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  1. #61
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    Quote Originally Posted by Coober View Post
    So does the tube HAVE to be high-tensile steel and must it be welded in place?
    I take it that it is necessary that the tube be a tight fit within the chassis.

    Presumably this is only required if the bolt passes through both sides of the chassis.
    High tensile is a bit undefined.
    You will get mild steel up to about dia40mm where 25mm round drilled to take a M12 should be sufficient (per bolt).
    High tensile is 350Wa and the starts at 45mm round.
    For a shear connection, yes welding would be required but not so for friction grip.
    I would advise all diy people trying to do this to rather speak to a structural engineer and get good info regarding this if you have no engineering background.

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    Quote Originally Posted by Uys View Post
    It is actually quite easy to calculate.....
    This is a great post and really helpful thanks Uys!

    I have just bought the frontrunner recovery point for the rear and have been going through your calculations to check it out. Its 10mm thick, and so inline with your calculations its cross sectional width should be (300mm2/10x2)) = 15mm, it is infact 16mm, With the loop diameter being 54mm.





    My question stems from one earlier, if my chassis has only 2mm thick walls is this not the weakest point? How do you strengthen this section of the chassis? do you need somebody to weld on thicker sections? If so surely these welds then become the weakest link?

    My second question is the calculations then call for the use of 4 M16 bolts, would the structural integrity of the point be compromised if you drill in 4 longitudinal bolt holes (see picture)? is this better than just having two?



    My third question is how do you attach this point to the chassis?

    could you bolt two plates around your chassis with 4 bolts, and then bolt the recovery point to one of these plates (with however many bolts depending on the above answer) like this, or will they not be secure enough?



    or would you have to bolt through the chassis like this to plates on each side? (then only 2 bolts can be used i think), would the weakest link then not be the bolts running through the 2mm chassis?



    I feel a little bit silly for buying this thing without knowing how or where to attach it to...
    Last edited by OwenD; 2009/11/11 at 08:23 PM.
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  3. #63
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    In both your proposed mounting configurations you will be inducing quite significant bending moments into the member/chasis which multiply the forces (by about two, looking at the dimensions of the member and bolt spacing).

    If you can, rather find some way to attach the member so the force applied will be inline with the bolt holes (ie if you project a line in the direction the force will be applied to the "end ring" by the shackle/strap etc, it runs directly through the bolt holes). That would result in the lowest possible force on the bolts and the chassis.

  4. #64
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    OwenD
    The last one has the strength of half of one bolt.
    The outer bolt serevs as a pivot point and the inner one sees double the forces on the eye. Not very good.
    Mount the lug as close to horisontal as possible with two M12 Gr. 8.8 bolts.
    Use doubler plates of minimum 6mm on inside of chassis.

    Take your time in making a plan, do more sketches and show them here.
    We will tell you what might work.

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    Thinking about this a bit more...

    Of the two configs you show the 1st could actually be workable provided:

    I'm going to call the part you bought the "shackle mount":

    1) The clamping plate is square, ie the distance between bolt holes either side of the chassis member in the vertical direction is the same as that in the horizontal direction. Use the bottom two clamping bolts to bolt the shackle mount to the clamping plates (ie you minimise the offset between the shackle member and the center of the chassis member). If you need to off set the shackle member more so that you have space to attach a shackle to it, then make sure that the distance from the shackle member bolt holes and the clamping bolt holes at the top of the clamping plate (in the vertical direction) is the same or less than the space between bolt holes in the horizontal direction on the clamping plates. These two provisions should reduce the multiplying factor on the clamping bolts to close to one.
    2) There is no gap between the clamping bolts and the top/bottom of the chassis member.
    3) The clamping bolts are at least as big as the bolts you intend to use to join the shackle mount to the one clamping plate and are equal or more in strength rating.
    4) You put a single large high strength bolt dead centre through the chassis and and both plates. (To stop the plates being dragged down the chassis)
    5) the clamping plates are substantial.


    The more I think about it, that is actually a very good, practical setup!
    Last edited by alanB; 2009/11/12 at 08:32 AM.

  6. #66
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    Quote Originally Posted by RudolfD2 View Post
    OwenD
    The last one has the strength of half of one bolt.
    The outer bolt serevs as a pivot point and the inner one sees double the forces on the eye. Not very good.

    Use doubler plates of minimum 6mm on inside of chassis.

