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Thread: L-21B tail wheel shimmy

  1. #1
    cubnut93's Avatar
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    L-21B tail wheel shimmy

    When I was restoring a L-21B in 2014 for a friend I was very aware of the tail wheel shimmy problem. I found many articles about this situation and I followed the rules as best I could to alleviate the shimmy. After flying the airplane for the past 6 years and recently installing new leaf springs, the shimmy seems to be back. It is not all the time, but just once in a while. The last time was a few days ago. I assume that it was quite noticeable because the short pieces of chain that connect the springs to the arm assembly disappeared.

    So I guess that I will be researching the problem again at this time. I have attached several photos that I hope will give everyone more information.

    The first photo is the tail wheel before I had installed the new leaf springs.

    I followed the advise of the person in this video---- https://www.youtube.com/watch?v=QtokU8mIDQk

    I have a question about the connector springs. I have attached a photo of these springs from a Aircraft Spruce catalog. Is one type of spring better than another? What is the "Maule Anti-Shimmy Connector Spring Kits" all about?

    I noticed that on several occasions the chain has come off of the connection spring. Is there a better way to attach the chain to this spring?

    Have I missed anything?
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  2. #2
    Grant's Avatar
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    Make sure the geometry of the caster angle is correct WITH weight in the airplane.... I.e. pilot/passenger,fuel etc.

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    BC12D-4-85's Avatar
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    In my experience the chain example in the pic are slightly too long and the spring is loose and sagging. There should be some spring tension. Remove a link from each chain would be my fix as a start. You might have to fine tune the spring extension with some more hardware.

    Maule compression springs increase tension at a greater rate when extended than what you have in my experience. I prefer what you have but others may have a different idea.

    Is there a water drain/vent hole(s) in the lower rudder next to the rudder post and just above the bottom tubing? If not get one as they can rust out.

    Gary
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    Crash, Jr.'s Avatar
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    Hard to tell from the angle of your picture but it looks like the tailwheel pivot is exactly perpendicular to the ground but would dip forward with weight in the back seat/baggage/ect. Don't forget that when you haul back on the stick you're also loading the tail pretty significantly on the ground.

    You might be able to eek a bit more out of the tailspring by putting the recommended piece of rubber or phenolic between the rear tailspring bracket and the tailspring. You need to have something soft in there to prevent the vibrations from the spring from fatiguing and cracking the welds in the tail post. As a bonus a 1/8" piece of rubber/plastic/phenolic in there will push the tailspring down in the rear and give you about another degree of castor angle.

    The steering springs do look a little slack so taking a link out wouldn't hurt. The idea is little to no tension on the springs at rest but not having things sagging. Check out this video of Steve Davidson (Paul Claus' mechanic) and how he sets up the tailwheel rigging: https://www.youtube.com/watch?v=u3qFwzPNBgE *EDIT* I meant this video: https://www.youtube.com/watch?v=QtokU8mIDQk but the previous link is really good too. Worth a watch whenever such an experienced mechanic is talking.

    The Maule "anti-shimmy" steering springs are in my experience pretty garbage. First off they are compression springs so if you turn the rudder far enough in one direction sitting still they will bottom out and then possibly bend the tailwheel's steering arm. In terms of their claim to prevent tailwheel shimmy, that is about 90% the job of the castor angle and 10% the condition of the thrust washers/springs inside the tailwheel head. The steering springs and steering rigging have no effect on shimmy as they are just along for the ride and only give the tailwheel the suggestion of how/when to turn and how to act on the ground. Watch a video of a tailwheel shimmying on landing and you will just see the chains and springs flying all over the place being basically useless.
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    BC12D-4-85's Avatar
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    Tailwheels can provoke threads like oil matters. But unlike oil, fixing isn't as simple as changing and often takes renewing. They wear...internal parts, the two main housings, and without periodic disassembly and inspection other components of the system can be called prematurely to question. When I've had problems I took them apart and replaced the visibly worn or damage stuff. The parts are available from Airframes for example (https://www.airframesalaska.com/Tail...rts-s/1814.htm). Once repaired then assess the spring, angles, and of course the spring and rudder yoke/steering arm integrity in the rudder.

