Tail wheel shimmy

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

bob turner said:

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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Not that high of pressure on the baby bushwheel. Standard tire yes


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hotrod180 said:

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.

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Only if it is messed up. You normally have to press them out.

Steve Pierce said:

Only if it is messed up. You normally have to press them out.

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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

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.

I think Gordon was looking for this video Steve Davidson did when he worked at Alaskan Bushwheel in Joseph.

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.

hotrod180 said:

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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I do it by feel, some people like them tight/stiff. I like mine loose and easy to turn.

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.

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.

Has anyone re arched their springs on there own, cold?

Local spring shop says they don’t have dies that small for there press. I have a press brake in my shop, but it’s a guillotine style. I can progressively apply brakes to it along the arch, but I’m worried it might stress/kink the spring metal in concentrated areas.


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We did that successfully to J3 springs. You need something sort of round to distribute the load. Local metal store sells scrap steel discs, we sawed off a crescent shaped piece.

I did recently, by 5 deg. I concentrated the change near the tailwheel to reduce elevating the tail (AOA). It's working well.

As the tail wheel wears - head to spring and head to fork assembly - the AOA of the mating surfaces can decrease. Shimmy starts happening - wow!. Then larger loads for the fork like big tires or tail wheel skis are added which complicates the geometry. Adequate dampening friction and correct AOA are important. The tail of the plane is an itch that need periodic scratching without complaint.

Gary

ADK_Cub said:

Has anyone re arched their springs on there own, cold?

Local spring shop says they don’t have dies that small for there press. I have a press brake in my shop, but it’s a guillotine style. I can progressively apply brakes to it along the arch, but I’m worried it might stress/kink the spring metal in concentrated areas.


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I have rearched but prefer to have a sping shop do it. Make a template and go slow making small changes and make sure you have good radius.

You say shimmy like it’s a bad thing.