Airhead tank slapper

Discussion in 'Airheads' started by davemon, Mar 16, 2010.

  1. davemon

    davemon High Speed Airhead

    Joined:
    Jun 19, 2007
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    140
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    NC
    Just did a 1200 mile ride on my 1977 R100S loaded with gear and camping eq. on the ride down road like a champ. Added my coat and riding pants onto the gear in the back and at 25 MPH the handlebars flapped like the tire was square. Repacked the load and same thing just not so bad. Was able to accelerate through the slap mode and cruised fine at highway speed. Made it home removed the load and same dance at low speed. Had the forks rebuilt about a year ago new rear schocks and new tires. Anyone else had this problem? Whats the fix?
    Thanks
    Dave
    #1
  2. jtwind

    jtwind Wisconsin Airhead

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    My r80 with an S fairing did this. Not uncommon on airheads with handlebar or fork mounted fairings and windshields. On mine it was steering bearing adjustment that fixed it. But could as easily be the fork, tires, tire pressure, fairing or windshield attitude, poor shocks or loading/weight issues. I'd just work on making sure each one was right and then move on to the next if it doesn't seem to improve.

    If you rode it similarily loaded before you did the forks, shocks and tires and it didn't do this, I'd start there.
    #2
  3. Foot dragger

    Foot dragger singletracker

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    The local BMW dealer is an airhead guy,he helped me put new races and bearings in the steering head on a R75/6 I brought back from the dead,he really stressed the adjustment on the steering head bearings. He used a torque measurement device suggested by BMW. He also said my forks would never work rt with out expert assembly and new stock parts.
    Evidently the ft end on these older beemers is touchy about adjustment so maybe check all that over closely.
    #3
  4. BKMLWR

    BKMLWR Wondering around...

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    been riding a 77 R100s since I bought it new in 77, over 170k miles never had a tank slapper except for hitting washboard road once, riding includes 2up cross country trips

    Check tire pressure and steering head brgs
    #4
  5. Bigger Al

    Bigger Al Still a stupid tire guy

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    15 years on an R90/6, and the only time the front wags is when the tire reaches the end of it's life span. Once the shimmy begins, I replace it and things go back to normal. This is irrespective of brand or tread.

    Still a good idea to check out the bearings, though.
    #5
  6. chasbmw

    chasbmw Long timer

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    low speed wobble is almost always caused by too loose steering head bearings, move the adjuster nut in small increments until the wobble goes away, but not so much that the bike feels as if you have wound down a friction steering damper.

    Charles
    #6
  7. dilandau

    dilandau Been here awhile

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    what he said - at least that fixed my wobble. the adjuster nut and the top nut sort of work together to make a friction damper so in my experience it was a bit of trial and error, moving the adjuster nut, then tightening the top--- testing how that was, then doing it again and again- maybe 4 or 5 times until it felt firm but the front end moved easy and smooth side to side.
    #7
  8. anonny

    anonny What could go wrong?

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    So.... how is your front tire? ..... check real close for cupping.
    #8
  9. mphoppe

    mphoppe Hops

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    Tucson, AZ - Dirt bike heaven!
    #9
  10. BadBob

    BadBob Riding or paddling

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    Any empty French road
    Yo Dave, nature of the beast!

    Airheads, untill you`ve felt it, you would not believe!

    Mainly because of the engines design.

    The airheads are VERY intolerant of ;

    Tyre wear/adjustment

    Headstock/swingarm adjustment

    Any loose frame/rack bolts!....one loose bolt on my rack, while touring in France. Tank slapper so bad, thought my frame had broken!

    If you look at the position of the headstock relative to the rest of the frame, (with the tank off)

    Its no wonder they somtimes shake there heads!

    I welded two tabs on the frame, and attached a strut, on each side. (as I`d seen on endurance BMWs)

    If you hold a inch dia. tube in position without removing anything.
    just under the tank at the front. just above the S/Arm at rear. Its clear where to attach.

    The change in handling is astounding!

    From BMW to Italian motorcycle!!!

    Ride safe, roll on summer!

    BB
    #10
  11. davorallyfan

    davorallyfan Commonist

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    BMW's like yours are bad handling things and extremely sensitive to front tyres - change out - check pressures - my 90S was a shocker - I used to change out half worn front tyres to make it handle - ME33's.

    They came with dampners but..

    A new tyre should fix it steering head bearings - yes but its usually front tyre related
    #11
  12. slash5swb

    slash5swb n00b

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    To get the steering head adjusted right, you have to lift the bike so the front end is hanging free. Starting with the wheel in the straight ahead position, you start to push the bars to one side or the other (doesn't matter which direction.) As you start to push, the bars front wheel should gently and slowly fall the rest of the way to the steering stop. This slow fall represents the right amount of friction from a properly tightened steering head. If the wheel falls quickly and bangs to the stop, it's too loose. if the wheel won't "fall" over on it's own (you have to push it all the way to the stop) it's too tight. It might take several adjustments to get it there, but it works, and it will make the front end very stable. (NOTE: Try to account for cables and wires that impede movement. Test fork in both directions and try to account for this in your estimation.)
    #12
  13. charliemik

    charliemik Been here awhile

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    When you're tweaking the steering brg adjustment, you'll know you've gone too tight if your bike starts to do a slow weaving back and forth on the road.
    #13
  14. Wirespokes

    Wirespokes Beemerholics Anonymous

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    Exactly! With one hand I hold the front of the fender and the other a handlebar. Then turn the front end slowly and feel for either too much free play or notchiness. Yes, notchiness! Too tight and it'll feel like bad bearings.
    #14
  15. P B G

    P B G Long timer

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    And when you tighten it up the top nut will change things a bit.
    #15
  16. Bigger Al

    Bigger Al Still a stupid tire guy

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    And that seems to be more pronounced when you have an aluminum top triple installed, at least in my experience.
    #16
  17. Rucksta

    Rucksta SS Blowhard

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    This can be due to not releasing the fork tubes from the top clamp prior to adjustment.
    #17
  18. pommie john

    pommie john Long timer

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    If the problem arose since the new tyres, it's worth checking they are on the rim true and properly balanced.

    I had a very similar problem straight after I had new tyres fitted to my old Honda and they had not seated the front tyre properly.

    Put it on the centre stand, prop up the front and spin the wheel. The tyre should run perfectly straight. Any wiggling from side to side, and that's likely the problem.
    #18
  19. Beemerboff

    Beemerboff Long timer

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    In my Brooklands Gold Portfolio of old Beemer tests they found that unequal loading of the panniers caused a wobble.

    Wobbles were much more common 50 years ago when tires had a flatter profile, and I have found the more rounded the tire the more stable my airheads are.

    So dont dick around with worn tires / flat profile tires.

    Lasertechs or Distanzas both have a good round profile and work well on airheads.
    #19
  20. davemon

    davemon High Speed Airhead

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    Oh what a difference a tightening of the knurled knob makes. Used the spacer trick under the locknut thanks to Alton. Did the tighten up and all is well in the handling department. Even loaded up the camping and parachute gear to check it out and straight as a die.Thanks for all the great advice.
    Dave
    #20