Bent forks = bent frame?

Discussion in 'Airheads' started by civhatch90, Jun 16, 2013.

  1. civhatch90

    civhatch90 Been here awhile

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    Simple question. I recently bought a 90/6 and after what started as a fork seal replacement I noticed both forks are bent. I do not know how this happened since the bike came this way although there are no signs of a crash.

    Anything to check on the frame? I'm looking for others experiences as I'm sure this is rather common.
    #1
  2. Kai Ju

    Kai Ju Long timer

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    Not necessarily, I've seen bent forks with no frame damage at all. It all depends where the forks were in their travel when they took the hit that bent them.
    Take a look at the two plates that flank the steering head tube. They should be perfectly flat. If they are bowed at all, your frame is bent.
    That's the quick and dirty check.
    If your forks are bent, then the triple clamps might be as well. That will reveal itself when you try and reassemble the forks.
    #2
  3. Big Bamboo

    Big Bamboo Aircooled & Sunbaked

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    Before you mount the forks, slide the new tubes in the lower clamp and check if they are in the same plane (plate of glass) After you assemble the bike you can do a simple string test to see if the wheels are in the same plane. More info on Duane Aushermans website.
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  4. ME 109

    ME 109 Long timer

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    You have good eyes if you can see bent forks, or you have REALLY bent forks.
    Remove forks and roll them together on a flat surface. There should be zero gap between the forks.
    If you buy new forks (or before you pay for them) perform the same test to ensure straightness.

    When you have known straight forks, you can then test the triple tree for straight.
    Bent forks tell lies.
    #4
  5. chasbmw

    chasbmw Long timer

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    I bought a pair of Italien made fork stanchions from Wemoto in the UK, cost £99 each inc tax and delivery. About as cheap as getting the stanchions straightened and re chromed.

    They do deliver abroad.


    Fork stanchions do take on a bit of a bend over time, well worth checking if you ave stictions problems.
    #5
  6. Pokie

    Pokie Just plain Pokie.

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    What I would do.

    Personally, I wouldn't worry about the frame, at least not yet. The fork tubes can be straightened. My test to see if the tubes are straight enough to use is (when the forks are apart) remove the lower nut (yes, the big one) on the bottom of the lower leg. If the tube is straight enough to pass through the lower leg (all the way through), then it's good enough to use. This test allows the fork tubes to be as much as .003" to .005" out and still function.

    When re-assembling, leave the fork springs out and keep testing the forks by hand until all is assembled. If at any point the fork stiffens up, do what-ever "tuning" it needs to move freely again.

    After the bike is once again ridable, choose a flat piece of road, get into at least second gear (or anything above 30mph) and remove your hands from the handlebars. If the bike tracts reasonable straight, the frame is fine. If the bike tries to dive one side or the other, the frame is likely bent. At this point you have to decide if it's too bent to ride or if it's just fine for you to put up with until winter.
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  7. supershaft

    supershaft because I can

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    I have seen lots of bent tubes not bend the frame. Checking the steering head gussets is a good indicator. Some of them do have an ever so slight bow and are still straight according to BMW's own specs and frame jig.

    If the tubes are bent, the top triple tree is almost always bent. Usually it's the steering stem hole pulled out of round.

    Fork tubes do not bend with normal use. They flex with normal use. Bent is bent and they need to be straight.

    No hands on any perfectly setup bike between 30 and 45mph is a good way to get into a tank slapper. Beware!
    #7