• 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!

acme shocks

hotrod180

FRIEND
Port Townsend, WA
A friend of mine bought a used Carbon Cub not long ago.
It has a set of Acme Aero black ops shocks in place of bungees.
The MLG wheels are splayed out quite a bit,
in order words the bottom of the wheels are farther outboard than the tops.
This seems backwards to me,
as when the gear is deflected they'd splay out even more.
I mentioned it to him, but he is not very mechanical or hands-on
and has the attitude of "it seems to work OK".
I'm wondering if the shocks are too long for the application, like maybe they were sized for extended gear,
or if maybe there's a length adjustment that could correct this condition?
 
They are sized for specific gear length. There is an adjustment at the bottom to fine tune. Maybe they just need to be serviced. There’s a valve stem at the top. Call Eric (Acme) for recommended pressure. Steve probably knows too.
 
They are sized for specific gear length. There is an adjustment at the bottom to fine tune. Maybe they just need to be serviced. There’s a valve stem at the top. Call Eric (Acme) for recommended pressure. Steve probably knows too.

Acme Aero Black Ops shocks are coil spring with oil damping. The gas only controls the damping oil. (open to correction if someone knows better)

Acme now provides a specific Black Ops set-up for the FX-3 as it is now a CubCrafters factory option. It's possible that earlier shocks were not the same spring rate or length.
 
Turning into parking will splay the tires. When setting the camber I roll the plane forward and aft to normalize it. My shocks are totally controlled by gas pressure (every gas shock I’ve ever owned has been) and my tires splay a bit when parked. My suspension is set a bit soft so that I can run a bit higher pressure in my tires.
 
A friend of mine bought a used Carbon Cub not long ago.
It has a set of Acme Aero black ops shocks in place of bungees.
The MLG wheels are splayed out quite a bit,
in order words the bottom of the wheels are farther outboard than the tops.
This seems backwards to me,
as when the gear is deflected they'd splay out even more.
I mentioned it to him, but he is not very mechanical or hands-on
and has the attitude of "it seems to work OK".
I'm wondering if the shocks are too long for the application, like maybe they were sized for extended gear,
or if maybe there's a length adjustment that could correct this condition?

He may just have a set that someone got off another and plane stuck on the plane or not properly adjusted. Logbook and a call to Acme Aero should answer the questions, they have a form you fill out for proper size and settings for individual planes. Increase in camber in or out is going to effect tire wear especially bushwheels. If you do not rotate the tires on the rims of Cessna spring gear they will wear to the cords quicker on one side due to camber issues. AOSS are ordered for the gear you run and also have adjustment so they set vertical. Let the plane roll forward for a bit to make sure it is not splayed from a turn.
DENNY
 
Acme Aero Black Ops shocks are coil spring with oil damping. The gas only controls the damping oil. (open to correction if someone knows better)

The Black Ops shocks appear to be a hybrid design. The extension is controlled primarily by a steel coil spring and oil damper. When the strut retracts the end travel seems to be controlled by the gas pressure.

If that's correct then suspension sag or wheel camber would be influenced by gas pressure. Aircraft weight and gas pressure will be trying to extend the strut while the coil spring is trying to retract the strut.

In a conventional air shock increasing the gas pressure reduces sag. I would expect increasing the gas pressure on Black Ops shock would cause the strut to extend and for the wheels to be more splayed out. Is that true?
 
This discussion reminded me I needed to get my Black Ops fitted to my FX-3. They have been sitting in the hanger for months and I felt no urgency to fit them as the bungee gear seemed to work fine.

Anyway, shocks specified for FX-3 Carbon Cub and fitted with no adjustment. After rolling the aircraft backwards and forwards several times to relax the tires I find the wheels sit with close to zero camber. That was with about 1/4 fuel and some stuff in aft bagage. I doubt my 160 lb will make much difference.
 