    Take your time in making a plan, do more sketches and show them here.
    We will tell you what might work.
    Thanks I was wondering about pivots. The transfer of energy through this whole system is quite alot of physics/vectors/mathematics so I shall definately be running things through you guys before I go out and do anything, The last thing we need is a monkey driving around with rubbish points.

    how exactly do you get plates on the INSIDE of a chassis? My bumper is on the end of the chassis rails so this recovery point will be about half a meter before that attaches so getting plates inside is a problem...

    alanB I have had a go at trying to draw what you suggest. Is this what you had in mind?



    WRT the large centre bolt, is all the force not being applied to the chassis through this single bolt? And is it not just acting against just the 2mm + 2mm chassis thickness regardless of if the clamping plates are 10mm?



    Speaking of clamping plates I was thinking of using towbar drop plates from outdoor warehouse which I think are 10mm, but will check.

    Does this setup not mean that the shackle mount is only connected by two bolts?
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    How the hell do you make such nice drawings so quickly? Yes that exactly what I was suggesting.

    Regarding your point about the centre bolt tearing the chassis member:

    It is a concern, with the following mitigating factors:

    1) The twisting force induced by the off centre placement of the shackle member will tend to make the whole arrangement twist on the chassis members and thus lock themselves there with friction - provided the chassis member doesnt buckle/collapse in on itself. So the shear force on the central bolt will be spread onto two of the clamping bolts in that manner.
    2) The thing I'm starting to like about this arrangement is that it seems be inherently safe, in that the failure of any one member (apart from the two bolts joining the shackle member to the clamping plate) should not allow parts to go flying. And it spreads the load on the thin chassis member quite effectively.
    3) The large plates clamped to either side of the chassis member will carry quite a lot of shear force through friction between the plate and the chassis member

    If you are worried about only one bolt use two, but make sure their spacing is at least as much as the vertical distance between them and the shackle member. Personally I would prefer them on the centre line of the chassis member (that way you dont weaken the structural strength of the chassis).
    Last edited by alanB; 2009/11/12 at 09:47 AM. Reason: keep thinking of new points

  8. #68
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    hehe thanks, its the joys of procrastination that enables me to draw them so quickly - I would rather draw them than do the work that I am supposed to be doing, student life is good to me.

    I have taken the above 'ideal' drawing and tried to measure it out but have run into some snags that I would like your opinion on. I think it may not be possible with my shackle member.

    Most importantly with this setup the shackle will not fit on the hole because it is flush with the chassis rail, so it seems like this ideal design will not work.

    Secondly with this setup the length of the shackle member restricts the horizontal distance that the holes can be drilled as in order to keep 16mm distance from hole to edge the maximum distance between bolts would be 85mm, vs the 108mm vertical distance determined by the height of the chassis rail. But I am not sure this is worth worrying about since the shackle will not fit on...



    Again it seems like my first drawing would be the only viable option with this kind of clamping plate design. The good thing is now that the shackle member is bolted on separately to the clamping plates you can get them nice and squarely spaced from each other. The problem is the force may be too far below the chassis rail, is this a valid concern?



    or should I go back to the drawing board?
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  9. #69
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    before you comment hang on I have another idea...

    If I put a thick spacer into the 'ideal design' it might give enough space between itself and the chassis rail for the shackle loop to fit in and keep everything on a similar plane. And allows for more optimal bolt placing.



    This now brings us to these measurements, which makes everything look a lot more symmetrical.



    Now the bolts holding the shackle member can be bolted through the centre of it rather than offcentre like without the spacer as you no longer have to worry about covering the hole with the chassis rail.

    As I said the length of the shackle member does not allow for a proper square separation of the bolts vertically and horizontally.

    What do you think of the spacer idea to resolve some of these issues?
    Last edited by OwenD; 2009/11/12 at 12:33 PM.
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  10. #70
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    Hi OwenD
    If a spacer is used it should be welded to the main plate or you must use the typical friction type connection.
    But the friction option would not work in your case.
    The centre bolt has almost no function without a doubler plate or a crush tobe except for keeping the two main plates from warping due to the load placed by the bolts.
    I would suggest making the main plates of at least 12mm plate to prevent bending and thus loosing the grip they have on the chassis.
    Good luck!!!

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  11. #71
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    Hi Owen

    Sorry unfortunately work tends to intrude on interesting discussions.

