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Need input: Is my Acme Stinger movement excessive?

I suspect people don’t see issues because they don’t jack up the tail and hand-exercise the assembly….just my hunch.

I have seen “issues” with all mount holes now exhibiting elongation, as documented in the above caliper pictures.

My first conversation with Acme was largely one sided. My second conversation with another individual at Acme was very cordial, with a good exchange of information. I was promised a set of custom, interference fit mount parts.

Side note: Promising me a one-off set of new parts doesn’t address the potential issues others might encounter.

After being promised new, interference fit parts, I followed up over the course of about 6 weeks with four phone calls to get shipping status. I never received a call back to my last two calls/voice messages. I never received any parts.
 
It is too late for “interference” parts. The holes in the structure are elongated and therefore the structure must be replaced or repaired and then proper fitted components installed. If left as is, the sloppy conditions will just get worst. PM me if you want to discuss this a little more.

Jim
 
I spoke to Acme staff after sharing the above video links via email. The response, “… don’t see an issue with the videos you sent.”

Do others have movement in their stinger assembly? Am I nuts to be concerned?

If you lifted up on the tip of your wing and the fork and strut attach fitting had that much play would you call it a day if the strut manufacturer said all is good?:roll:

While not an engineer, I agree with Tcraft, the elongation is there, and not going to fix itself, and no matter how much you over tighten it, with the tail loaded, unloaded or standing on your head with washers jammed in between, the elongation is going to continue to get worse until the point of failure. That may take a million more hours, or it may happen next week, that is for folks like Tcraft to determine. As is, either the part should be deemed a consumable item and have a wear limit assigned to it, redesigned, or it will ultimately fail.

If you have a catastrophic issue as a result of a failed part you were having issues with I would think they would have to make good on it

That is a wonderfull notion .... if you dismiss the places this cub has already been, and will continue to go. I was on the ground in a saddle the outfit Ted used to? fly for in the Wrangles serviced to some degree, when one of the original T3's lost half of it stinger in a totally benign landing. The manufacturer stood by it, and 'made it good' (once he got back to the L48 on a pawnee stinger I flew in to him).... I hate to imagine how that would have played out if there wasn't another aircraft on the ground? I'm not being melodramatic, I guess now days most that venture that far out carry a sat phone, but all that country was 'airplaned' for years without the need of a satellite overwatch in the name of experimenting.

I generally think experimental owners need to cut manufacturers more slack than the certified crowd. After all, they are 'experimenting' which infers they will have some experience to gain along the way. But I also happen to think that when an experimental owner brings a valid concern to the table with a manufacturer, the manufacturer should probably be a little (or in this case a lot) more pro active. Because like Steve P alluded to there is a bunch already riding on it. It is my understanding that the T3 stinger (at least on that model) was addressed by a change in the alloy or design of that stinger.

Like the T3 failure, the one coming here is foreseeable, and preventable. permanently repairing what's there, is probably not in the cards, although maintaining (temporary and continuing fixes) what's there until it ultimately 'cycles out' may be.

Relying on 'squeezing' the ears to capture the heim will not solve the problem, and will likely lead to a new problem of compromising the ears. Not squeezing the ears, but simply relying on a press fit of the heims in between them is not going to solve the problem, it is only going to delay it some.
As long as there is any slop anywhere in the assembly (even .0001"), be it between the bolt and the fitting, the bolt and the heim, or the heim and the fitting, the problem will show again. If the slop needs to be there for manufacturing, or design, the only thing that will negate this problem is for the fittings to be of the same hardness as the bolt, and that just slows the process and spreads the wear over two items.

This failure is as easy to predict as the one that took out the original T3 stinger. To be fair, in that case it was partly the owners failure to realize the importance of maintaining an appropriate spring rate to the load so as to not bottom out the spring violently. which in turn tried to make a spring out of the swing arm. In this case, softer aluminum vs hardened steel, slop, and jarring. You don't even need to land tail first for this to happen, the jarring of taxiing will eventually do it, or there would be no need for a shock back there.

