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Thread: Tail wheel shimmy

  1. #1

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    Tail wheel shimmy

    Hi forum! Getting bad tail wheel shimmy when someone is in backseat. Tire pressure fine. What are diagnosis steps? Thanks!


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  2. #2
    TirolCub's Avatar
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    ... have you checked your tailwheel spring ?
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  3. #3
    Kodiakmack's Avatar
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    I’m having g the same problem. Went down the tailwheel shimmy rabbit hole. This is a great article. There’s several things it could be but the first and most glaring I found is that my spring is toast and I have bad caster. New spring is in the mail. Also, check that your chain/spring tension is correct, and correct is different than you may think....

    https://glasair-owners.com/glastar-s...-wheel-shimmy/
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  4. #4

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    For those of you who wheel it in to a shop for work did you notice that it shimmies right after the annual? Go easy with that grease gun.
    The harmonic motion equation has damping in it, which for us is friction. On the big Scott it is adjustable - it is that castellated nut on the bottom. But grease makes that less effective.

    For the smaller Cubs I converted to the cute little API tailwheel. No damping, so I made an .032 Bakelite washer for the bottom of the kingpost, and all shimmy stopped.
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  5. #5
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    HAVE FUN. DON'T DIE.

  6. #6
    Kodiakmack's Avatar
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    Wondering if the previous owner over lubed. What’s the fix if this is the case?
    HAVE FUN. DON'T DIE.

  7. #7
    mvivion's Avatar
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    Quote Originally Posted by Kodiakmack View Post
    Wondering if the previous owner over lubed. What’s the fix if this is the case?
    Disassemble, clean all parts, lube and reassemble.

    MTV

  8. #8
    Kodiakmack's Avatar
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    Sounds good. She had an annual done before purchase but due to it coming due during fire season, and the fact that I don’t think it was exactly thorough, my dad and I will be doing a deep annual in November. Will do this as part of that.
    HAVE FUN. DON'T DIE.

  9. #9
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    Spring is flat.


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  10. #10
    Kodiakmack's Avatar
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    ��
    HAVE FUN. DON'T DIE.

  11. #11

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    Mack - I think you already said your angle is wrong. From those photos - wow!

    While you wait for new springs that may or may not fix it, machine an aluminum wedge to fit between spring and tailwheel. And remember - these springs can be re-arched cold.
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  12. #12
    Kodiakmack's Avatar
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    I’m time and resource limited so I’m making a flight with one gas stop to where she will get spruced up. It’ll just be me and the pupper onboard so hopefully it shouldn’t be a problem.
    HAVE FUN. DON'T DIE.

  13. #13
    BC12D-4-85's Avatar
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    In my experience after lubing old grease is extruded and new seen....then rotate the fork assembly 360* several times when unloaded to distribute and then expel any excess lube. But really what needs to be done is it disassembled and any worn parts replaced. Copper spacers and metal contact points wear. If not we tighten the main lower bearing nut and hope it recovers....but eventually the lower tapered roller bearing tightens but the components above can't because of wear so any buffering friction against shimmy decreases.

    Gary

  14. #14
    mvivion's Avatar
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    And, as you have no doubt figured out, shimmy is a VIOLENT event, and hard on things besides the tailwheel.

    With an empty back seat, it should be pretty easy to do a tail low wheel landing, and keep the tail off til almost stopped.

    The tailwheel: Arguably the most neglected part of any airplane. Being a tailwheel is a tough job.

    MTV

  15. #15

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    What he says. It can damage your fuselage. When it starts, try to unload the tail with a little forward stick. Be careful, and never jump the brakes at slow speed - use only enough forward stick to stop the shimmy. You will be fine.

  16. #16
    Kodiakmack's Avatar
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    Thanks fellas. It definitely was not a pleasant sensation. I’ve had really bad tailwheel shimmy in the turbine beaver before that contributed to repairing bulkheads back there(beating them up on glaciers all summer didn’t help). With this old of an airplane though, and with my wallet taking the damage, I plan to have it fixed before I do any more real flying.
    HAVE FUN. DON'T DIE.
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  17. #17

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    My two cents worth..., I looked at Steve's videos after I had some shimmy problems and went through everything... I went from no shimmy to bad shimmy overnight!... okay, I put on a baby bushwheel and then I had shimmy!
    After everything seemed in order, I realized the connector springs were pretty loose.... the chain connector links were stretched! I compared some new links with what was on the airplane and there was roughly an inch of difference in 5 chain links!!!!
    New chain..., no shimmy! Easy Peasy.
    Last edited by av8rtom; 10-05-2020 at 12:05 AM. Reason: confusing
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  18. #18
    Gordon Misch's Avatar
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    I've been looking for the link to Steve Pierce's article on tailwheel setup and haven't found it. I'd appreciate any help - thanks. FWIW, I re-arched my springs for a slightly positive caster angle and brought tire pressure up to spec. Shimmy was greatly reduced but not elimated.