    I would avoid the spacer idea just to keep things as simple as possible.

    If you have to revert to shifting the shackle member down to enable sufficient space to attach a shackle, which I think is unavoidable (but obviously no more than absolutely necessary), then just increase the horizontal distance between bolt holes on the clamping plates by the same amount (ie the vertical distance between the bolt holes for the shackle member and the top clamping bolt holes should be less than or equal to the horizontal clamping bolt hole spacing distance.

    I would add the central bolt as a fail safe.

    Plate thickness should be at least equal to that of the shackle member.

    I know you have already bought the shackle members but another really neat idea is to have the one clamping plate simply shaped to include a big enough hole in the corner to attach a shackle, that would be as simple as it gets!

    PS Scanned your post too quickly and didnt see your question about drilling the holes in the shackle members slightly off centre which I wouldnt think was a major issue, provided the holes arent too close to the side of the member so that it loses strength, 10mm is on the limit but OK IMO.

    PPS The point Rudolf makes about a crush tube is valid. A severe recovery could potentially result in the plates bending and/or the chassis members distorting/bending allowing the plates to shift on the chassis members, losing their friction grip. The central bolt will stop any massive movement but a crush tube will help everything stay in shape far better.

    The fundamental problem from which there is no getting away from is that the chassis member is very thin and not really designed to take the sorts of forces that will be applied.

    The large clamping plates do a lot to help spread that load, but ultimately they will shift and the chassis members will distort/bend if enough force is applied.

    But this is a better arrangement than simply bolting the shackle member to the chassis with a large washer IMO.
    Last edited by alanB; 2009/11/12 at 03:32 PM.

  12. #72
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    What a busy day...spamming this whole thread with doodles
    oh well at least I am learning something!

    I will give the spacer a miss...

    To try keep it simple I have got hold of some towbar drop plates, which are 16mm thick. I shall have to drill in some new holes but they are spaced far enough from the others to make much difference.

    The drop plates look like this



    and I shall drill holes where the tape is in order to fit them over the chassis rail

    I shall then drill holes into the shackle member and attach it to the bottom holes in the drop plate like this



    I understand what you are saying about making the horizontal distance between bolts equal or greater than the vertical distance, the drop plates fall short of this goal by quite a bit (around 50mm)



    I would really like to use the drop plates even though the angles aren't perfect...or should I rather get some plates made to measure? (I have no idea where I could get this done in CPT!?)
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  13. #73
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    What is the vertical distance between the shackle member bolt holes and the central bolt hole?

    If that is more or less the same as the diagonal distance from any clamping bolt hole and the central bolt hole, then I think you are OK.

    This is because the whole thing is going to tend to rotate around the central bolt hole.

    The bending moment will be the force applied to the shackle member times the distance to the central bolt hole.

    If the diagonal distance from a clamping bolt hole to central bolt hole is the same as the vertical distance from shackle member to central bolt hole, then the clamping bolts, which have to resist that rotation, are carrying the same shear force as the force applied to the shackle member, not more. If the diagonal distance is less, then the clamping bolts will be carrying proportionally MORE force (half the distance = twice the force etc). If it is larger the reverse applies (so the longer you can make the plate in the horizontal direction, the lower the bending force on the chassis as well as the shear stress on the clamping bolts).

    Without the central bolt, the whole arrangement rotates about the top clamping bolt and the design forces change according to the various distance ratios for that configuration.

    The central bolt carries the same shear force as that applied to the shackle member.

    This estimation of forces ignores friction between the plate and chassis which will tend reduce the forces in practice. For that reason when you bolt it all up, ensure that there is no dirt on the plates or the chassis to ensure you get good friction.

    I think it will work quite well.

    PS If you want to get plates cut for you try and find a laser or water jet cutting firm, look in the yellow pages maybe, they will cut it all to size including holes etc according to your drawings. In that case I would dispense with the separate shackle members and incorporate a suitable hole for the shackle into the clamping plate design. Temped to do this myself!
    Last edited by alanB; 2009/11/13 at 09:40 AM.

  14. #74
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    Quote Originally Posted by alanB View Post
    What is the vertical distance between the shackle member bolt holes and the central bolt hole?
    The vertical distance between shackle member bolt holes and central bolt hole is 90mm. The diagonal distance between the clamping bolt hole and central bolt hole is 70mm.