I will watch this one with interest. Mostly because I like outfits with good customer service, but also because I like watching problem solvers, and regardless of how big or small anyone perceives this to be, there is a problem, and there is no doubt a solution.

Take care, Rob
 
If it were mine I'd open the existing holes and insert a drill bushing. https://www.mscdirect.com/browse/tn?searchterm=drill bushings&hdrsrh=true&rd=k

You need some clearance between the bolt and through-holes to allow movement. I'd think a standard AN bolt would be a good choice. Too tight and you're going to need lubrication and that's going to attract grit.

I don't know the overall width of the ears but you could probably line bore it with a chucking reamer https://www.mscdirect.com/browse/tn...g-Reamers?navid=2106128&rdrct=chucking reamer
the key being enough flute length to have engagement of the flutes through all three holes at the same time. It might have to be done in steps depending on the wall thickness of the bushing. A thin wall bushing would be best for this reason and also it would leave more of the original 'ear' material.

I made a close fit pin once to take out some slop on a front control stick pivot point. Like was mentioned above I threaded both ends and used two AN310 type nuts. Clearance was about .0005" with a little white grease.
 
If it were mine I'd open the existing holes and insert a drill bushing. https://www.mscdirect.com/browse/tn?searchterm=drill bushings&hdrsrh=true&rd=k

You need some clearance between the bolt and through-holes to allow movement. I'd think a standard AN bolt would be a good choice. Too tight and you're going to need lubrication and that's going to attract grit.

I don't know the overall width of the ears but you could probably line bore it with a chucking reamer https://www.mscdirect.com/browse/tn...g-Reamers?navid=2106128&rdrct=chucking reamer
the key being enough flute length to have engagement of the flutes through all three holes at the same time. It might have to be done in steps depending on the wall thickness of the bushing. A thin wall bushing would be best for this reason and also it would leave more of the original 'ear' material.

I made a close fit pin once to take out some slop on a front control stick pivot point. Like was mentioned above I threaded both ends and used two AN310 type nuts. Clearance was about .0005" with a little white grease.

This is most likely how I'd approach this as well, mostly because I am just not the guy who complains about my steak being burned to shoe leather when I ordered it medium rare... *but I do have a memory ;-)


Having said all that, we are looking at this through our own eye's and not the eyes of the average consumer pilot / builder. And in your case, those would be the eyes of a world renowned master machinist.

If I were to take a wild @$$ guess, I'd be willing to bet the average builder would stand a better chance at a line dance, than line boring anything.This is not how I perceive Ted's fabrication skills, I'm just saying the average consumer guy... SO what does that guy do? Hire it out? does that sound like a reasonable expectation with so little time / cycles on it? I dunno...

If I were the manufacturer, at a minimum, I'd want to see Ted's assembly. Maybe he got one from a defective run, since some, but not other's have seen this?
I'd also consider bushing them in the manufacturing stage. Then you have the clearance (slop) at the bolt riding on a hardened surface. Maybe those will be the 2.0 models? Maybe Ted's is free of charge since he contributed to the R&D? I dunno?

Take care, Rob
 
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I think I would message this thread to Matt McSwain who owns Acme and see what he says. He can be found on Facebook very easily. It would be interesting to measure a new one and see how it compares. I have not had any issues with an Alaska Bushwheels tail spring myself. I know a lot of people like the tail suspension but have seen a need for it in the places I go.
 
I’m not an expert on the Acme device, and I am most certainly not an engineer, though I can spell it, most days.

But I can say without hesitation that ANY “slop” in a tailwheel attachment can have VERY ugly consequences. Acme should be on this big time. Hopefully they will.

MTV
 
I bought a Stinger and then watched Ted’s YouTube video on this issue and talked to him on the phone. My bolt holes in my new, never used Stinger, measured the same as what Ted posted for his new Stinger. So I assume the hole size is consistent and as designed. I don’t know why Acme is resistant to making a simple change to the reamed hole diameter. Easy fix. Maybe it has to do with the STC process being well under way and not wanting to slow the process by making a change. I may bush the holes if Acme doesn’t come up with their own fix. My project isn’t flying yet so no wear on mine.
 
