Seeking advice regarding vertical pop-top canvas + stability

[PathNotTaken]

working on a tall pop-top and have most everything worked out but a couple questions for those with experience. The way I am attaching my canvas allows me to control the tension quite easily. How tight did you fit your canvas? I could go anywhere from just firm to extremely taught. I obviously don't want to overly stress the canvas, but want to minimize flapping. There will also be 2 bungees going around the center which can also create some tension..

As a follow up question, how much extra stability in the lateral direction did your pop top have after the canvas? I don't have the skills to calculate this so looking for some anecdotal information... I assume the taught canvas is helping some but how much is the question. thanks in advance!

 

[Fenderfour]

I've got a build thread for my camper with a 30" pop up. I attached my canvas using RV screw rail to a flat surface. The canvas went from the inside of the rail to the outside. On the bottom, I used a 90 degree extrusion, on the top, I used a flat extrusion. For canvas, I used sunseeker UV & flame treated cotton. I've made tents out of Vinyl Coated Polyester (VCP) before, and it's great for large structures, but I wanted more breathability in this tent.

I didn't fit it especially tight. The way I installed created 90 degree folds to the outside all the way around. This created a significant stress point at the corners, with extrusion pushing in to the canvas. I didn't want to over stress this. It's also not easy to install the canvas very tight with my attachment method. Like you, I used a bungie all the way around. This helps reduce flapping in the wind and makes it easier to keep the canvas in the camper whenI drop the top.

I don't think canvas will give much lateral stability unless it's very tight and doesn't stretch. Wind loading, even with a modest breeze could/would overcome any tensegrity type stability created by very tight canvas. A very strong set of gas struts might provide enough tension, but that creates other problems.

 

[ITTOG]

I believe my pop top (build thread in signature) rises about 36". I made my fabric as tight as I could get it. The connection method uses 3M Dual Lock Reclosable Fastener. I also have one bungie around the canvas.

Adding the canvas does stabilized the top some but in my opinion it is not safe to just rely on the canvas. Thus, my gas struts run north/south on the camper and I added lift stabilizers at each end going east/west. So now the top can be locked out and can't fall down on me and the canvas is tight enough to not flap. I have had it in winds about 20 mph with no flapping. If you are using linear actuators to raise the top then the lift stabilizers may not be needed. Do you plan to camp in windy area's?

 

[Ozarker]

Your canvas is not much of a structural member, very little and would only become significant just prior to the roof being displaced by wind or similar stress. It also depends on the material you are using. I've had pop tops and still have a tent trailer, canvas on frame.

Hard to describe but you don't want canvas to rip out of its attachment to the body or roof, so tight that there is no flex, but not so lose with excess material that it slaps the frame in a breeze.

So, what is the weight of the canvas? And, I'd suggest going heavy, I have PVC coated canvas, military grade, fantastic stuff!

Pics might help.....

 

[PathNotTaken]

Thanks for weighing in everyone! To clarify, I am not relying solely on the canvas, just curious if it adds a measurable amount of stability. My lifting mechanism consists of an actuator mounted in each corner, which provide great support but not any lateral stability. I have a total of 8 gas struts, 2 per corner, that provide all of the lateral stability.

The issue I am running into is that in order for my struts to provide the lateral stability I desire, they end up putting a lot of vertical pushing force and therefore loading the actuators more than I intended. I calculated this all out and my actuators will essentially be most loaded for the first few inches of lift (since the struts are near horizontal here) and for the last 10in when lowering (working against the struts pushing up). I'm still sitting around 52% of actuator capacity, it just all seems a little counter productive for the actuators to be loaded 50% to overcome the struts pulling the heavy top DOWN. I cannot mount the struts at any better angle to provide more lateral force and less vertical lift. So my options are either to have a less stable top, or load the actuators more with strong struts.

The canvas is 34lb total, dual layer marine awning material with insulation in between. I plan to camp in extremely windy conditions as we seem to face them just about anywhere we go.. obviously we can always lower the top, but I don't want to have to worry about it. Maybe I'm going a little overkill here but I want to be able to push really hard on the top when fully raised and have near zero movement.

