Tow bar design calculations

Discussion in 'General Discussion' started by welland99, May 11, 2017.

  1. welland99

    welland99 Hardcore MB Enthusiast

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    I want to make a tow bar and need some help with calculations to make sure I choose suitable steel profiles. The tow bar won't be for towing, but only for attaching a bicycle carrier.

    The current idea is as follows:
    1) two pieces of channel pointing front to rear. These will have the open side downwards, so they look like a "n". They would be about 80 cm in length and the overhang at rear would be about 40 cm.

    2) resting on these two channels will be a piece of square or rectangular hollow section, that is transverse mounted. The hollow section will be 129cm in length, supported only on its ends by the two channels. The hollow section will be bolted to the channels. A flange tow ball will be bolted to the rear face of the hollow section.

    I estimate that the dead weight of bicycles, bicycle rack, hollow section and tow ball to be no more than 90kg; ie 45kg resting on each channel. (Plus more due to bouncing while driving).

    So the big question is what size profiles would be needed?

    1) channel: how about 51x38x5?
    2) hollow section: 60x60x5?

    I have no idea how to do the necessary calculations, so any help would be appreciated.
     
  2. whitenemesis

    whitenemesis MB Club Veteran

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    I'd be surprised if some sort of approval/testing/certification wouldn't be required, especially for insurance purposes ..
     
  3. a111r

    a111r Hardcore MB Enthusiast

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    The material dimensions are suitable, indeed overkill for the loading.

    Why not an 'off the peg' tow ball mounted bike carrier, even second hand?
     
  4. OP
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    welland99

    welland99 Hardcore MB Enthusiast

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    Thanks. The tow bar mounted carrier itself will be off the peg, but the tow bar needs to be bespoke.

    I agree that the size of the transverse hollow section may be overkill for a static load with purely vertical force, but it will be subject to some torsion forces, and I have no idea how to check the size required to withstand this.
     
  5. Pontoneer

    Pontoneer MB Club Veteran

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    Depends on the age of the car : after a certain date of first registration ( which I can't recall ) all towbars have to be EC type approved , bear the plate , and must fit to the vehicle using all the manufacturers fitting points .

    With earlier vehicles , you can pretty much do as you like - as the OP proposes .

    Edit - vehicles first registered after 1st August 1998 must be fitted only with EC type approved brackets .

    Towbar EC Type Approval

    The OP's S210 would fall into this category , but plenty of brackets available , and a breaker local to me had one in with the factory towbar when I saw it recently .
     
    Last edited: May 12, 2017
  6. MrMotul

    MrMotul Hardcore MB Enthusiast

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    Towbar

    I have just bought an S210 with a towbar which comes with what looks like overkill frames for mounting bikes. It should be coming off within the next week or so, I'll get some pics of it later, and if it suits your need please let me know. There is also a light board as well.
     
  7. OP
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    welland99

    welland99 Hardcore MB Enthusiast

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    Thanks for the suggestions. If it was for my 210, i wouldn't be doing this: I'd get one designed for the car. For my application, there isn't one available anywhere, so my only option is to build it myself.
     
  8. OP
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    welland99

    welland99 Hardcore MB Enthusiast

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    Would be very interested in seeing pics, Please, but it wouldn't fit directly in my application.
     
  9. whitenemesis

    whitenemesis MB Club Veteran

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    What vehicle is this for? Is it type approved for towing ( I understand you don't intend towing but..). The fact there isn't a tow bar fitting available might suggest it isn't?
     
  10. OP
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    welland99

    welland99 Hardcore MB Enthusiast

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    It's to fit to the rear of my caravan so I can carry bikes whilst towing, without using a roof rack. This solution will allow me to use the bike carrier on the car when towing has finished.
     
  11. mat8n

    mat8n Banned

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    Put the bikes inside the caravan while towing?
     
  12. whitenemesis

    whitenemesis MB Club Veteran

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    Cool..