    Gary

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    Looks like in your pic the tailwheel angle is negative.. slightly. Try a rubber or thick leather pad under the tailpost between the spring and tail. it might fix your super tail shake, but I would just take the spring off and take it to a local spring shop and have them re arc it, thats your shake. Get some positive in that, slightly, not too much or it will be hard to steer and break free.

  7. #7
    BC12D-4-85's Avatar
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    The pic is prior to the new tail springs were installed.

    Gary

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    Steve Pierce's Avatar
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    Put the Airframes PA18 spring on and the angle problem is solved and they do not loose their arch like Pawnee springs do in a year or so. Also use the steering springs Piper Used, available at Airfraes and Univair. Univair under the part number in the PA18 parts manual. I get in Super Cubs that people have put these wimpy POS springs in and you have to keep stabing brake to steer the thing. I find it majorly annoying. When I push the rudder I want the tail wheel to move. I like them where there is no slop in the chain.
    Steve Pierce

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    cubnut93's Avatar
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    Quote Originally Posted by BC12D-4-85 View Post
    In my experience the chain example in the pic are slightly too long and the spring is loose and sagging. There should be some spring tension. Remove a link from each chain would be my fix as a start. You might have to fine tune the spring extension with some more hardware.

    Maule compression springs increase tension at a greater rate when extended than what you have in my experience. I prefer what you have but others may have a different idea.

    Is there a water drain/vent hole(s) in the lower rudder next to the rudder post and just above the bottom tubing? If not get one as they can rust out.

    Gary

    ************************************************** ***********************

    According to this video there is supposed to be some slack--https://www.youtube.com/watch?v=QtokU8mIDQk

    Please correct me if I am wrong.

  10. #10
    cubnut93's Avatar
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    Quote Originally Posted by BC12D-4-85 View Post
    The pic is prior to the new tail springs were installed.

    Gary

    *********************************************


    I will be going to the hangar Tuesday and I will get some better photos.
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  11. #11
    cubnut93's Avatar
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    Quote Originally Posted by Crash, Jr. View Post
    Hard to tell from the angle of your picture but it looks like the tailwheel pivot is exactly perpendicular to the ground but would dip forward with weight in the back seat/baggage/ect. Don't forget that when you haul back on the stick you're also loading the tail pretty significantly on the ground.

    You might be able to eek a bit more out of the tailspring by putting the recommended piece of rubber or phenolic between the rear tailspring bracket and the tailspring. You need to have something soft in there to prevent the vibrations from the spring from fatiguing and cracking the welds in the tail post. As a bonus a 1/8" piece of rubber/plastic/phenolic in there will push the tailspring down in the rear and give you about another degree of castor angle.

    The steering springs do look a little slack so taking a link out wouldn't hurt. The idea is little to no tension on the springs at rest but not having things sagging. Check out this video of Steve Davidson (Paul Claus' mechanic) and how he sets up the tailwheel rigging: https://www.youtube.com/watch?v=u3qFwzPNBgE *EDIT* I meant this video: https://www.youtube.com/watch?v=QtokU8mIDQk but the previous link is really good too. Worth a watch whenever such an experienced mechanic is talking.

    The Maule "anti-shimmy" steering springs are in my experience pretty garbage. First off they are compression springs so if you turn the rudder far enough in one direction sitting still they will bottom out and then possibly bend the tailwheel's steering arm. In terms of their claim to prevent tailwheel shimmy, that is about 90% the job of the castor angle and 10% the condition of the thrust washers/springs inside the tailwheel head. The steering springs and steering rigging have no effect on shimmy as they are just along for the ride and only give the tailwheel the suggestion of how/when to turn and how to act on the ground. Watch a video of a tailwheel shimmying on landing and you will just see the chains and springs flying all over the place being basically useless.
    ************************************************** ************************************

    Thanks for your reply--

    One thing that I was wondering about is the difference between the two spring types where they attach to the chain. The springs that are currently on the airplane don't seem to have a secure attachment to the chain. Any shimmy and the chains unattach themselves from the springs. Is there a better way to secure the chain to this type of spring?