    I did find the Scott document on spring tension. Puzzled for a while re why the preload is necessary, and now think I might know why. Oscillation requires a restoring force. If the two spring forces are balanced that is eliminated, at least for relatively small displacements.

    I have a slight (not 1") preload on my springs. I will take up a link and see how that goes.

    Gotta love the wisdom available here!
    Last edited by Gordon Misch; 10-05-2020 at 12:28 AM.
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  19. #19

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    Mine run loose. Keeps friction down at the lower rudder hinge. I don't think the rudder connect springs are a really large part of the equation.
    Yours is probably the 3200 - as above, the clutch needs to be engaged before those bearings get pre-loaded.
    Easy to rebuild. Next time you are down one of my students is an expert - the guy with the dark green J3.
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  20. #20
    Gordon Misch's Avatar
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    Bob, that sorta correlates with the guys that free-caster their tailwheels. Interesting (and mysterious) stuff! Maybe, along with preloading, I should experiment with taking the springs off
    .
    Gordon

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  21. #21
    hotrod180's Avatar
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    Quote Originally Posted by bob turner View Post
    ..... the clutch needs to be engaged before those bearings get pre-loaded......
    I don't understand this comment.
    Cessna Skywagon-- accept no substitute!

  22. #22
    hotrod180's Avatar
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    Kind of a sidetrack to this thread....
    I see that The Landing Gear Works sells hub bolts (what I call the kingpin bolt),
    in standard size plus several over sizes, for the 3400 series tailwheel.
    For when the hole in the tailwheel hub (what I call the t/w head) wears.
    Has anyone had trouble with this?

    I get an occasional t/w shimmy in my 180-- done a bunch of tinkering to eliminate it,
    but it always eventually happens again.
    I had the tail hoisted up to see if the stinger was loose in it's mounts (it wasn't),
    but a friend noticed that the kingpin head seemed to move a bit when I grabbed the fork/wheel & shook the whole works.
    The replacement kingpin bolts (all sizes) are about $200,
    and I imagine that the hole in the t/w head has to be machined out to a proper fit,
    plus the tailwheel head would have to be removed from the stinger.
    So it wouldn't be a cheap or easy thing to do,
    but if it did the trick it'd be worth it.
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  23. #23
    wireweinie's Avatar
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    It's a common item to check for. Any looseness in this bolt is an issue. The only cure's that I know of are to replace the head assembly (yeah right, lol $$), replace the pin with an oversized one, or knurl the existing pin and re install it.

    Pressing the pin in or out is easier when the unit is off the aircraft but it is doable with some imagination, while still installed. Pressing in an oversized pin does not mean the head needs to be machined. If the hole is that large/misshapen then it is probably replacement time for the head.

    Web
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  24. #24
    BC12D-4-85's Avatar
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    The bronze bushing pressed into the fork the hub bolt passes through wears. When it gets loose the fork can do bad things despite preloading the assembly. From my past experience.

    Gary

  25. #25

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    I have seen several older heads that part being loose people have cleaned the parts well and used a two part epoxy to reset hub bolt. Not sure what they used but it worked well. Gordon, here is a link. http://www.pierceaero.net/tws.php
    DENNY
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  26. #26
    hotrod180's Avatar
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    I always thought that kingpin bolt was a press fit, but I just watched Steve Pierce's YouTube about reassembling a 3200 & his kingpin just pops in & out freely.
    Cessna Skywagon-- accept no substitute!

  27. #27
    hotrod180's Avatar
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    Quote Originally Posted by BC12D-4-85 View Post
    The bronze bushing pressed into the fork the hub bolt passes through wears. When it gets loose the fork can do bad things despite preloading the assembly. From my past experience.

    Gary
    I believe the bushing you're referring to fits into the fork, not the t/w head.
    And it is on the exploded drawing for the 3200 series, but not the 3400 series.
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  28. #28
    BC12D-4-85's Avatar
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    Thanks for noting that the 3400 lacks a bushing in the fork. It must use the upper and lower roller bearings to center the hub bolt. Never had a 3400 apart just the 3200's after years of apparent neglect. Neglect includes repeated annual inspections with a tuneup via grease gun only. Like painting instrument panels during restorations that end of the plane gets put off or never properly done.

    Gary
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  29. #29

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    What do you guys think of this one? I have an occasional shimmy. Click image for larger version. 

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    Only felt in pavement, most times starts about 1/2 through the roll out

  30. #30

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    Shouldn't. These are bulletproof tailwheels. Your angle looks good. Tighten the friction nut and go for 50-60 psi.

    Even the Maules use this tailwheel! I have purchased several new, before the price went through the roof.