 

[PathNotTaken]

I believe my pop top (build thread in signature) rises about 36". I made my fabric as tight as I could get it. The connection method uses 3M Dual Lock Reclosable Fastener. I also have one bungie around the canvas.

Adding the canvas does stabilized the top some but in my opinion it is not safe to just rely on the canvas. Thus, my gas struts run north/south on the camper and I added lift stabilizers at each end going east/west. So now the top can be locked out and can't fall down on me and the canvas is tight enough to not flap. I have had it in winds about 20 mph with no flapping. If you are using linear actuators to raise the top then the lift stabilizers may not be needed. Do you plan to camp in windy area's?

I'd be curious if you know how heavy your roof is, and the angle/force of your struts at full extension. My top has right about the same amount of lift height, except no stabilizers east/west, just more struts. How solid is the top to movement against the struts?

 

[ITTOG]

I'd be curious if you know how heavy your roof is, and the angle/force of your struts at full extension. My top has right about the same amount of lift height, except no stabilizers east/west, just more struts. How solid is the top to movement against the struts?

I do not know the exact weight. I believe it is about 200 pounds. My struts are undersized at 50 pounds of force each (200 total). I tried 100 pounds of force each (400 total) and I was not able to pull the roof down. I plan to purchase 80 pound struts (320 total) because I could use more support. When my top is up and the lift stabilizers locked out there is zero movement in the roof. Even in high winds or if I am standing on it the roof does not move (well for standing on it, the roof may flex a bit when I am in the middle).

 

[PathNotTaken]

I do not know the exact weight. I believe it is about 200 pounds. My struts are undersized at 50 pounds of force each (200 total). I tried 100 pounds of force each (400 total) and I was not able to pull the roof down. I plan to purchase 80 pound struts (320 total) because I could use more support. When my top is up and the lift stabilizers locked out there is zero movement in the roof. Even in high winds or if I am standing on it the roof does not move (well for standing on it, the roof may flex a bit when I am in the middle).

I was having trouble finding in your build thread where you go into your mechanism, specifically the lockout. Do you have a post # handy? I'm curious if your lockout mechanism is also providing stability in the north south direction, otherwise I would think your top should have a similar amount of play as mine in that direction, considering I currently have 60lb struts. I'm considering moving up to 100lb, but it's quite a lot of vertical lift having 8 of them! Still 50% load of my actuator at maximum as I mentioned.

 

[ITTOG]

I was having trouble finding in your build thread where you go into your mechanism, specifically the lockout. Do you have a post # handy? I'm curious if your lockout mechanism is also providing stability in the north south direction, otherwise I would think your top should have a similar amount of play as mine in that direction, considering I currently have 60lb struts. I'm considering moving up to 100lb, but it's quite a lot of vertical lift having 8 of them! Still 50% load of my actuator at maximum as I mentioned.

The best post is at https://forum.expeditionportal.com/...6-x-12-trailer-conversion.199414/post-3126823. A few posts later I talk about the front locking mechanism which is just like GFC's.

It does provide some north/south stabilization as well. The hinges prevent the stabilzers from moving beyond a certain point. If I were to push north or south on the top with only one end of the roof up it may move 6" but when both ends are up and locked up I would estimate it only moves about 1" or 2". I think I would have to push pretty hard though.

How much does your top weigh. It better be very heavy to overcome 800 pounds of force.

 

[PathNotTaken]

The best post is at https://forum.expeditionportal.com/...6-x-12-trailer-conversion.199414/post-3126823. A few posts later I talk about the front locking mechanism which is just like GFC's.

It does provide some north/south stabilization as well. The hinges prevent the stabilzers from moving beyond a certain point. If I were to push north or south on the top with only one end of the roof up it may move 6" but when both ends are up and locked up I would estimate it only moves about 1" or 2". I think I would have to push pretty hard though.

How much does your top weigh. It better be very heavy to overcome 800 pounds of force.

