Homers Single Wheel Pull Behind Trailer Build

Discussion in 'Some Assembly Required' started by Homer GSA, Oct 30, 2016.

  1. Homer GSA

    Homer GSA R1200GSA 2008

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    Hi all

    I am going to document my single wheel bike trailer build and would love input from anyone who is also interested/experienced in this sort of stuff.

    First up. If your only input is "Bikes shouldn't tow trailers" or "Your'e carrying too much stuff" move along, nothing of interest here. That argument has been done to death. I get it, you don't like it, I respect your view, have listened to it and have decided to build it anyway. :-)

    WHY
    So the first question is why would I want a bike trailer? My R1200GSA is perfect for touring one up and can take all my gear using soft panniers, top box and strapping some gear to these. But the biggest issue I have is sleep. I hurt my back when I was 25 and now at 50 I just cant get any kind of decent sleep unless I have a decent bed. So over the last few months I have been borrowing all manner of sleeping pads/mattresses etc including the Thermarest and Big Agnes ones. Sadly, none of them suited. The only mattress that works is self inflating foam filled type that are pretty bulky. Over in Oz they are called 4wd mattresses and are made by companies such as 'Dune'.

    My wife has also started to ride with me and we love the outdoors and staying in National Parks and the like. So the storage space needed has effectively gone up at least a 1/3 if not a half. We would also like to tour Nth America and Europe when we retire in ten years and the bike is the ideal way. But at 60 I reckon my back will be worse so getting this nailed right before then would be good. Though we have an BMW GS most of our time will be on the blacktop, secondary roads and national parks. The badlands of Upper Mongolia are not on the horizon.

    So the brief is more about accommodating the bulk (not weight) of the sleeping gear.

    WHICH TYPE OF TRAILER
    I decided on a single wheel trailer as it keeps the wheels and the body in the bikes line. The two designs that seem the best, or at least that I like, are the http://n-line.com and the eaglemate style that is like a hand trolley but with one wheel. I have one of those spacecase's like the army uses that I want to put in the design as its lockable, tough, light structurally sound and I have one that I can use in the build and replace it with a new one later. I have attached a pic.

    ENGINEERING
    I am trying to work out how this single wheel type of trailer distributes the weight. I have a skp of the bike and the trailer with some measurements. Basically the centre of the rear wheel is about 300mm from the ground, the attachment point to the rear of the bike is about 1000mm and the overall length is about 2000mm. If the load weight is taken to be in the exact middle of the trailer what would the weight over the rear wheel and the bikes rear wheel be? I would have thought 50% of the weight on each. However, my son who is an electrical engineer suggested that because of the angle this may be incorrect. (sadly as there are no wires he couldn't help much more and was turning his mind back to yr 1 engineering).

    So thats the first question if anyone has some thoughts.

    Cheers

    Homer

    Attached Files:

    #1
  2. Homer GSA

    Homer GSA R1200GSA 2008

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    Here are some pics of the very basics.

    As can be seen from the picture from the front of the bike the case etc is invisible from the front.

    The case is 900mm long, 500mm wide and 400mm high.

    cheers

    Homer Bike trailer 7.jpg Bike Trailer 6.jpg Bike trailer 5.jpg Bike trailer 4.jpg
    #2
  3. boatpuller

    boatpuller Long timer

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    Good of you to be thinking about the engineering part of pulling a trailer. I have two, an n-line single wheel, and a common double wheel. The tongue weight on the single wheel is a lot. I'm surprised how well my big heavy touring Harley does with about 70 pounds of tongue weight. I'm not kidding about the tongue weight. It's enough to make me cringe every time I hit a big bump, as I wonder is this the time....? So I do my best to put the heavy stuff in back, but still....

    So, I will likely stretch the tongue to reduce the weight on the hitch. If the weight is right in the middle, then it's 50/50 wheel and hitch. make the weight 1/3 of the way to the wheel, 2/3 the way to the hitch, and it's 67/33 wheel and hitch, and so on.

    Another thing to consider, as you are building this, is creative place to put the hitch. Best would be to turn the tongue into a big U and hook it in two places on either side of the motorcycle well ahead of the rear axle. You might be able to find mounting points around the foot peg brackets, either passenger's or rider's. This does great things for you: lengthens the tongue to reduce weight on it; transfers the tongue weight to both front and rear, not just rear. This is how goose neck and fifth wheel hitches for trucks are designed.