    Quote Originally Posted by alanB View Post
    The bending moment will be the force applied to the shackle member times the distance to the central bolt hole.
    Using this logic the force on the clamping bolts will be 1.3xForce applied to the shackle member.

    To use the calculations that Uys suggested then the following bolts would be needed.

    80 000N*1.3/171 = 608mm. 608mm/4 = 152. D = sqrt(4x152/pi) = 14.14mm = 16mm MS

    or 80 000N*1.3/293(8.8HT bolt) = 354mm. 354/4 = 88mm. D=sqrt(4x90/pi) = 10.95 = 12mm HT

    I tried to do something similar to work out force using vectors, but probably messed it up. will show you anyway.



    Tried to factor in the rotational effect around the centre bolt hole as well as the horizontal force applied to the shackle member. The vertical distance was used for the shackle member bolts AS WELL as the clamping bolts (instead of the diagonal distance like is probably more correct). I then tried to account for the lever effect around the pivot point to the top bolts by making the horizontal vector 2N compared to N values below the pivot.

    At least if it is wrong, I think its wrong in the safe direction (indicating MORE force than expected rather than less).

    Either way with THESE values (which are most likely over estimates rather than underestimates) I calculated the following bolt requirements.

    Mild Steel bolts 171MPa

    A = 80 000N x 2.75 / 171 = 1286mm. 1286mm/4 = 321.5mm. d = sqrt (4x322/pi) = 20.24mm. Would need a bolt in excess of 20mm!

    HT bolts (8.8 ) = 880/3 (safety factor) = 293MPa

    A = 80 000N x 2.75 / 293 = 751mm. 751mm/4 = 187.75mm. d = sqrt(4x200/pi) = 16mm

    Either way I think I shall be safe using 16mm HT bolts. It is just the bending force of the relatively narrow plate on the chassis rail that I will have to be weary of, but at least it shouldn't be dangerous.

    This has been pretty fun indeed!
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    Dont have time to go through your vector analysis, just dont forget the central bolt will be carrying the full load applied to the shackle member in shear.
    Last edited by alanB; 2009/11/13 at 01:16 PM.

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    Even with all the load diagramms the chassis member is still going to be the weak link in the chain.
    The centre bolt will just tear throught the 2mm side wall as there are no friction keeping it there.
    The outer point could just crush/bend the chassis tube as it's not designed for that.
    The centre bolt will only be a pivot point for a moment until the chassis start to deform.
    The load distribution could change and bend the frame.
    I still dont like this setup.

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  17. #77
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    Quote Originally Posted by RudolfD2 View Post
    Even with all the load diagramms the chassis member is still going to be the weak link in the chain.
    The centre bolt will just tear throught the 2mm side wall as there are no friction keeping it there.
    The outer point could just crush/bend the chassis tube as it's not designed for that.
    The centre bolt will only be a pivot point for a moment until the chassis start to deform.
    The load distribution could change and bend the frame.
    I still dont like this setup.
    I must admit it does seem to appear as if I have actually just built a tube bender, almost purpose built for bending chassis members with other vehicles...

    For interest how does frontrunner EXPECT you to attach their recovery point?
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  18. #78
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    Quote Originally Posted by OwenD View Post
    I must admit it does seem to appear as if I have actually just built a tube bender, almost purpose built for bending chassis members with other vehicles...

    For interest how does frontrunner EXPECT you to attach their recovery point?
    Usually from the open end of a chassis member.
    Doubler plates and/or crush tubes are the way to go.
    Post photos of the chassis and we could make a reccomendation.

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    Hi Rudolf

    Everything you say is ultimately true if enough force is applied. There is no getting away from the fact that the chassis is a hollow 2mm section.

    However my question is: is the above set up better or worse than simply bolting the shackle member directly to that thin hollow section with two bolts with large washers which is the normal way that recovery points are mounted? They would tear out just as easily (more easily I think because the bolt hole spacing is usually far less than suggested here), so the forces will be higher.

    Without somehow inserting some form of reinforcement into that section which would be ideal, this is the next best option IMO.

    I was originally attracted to the idea of clamping plates that are longer horizontally than they are high, to reduce the bending moments and spread the load along the chassis member.

    What would you suggest?

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    duplicate post
    Last edited by alanB; 2009/11/14 at 07:13 AM.

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