Took the FX-3 out today to see if I liked the Acme Aero Black Ops. I was worried they would feel soggy and wallowy compared to the bungee gear but, no, aircraft felt solid during taxi and just like the bungees for zero sink 3 point landings. I tried to provoke them a bit with some intentional firmer landing and they were very well behaved. I think I'm going to like them.

Anyway, when I pushed it back into the hangar with an extra 30 gallons of fuel, the wheels still sat at zero camber. I don't know how much work Acme and CubCrafters put into specifying these shocks but they seem to have got it just right.
 
Anyway, when I pushed it back into the hangar with an extra 30 gallons of fuel, the wheels still sat at zero camber. I don't know how much work Acme and CubCrafters put into specifying these shocks but they seem to have got it just right.
Wouldn't this have more to do with the amount of toe-in/out than the type of shock system?
 
I don't know how much work Acme and CubCrafters put into specifying these shocks but they seem to have got it just right.
Cub Crafters didn't put anything into until it until recently. The dealers were having to take the new Carbon Cubs to Tac Aero on bungees and have them install the Acme shocks. After years of this the factory finally tested them and started installing the Acme shocks. Tac Aero had piles of bungee shock struts.
 
Cub Crafters didn't put anything into until it until recently. The dealers were having to take the new Carbon Cubs to Tac Aero on bungees and have them install the Acme shocks. After years of this the factory finally tested them and started installing the Acme shocks. Tac Aero had piles of bungee shock struts.

I am aware that CubCrafters engineering approval, and the availability of the Acme shock option, was quite recent. At what time did Acme have a specific build configuration for the FX-3?

When I called to order mine I expected to have to make a choice of spring rate, damper porting, unloaded length, max extended length, etc but was told they had a build configuration specific to the FX-3.

I wonder what stock configurations Acme now has for the Black Ops. I doubt there is only one build configuration they sell to everyone.
 
Last edited:
Wouldn't this have more to do with the amount of toe-in/out than the type of shock system?

I would expect toe in/out to give a different camber angle depending on whether the aircraft was rolled forwards or backwards. I see no obvious difference.

Camber angle doesn't depend on the type of shock system. For Cub type gear aircraft weight tends to splay the wheels out and shock force tries to pull the wheels in. At some point the forces balance and define the camber angle. That force could come from any type of shock and, even for one type of shock, there is probably an infinite number of combinations of spring rate and unloaded length that would give zero camber angle.

What I didn't want my gear to have is large tuck in when unloaded and large splay out when loaded. BushWheels are too expensive to scrub off a load of rubber on every pavement landing.
 
Last edited:
When I put Acme shocks on my Carbon Cub the camber was out and they sent me shorter rods, I'm guessing 3/4" shorter. I believe the rods are to long.
 
I ordered Acme shocks for my EX-3 18 months ago, first flight was last March. I never tried the bungees. I instantly fell in love (20 years in Skywagons may have had an influence). You can just glom it onto the runway however you want. I have about 150 pavement landings on 29s since then and the tires are holding up so far.
 
I took a few measurements on my FX-3 Acme shocks today. All measurements with aircraft weight on wheels after pull back into hangar and with no obvious wheel camber.

Exension when loaded (length of shaft dirt/dust free zone from lower end of shock body) - 32 mm
Length bolt center to bolt center - 427 mm (average of both sides)
Length of shock body cylinder (shoulder to shoulder) - 335 mm

May also be worth noting that these shocks do not have an exposed thread and jam nut where the shaft mates with the rod end bearing. A long sleeve butts against the rod end bearing and this may be a custom jam nut that covers the threaded part of the shaft.
 

Attachments

  • Acme rod end.PNG
    Acme rod end.PNG
    395.9 KB · Views: 63
Last edited:
You can just glom it onto the runway however you want.

No doubt the Acme shocks are far more forgiving than the stock hydrasorb/bungee gear. My first flight in an FX-3 was the local dealer's demo. I landed it tail first and expected it to bounce. It simply attached itself to the runway and rolled out.