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    It looks good in the picture, however, I believe it is an empty plane. Put two people, gear and full fuel in and then look at it. I suspect the head caster angle will be off. The reason you feel the shimmy in the last half of the roll out is because the wings/tail are still supporting some of the weight. A simple re-arch should make it better. I run two different springs sets depending on the mission. I have had to re-arch both in a single year not uncommon for a hard working cub or a new pilot doing a lot of tail first landings. DENNY

  32. #32
    RaisedByWolves's Avatar
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    Quote Originally Posted by bob turner View Post
    Shouldn't. These are bulletproof tailwheels. Your angle looks good. Tighten the friction nut and go for 50-60 psi.

    Even the Maules use this tailwheel! I have purchased several new, before the price went through the roof.
    Not that high of pressure on the baby bushwheel. Standard tire yes


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  33. #33
    Steve Pierce's Avatar
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    Quote Originally Posted by hotrod180 View Post
    I always thought that kingpin bolt was a press fit, but I just watched Steve Pierce's YouTube about reassembling a 3200 & his kingpin just pops in & out freely.
    Only if it is messed up. You normally have to press them out.
    Steve Pierce

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  34. #34
    hotrod180's Avatar
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    Quote Originally Posted by Steve Pierce View Post
    Only if it is messed up. You normally have to press them out.

    That has been my experience--
    even with the kingpin bolt on my 3400 which seems to rock a bit in the t/w head,
    I had to drive it out with a hammer & a block of wood.
    But at about 3:10 into this video, the kingpin bolt appears to have just "come out" and you just pop it back in.

    https://www.youtube.com/watch?v=8MGri0D50hg
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  35. #35
    Steve Pierce's Avatar
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    It is a press fit. That was my tailwheel and I should have addressed that issue in the video. I did have the pin knurled and we heated the head and dropped it in but it got loose again and I replaced the head with a double bolt head because I was tired of the head getting loose on the tail spring. I did find out that Bushwheel was knocking those pins in with a hammer early on and then later started using an arbor press.
    Steve Pierce

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  36. #36
    Steve Pierce's Avatar
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    I think Gordon was looking for this video Steve Davidson did when he worked at Alaskan Bushwheel in Joseph.
    Steve Pierce

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  37. #37
    hotrod180's Avatar
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    I've yet to see a video which shows / explains how to properly set the tension on the kingpin nut the first time around.
    It's always been a trial-and-error show for me.
    Initially I usually snug it all the way down, then back it off about 4 flats worth.
    When disassembling, I count the flats required to snug it down,
    then use the same number of flats when I reassemble.
    But it often needs a flat one way or the other to fine tune.
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  38. #38
    Steve Pierce's Avatar
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    Quote Originally Posted by hotrod180 View Post
    I've yet to see a video which shows / explains how to properly set the tension on the kingpin nut the first time around.
    It's always been a trial-and-error show for me.
    Initially I usually snug it all the way down, then back it off about 4 flats worth.
    When disassembling, I count the flats required to snug it down,
    then use the same number of flats when I reassemble.
    But it often needs a flat one way or the other to fine tune.
    I do it by feel, some people like them tight/stiff. I like mine loose and easy to turn.
    Steve Pierce

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  39. #39

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    I am with Steve - if it is in good shape and the kingpin is vertical, only tighten if it shimmies.

    On the Bushwheel? If you are talking about the big balloon tire - we couldn't get it to not shimmy on pavement. It needs a locking mechanism if you like to operate on pavement. Of course, that was just one assembly, but it was brand new.

  40. #40
    TVATIVAK71's Avatar
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    We had a shimmy issue only on last part of rollout on pavement (especially with wheel/tail ski) from the time we got the plane. It was a fresh rebuild with a new Baby BW tailwheel. Spring and king pin angle seemed right. Just decided to live with it because many others seemed to have the shimmy issue so just rode the brakes with tail up. We also have a C-140 that doesn't have any shimmy or tailwheel issues but I disassemble/clean it every two years. Landing the Cub consistently tail up nearly to a stop was masking a big problem. While pushing it around by hand to its tie down the tailwheel locked to one side and refused to move. This woulda sucked on landing. Pulled it off and it was super packed with grease, all three spring pins from thrust plate were sheared off and two springs remained with broken pins inside and the other three pins and springs were ground to pieces floating around in the tightly packed grease till some bits got jammed under/above spring and brought the whole operation to a halt. Other than the rollout shimmy it seemed to work just fine till it didn’t. Looking at the pics you would think a grinding would have been felt when rotating it by hand. That was not the case, it was smooth. Now all tailwheels get taken apart/cleaned/lubed at every annual. New guts and proper lubing did away with our shimmy in this case. Just cause it looked/felt good (Other than occasional shimmy) was no excuse for waiting 2 yrs to disassemble it.
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