Gas Struts

[Larryjb]

You know, a torsion bar in a carefully engineered cone shape could provide a "variable" lift assist, perhaps similar to that of a gas strut. Remember the old torsion bars for trunk lids in the 1960s-1980s? They provided fairly even lift that was long lasting. There's also the old coil spring they used to use for hoods. They lasted forever. Now the gas struts only last a comparatively short time.

My background is chemistry, not physics, but I imagine a torsion bar that is slightly narrower at one end could continue to provide some lift assist near the end of its twist, but as the shell is lowered, the thicker end begins to twist to provide further list assistance from the closed position.

Or, perhaps a hybrid system using a smaller gas strut to provide just a little extra lift at the beginning, but also uses the torsion bar for the majority of the lifting.

I suggest this because I can see gas struts needing periodic replacement, and be more expensive.

[rickst29]

Bill's picture shows show the compressed strut completely vertical (with maximum lifting power), but I think that there isn't ground clearance to provide for that. That placement would also probably run out of working distance in the vertical position. (The 8" limit for maximum motion provides a total of 20" from the connection point). I assume that we're talking about a connection to the existing struts, and not a completely new structure. That makes this a 2D "math problem ", rather than a 3D situation.

I looked at the front-most struts of my 2619 with a tape measure (in the garage, closed. The gas strut is only effective for lifting the shell (a twisting when its own lowered position contacts the the lift arm at an angle. The limiting factors on placing the lower fixed" connection seem to be as follows:

1. The maximum length of the extended strut is 19.7 inches. This creates a limit (in the shape of an arc) for the maximum distance from the strut lower mounting point.
2. The compressed strut also attaches the lift bar at a distance not less than 12" minimum from it's lower mounting point.
3. In closed position, the strut should have the largest possible angle from the horizontal bar.

Rule #1 creates a limit in "open" position, while rule #2 limits the "closed position. The pair of curves, and the desire to keep the fixed mounting point low, seem to land together at a point about 3-1/2 inches below the "lower" screw of the inner pair of screws which anchor the lift bar to the torsion arm. the torsion bar. The lift bar attachment would be about 14" from the very end. I attach photos, holding the socket extender end in the air at the approximate mounting spot. The second photo shows a black twist-tie at the location of the other end.

[rickst29]

In closed position, and at the beginning of motion, a majority of the pressure (the cosine of the angle, IIRC) is trying to push the lift bar out from its joint with the torsion bar, in a sideways direction. This worries me a bit.

As the shell becomes lifted, the weight of the shell pushes in the opposite direction, somewhat balancing that. But the very end of motion, the strut begins to "pull in" (rather than push out) adding to the weight on the lift arm. To the extent that the strut has not reached 90 degrees, this "pull-in" pressure also affects horizontal motion a bit - trying to pulling the shell "inwards" from full open position, which may or may not be completely desirable.

In my own "guessed at" positions for installing these, the angle for the initial lift seems to be 15-20 degrees, so about 30% of that rated power would be working in the vertical direction. The 200 lb bars would be maybe 50-60 lbs of assistance in the desired vertical direction.

Anyway, the lower mounting structure "in empty air" might be difficult for me to build and install, I have no welding experience.

[Bill]

That's exactly the kind of more detailed thinking that we need. Thanks for putting it up, Rick. Before anybody even thinks of going anywhere with the idea, we need a lot more of this. I'll make a few more sketches, with your comments in mind, and see if anything looks even a bit do-able. Not that I intend do do anything like it, but it is fun to think about. Yes, I've got to do something about the very high initial lift load for the front shell of my TM. And no, I can't weld either.

It occurred to me that a gas strut arrangement such as we are thinking about would put the strut out in the road grime and weather. I'm not sure that is a good idea ...

Bill

[jagizzi]

There are quite a few options for length, stroke and force, along with mounting methods. McMaster Carr lays it out in a pretty nice chart at this link: https://www.mcmaster.com/gas-springs/gas-springs-7/

[Larryjb]

Originally Posted by Bill

...
It occurred to me that a gas strut arrangement such as we are thinking about would put the strut out in the road grime and weather. I'm not sure that is a good idea ...
Bill

Bill, don't forget that shock absorbers (or struts if they are assembled with the spring) are not so different from the gas strut we are looking at, and they are definitely exposed to road grime. However, they usually have a protective sleeve on them.

As for my previous comment, I realize now that if shock absorbers can last as long as they do, surely they should be able to make a gas strut last longer than a few years. I'm sure they can, but the real question is would we be willing to pay for such a gas strut?

[mickmanor]

and I was an A Frame camper through the few years of the cycle.
It happens that each corner of the A Frame camper has a small
"torsion bar" in it that supplies the spring lifting power for the two
roof panels to lift, almost by themselves. Well, that was the original
lifting mechanism. Keep in mind, understandably, that those torsion
bars lose some lift power, over time.