Steve, as noted in my original post, in February 2022 staff at Acme viewed the same videos as seen above. Staff at Acme said, “… don’t see an issue with the videos you sent.” If you or someone else doesn't believe me regarding this statement, I have a screen-shot of the text message reply.

My point was, and remains, staff at Acme are fully aware of my hole clearance issues.

When I ordered my stinger, I actually ordered a 2[SUP]nd[/SUP] stinger as well for a friend (to save on shipping). I measured each of the mount assembly holes on that 2[SUP]nd[/SUP] stinger—the holes exhibited the same clearances as on my stinger. Exactly the same.

I really like Acme's Black-Ops shocks. I think the design of the Acme stinger is awesome. I truly--genuinely--want to see Acme both succeed and to continue to innovate. I expressed my desire to help Acme in my original, February 2022, email to Acme. But--in no uncertain terms--I was told there is no issue. Maybe I am nuts to be concerned—that’s why I asked the question in my original post above.

I’m thankful for at least one other person on this thread having measured and reported on their stinger hole clearances.

As much as I’d like new, properly manufactured, interference fit parts, sending me new parts does nothing to address potential hole clearance issues for anyone else who finds themselves in the same situation.
 
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Maybe Ted's is free of charge since he contributed to the R&D? I dunno?

Take care, Rob

For the record, I paid full retail price. And as noted above, I bought not one, but two (!!) stinger assemblies [one for a friend to save shipping].
 
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Knowing the guys at ACME I'm sure they will make it right. I'd just send it to them and let them take a look. Emailing and texting leave a bunch to get lost. . . . .
 
Steve, as noted in my original post, in February 2022 staff at Acme viewed the same videos as seen above. Staff at Acme said, “… don’t see an issue with the videos you sent.” If you or someone else doesn't believe me regarding this statement, I have a screen-shot of the text message reply.

My point was, and remains, staff at Acme are fully aware of my hole clearance issues.

When I ordered my stinger, I actually ordered a 2[SUP]nd[/SUP] stinger as well for a friend (to save on shipping). I measured each of the mount assembly holes on that 2[SUP]nd[/SUP] stinger—the holes exhibited the same clearances as on my stinger. Exactly the same.

I really like Acme's Black-Ops shocks. I think the design of the Acme stinger is awesome. I truly--genuinely--want to see Acme both succeed and to continue to innovate. I expressed my desire to help Acme in my original, February 2022, email to Acme. But--in no uncertain terms--I was told there is no issue. Maybe I am nuts to be concerned—that’s why I asked the question in my original post above.

I’m thankful for at least one other person on this thread having measured and reported on their stinger hole clearances.

As much as I’d like new, properly manufactured, interference fit parts, sending me new parts does nothing to address potential hole clearance issues for anyone else who finds themselves in the same situation.

I don't doubt anything you have posted. My comment about maybe reaching out to Matt was because I know sometimes employees can sometimes not share the same concerns as the owner. I can tell you that I will be looking the next Acme Stinger over very closely that comes in my shop.
 
For the record I have the stinger on my M6 and fittment is inline with Ted’s video. I haven’t reached out to Acme but agree that the clearances are sloppy at best.


Sent from my iPhone using SuperCub.Org
 
I still have a regular leaf spring on mine, going strong and flawless.



T3 seems to have a similar problem a couple of years ago. Have they solved it?