    So the up down movement of the tongue would be at the pegs, and the side side movement would still need to be behind the rear the rear tire, as close as possible. So you'd need to have a vertical hinge pin in the tongue behind the bike, and horizontal mounting pins at the foot pegs. You'd also need to make the U sized to miss all components and panniers you'd want to carry on the cycle. It's possible you could do it with a single side arm, but I've not done the engineering on that.
    #3
  4. PHX_Joe

    PHX_Joe Adventurer

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    Interesting problem, I'm not technically skilled enough to offer anything other than support for the idea. Pondering such a trailer for a versys 650, total weight with me is about 650# so I think I could pull 100# without any issue, and the weight would be low and off the bike's suspension instead of high and overloading the rear. My unresolved concern is the mounting point - not sure I could mount to the swingarm (best I think), or build some form of drop down hitch from the rear end trellis. My concern is the geometry of a loaded trailer providing a lateral 'push' force to either the rear wheel or the tail in emergency braking. In certain circumstances, particularly wet or gravel, I could see that being tricky. That might be fully resolved by reducing the weight of it all. Even 50# of gear, presuming a 50# net weight trailer assembly, is a both a lot of gear and a lot of mass to take off the top of the bike's back end. Have you made any progress on the project?
    #4
  5. k-moe

    k-moe Long timer

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  6. Dan Diego

    Dan Diego Long timer

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    Interesting indeed. I found this gem while search for KLR info:

    Norwegian KLRista Jørgen poses with his KLR650 and the trailer he had custom made (for less than $100).

    So I know it's do-able IMG_0162.JPG
    #6
  7. PHX_Joe

    PHX_Joe Adventurer

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    Great heads up, thanks.
    #7
  8. Homer GSA

    Homer GSA R1200GSA 2008

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    A bargain buy on a pair of Softail Alloys with good tyres came up so grabbed them. I like the fact they are lightish, look ok and use a simple 3/4" axle to fit them.

    Started playing around with a very basic setup using our old moving trolley really just scoping out how things worked and was really astounded by the results of moving the weights and angle around. There is a little bit of weight past the wheels but it couldn't be much, maybe 1/2 kg or so.

    The trolley weighs 14kg. (30lb)
    The front wheel weighs 17kg. (37lb)
    The rear wheel weighs 19kg. (42lb)

    I laid the trolley on the ground, put the front wheel in the middle to replicate the load (combined weight 31kg (68lb)) and the trolley handle on the scales. I dont think it was quite in the middle and the weight showed 9kg (20lb). Which is half the of the load. So the actual weight of the 'trailer' has little bearing on the 'hitch' end. This is most probably down to the trolley having most of its weight down the back similar to the eagle style of trailer, which from here on I will call the "trolley trailer"

    Test 3.JPG
    I then moved it to the rear of the trolley and the weight dropped to 6kg (13lb).
    Test 5.JPG
    So interestingly simply moving it to the rear dropped the weight at the 'hitch' by 30%. Then I added the rear wheel which made a combined load of 36kg (80lb) plus the trolley at 14kg so 50kg (110lb) all up. The hitch weight showed 23.6kg (52lb) which sounds correct at a little under 50% of the weight.
    I moved the weight to the rear of the 'trailer';

    Two wheels flat.jpg

    Next came the interesting part. I raised the front of the trailer about 450mm off the ground;
    Two wheels raised.jpg

    And the weight at the hitch DROPPED to 11.6kg (25lb). I then raised it to 600mm and the weight at the hitch DROPPED to 8.2kg (18lb). At this point the angle of the 'trailer' was 19degrees. At this point the mozzies began to feast so I packed it in for the evening. I will redo the test to check things are make sense but based on this the theory of many that hitching a trailer on a bike puts too much down force is most probably not correct.

    The Trolley trailer is looking like a possibility.
    #8
  9. PHX_Joe

    PHX_Joe Adventurer

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    Great test - well done. Not sure I'd like to lift the hitch up that high, but that's a gut reaction, not scientific. Love the KLR pic above, Don Diego.
    #9
  10. Homer GSA

    Homer GSA R1200GSA 2008

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    Thats my gut instinct too!. Will have to think through a scientific way to work out the pros/cons.

    cheers
    #10
  11. DRjoe

    DRjoe Long timer

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    With the hitch up high it'll want to pull the bike over when you go round a corner.
    #11
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  12. Homer GSA

    Homer GSA R1200GSA 2008

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    HI DRjoe

    One of the things that I am trying to do in this thread/build is determine facts scientifically, not what has been doing the rounds for years.

    Don't take this the wrong way, but do you have anything to support this? (Again, not argumentative just questioning).
    As you can see by the tests so far the theory that a trailer puts too much force over the rear wheel is not supported.

    Riding in to work today I was thinking of this exact issue. If my 70kg pillion, sitting on the rear seat doesn't pull the bike over would 10kg 100mm further back pull the bike over?
    #12
  13. DRjoe

    DRjoe Long timer

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    Ok an easy way to see what I mean is for you to sit on the bike with your feet down now get someone to pull the top of the bike at a slight angle sideways.

    Obviously the more extreme the angle of turn the more effect it will have on the bike.

