• If You Are Having Trouble Logging In with Your Old Username and Password, Please use this Forgot Your Password link to get re-established.
  • Hey! Be sure to login or register!

Need input: Is my Acme Stinger movement excessive?

Not sure you can ask for much more than that.

When I saw that AcmeAeroFab had commented I was hoping to see an explanation as to why their through bolt torque spec was correct and appropriate. When I suggested using the Acme torque spec I was told it wouldn't be appropriate and wouldn't fix the problem.
 
Yah, I thought the response was more defensive than explanatory. But that's just me, the perennially skeptical guy. As I understand it, they say camber adjustment causes bolt misfit, but isn't that an inherent flaw in the case that camber adjustment isn't required? Thing is, I know from experience that too-loose fits cause parts to beat themselves to death from shock-loading at each cycle.
 
Hole tolerance, as designed, results in eccentric hole elongation over time, as I documented with caliper pictures in my first post in this thread.

To be clear, the above “hole tolerance design” issue was never mentioned in several phone calls and text messages with both Matt & Eric in February 2022 (when I first reported possible hole tolerance issues).

I’m thankful for Acme’s unconditional money-back guarantee.

When I return from overseas at the end of April I will be returning my stinger assembly. I will report back in this thread to hopefully confirm the success of my return.
 
I have been thinking a lot about this thread and happened to have had to talk to Matt at Acme on another matter and we discussed this thread. Like I posted before, I maintain several Carbon Cubs with the Acme Stinger and one in particular has a hard life and have not noticed any issues. I started looking at the Stinger design and the load paths. The main load paths does not travel through this hole that is in question. Most of the load goes through the shock, that is the whole purpose in having a shock. Now that brings me to my good old tail spring. It has an enlarged hole as well. Pretty big and have never seen an issue. I also have to think about Acme's experience. They build suspensions for NASCAR, designed, built, tested and certified the Stinger with the FAA. I gotta give them credit, they are way smarter than me. I will keep an eye out for an issue because of what Ted has pointed out but so far everything I am seeing has not had an issue. I am also sure that if there is an issue the Acme guys will help.
94132.jpeg


94133.jpeg


94134.jpeg



94135.jpeg


94136.jpeg


94137.jpeg
 

Attachments

  • 94132.jpeg
    94132.jpeg
    94.7 KB · Views: 886
  • 94133.jpeg
    94133.jpeg
    83.1 KB · Views: 885
  • 94134.jpeg
    94134.jpeg
    78 KB · Views: 883
  • 94135.jpeg
    94135.jpeg
    93.7 KB · Views: 883
  • 94136.jpeg
    94136.jpeg
    97.5 KB · Views: 883
  • 94137.jpeg
    94137.jpeg
    66.3 KB · Views: 887
I think the real problem here is that damn sticker Univair puts on new tailsprings!! I'd pay double if they would leave the thing off. I cuss them every time ;-)
 
At the risk of thread hijack I'll give an update on my tailwheel shimmying. To be clear I'm not saying this is an ACME tailwheel stinger problem for sure, although I guess it might be, I can't really know until/unless I change it out for a leaf spring/t3/tk1 I guess.

I paid a tailwheel experienced A&P IA disassmble my ABI-3200 with the Acme Stinger and 2.50/2.80-4 6 ply tire during condition inspection, on the Carbon Cub FX3. We tore the whole thing down and built it back up, relubricrating everything I.A.W. recommendations and replaced the ABI compression spring & spacers in the steering mechanism which the mechanic said were too soft and becoming dented on the edges. We used scott parts to replace them, iirc, not sure, he bought them.

We cleaned and relubed everying, and had the mechanic reinstall it all. We serviced the Acme strut to 300 PSI using nitrogen and checked torque to ACME spec on all the stinger bolts. Mechanic said everything looked good to him while checked it out in the hangar. It still shimmys on pavement pretty hard.

Last thing I guess I can try is going to the most forward caster angle on the acme strut adjustment, even though the caster angle looks OK (tailwheel steering axis of rotation points slightly forwad) at the middle setting of the 3 where it currently sits. This will give up some of the AoA especially on 8.50's and make the shock firmer, if I understand correctly. Then I guess I will try a leaf spring if that doesn't work. If I hold the tail off until the last possible second the shimmy isn't horrible, but it's still annoying for those last 10 MPH. Maybe I will wait until I am back on 31's for summer to see how that affects things too (currently on 8.50's with penetration skis).
 
I have shimmy issues also, even with new Airframes leaf spring. I've found that fully inflating the tailwheel tire to at least 50 psi (70 psi is max, I think) minimizes the issue. It also seems a tire that is an "experienced" shimmy-villain seems to get worn in a manner that worsens the problem, though this might be my imagination.
 
I have shimmy issues also, even with new Airframes leaf spring. I've found that fully inflating the tailwheel tire to at least 50 psi (70 psi is max, I think) minimizes the issue. It also seems a tire that is an "experienced" shimmy-villain seems to get worn in a manner that worsens the problem, though this might be my imagination.