    BTW--I will have better photos (with the newer leaf springs) soon.
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  12. #12
    BC12D-4-85's Avatar
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    There's always an implied disclaimer when posting opinions or experience, at least for me but likely some others. As in "this works for me or this doesn't work for me". Can be past tense or history for experience. There's rarely a simple right or wrong just what works best for you.

    I prefer slight spring tension versus slack. I leave the why of it to personal experience having tried several setups before You Tubers populated the Internet. But that assumes an airworthy and slightly tight tolerance tailwheel and rudder yoke or bets are off.

    As far as chains detaching I have put a wrap of safety wire over the chain-spring connection and tighten. It can help hold them in firm contact and discourage excessive movement.

    Gary

  13. #13
    Steve Pierce's Avatar
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    The Piper springs take internal pliers to spread out and get the chain installed. I too like a bit of tension.
    Steve Pierce

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    Crash, Jr.'s Avatar
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    Cubnut, you're right the original Piper style springs have that opening that can cause the spring to fall off. First I would get your spring tension a little tighter by removing a link. Secondarily I have seen some people throw some safety wire around the chain/spring connection to keep it from popping off. Losing steering can be bad depending on the situation so you really don't want that thing coming off.

  15. #15
    Steve Pierce's Avatar
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    The Piper springs I have worked with require spread apart with a tool to get the chain on. I have seen the tear drop shape links safety wired. Really like the threaded chain links with safety hole that Bushwheel use to sell, made it easy to install and remove. I believe Maule still sells them. I really think if you replace the spring your issue will be solved, has worked in every single case of tail wheel shimmy in every kind of tail dragger I have dealt with over the years.


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    Steve Pierce

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    mvivion's Avatar
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    I asked Airframes why they quit selling those......man, those things are a work of art. The response was they cost like $6 or $7 each to have made. I told em id pay that.

    MTV

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    JWE's Avatar
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    Hate to be a dumba$$, but what is that tool called, Steve. I need one.

  18. #18
    Steve Pierce's Avatar
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    Quote Originally Posted by JWE View Post
    Hate to be a dumba$$, but what is that tool called, Steve. I need one.
    Not a dumb question. I don't even know where I got them but I figured out a good use for them and now they live in my tail wheel service box. Got tired of getting up and down from under the tail messing with the tail wheel and put everything I needed in a tool box that had to do with the tail wheel down to the chain and cotter pins. Mine are Snap On 70A which there are a pair on eBay right now https://www.ebay.com/itm/123504278987 but MSC has these. https://www.mscdirect.com/product/details/84971332 They are an external snap ring plier. Very handy for spreading those springs apart.
    Steve Pierce

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    Crash, Jr.'s Avatar
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    Quote Originally Posted by Steve Pierce View Post
    The Piper springs I have worked with require spread apart with a tool to get the chain on.
    I'm looking at a newer stock of the U3239-001 springs here and they all have about a 3/32 to 1/8" gap in all of them. The ABI-3239 (knockoff) springs have a healther gap in the 1/8" range for all. I do remember seeing the older Univair springs with little to no gap but the newer ones seem to have a gap.

    The quick links were nice, I'll have to dig around and see if I can find some old stock somewhere....

  20. #20
    Steve Pierce's Avatar
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    Takes quite a bit of stretch to get the chain in there. Wish I had stashed more quick links. Gotta call Maule and see if they still have them.
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    Steve Pierce

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    JWE's Avatar
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    Thanks, Steve. I ordered one.

  22. #22
    Steve Pierce's Avatar
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    I'm a tool junky.
    Steve Pierce

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    Eddie Foy's Avatar
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    Did the same thing with my old Cub.

