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Steel tube problem

  1. May 4, 2011 #1
    Im making an A frame to tow a quad bike behind a car, i need to know something because it affects the design.

    Imagine a steel tube of an arbitrary length of 1 inch diameter, and 2mm thick walls.. It will have a specific inherent strength before load causes it to buckle or deform.

    now drill a 6mm hole all the way through at some point along it. You have now created a weak point, which will fail before any other point on the tube when under excess load, generally by the hole deforming inwards on itself, or the steel between the holes crumpling up.

    if i now fill the holes by placing a 6mm steel bolt of equal strength steel as the tube is made from, to what extent do you now restore the strength and load bearing limits of that point in the tube. I reason it must now be at least the same as the rest of the tube, since the tube has no free movement within the hole to deform into, and the amount of material at a cross section of the hole is now more than an undrilled and filled point on the tube.

    To put this into practical context: if i drill and place a 6mm bolt in one wishbone arm on the quad bike, do i fatally weaken the 32mm dia arm ?
  2. jcsd
  3. May 4, 2011 #2
    If your bolt is perfectly welded without any welding errors, the joint would have at least 90-95% strength of the parent metal.
    the cause of remaining 5-10% is because of the changed microstructure at the joint.

    To add up, more the imperfections in the process of joining, lesser would be the strenght..
    Hope this helps..
  4. May 4, 2011 #3
    I'm not quite sure of what your trying to do here, However, drilling a 6mm hole through a 32mm tube in a quad bike suspension is not something I would recommend doing.

    Placing a bolt through the hole will not remove the stress concentration caused by the drilled hole in the first place.

    If you could describe more in detail of what your trying to do maybe we could suggest better methods than drilling holes through suspension members of your quad. A wishbone is not something i would like to have fail when riding along at high speeds.
  5. May 4, 2011 #4
    im just pondering ways to clamp the A frame to the wishbones, and the relative merits of several ways i can think of doing it.

    One way i thought of requires a 15mm radius U bolt made of 6mm steel being inserted into the front leg of the wishbone on each side (theres 4 legs each side remember), this then allows me to create a horizontal U shape which i can close round the leg U bolt with a pin, and that would provide enough free play to take any bouncing or spreading of the wishbone on bumpy ground.

    The alternative would be simply to close the A frame U around the wishbone arm, but that requires a larger assembly,
  6. May 4, 2011 #5
    So are you trying to create an a-frame so you can tow the quad behind a car like a conventional trailer? I cant see this as a very safe or efficient or legal way of transporting a quad. With all four wheels on the ground, the quad would not tow very well.

    If the quad is to be towed only short distance, maybe consider something without an a-frame as it will not have to take much weight. without knowing details of your quad this is hard to to.

    A suggestion could be bolting some type of frame where there are already bolts on the quad?

    Sorry if im not being much help.
  7. May 4, 2011 #6

    Well, i rang the police station to ask, and some female said that as long as the quad cant wander about laterally behind the car, they she cant see a problem. Mind you, i dont think she was the font of all knowledge on the subject.

    When you tow with a single bar, you have to have someone steering. If you triangulate with an A frame, the towed vehicle cant really go anywhere but where the apex of the towing frame goes. Remember this is a bike steering, not even rack and pinion, just two steering arms attached to a crank plate at the bottom of the steering shaft.

    ive come up with a way to attach the A frame to the wishbone front upper arms, its not snap fit, basically you can use two exhaust U bolts round the arms, with wing nuts to spin on then drill the ends of the U bolts and insert a hitch pin to stop it unscrewing.

    I would like to stop a traffic cop and ask, they are the most likely to know.

    Towing a car behind a Motorhome is the same problem, i found this reply on a Motorhome forum:

    I towed my Panda with our previous Motorhome, and will do the same with our new one when I can sort out fitting a towbar.
    I use a "Car-a-tow" A frame which are advertised in most of the magazines. A bracket is premernantly attached to the front of the car to be towed, to which the A frame can quickly be coupled to. The other end goes on to a normal tow ball on the back of the motorhome.
    The way I connect my electrics is to wire a female 7 pin socket into the appropriate wires on the tow car, so that the original rear lights, indicators and brake lights work as normal. Then you just need a male to male link lead from the standard motorhome socket to the one in the towcar.
    All that is left to do then is to put a pair of red triangles on the back of the tow car to show it is a towed vehicle (trailer) and the registration plate of the motorhome to cover the tow car plate. There are lots of ways to do this, Car-a-tow supply some nice brackets for this.
    There are many differing opinions on towing cars in this way, I have never had any problems in the UK, but this method is not approved of by most of the continent.
    The only legal way to tow abroad is with your car on a trailer. I can assure you, and any one else who trys the " its legal in the UK so it must be legal here" attidude, it doesn't work, as the guy I bought my Car-a-tow off told me. (His tow car was impounded and cost a fortune to sort out getting it home).
    Hope this may help,


    I just found this - A VW towing frame:


    I came up with almost exactly the same shape. :D
    Last edited: May 4, 2011
  8. May 4, 2011 #7
    I see what you are attempting now, excuse my ignorance.