Good point, I did recently replace the tire about a month and ~ 20 landings ago and I'm running it at 60 psi last I checked. My old tire had definitely been deformed with a visible flat spot. I've watched all the Steve Pierce vidoes and have run through everything on the Airframes Alaska list of reasons shimmy happens. I will say that it seemed to shimmy less when I took almost everything out of the aft baggage and operated with a forward CG on the test flight after condition inspection for obvious reasons. I usually carry 30-40 lbs of engine/wing covers and a bunch of survival gear with snowshoes in the aft baggage.
 
I've found that fully inflating the tailwheel tire to at least 50 psi (70 psi is max, I think) minimizes the issue.

At least some of those experiencing this problem are using the Baby Bushwheel. The sidewall of mine is marked 25 psi max. I've run mine between 12 and 20 psi and seen no obvious correlation between pressure and shimmy resistance.

Increasing pressure increases tire stiffness but reducing tire pressure increases the contact patch size and increases damping. There may well be a sweet spot for any given tail weight.
 
I didn't clarify - I'm using a standard 8" tailwheel, not BBW.

I'm not sure the tire is actually sliding on the pavement significantly? Decreasing pressure also increases flexibility resulting in increased amplitude for a given input, and decreased spring rate resulting in decreased natural frequency. It's all witchcraft. :-?
 
I'm not sure the tire is actually sliding on the pavement significantly?

The scuff marks on my BBW show that when it shimmies it rotates +/- 90 degrees. That means the contact patch is rotating on the pavement. The scuff marks extend well beyond the normal contact patch and are very visible when I get back to the hangar. I doubt they would be as obvious on a tire with a flat tread profile.

The only damping would seem to come from friction in the pivot axis of the tailwheel fork and the contact patch of the tire. Airframes will sell more damping for $1,095 +tax.
 
The scuff marks on my BBW show that when it shimmies it rotates +/- 90 degrees.

I thought about that some more. The scuff marks are parallel to the tail wheel spindle but does not mean the fork was rotated 90 deg. The tire is both displaced laterally and rotated when the assembly shimmies. Someone smarter than me may be able to describe the vector diagram.
 
Final solution for the tailwheel shimmy on my Acme Stinger/Scott 3200 with the 2.80/2.50 aero classic tire was moving the bottom shock heim attachment to the aft-most (tail highest/highest rate) of the 3 tail-height options. As is well documented, the resulting forward movement of the tailwheel caster angle is probably what stopped my shimmy.

Unfortunately the aft part of my burl's tail ski scrapes the ground if I leave it attached with the Acme stinger at the highest position, so I had to take the tail ski off. I also lose a fair bit of AoA for takeoff and/or more easily hit tail first on landing, which isn't great especially on the 8.50's I use while I'm on penetration skis. Not sure why it seemed to work for a little while in the mid-height position but now it wants the tallest setting to avoid shimmying. On the upside ski season almost over and the tail-high shock position gives better forward visibility.
 
So I just finished the condition inspection on the FX3 serial #2 which has just over 100 hours on it. This airplane is used for demos by the local Cub Crafters dealer, has been to Alaska once, Idaho several times as well as Arkansas. I actually inflicted some damage to it in Arkansas a few years ago. I gets rode hard and put up wet so to speak. There was some play in the Acme Aero Stinger when I jacked it up. Sorry I did not video the play but it looked like what Ted posted. I disassembled the unit and found wear in these bushings. I contacted Matt at Acme Aero and sent him the video. He looked at the video and said the bushings did their job and sent me some new ones.
This video is play in one of the four bushings, this was the worst one.
I installed four new bushings and I have no more play other than the normal play between the head and the fork on a tailwheel I just disassembled, cleaned, inspected, lubricated and reassembled.
Matt did say that he did refund Ted's money 100% on the Acme Aero Stinger he sent back.
 
Last edited:
Yes, last week I received my refund from Acme Aero.

I had 208 landings on my Acme stinger.

My posted caliper pictures (above, in this thread) show elongated stinger mount assembly holes. There is no arguing with the caliper pictures.

Thank you for your input Steve.
 
My posted caliper pictures (above, in this thread) show elongated stinger mount assembly holes. There is no arguing with the caliper pictures.
I am not seeing the caliper pictures, where are they? I still have the airplane here and can easily compare.
 
Final solution for the tailwheel shimmy on my Acme Stinger/Scott 3200 with the 2.80/2.50 aero classic tire was moving the bottom shock heim attachment to the aft-most (tail highest/highest rate) of the 3 tail-height options. As is well documented, the resulting forward movement of the tailwheel caster angle is probably what stopped my shimmy.

Unfortunately the aft part of my burl's tail ski scrapes the ground if I leave it attached with the Acme stinger at the highest position, so I had to take the tail ski off. I also lose a fair bit of AoA for takeoff and/or more easily hit tail first on landing, which isn't great especially on the 8.50's I use while I'm on penetration skis. Not sure why it seemed to work for a little while in the mid-height position but now it wants the tallest setting to avoid shimmying. On the upside ski season almost over and the tail-high shock position gives better forward visibility.