    It's not the weight acting on the bike but more the drag of the trailer.
    Remember that old guy and his silly law about opposite an equal forces.

    The other factor from towing up high like that is when accelerating it will lighten the front wheel and decelerating will weight the front more so than if the pull of the trailer was lower.
    Think about it as pivots and levers
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  14. Homer GSA

    Homer GSA R1200GSA 2008

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    At walking pace you may be correct. But that would be no different to a pillion. Well the pillion would be 60kg worse that 10kg of force from a trailer. Once the bike is moving centrifugal keeps it from falling over. The centrifugal force also acts on the wheel on the trailer so it will also want to stand up.

    "The other factor from towing up high like that is when accelerating it will lighten the front wheel and decelerating will weight the front more so than if the pull of the trailer was lower."

    Not so sure. There is no leverage off the trailer just the force over the rear wheel. There is nothing to leverage off with regards to that force. So it should be no different to a pillion, only much less. The factor for this being an issue is how the force of pulling the trailer from a standing start affects the force on the rear of the bike.

    Thanks for the input. More to try and study.
    #14
  15. MotoBoss

    MotoBoss Old Dog

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  16. DRjoe

    DRjoe Long timer

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    Sorry I'm not really very good at explains things so other than drawing some diagrams or your bike to point out how it's to do with the pull of the trailer rather than the weight then I think the only way for you to learn is to try towing something (anything) from high up on your bike.

    As soon as the pull forces from the trailer go from being in line with your bike (pulling back on it) to being at an angle when going round a corner (more evident on slow tight corners) it will start pulling at an angle so same forces as if someone walked up an pushed you bike from the side.
    Your bike would want to tip over.

    Again it's not a weight of the trailer pushing down on the bike that's the issue, its the pull and push of the trailer acting on the the bike.

    That is until the bike is pulled off its natural angle of lean then the weight of the trailer, bike and rider will start pushing down resulting in what is know in the scientific world as a crash.


    Same for my second part.

    Simplistically look at the rear axle as the pivot (in real life the bike will want to rotate round a point further forward and slightly higher) draw a line up from the pivot to your tow point.
    That's your pivot and that's your lever.
    Pulling back on the lever will want to lift the front of the bike and pushing forward on the lever will want to lower push the front down.
    If that levers shorter the effect the pain and pull of the trailer will have much less effect on the bike.

    Again that's a simplistic way of looking at it.
    Reality is the bike will actually pivot around a point further forward and slightly higher but the effect will be the same.

    Just remember its not just the weight of the trailer paining down on the bike you need to be concerned about.
    It takes a force to accelerate and decelerate that trailer and for the force the bike exerts on the trailer to accelerate or decelerate it the trailer exerts the same force back at the bike.


    And if that's not a scientific enough expiration then try an experiment to test the idea.

    Tie a rope to your top rack and the other to bag of sand or some bricks the tow them around your yard.
    This will simulate the drag of a tailer under acceleration wilst doing contrlable safe speeds.
    This will also take weight acting on the trailer out of the picture.

    Going straight you'll feel the load on the bike pulling the rear down and the front up then if you go round a corner you feel as the direction of pull moves off centre from the bike and start to try and pull the bike over.
    #16
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  17. goforth

    goforth Adventurer

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    I just designed and built a small pull behind single wheel trailer. Well a friend and I did together and we have one for each of us. One thing that we learned is that with a single wheel trailer there is significant lateral forces placed on the hitch from the trailer when you change direction. Imagine a small passenger ( the load of your trailer + gear so 100lb child ) pushing as hard as they can on the inside peg after you tip into the corner and then pushing as hard as they can on the outside peg once you stand it up coming out of the corner. This is the same force the front wheel has on the steering head of a motorcycle taking the front and rear wheels out of line. If you have ever ridden old flexy bikes hard you will know the feeling.

    So my concern with mounting a single wheel trailer up hi on the sub frame is not that the centre of gravity is too high but is the sub frame stiff enough laterally to hand the inputs from the trailer. Mounting to the swing arm has an advantage because the frame and swing arm of a modern motorcycle is quite stiff laterally. The sub frame on some bikes may be stiff enough but I'm betting none of them where designed for lateral stiffness the way a main frame and swing arm were.
    #17
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  18. Homer GSA

    Homer GSA R1200GSA 2008

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    Thanks DrJoe and goforth for your input. Much appreciated.

    Luckily as I can do my own fabrication I can muck around with different setups. The kids have a couple of old pushbikes so I might pilfer the frames and wheels and make up a system to check out the forces.

    cheers all
    #18
  19. Homer GSA

    Homer GSA R1200GSA 2008

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    Any chance you could post a pic and check the hitch weight?
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  20. Homer GSA

    Homer GSA R1200GSA 2008

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    #20