The next step was the use of an electric actuator bar with a mini motor
in it that used a mini drive and worm gear to raise the roof panels.
These became problematic, due to corrosion damage.

The next step was the use of gas struts, and when installed correctly,
they worked good, and are still being used.

I analyzed posts from others on A Frame sights, and installed gas struts.
After a minor adjustment, 1/2" will make it or break it, they worked great.

So, the basic concept of torsion bar spring lifting power needing auxiliary
help is the same. The lifting mechanism of the TM shells is more complicated
and the gas struts must be perfect in alignment and geometric direction.

The best of plans will still require some experimentation.

Luckily, my torsion
bars on the TM are still good.

I wish you luck, as I am senior, as well.

[FlyboyTR]

I was getting ready to post this when I saw mickmanor's post about the Aliner. Way back, we played with an Aliner for some rough off-road adventures. I ended up installing four gas struts which helped in raising the roof panels. They also use a torsion spring system.

I followed the info provided in this attachment. The author goes into great account as to the physics as to placement, etc. Maybe there is some info here that can help push the thoughts of adding gas struts to the TM.

[rickst29]

Originally Posted by Bill

That's exactly the kind of more detailed thinking that we need. Thanks for putting it up, Rick. Before anybody even thinks of going anywhere with the idea, we need a lot more of this. I'll make a few more sketches, with your comments in mind, and see if anything looks even a bit do-able. Not that I intend do do anything like it, but it is fun to think about. Yes, I've got to do something about the very high initial lift load for the front shell of my TM. And no, I can't weld either.

It occurred to me that a gas strut arrangement such as we are thinking about would put the strut out in the road grime and weather. I'm not sure that is a good idea ...

Bill

Thanks for the kudos. Road grime is probably not an enormous factor, because the strut rod is fully compressed during travel. But your issue will apply during take-down, any grime or moisture on the rod will be pushed into the seal. If the seal isn't very good, residue ends up in the body. Because TM lift arms are exposed to rain, an oil-filled strut would probably be a bad idea - better stay with nitrogen qas. Except for the case of heavy rain, I think that you could simply wipe them clean as a new step in the lowering process.

Just talking, without looking:
I can imagine a reinforcing plate on the thin steel "L" member which hangs down from the outside front-to-back edge of the TM floor, but that alone would leave the corner of the L-member prone to cracking. The plate is anchored to the floor with only a few screws. The torsion bar hole (within the vertical portion of the present "L" plate) seems to be a kind of "limiting" guide for the horizontal portion near the outside bend of the lift bar.

At build time, each the "L" plate was put onto the torsion bar end before attaching the lift barb. In order to completely replace the current (thin!) L-plate with something strong enough to handle the additional force from the compressed strut, it would need to have the lift arm removed (so that you could pull the detached plate over the bar end. At that point, you've basically done more than half the work involved to in replacing the torsion bar -- and torsion bar upgrade is a far superior fix. (If you can support the armless shell, and handle the effort to apply initial bar torque, and pay the really high "truck freight" shipping costs for new bars).

I therefore think that replacement (of the thin-walled L plate) makes almost no sense at all. But It could maybe be augmented, on the portion away from the torsion bar hole, by using 2 new parts. In this description, I'm describing the street side front corner:

A new L bar, perhaps 3/16 steel and maybe 1.5" wide, needs to come down from "further back" with the 1.5" edge mostly vertical. There is possibly some overlap with "rear" portion of the current plate: in that case, this L bar is placed along the outside of that plate , and a screw may be used to bolt al the way through to the new "inner" L bar). This is angled, high at the rear and low at the front, so the that the shorter arm of the ""L" sticks out from the TM body to attach the the cylinder end fitting. 45 degrees is optimal for the strut range of motion, but a lower angle is needed - to provide more contact length and area for between the angled outer "L" and the flat-against-the-floor-edge horizontal/vertical bar.

The second new L bar, a plate very similar to the current plate, doesn't need to be that strong (even 1/4" "feels" like overkill for that one). It is mounted like the current plate but further to the rear. They may overlap for some portion, In that case I simply removed the "rear-most screw" of the current plate, put on the new L bar, and then reaplce that screw with a taller one. This is perhaps 10" long, to provide for multiple screws into the bottom corner of the floor. The downwards width may be 3-4", and the "against the floor" width should be the same as the current plate.

The custom plates will cost a lot more than the struts, but I'm a long way from South Dakota - and my HOA would "severely unhappy" if I began assembling open shell supports for a disconnected front shell in my driveway (to do a torsion bar replacement at home).

[larsdennert]

Probably better to take it to a dealer and have them adjust the bars. If the bars are worn out, get new ones.