Most any taildragger will be able to slow down on final so the tailwheel is below the mains at landing. A cub can get a foot below a set of 31 in bushwheels pretty easy. The key to avoiding a tailwheel first landing is reading up on the MAF technique. Do the normal flare allowing the tailwheel to go below the mains but release back pressure right before you touchdown (takes practice) and let the tailwheel move above the mains. It works great is a Cessna 180 and fine in a cub. I am a big fan of a Bushwheel/stock tail spring but even with that I do the MAF stick/yoke release to avoid a tailwheel strike and then pin the mains with brake or stick (also dump flaps). I would like to say a shocked tail spring is totally useless but I have see 3 year old tail sections broken in a working cub!! If you can't avoid tail first landings for whatever reason you will have to live with early replacement/repair of the units.
DENNY
 
Most any taildragger will be able to slow down on final so the tailwheel is below the mains at landing. A cub can get a foot below a set of 31 in bushwheels pretty easy. The key to avoiding a tailwheel first landing is reading up on the MAF technique. Do the normal flare allowing the tailwheel to go below the mains but release back pressure right before you touchdown (takes practice) and let the tailwheel move above the mains. It works great is a Cessna 180 and fine in a cub. I am a big fan of a Bushwheel/stock tail spring but even with that I do the MAF stick/yoke release to avoid a tailwheel strike and then pin the mains with brake or stick (also dump flaps). I would like to say a shocked tail spring is totally useless but I have see 3 year old tail sections broken in a working cub!! If you can't avoid tail first landings for whatever reason you will have to live with early replacement/repair of the units.
DENNY

Thanks Denny!

I just thought I would add my final troubleshooting with my tailwheel shimmy was to take the Burl's Tailwheel Ski off as Frequent_Flyer suggested. I did this because I was still getting occassional shimmy even with the ACME shock adjusted to the tallest setting with the best caster angle. I never had it shimmy once with the Burl's tail ski off. I will try a different tail ski next season.
 
So I stopped by my machinist buddie's shop this morning on the way to the hangar. I have been privileged enough to know two very good machinists in my lifetime and have learned a lot from them. One thing they taught me was the best way to measure a hole is with a pin gauge. I borrowed my buddies pin gauge set and removed the bolt from the Acme Aero Stinger to measured it. I used these gauges when I found the sloppy struts attachment holes on the Carbon Cubs several years ago.
PXL_20230627_121723120.jpg


This is a video like what Ted posted of the bolt in the Stinger. Those holes are .380" but as you can see in the video, you need an oversize hole to be able to get the bolt through all three holes. It is not like the main bearings of a crankcase with bearing bores.
Here are the dimensions of the bushings, 1000 hours or so on top, new on the bottom.
PXL_20230627_121607707.jpg


PXL_20230627_121716909.jpg


After studying the part and a discussion with my machinist friend neither of us believe the parts would fit together with a tighter hole tolerance for this type of assembly. I am sorry that Ted's Stinger did not work for him but I do appreciate Acme Aero's satisfaction guarantee and the fact that they took the Stinger back and refunded his money. I have learned a lot going down this rabbit hole and now have an appreciation for what went into this design and now have the wear parts on hand as the need to replace them arises. Hope this all helped others as well.
 

Attachments

  • PXL_20230627_121723120.jpg
    PXL_20230627_121723120.jpg
    130.8 KB · Views: 278
  • PXL_20230627_121607707.jpg
    PXL_20230627_121607707.jpg
    96.7 KB · Views: 280
  • PXL_20230627_121716909.jpg
    PXL_20230627_121716909.jpg
    99 KB · Views: 279
This is a video like what Ted posted of the bolt in the Stinger. Those holes are .380" but as you can see in the video, you need an oversize hole to be able to get the bolt through all three holes. It is not like the main bearings of a crankcase with bearing bores.

After studying the part and a discussion with my machinist friend neither of us believe the parts would fit together with a tighter hole tolerance for this type of assembly.
Steve,
I'm not a machinist, but that video is indicating to me that the three holes are not aligned. Thus at least one of them needs to be oversized in order to insert the bolt through all three holes. If all three holes were line bored to the same size, the bolt ought to slide all the way through without the slop as shown. Is the O.D. of the bolt a constant dimension from one end to the other? It should be.
 
Steve,
I'm not a machinist, but that video is indicating to me that the three holes are not aligned. Thus at least one of them needs to be oversized in order to insert the bolt through all three holes. If all three holes were line bored to the same size, the bolt ought to slide all the way through without the slop as shown. Is the O.D. of the bolt a constant dimension from one end to the other? It should be.

It is a standard AN6 bolt. I cannot imagine holding a tolerance on keeping those three bolt holes aligned over the span of that part, bolted to the tail spring pad. The whole reason for the heims joints is to correct for tail spring pads that are no level and then you start torquing that fitting down. I had not noticed any issues with the Stingers I maintain until this one. You could hear a click when you pushed it backwards. When I jacked it up, that bolt was tight through the fittings but the heims joints were moving. Found the worn bushings, replaced them and all is tight and no more play. Acme told me the aluminum bushings did their job, sacrificial I guess. Now I have a bag of them.
 
Back
Top