 
I lifted the tailwheel on my Carbon Cub FX3 up today and tested for the type of slop/play in Ted's original video and didn't really notice any. There is a little bit of fore/aft play before shock compression begins, the rotation appears to be the heims rotating around the bolt just a little bit, not the bolt moving up & down, fore & aft inside the bolt holes in the A-arm like Ted's. but that seems to be it, nothing side to side. I didn't take the shock apart to see how snug the bolts were in the holes. Not sure if it is helpful but here is the video. What I did notice is that my main AN7 stinger-to-airframe bolt seems to spin very easily? Definitely doesn't seem like 33 ft lbs per ACME spec or 25 ft lbs per Cub Crafters spec although I don't have a torque wrench that can fit in there without disconnecting the shock. I tightened it up a little bit with a thin 5/8" wrench, but only a turn or so. The whole setup was installed at cubcrafters with torque seals and nothing had rotated, except I didn't see a seal on that AN-7 main bolt. I will investigate that main bolt more before I fly again.

This is at 170 hours of tach time and about 400 landings.

 
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Thank you for your post and video Narwhal.

That up & down movement on your swivel sure seems like more than Heim joint movement, but what do I know.

I still don’t understand if industry specs say an AN-6 bolt torque is 13.3-15.8 Ft lbs, why Acme says to over-tighten by over 100% and how CubCrafters came up with 25 Ft lbs. And what bolt manufacturer or industry references do they both use to justify exceeding specs?

Matt is who I first emailed then talked to about these issues over a year ago.

After over a year since alerting Matt & Eric to these issues, no one has followed up
 
Thank you for your post and video Narwhal.

That up & down movement on your swivel sure seems like more than Heim joint movement, but what do I know.

I still don’t understand if industry specs say an AN-6 bolt torque is 13.3-15.8 Ft lbs, why Acme says to over-tighten by over 100% and how CubCrafters came up with 25 Ft lbs. And what bolt manufacturer or industry references do they both use to justify exceeding specs?

Matt is who I first emailed then talked to about these issues over a year ago.

After over a year since alerting Matt & Eric to these issues, no one has followed up

You might be right, maybe that fore aft movement is related to my AN-7 tailwheel to fuselage bolt being loose, or maybe the an-6 A-arm heim bolt holes are getting bigger like on yours. I'm going to tighten the an-7 up tonight.

To be clear 25 ftlb is the cc spec for the an-7 tailwheel leaf spring to fuselage bolt not supplied by ACME although they spec the 33 ft lbs for it - CC doesnt really address the Acme stinger possibility in the maintenance manual. I will say that an ~8" long 5/8" wrench can currently move that bolt on my rig with 2 fingers worth of pressure. I recently developed a shimmy which is why I'm looking at all this.
 
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It is not the bolt torque it is the clearance between the bolts and the structure that is the problem

Jim
 
I tightened that AN-7 main tailwheel spring bolt attachment that goes through the forward part of the ACME stinger attach plate up to 35 ft - lbs (was reading/hearing ranges all over the places from 23 to 75 ft-lbs) and it seems to have greatly reduced my shimmy, along with trying 60 psi in the tailwheel (in the hangar, which probably equates to 50 psi outside in the 15F weather). Also 10 pumps of fresh mobilegrease into the zerk at the top of the 3200. Mostly it got rid of the shimmy, but not completely. Out of 7 landing I did have one shimmy where I 3 pointed it, but much less violent than previous. Luckily first condi inspection is in 2 weeks and we're going to give it a thorough going through. I'm thinking at least replacing all the nuts, bolts, and washers might be a good move, but I'll see what my IA recommends.
 
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Did the shimmy start when you fitted the tail ski?

Nah, it was fine until last week, seemed to develop after a few hundred landings with the ski.

Caster Angle seems good, been doing a lot of research on Steve Pierce's info about it.
 

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Off topic.. What? a few hundred - you are way behind on YT vids ;)

Nah, it was fine until last week, seemed to develop after a few hundred landings with the ski.

Caster Angle seems good, been doing a lot of research on Steve Pierce's info about it.
 
Off topic.. What? a few hundred - you are way behind on YT vids ;)

Mightve been all the ice that finally snookered my tailwheel. I tried merrill ski strip before they closed it this week and it was an ice roller coaster, went around and landed pavement.
 
@tedwaltman1 , do you have any shimmy if you ever land on pavement? I think you have the BBW so I doubt you do much of that.