I have about 250 hours and 260 landings with the FX-3 on the bungee gear. After the first few embarassments I learned to land it 3 point or wheel without bouncing and came to like the gear. Even with only 12 landings on the Acme gear I can see significant potential for shorter landings because the need to finesse the landing goes away.

The downside is that the ability to land a taildragger with less forgiving gear may be lost.
 
Last edited:
A friend (air guide) once told me that a Cub with 35s and TK-1/Acme shocks could land on a strip littered with 55 gallon drums. After pooching a few landings in my Cub? I agree with him. 35s and gas shocks may be the most significant improvement to Cubs in the last 50 years.
 
A friend (air guide) once told me that a Cub with 35s and TK-1/Acme shocks could land on a strip littered with 55 gallon drums. After pooching a few landings in my Cub? I agree with him. 35s and gas shocks may be the most significant improvement to Cubs in the last 50 years.

Want to really appreciate your shocks? Fly a cub with bungees again. It will have you questioning if you can land an airplane


Sent from my iPhone using Tapatalk
 
The Black Ops installation manual says:

"Tighten AN bolts to 37ft lbs non-lubricated and 47ft lbs if lubricated."

I have no FAA mechanic certification but all the time I have been turning wrenches I have known (or at least believed) that to get the same bolt stretch a lubricated thread must have a lower torque applied than a dry thread.

Was I wrong all these years or did Acme Aero get it wrong in their instructions? - https://acmeaerofab.com/wp-content/uploads/2020/07/BLACK-OPS-Instruction-Manual.pdf

(No, I didn't use either of their torque values when I installed my struts.)
 
I too am curious about your shocks frequent_flyer, as well as what the latest and greatest cubcrafters spec'd setup is. I have the "gen III" acme black ops on my FX3. During build, I discussed optioning the "gen IV"/"pro series" (same thing from what I can tell) with B.D. for a few minutes and he dissauded me from doing so, citing 5-10 lbs of additional weight on the gen 4 iirc. I flew the gen IV's for about 15 hours at Tac Aero in 2 different FX3's and they definitely felt more pliant than my 3's, although that might've also been a function of tire pressure differences on the 31's. The IV's use dual rate springs (2 different springs in each shock?) from what I understand, and the III's do not.

Going from memory, Acme told me to service the gen III as well as the tailwheel shock to 300 psi with nitrogen during condition inspections. The manual says the pressure depends on the application, which probably explains the 250 psi in the video.
 
I’ve run both Gen 3s and Gen 4s on the same cub. There is a difference. If your a 25 hr a yr guy, it won’t matter
 
I’ve run both Gen 3s and Gen 4s on the same cub. There is a difference. If your a 25 hr a yr guy, it won’t matter

Yeah, I had some questionable landings during my tac aero checkout but I don't remember theirs with the pros ever bouncing. I can definitely bounce mine on the 3's without trying too hard, especially if I'm running 10 psi on the 31's for pavement ops. I did spend 2021 in a flapless spring gear citabria so I do know what it can really be like....
 
I’ve run both Gen 3s and Gen 4s on the same cub. There is a difference. If your a 25 hr a yr guy, it won’t matter

What difference did you observe?

If the Pros are set up for STOL contesters I would expect the main spring to be a little lower rate than the non-Pro and the shorter spring to be higher rate. This would make the shock more compliant on the intial hit but, when the long spring becomes coil bound, the shorter spring comes into effect and prevents bottoming out.

Acme winds their own springs so I'm surprised they don't wind true mult-rate springs for the Pro series. Is there an additional damper valve between the the two springs?

I just completed my weight and balance revision. The removed bungee gear with fairings weighed 13.88 lb. The installed Black Ops weighed 15.36 lb.

No enough flight hours to be sure yet but the aircraft seems to be slighly faster for the same power settings and fuel burn. I was expecting it to be a little slower.
 
Back
Top