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Process of designing a hitch for my motorcycle

  1. Nov 15, 2009 #1
    I am in the process of designing a hitch for my motorcycle.

    Realistically it is strong enough unless i pull more weight than i really should. That said I still should make it stronger in case of the foreseen, like when i got rear ended.

    Before i get to the pictures, what I have now is strong enough to pull a 580 lb bike behind my bike, as well as the 40 50 lb trailer. Its a bit too much weight, however this is the problem. It can hold the weight just fine, but if there is much force applied in a forward motion, for example when i got rear ended, or when i had to push start the bike while attached via trailer the 2 lower arms get bent thus demolishing the hitch.

    Mainly what im asking is for some advice as to design it. There really isent anywhere to attach it other than the subframe (seat) and bolted into where the rear foot pegs bolt into. With the new design (created in photshop) I worry about 2 things. 1, the part of the hitch that holds the ball will no longer have as much support, 2, the old design had 2 square bars holding it from moving left and right, now i have a plate above the wheel that may or may not be enough to hold it from moving left and right.

    Thank you for your help.

    Current design (currently demolished by front and back motion)
    oldhitchdesign.png
    IMAG0002.jpg

    This is the proposed design, im also thinking of goign to the metal supply for grade 8 steel rather than whatever home depot is selling.
    remember, photoshoped so doesent look perfect.

    newhitchdesign.png
    newhitchdesign1.png
    newhitchdesign2.png

    Thanks again for your advice.
     
  2. jcsd
  3. Nov 16, 2009 #2
    Re: Motorcycle

    That is WAY too much for a motorcycle-pulled trailer. The weight of the loaded trailer will easily pull you off-balance. If nothing else, your clutch and brakes will be fried in no time.

    You should check with commercial builders for advice on hitch design, tongue weight, and max load.

    My father had a trailer on his bike, and the hitch was attached to the swingarm so it didn't interfere with the bike's suspension.
     
  4. Nov 16, 2009 #3
    Re: Motorcycle

    I would also be concerned about the effects of you slamming on the brakes:
    1) - they aren't going to slow you down nearly like they do w/o the trailer
    2) - the trailer & the towed bike could unload your rear tire (even less braking)
    3) - the trailer & the towed bike could induce some side force on your rear end, especially if you brake hard in a corner

    This looks like a seriously dangerous project to me.
     
  5. Nov 16, 2009 #4
    Re: Motorcycle

    I agree with you there. the most weight that you should pull with a vfr is going to be around 300lbs.

    the average that I am pulling is around 50lbs.

    I am surprised at how well this did work going down the highway even.

    my issue still stands however. I need to make this hitch stronger. for safety reasons. if I get hit at a faster speed I do not want the lower arms to end up in the rim and lock it up going down the road.

    as for the swing arm. this bike has a single sided swing arm and there is nowhere available to clamp the hitch to. the only option would be tig wielding, but I am not prepared for that quite yet. my current design does not require any modifications to the bike.

    but the hitch still needs to be made a bit stronger before calling it finished.
     
  6. Nov 16, 2009 #5
    Re: Motorcycle

    Well. first, pulling the bike isent something i plan on doing on a regular basis. at the least not one this heavy. I would be doing this once more for the fun of it than anything. It is simply too much weight.

    For a bigger bike than a vfr pulling another bike isent a big deal, however trailer brakes would be a must.

    As for the side to side force, this is a pretty big problem with this amount of weight, thus the reason its too much weight for this bike. the most i typically pull is about 50 lbs including the trailer.

    so the weight isent the problem. the problem is the two lower arms bend when forward force is applied, for example, slamming on the brakes as you mentioned, being rear ended.

    this is the problem that i need to fix. and i guess hardening the steel wont keep it from bending, thus my next one will be built with either 1/2, or 1/4 thick steel rather than 3/16.

    My question is what can i change in the design to make it stronger to forward force so that the bottom peices will not bend or bow when forward force is applied.

    suppose i could slightly modify the bike slightly and the 2 arms comming down from the top could be angle iron, this would help prevent the bowing of the 2 bottom arms?
     
  7. Nov 17, 2009 #6
    Re: Motorcycle

    I would use tubing. You'll get much more stiffness with less weight.
     
  8. Nov 17, 2009 #7
    Re: Motorcycle

    What type of tubing are you referring to. I ask because you mention less weight, wouldn't tubing have more material thus more weight.

    with square steel tubing i am curious about the best way to attach it. using flat stock i can put a bolt right though, but if i put a bolt through a 1inch square tube then into the frame wouldn't bolting it through the tube give the bolt room to wiggle and eventually brake?
     
  9. Nov 18, 2009 #8
    Re: Motorcycle

    This is just for a comparative example of bending strength for equivalent weight of material. I do not know if these materials are adequate for your needs.
    • Solid steel strip, 1.0" x 0.25" x 24" = 1.7 lbs (approx.)
    • Square tubing (16ga), 1" x 0.063" x 24" = 1.65 lbs (approx.)

    Some quick calculations using http://www.google.com/search?q=beamboy+freeware":
    Maximum Deflection with 200 lb point load (centered) for 24 inch span:
    • 1", 16ga Tubing = -0.0553 in.
    • 1/4" Strip (hard way) = -0.0922 in.
    • 1/4" Strip (easy way) = -1.47 in.

    Since the tubing is equally strong on both X and Y planes, it will resist buckling.

    Here's how I do it: Find some round steel tubing just large enough to fit over your chosen bolt diameter. Cut a length of it to a snug fit inside the square tubing, and use it to reinforce the bolt hole. Alternatively, you can cut the round tube longer, so it extends all the way through the square tubing, and weld it in place.
     
    Last edited by a moderator: Apr 24, 2017
  10. Nov 18, 2009 #9
    Re: Motorcycle

    Thanks for the advice. This is what i am going to do.

    I bought some cold roll flat stock for the top peice (not sure if ill go straight down or simply go at an angle all the way to the hitch since i have to have room for crossmembers to keep it still left to right), and some 1 inch by 2 inch square tubing for the two bottom peices. I did up the flat stock a little, and will have to move the rear foot pegs out conciderably more, but the square tubing will resist buckeling.

    the proposed design might not be feasable because there wont be anywhere to put the corssmembers which keeps the hitch from moving left to right, the round plate i designed wont be safe as it could get shoved into the wheel.

    Main problem is solved however, the square (recangle actually) tubing will resist buckelig.
     
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