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tank to frame attach question

Discussion in 'Technical and Troubleshooting Torque' started by vagrant, Jun 22, 2007.

  1. This is a purely out of interest question:

    What is the original intent behind the flexible joint between tank and frame on modern sportbikes?

    I have some ideas:
    1) one being to damp/reduce the affect of fuel slosh on cg. nice idea but maybe a little over the top.
    2) reduce stress on joints due to engine vibration transmitted through frame. dont really see this as an issue in modern inline 4 bikes...(?)

    Any other ideas?


  2. Mines rubber mounted to absorb vibs I'd say. Steel + vibration = cracking. What are you talking about?
  3. what do you mean, what am I talking about? the question seems to be pretty concise to me.....

    i hadnt really thought it through before posting it, just wondering if anyone knew the exact reason.

    wow, really? :grin: that is a very general statement though as it depends on the amplitude of the load and the number of cycles. As I said, hadnt really thought it through. I was thinking "pretty low amplitude compared to say a V-twin, so shouldnt be a problem...." what I didnt consider was the huge number of cycles (squizillions) over the intended lifetime of the joint.

    So yeah, fatigue could be a reason, as I initially eluded to in the OP.

    Just bored on a friday afternoon, and wondered if some old school mechanic out there could confirm exactly why......

    maybe it was a stupid question.......... :?
  4. Sorry I really meant "is that what you're talking about"?

    Ideally the tank would be fixed solid to the frame or like Buel do make the tank part of the frame but the compact nature of sportsbikes means the airbox is usually up under the tank so removal is necessary.
    All this means the tank must be isolated as much as possible from any vibrations. Steel tanks will last longer than aluminium in a vib environment but they will all "work harden" (trade term) and crack if not isolated or even mounted incorrectly.

    Call it a general statement if you like but I spent 15 years in the welding/boilermaking/sheetmetal game.


    EDIT: It's not a stupid question.
  5. fatigue cracking is the reason for rubber mounts to the tank.
    i see you mention amplitude etc. with regard to your understanding of the effects of vibration and fatigue.
    remember that high amplitude + low frequency imparts as much stress to steel (on a micro level) as low amplitude high frequency does.

    you may also notice that the tank is "all but" isolated from the frame, and mostly on modern bikes, it is completely isolated from the frame.
    modern fames are made with alloys that can chemically weld to steel over time. this would cause failure under exposure to vibration.
    another consideration when "rubber mounting" a tank would be to remove it from the grounding circuit in the electrics, for obvious reasons.

  6. The vibration may not be due to engine but road impacts.

    Plus metal on metal noises sound bad...
  7. Hey guys, all great points. Thanks alot.

    2wheelsagain, sorry mate if I sounded a little stroppy with my response. didnt really mean it that way either! :LOL:

    regarding freq vs amplitude thing - joel, thats pretty much what i was geting at, just didnt explain aswell as you!

    dirty, fair point, though i reckon its more likely engine vibration.