Africa Twin - Another broken rear rack

Discussion in 'Japanese polycylindered adventure bikes' started by stou, May 14, 2017.

  1. tremor38

    tremor38 Long timer

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    This ^^^^^^



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  2. tremor38

    tremor38 Long timer

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  3. twinrider

    twinrider Pass the catnip

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    My SW-Motech setup's been fine for the past 14 months, use a 45 liter top box every day. I reckon yours will survive till you get your SS rack...
    #43
  4. twinrider

    twinrider Pass the catnip

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    That's not the SW-Motech setup I have. My aluminum rack bolts directly to the OEM rear rack square over the original holes.
    #44
  5. tremor38

    tremor38 Long timer

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    Interesting that those pieces are available for the Alu-rack...or maybe that's their steel version, as I haven't seen those bits available for the Alu-rack. In any case, not sure I like the idea of the rack mounting further aft.

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  6. Bhart89

    Bhart89 Long timer Supporter

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    I remember learning about the harmonic impact of vibrations in physics class.

    #46
  7. G-van drone

    G-van drone Been here awhile

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    Almost. To simplify things, the basic characteristics (i.e., the natural frequency of the system) are determined by the stiffness of the "cantilever" ("k") divided by the mass it supports. Adding the topbox ever so slightly changes the stiffness of the rack/cantilever, but adding mass lowers the natural frequency, but also slightly changes the amplitude of vibration as well. This increases the stress in a weak point, and adding up all of these stress cycles over time, induces a failure (think s-n diagrams, fatigue failure). There are no "waves to get out of sync". Simple fatigue failure. The other factor in the OP event may have been failure due to a single event - a huge impact that overloaded the rear OEM rack. The same phenomena occurs when mounting a GPS and mount to the w/s support cross-bracket. All those stress cycles add up over time to produce a crack someplace.
    #47
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  8. tremor38

    tremor38 Long timer

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    And if the vibration happens to be in the downward phase when a sudden, strong force is exterted in the same phase/ direction, you will have an additive effect.

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  9. G-van drone

    G-van drone Been here awhile

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    The relevance of that is unknown in this case. The impact force/time history (not quite a squarewave, but close) is much, much lower frequency than the natural frequency of the rack/topbox system - which means the impact force covered many cycles of the racks motion. The rack undoubtedly failed in tension - could have been on the "top" of the cantilever beam (the rack), or the "bottom". It depends on where the actual crack initialized - the rack & topbox moves (vibrates) up & down, and could have failed with the topbox moving in either direction, up or down. The stress in tension at any location is largely influenced by the distance of that location from the neutral axis (a property of the cross-section / design of the beam/rack) of the rack, so a thin cross-section on the underside of the rack (or a spot where a stress riser, a notch or sharp edge is located) could easily fail in tension (at the uppermost movement of the topbox). A small crack initiates, quickly propagates, we pick up the pieces, and then blather on about basic engineering principles on the internet.
    #49
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  10. michaeln

    michaeln What're YOU lookin' at? Supporter

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    All the science boils down to this.... don't carry weight high and far back.
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  11. G-van drone

    G-van drone Been here awhile

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    It may be a case of Honda guessing poorly at the magnitude of "normal" in-use impact loads when designing the rear rack. Warning labels aside, I don't think most of us would consider carrying 17 lbs on the rack as abusing the bike. If the OP could continue to ride after the impact, it couldn't have been that severe. He didn't mention loss of control, bent rims, subframe failure, etc. This also could have been a unique event, in that the part may not have been manufactured correctly - it already had some inherent defect - I suspect this is the case, since there have been so few reports of other failures of this type. Plaintiff's attorneys hate me, btw.
    #51
  12. twinrider

    twinrider Pass the catnip

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    That's what I was thinking as well, since we've only heard of two breakages. I'm sure Honda calculated the strength when they built this rack, they've been in the biz a while now.
    #52
  13. MrKiwi

    MrKiwi backwards & upsidedown Super Supporter

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    They have, but it is rated for 10 kgs max only...
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  14. G-van drone

    G-van drone Been here awhile

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    Undoubtedly the 10 kg includes a safety factor. Seems as though there are numerous examples of folks exceeding 22 lbs without issue - but without a leg to stand on if a failure does occur.
    #54
  15. twinrider

    twinrider Pass the catnip

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    The 10kg limit is nothing unusual. Pretty sure all the bike makers have that kind of sticker on their racks.
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  16. stou

    stou Been here awhile

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    I went to the dealer Friday and no news from Honda.
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  17. stou

    stou Been here awhile

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    This was on a public dirt road, nothing verry hard on the bike. No bent rims or anything else. Yes it was a big pot hole but the suspension worked fine. I transfered my weight at the back to free the front a little bit and I passed through very well.
    #57
  18. DirtyRoadie

    DirtyRoadie Been here awhile

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    Ha ha ha , I am happy to be "almost" right ! It's been a looooong time since I took a physics class. But I do frequently read and sometimes retain Kevin Cameron in Cycle World. Probably my favorite part of that magazine now the Peter Egan retired.
    #58
  19. Motomochila

    Motomochila Moto Scientist and time traveler

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    Me too, so I sawed mine off. New stainless handles being fabricated as we speak.
    IMG_4410.JPG
    #59
  20. G-van drone

    G-van drone Been here awhile

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    Can you give us an overall and a close-up of the x-section of the material where you performed the surgery? I'd like to have a look.
    #60