    Quote Originally Posted by Steve Pierce View Post
    Takes quite a bit of stretch to get the chain in there. Wish I had stashed more quick links. Gotta call Maule and see if they still have them.
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    "Put out my hand and touched the face of God!"

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    Not that my opinion matters but I rigged my tailwheel with a slight tension. Figured if I have a steerable tailwheel I would like positive not sloppy responce.


    Sent from my iPhone using SuperCub.Org
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    cubnut93's Avatar
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    Quote Originally Posted by Crash, Jr. View Post
    Hard to tell from the angle of your picture but it looks like the tailwheel pivot is exactly perpendicular to the ground but would dip forward with weight in the back seat/baggage/ect. Don't forget that when you haul back on the stick you're also loading the tail pretty significantly on the ground.

    You might be able to eek a bit more out of the tailspring by putting the recommended piece of rubber or phenolic between the rear tailspring bracket and the tailspring. You need to have something soft in there to prevent the vibrations from the spring from fatiguing and cracking the welds in the tail post. As a bonus a 1/8" piece of rubber/plastic/phenolic in there will push the tailspring down in the rear and give you about another degree of castor angle.

    The steering springs do look a little slack so taking a link out wouldn't hurt. The idea is little to no tension on the springs at rest but not having things sagging. Check out this video of Steve Davidson (Paul Claus' mechanic) and how he sets up the tailwheel rigging: https://www.youtube.com/watch?v=u3qFwzPNBgE *EDIT* I meant this video: https://www.youtube.com/watch?v=QtokU8mIDQk but the previous link is really good too. Worth a watch whenever such an experienced mechanic is talking.

    The Maule "anti-shimmy" steering springs are in my experience pretty garbage. First off they are compression springs so if you turn the rudder far enough in one direction sitting still they will bottom out and then possibly bend the tailwheel's steering arm. In terms of their claim to prevent tailwheel shimmy, that is about 90% the job of the castor angle and 10% the condition of the thrust washers/springs inside the tailwheel head. The steering springs and steering rigging have no effect on shimmy as they are just along for the ride and only give the tailwheel the suggestion of how/when to turn and how to act on the ground. Watch a video of a tailwheel shimmying on landing and you will just see the chains and springs flying all over the place being basically useless.
    **************************************************

    I have attached new photos that were taken today. These photos show the new springs which were purchased from Univair. The caster angle is about 3 or 4 degrees positive. I know that will change when more weight is placed in the airplane. I do not have any shim in the rear mounting bracket. Is that the only way to increase positive caster?

    When I was checking the tail wheel out today I found that the tail wheel tire was very low on air pressure. Undoubtedly that was the main cause of the shimmy. But I would like to see more positive caster see I will install a 1/8" shim on the rear mount. Won't rubber just smash down after a few landings? Will anything else work? How about a piece of aluminum?
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  26. #26
    Crash, Jr.'s Avatar
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    Okay, that castor angle looks really good; that should fix your shimmy issue if not completely then at least diminish it. The rubber "shim" between the spring and tail is not so much for angle (you already have plenty) but to isolate harmonic vibrations from the spring from traveling into the fuselage and damaging welds in the tail. It is HIGHLY advised to have some sort of soft material like rubber, plastic, or phenolic between the spring and rear mounting point to prevent damage to the fuselage.

  27. #27
    BC12D-4-85's Avatar
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    Engine baffling material is tough and works as an insulator but should be replaced as required...Bellanca used an aluminum shim between the spring and frame. Keep the tire aired up. Keep the tailwheel serviced/lubed and internal tolerances tight.

