m wide (single axle, without brakes).
    Thanks.
     
  2. I would say for the size of trailer you are building, how you load it will have more of an influence on weight distribution than anything.

    Nose weight is the important thing, not too much and certainly not negative.
    Look at most trailers and the wheel is centred under the load bed. For caravans they are centred under the body. If building twin axled the centreline of the two axles is about the centre of the body/load bed.

    The distance form the towbar to the axle will have an influence on the dynamics of your car. Best to try and minimise this if possible.

    If you are building a trailer without brakes the maximum all up weight is 750kg. For smaller cars even this may be too much.
     
  3. rtbcomp

    rtbcomp Moderator Staff Member

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    Your car manufacturer should specify the maximum nose weight on the tow ball. You can get gauges (Towsure &c) to measure nose weight as you load the trailer.

    As for the length of the drawbar make sure it's long enough to allow you to turn corners without catching the trailer on the car.
     
  4. zzr1200

    zzr1200 Working at 650 ft on open steel work

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    Which type of suspension are you thinking of using, this affects the answer by about 4 to 6 inches????

    Plus do you wish to have a single box section draw bar or an "A" bar using angle iron?
     
  5. rtbcomp

    rtbcomp Moderator Staff Member

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    Regardless of the suspension type, align the centre of the bed with the centre of the hubs.
     
  6. Pete.

    Pete. Member

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    Yep center of the bed in line with center of the hubs and don't include the drawbar in that measurement. I always load a small trailer so that the drawbar can be hooked-on by hand, by one strong person with average effort.
     
  7. pauld Member

    Messages:
    138
    Southampton
    The trailer I have is about that size. It is a Caddy 640S, has an A-frame tow bar, with the coupling being about 1m from the body of the trailer. http://www.caddy-trailers.co.uk/caddy640s.php
     
  8. Bluekoop Member

    Messages:
    24
    australia
    Abour 50mm behind centre of the bed is what i make them.
     
  9. zzr1200

    zzr1200 Working at 650 ft on open steel work

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    I disagree..

    If you can refer to the chassis drawing/plans Indespenion used to publish the axle beam was placed/welded in the centre of the load bed, the suspension unit arms partly controlled the nose weight because the wheel centres had been moved reward's.

    If you have the wheel centre in the centre of the bed and with a totally evenly distributed load, the only nose weight you will have is the weight of the tow bar. Thus if using an indespenion type unit the axle beam is welded in the centre of the bed the arm then moves the wheel backwards proving nose weight, if leaf springs are used the axle/wheel centre should be moved reward by about 4 inches from the centre of the bed. When a leaf spung trailer is loaded the axle will move backwards due to the compression of the spring, increasing the nose weight.
     
  10. Pete.

    Pete. Member

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    Yes, hence why you load the trailer in order to make sure you have weight on the drawbar. No matter where you have the axle (unless it's right at the tail) it's possible to have a negative-load on the hitch by loading the bed incorrectly.
    The weight of the drawbar & hitch is all that's needed for an empty trailer IMO. Moving the axle/hub centres rearward a little would be just fine - perhaps desirable - but moving it forward is a big no-no.
     
  11. rtbcomp

    rtbcomp Moderator Staff Member

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    Back to the original question.

    You need to know the following

    Minimum nose weight, M
    Weight of the trailer (excluding hubs & wheels), W
    Horizontal distance from centre of gravity to hubs, H
    Horizontal distance from towball to C of G, B

    Take moments about the centre of the hubs,

    M * B = W * H
     
    Last edited: Feb 14, 2009
  12. rtcosic

    rtcosic Member

    Typically car owner's manuals say 50-75kg nose weight. MAXIMUM I've seen myself is 100kg, but I stand to be corrected on that.
    Check your vehicle manual.

    Generally for light general purpose utility trailers I use the eyeball estimate of 'can I lift it onto the tow hitch' when loading them - which gives around 50kg.

    If you plan on loading the trailer bed evenly and have an unbraked 750kg gross weight then setting the axle centre line around 55/45 to 60/40 of the bed length is 'usually about right'. Unless you have an unusually long/heavy drawbar on the trailer.

    Remember loading a trailer correctly is very important.

    Go look at the cheapy ones in Allfrauds or the DIY sheds to see what I mean.

    But if this is a dedicated special purpose trailer - such as a boat/bike/jet ski trailer - then the weight distribution will NOT be even and you should do the whole moment calculation thing as descibed by rtbcomp.

    Our colonial cousins seem to spend an inordinate amount of time discussing this subject - put 'trailer' in their search function and spend two days reading!


    http://weldingweb.com/showthread.php?t=27031&highlight=trailer

    I will admit to having resorted to loading the bed of a trailer under construction with a known number of evenly laid out bricks of known weight (up to the planned capacity), putting the bathroom scales under the planned hitch and moving axle stands back and forth until I got 50-75 kg nose weight I wanted.

    'Bush mechanic' who can't be bothered to do the calculations method! Might be useful if this is dedicated jobbie and you put the intended load on the bed to do the trial and error testing.
     
  13. zzr1200

    zzr1200 Working at 650 ft on open steel work

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    Bad loading is always a hazard, but bad design is no excuse, I believe these peeps may know a thing or two about axle locations on trailers
     
  14. rtcosic

    rtcosic Member

    Indeed they do - best designed/built trailers I know, tow beautifully, but ye Gods they're heavy! Every farmer round here seems to use them and I shudder to think what loads they carry relative to their rated capacity!

    By the way this is how NOT to do it:

    http://picasaweb.google.com/robert.cosic/LandRover?feat=directlink

    Eldest son knocked up a trailer from an old caravan chassis - which are physically big but designed for relatively light loads. FAR too much nose weight and vast chassis deflection from a wildly overloaded rig.
     
  15. rtbcomp

    rtbcomp Moderator Staff Member

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    Not as bad as a friend of mine who built a trailer box out of chipboard and bolted the Indespension units straight onto the floor.
     
  16. zzr1200

    zzr1200 Working at 650 ft on open steel work

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    Indespension used to show a diagram of their units under load, they suggested that it took a 500% over loading to damage the unit...:o


    Farmer tonnes are a lot like the country mile....:laughing:



    Caravan chassis trailers are not worth the effort of cutting them up to start with let alone the welding, imo.

    rtcosic, are they Lawn Mower Racing????
     
  17. zzr1200

    zzr1200 Working at 650 ft on open steel work

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    Nice....Einstein Moment.....:clapping:
     
  18. rtcosic

    rtcosic Member

    Don't let eldest hear that!!!!

    He was the founder/team manager/project manager/chief engineer of:

    http://www.fullblueracing.co.uk/ Go to Media Gallery 2007

    This was their first ever attempt in 2007 at Formula Student Germany.
     
  19. zzr1200

    zzr1200 Working at 650 ft on open steel work

    Messages:
    3,560
    Location:
    Brimington, Chesterfield, Derbyshire
     
    Last edited: Feb 14, 2009
  20. ghound

    ghound Member

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    251
    norn iron
    I was a manager at indespension for a few years, and from experiance the guys at there head office can be quite helpfull if you give them a call, ask for the guys in axles or R+D they can supply the details you need.
    There suspension units are fine for light loads, but he reason most farmers / builders buy ifor williams is because they are on leafs and much cheaper / easer to repair at home, and they usually have them over loaded a good part of the time. The beam axles cannot be repaired, and cost a fortune.
    I can remember changing around 15 plant trailers under warranty to one company because of the issue of beam axles bending when the trailers are worked hard under load.
    If you are set on indespension beam axles, there doing deals on them on there website.
     
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