Thanks for sending this over!

My top is a little under 200lb currently, will be a little under 300lb when factoring canvas weight, solar, fan, etc. With 8x100lb actuators, this results in 660lb of vertical force when fully extended ~36in. At 10in extended is when the weight of a 300lb roof breaks even with the vertical force of 100lb struts. For the end of the travel where the struts are pushing the hardest, this is 660lb strut force - 300lb roof = 360 lb, / 4 actuators or 90lb per corner load on the actuators.

For the first few inches of lift where the actuators bare all the load with no help from nearly horizontal struts, they see 75lb per corner, so the 90lb isn't much more than that. And still far under their 180lb load rating.

 

[ITTOG]

Thanks for sending this over!

My top is a little under 200lb currently, will be a little under 300lb when factoring canvas weight, solar, fan, etc. With 8x100lb actuators, this results in 660lb of vertical force when fully extended ~36in. At 10in extended is when the weight of a 300lb roof breaks even with the vertical force of 100lb struts. For the end of the travel where the struts are pushing the hardest, this is 660lb strut force - 300lb roof = 360 lb, / 4 actuators or 90lb per corner load on the actuators.

For the first few inches of lift where the actuators bare all the load with no help from nearly horizontal struts, they see 75lb per corner, so the 90lb isn't much more than that. And still far under their 180lb load rating.

It seems you may be using the terms actuator and struts interchangeably. They are very different. Struts are usually gas filled and there is no motor. Actuators are not gas filled and use motors to lift straight up. In the second paragraph you mentioned 8 actuators. I will assume struts. At the end of that paragraph you divide the load by 4 actuators. Do you have four or eight? Not trying to question your setup but you have me curious.

Also, are you calculating the lift forces yourself or are you using a website? if a website I would like to know the URL please.

 

[PathNotTaken]

It seems you may be using the terms actuator and struts interchangeably. They are very different. Struts are usually gas filled and there is no motor. Actuators are not gas filled and use motors to lift straight up. In the second paragraph you mentioned 8 actuators. I will assume struts. At the end of that paragraph you divide the load by 4 actuators. Do you have four or eight? Not trying to question your setup but you have me curious.

Also, are you calculating the lift forces yourself or are you using a website? if a website I would like to know the URL please.

My bad. 8 actuators was the only typo! my setup is: 8 gas filled struts. 4 electric actuators, 1 per corner.

I was doing it manually but you can use something like this to calculate netX force and netY force. Just need the angle of the strut. https://www.inchcalculator.com/net-force-calculator/


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[Fenderfour]

Gas struts and linear actuators are designed for an aligned push. They are typically installed with clevises and ball joints to reduce misalignment and non-ideal loads. Misalignment, side loads, etc... are likely to lead to leaky seals and jammed actuators at a minimum.

You might consider a hinged lift mechanism or an actuator drive scissor situation to maintain alignment on the pushing parts and to increase lateral rigidity.

 

[PathNotTaken]

Gas struts and linear actuators are designed for an aligned push. They are typically installed with clevises and ball joints to reduce misalignment and non-ideal loads. Misalignment, side loads, etc... are likely to lead to leaky seals and jammed actuators at a minimum.

You might consider a hinged lift mechanism or an actuator drive scissor situation to maintain alignment on the pushing parts and to increase lateral rigidity.

Correct. The gas struts are mounted at an angle and only see push/pull as they are designed. The actuators push vertically and rely on the struts for rigidity. It is by no means weak as I have it now, but I’d like it stronger for increased winds.

A scissor driven mechanism would definitely be interesting, even if it was just using a gas struts. That way the strut can be mounted completely horizontal.

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[PathNotTaken]

Another thought I had: As I have it now when the pop top is fully extended, the struts are ~1in away from the end of their stroke. if I instead made them at max travel in the up position, when the top takes on lateral load the struts in one direction would act as solid rod instead of a spring, and I wouldn’t have to rely on the force of the strut fully. At the cost of strut life… however it seems almost all applications of gas struts use them as an end of motion stop.


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