    Gary

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    skywagon8a's Avatar
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    Quote Originally Posted by Steve Pierce View Post
    Wish I had stashed more quick links. Gotta call Maule and see if they still have them.
    Will this work? $0.30 ea.
    https://www.mfrexpress.com/quick-lin...nc-plated.html
    N1PA

  29. #29
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    I have not had any issues with damaged fuselages without rubber between the spring and the tail post pad. I have used a tractor trailer mud flap and cut a pad out of it to get more arch. Bill Duncan cut a bunch of them the last time he was here. My experience is that the Univair spring looses its arch over time depending on tail weight. Heavy Cubs in one year.
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  30. #30
    JWE's Avatar
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    Maule has the quick links. $6 each.
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  31. #31
    cubnut93's Avatar
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    Quote Originally Posted by Steve Pierce View Post
    The Piper springs I have worked with require spread apart with a tool to get the chain on. I have seen the tear drop shape links safety wired. Really like the threaded chain links with safety hole that Bushwheel use to sell, made it easy to install and remove. I believe Maule still sells them. I really think if you replace the spring your issue will be solved, has worked in every single case of tail wheel shimmy in every kind of tail dragger I have dealt with over the years.


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    ************************************************** ************************************************** ***********

    Thanks for the reply--

    I think that the shimmy problem happened because the tail wheel tire was flat (no air pressure in the tire--the gauge didn't even move when I checked it) I do like those quick links. I assume that I can't use the hardware store links since they don't have a safety wire hole? BTW could you please watch this video and tell me if it shows the CORRECT way to rig the spring and chains? https://www.youtube.com/watch?v=QtokU8mIDQk

  32. #32
    skywagon8a's Avatar
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    Quote Originally Posted by cubnut93 View Post
    I assume that I can't use the hardware store links since they don't have a safety wire hole?
    Don't you have a drill?
    N1PA

  33. #33
    cubnut93's Avatar
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    Quote Originally Posted by JWE View Post
    Maule has the quick links. $6 each.
    **********************************

    Can you give me a phone number or a link to their website?

  34. #34
    cubnut93's Avatar
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    Quote Originally Posted by skywagon8a View Post
    Don't you have a drill?
    **************************************
    Will that work on a certified airplane?

  35. #35
    skywagon8a's Avatar
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    Quote Originally Posted by cubnut93 View Post
    **************************************
    Will that work on a certified airplane?
    There are some things which are don't ask don't tell. This is one of them. Where do you think the chain comes from?
    https://www.acehardware.com/departme.../chain/5387188

    And this: https://www.acehardware.com/departme...essories/79101
    Last edited by skywagon8a; 08-04-2020 at 05:37 PM.
    N1PA

  36. #36
    JWE's Avatar
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    Maule Parts: 229-985-2045 ask for Kasey She said she wouldn't hold it against me since my bird is not a Maule

  37. #37
    BC12D-4-85's Avatar
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    I would think with some creative safety wiring an un-drilled quick link would suffice. Like full wrap wired from the raised portion of the threaded end over the barrel to the link's body just behind the turn barrel. If the barrel can't move away from the threaded portion's lip where's it going to go? I've used these for years in other applications and once well tightened with blue or red thread locker never had them release.

    Gary

  38. #38
    Steve Pierce's Avatar
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    Quote Originally Posted by cubnut93 View Post
    ************************************************** ************************************************** ***********

    Thanks for the reply--

    I think that the shimmy problem happened because the tail wheel tire was flat (no air pressure in the tire--the gauge didn't even move when I checked it) I do like those quick links. I assume that I can't use the hardware store links since they don't have a safety wire hole? BTW could you please watch this video and tell me if it shows the CORRECT way to rig the spring and chains? https://www.youtube.com/watch?v=QtokU8mIDQk
    Yes, that is actually a video done by Bushwheel that Wup has reposted.
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  39. #39
    fancypants's Avatar
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    I hitch the sash chain onto the spring with safety wire. I tried without and it just shakes its way off right when you need it to work. With the safety wire it can still slide as required, but it has enough friction to keep it on the hook.

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  40. #40
    cubnut93's Avatar
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    Quote Originally Posted by Steve Pierce View Post
    Yes, that is actually a video done by Bushwheel that Wup has reposted.

    *******************************************

    Thanks---so I assume that the chain should have soon slack as shown. Some people seem to prefer a taut chain. Am I better to have some lack? ALSO, when I set up the chain as shown in the video I ended up having one side longer than the other by one link in the chain--is that a problem?

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