    A single draw-bar would work if the front wheels were off the ground. I am thinking something like below.

    *EDIT* (found picture of what i was looking for. )

    This way the front wheels would be off the ground and no one would need to steer the quad. There a trailers built with this design instead of an aframe.

    I still cant see this being good a safe way of towing your quad long distances, especially on motorways etc. Although im not sure what type of quad this is, I dont think it would be too good for the quad being towed along with the transmission spinning at 100-110 km/h for extended periods. I think a conventional trailer would be your best choice for long trips.

    Attached Files:

  9. May 5, 2011 #8
    Ah yes.

    Fortunately, i dont need to tow it far, just locally.

    its a cheap chinese quad, i use it to go to work on, but it had to be taken to the garage a couple of time, and each time its cost me £25 to get a tow there with it (they take it in a specially adapted short wheelbase transit van) and then a taxi fare to go and collect it.

  10. May 5, 2011 #9
    ahh I see,

    It looks like the bolts on that front bumper could probably be used to affix some kind of tow hitch.

    All the best for your project, let me know how you get on or if you have any more questions.
  11. May 5, 2011 #10


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    I do not yet understand the details of the joint you are asking about. Would you be able to post a sketch of your proposed assembly? And could you post a dimensioned sketch of the joint details?

    Also, what is the mass of your quad bike? Is the picture in post 8 your exact quad bike? If so, does it have a mass of 160 kg?
  12. May 6, 2011 #11
    its virtually the same design as the VW A frame thumbnail i posted. The detail we are discussing is how to attach the frame to the bike. I wondered if i could insert U bolts into the lower front wishbone arm, so there was two towing lugs, in the same way a car has a towing lug on the front, hence the question, would inserting a 6mm U bolt (2 holes 50mm apart on each side) into a 30mm tube fatally weaken it.

    The other way i thought of would be to use exhaust U bolts large enough to go all the way round the front lower tube of the wishbone arms. This would be less elegant but prolly safer, and not as quick to attach/remove.

    That is the exact quad bike, according to the book its 292 Kg
  13. May 6, 2011 #12


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    I agree. The bolt does not restore the tube strength, in any way. This answers one of your primary questions.

    If I had to guess, without a detailed drawing, I would currently say there is perhaps a 10 % chance your proposed connection could fatally weaken the wishbone round tube, whether you drill through the tube, or clamp around the tube without drilling.
  14. May 8, 2011 #13

    Heres the Mark I version, i used clamps in the end, held by M8 bolts, with wing nuts spun on and then trapped in place so they cant shake off by hitch pins. Foolproof.........the holes through the tube where the M8 bolts go are lined with 10mm x 1mm steel tube to prevent the square frame sectionfrom collapsing under load.



    Edit by Borek: too wide image replaced by link
    Last edited by a moderator: May 8, 2011
  15. May 8, 2011 #14


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    (1) It appears you used carriage bolts. Carriage bolts are generally for wood or low-tension applications. They do not necessarily have a strong head, and often are not designed for high-tension steel joints. The head might be able to pop off. Use a regular, hexagonal-head M8 bolt in which you ensure the head is marked "8.8," which means ISO property class 8.8 steel. If you use a carriage bolt, ensure its head is marked "8.8."

    (2) Wing nuts are generally low-grade steel, and unmarked, if I recall correctly. If your wing nuts are not marked ISO property class 8, then change to regular hexagonal nuts marked ISO property class 8, which means they must have dots exactly in the positions shown here.

    (3) Usually use a flat washer under the bolt head and nut.

    (4) For locking, you could perhaps use or add a http://upload.wikimedia.org/wikipedia/en/4/42/Castellated_Nut_%28new%29.png [Broken], hexagonal, ISO property class 8 nut, and a split (cotter) pin.

    (5) Ensure little or no twisting force is applied to the joint when you fully compress or extend the wheel suspension.

    (6) My estimate for fatal damage to the wishbone tube is in post 12. Maybe I should increase the percentage slightly, but I do not know the exact tube wall thickness. I guess I will leave the estimate as-is, for now.

    (7) The rear weld on the towing A-frame might be overstressed, although I do not know. I am just mentioning this as a possibility, because no one quantified the loading conditions on that weld.

    (8) The safety chain or cable, and attachments, must be able to withstand the rated load capacity of the towing A-frame. In the event of failure, the safety chain or cable must prevent the towing A-frame from dropping to the ground.
    Last edited by a moderator: May 5, 2017
  16. May 8, 2011 #15
    ok, advice duly noted, ill change the bolts and nuts :D

    ty :D

    (btw, im the last of a long line of back-of-fag-packet engineers. My g'g'father was an Agricultural Engineer, My g'father was Chief Mechanic on the HMS Pegasus, My father built prototype Diesel generating Sets and Gas Turbines).
    Last edited: May 8, 2011
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