    I'm surprised no one has thought of designing a caravan mount that mimics the tow bar. There must be many caravanners in your position, wanting to use the bike carrier on both?

    Potential market for you? ;)
     
  13. Darrell

    Darrell Hardcore MB Enthusiast

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    Something that I was thinking.
     
  14. Druk

    Druk Administrator Staff Member

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    Anything, especially an overkill steel frame, should NOT be mounted on the rear of a towed caravan. If you absolutely have to, make a frame that sits across the drawbar so the weight is then in the correct place. Having it at the rear is asking for trouble.

    As said earlier a simple internal frame is the best bet situated over the axle where the additional weight won't matter ( unless it takes you over the MTPLM of the van).
     
  15. Pontoneer

    Pontoneer MB Club Veteran

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    There are rear cycle mounts for caravans , although permanent fixtures which would not translate to the solo towcar .

    There are also cycle carriers which can be mounted either to a swan neck bar , or behind a flange towball so that cycles can be carried even whilst towing .

    Two concerns , which needn't be a problem if considered and allowed for - are re weight distribution - hanging cycles off the back of the caravan will add mass just where you don't want it ; likewise putting them over the towball would effectively double the noseweight .

    When we used to take the bikes on caravanning trips with the W123 and W124 estates , I had roof bars and four Thule roof mounted carriers . Sometimes , as suggested , we just put a couple of the bikes inside the caravan and used the seat cushions ( with sheets over them ) to pack the bikes in place over the axles ( it was a six berth van so we had plenty of cushions ) .

    With most caravans these days being on Alko chassis , I'm mildly surprised there isn't anything .

    For your DIY bracket , go down to your builders merchants and look at getting some Unistrut channel - perfect for your application and all sorts of fixings available - cheap too .
     
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  16. BTB 500

    BTB 500 MB Club Veteran

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    Many good points already made, which I would echo.

    Total weight of channel sections, carrier, bikes, towball etc. would be maybe 100kg, which could easily put the caravan over its MTPLM unless you are incredibly frugal with what else is on board.

    Modern caravans are lightweight structures so attaching that sort of weight would have to be done very carefully.

    Will you be able to achieve a sensible noseweight with 100kg hanging off the back?

    If you do balance the caravan OK ... with lots of weight at each end (like a dumbbell) the moments of inertia may well cause stability problems if there's any kind of disturbance or snake while towing.

    IMHO as mentioned it would be much better to carry the bikes inside the caravan, near the axle, and have a separate carrier to use on the car's towball when not towing.
     
  17. OP
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    welland99

    welland99 Hardcore MB Enthusiast

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    Hi All,
    Thanks for your replies. Many good points have been made.
    1) I can't put bikes inside, as it's a folding caravan, so it's not tall enough.
    2) I do have sufficient payload capacity to add bikes, rack and counterbalance at the front, without breaching the Max gross mass.
    3)I understand the principle of balancing the trailer, and what will happen if it is too heavy at the rear. However, many other factors also affect the stability on the road, and it is not possible to quantify the extent to which I would be moving along the line towards instability. I have a heavy car, so this will help somewhat in maintaining stability.
     
  18. OP
    OP
    welland99

    welland99 Hardcore MB Enthusiast

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    DIY design tends to result in overkill, when the designer cannot verify the precise strength in materials. After all, I am not trying to shave off as much weight as possible, at the possible expence of strength.
     
  19. Bobby Dazzler

    Bobby Dazzler MB Club Veteran

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    Sounds like you're clear that you want to mount to the rear of the folding caravan, however have you considered whether it's possible fabricate a structure to hold the bike above it, like a roof rack for the folding caravan?
     
  20. OP
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    welland99

    welland99 Hardcore MB Enthusiast

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    That would be even more complicated, and less desirable than using the roof of the car. The caravan roof cannot take the weight, so any frame would need to be mounted the the chassis, and have supports up the outside of the bodywork.
     
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