I have been really going over my ACME tailwheel carefully, retorquing every bolt and checking shock inflation, making sure it has 300 psi of nitrogen. The shimmy is still rather nasty on pavement with on ACME with the 3200 & 2.50/2.80-4 tire (due to shoulder season ice/poor snow conditions at the airport I am forced onto pavement for a little while). I feel it might be related to the fore/aft slop in my video but can't be sure. The tailwheel does feel a bit clunky on pavement when taxiing over seams in the concrete. Additionally I talked to a high profile former user of the ACME (same type of airplane as you) who said he has switched back to the leaf spring because of the way impact forces are trasnferred to the fuselage, resulting in fuselage breakage rather than leaf spring breakage when exposed to strong impacts. If my mechanic can't help me get this solved at condition inspection I might consider doing the same, but I still haven't fully disassembled the tailwheel fork to check the steering/compression springs yet.

Edited to add: I will go check the bolt hole clearance on the bolt hole for the upper shock heim this afternoon.
 
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I do have some shimmy if I land on pavement, which I rarely do. Occasionally on hard gravel runways too. But shimmy is Indeed rare.

Yes, I have a BBW.

When you torque your assembly, do you do so to industry standards, or to the values in the Acme manual?

I plan to go back to the stock leaf spring when I finish up my annual this May. Acme isn’t acknowledging any issues with suspect manufacturing tolerances nor, as far as I’m aware, reaching out to existing owners to address the assembly mount hole issues already in the field.
 
I do have some shimmy if I land on pavement, which I rarely do. Occasionally on hard gravel runways too. But shimmy is Indeed rare.

Yes, I have a BBW.

When you torque your assembly, do you do so to industry standards, or to the values in the Acme manual?

I plan to go back to the stock leaf spring when I finish up my annual this May. Acme isn’t acknowledging any issues with suspect manufacturing tolerances nor, as far as I’m aware, reaching out to existing owners to address the assembly mount hole issues already in the field.

I'm presently using the Acme Spec from the stinger install manual on all the bolts. Everything I've done thus far seems to not have made too much difference though.

Edited to add: I'm not sure what Cub Crafters used when they put mine on at the factory during the initial build, but the torque seal marks seemed to line up pretty closesly.
 
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We don't usually post on forums but it has come to our attention there is a need to offer an explanation as to the design of the Stinger Mount and more specifically the through bolt hole ID. The Stinger was designed to address any offset camber issues in the fuselage that account for shimmy issues. In the event that the Heim joints require misalignment to acheive zero camber it is virtually impossible to install the bolt if the tolerances were any tighter. THE TOLERANCE IN THE BOLT HOLE IS INTENTIONAL. As outlined in the instruction manual, when the alignment is correct and the bolt is installed to the proper torque spec there is ZERO issues.

During 4 years of STC testing and with almost 1,000 units flying, there has never been a single issue with this concern. As always, we offer a money back guarantee that is communicated to EVERY customer. If anyone is not satisfied with construction, performance or appearance of ANY product that item can be returned with no questions asked.
 
We don't usually post on forums but it has come to our attention there is a need to offer an explanation as to the design of the Stinger Mount and more specifically the through bolt hole ID. The Stinger was designed to address any offset camber issues in the fuselage that account for shimmy issues. In the event that the Heim joints require misalignment to acheive zero camber it is virtually impossible to install the bolt if the tolerances were any tighter. THE TOLERANCE IN THE BOLT HOLE IS INTENTIONAL. As outlined in the instruction manual, when the alignment is correct and the bolt is installed to the proper torque spec there is ZERO issues.

During 4 years of STC testing and with almost 1,000 units flying, there has never been a single issue with this concern. As always, we offer a money back guarantee that is communicated to EVERY customer. If anyone is not satisfied with construction, performance or appearance of ANY product that item can be returned with no questions asked.

Not sure you can ask for much more than that. Pretty awesome to see a company stand behind their products like they do. Rare in today's environment.

I sure do enjoy flying their gear and stinger. Total game changer for me.
 
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