I had the same problem. When I added the 4" memory foam the bed was heavy to lift and would not stay up on it's own. I replaced with 150lb 26" props. The plywood was going to shatter if I forced it closed! I put the mattress back on which was hard in the up position. Still no go. I finally put the foam on also which was near impossible in the up position. All of a sudden with all the weight distribued evenly all was GREAT! It closed and opened easily and stayed open with no problem. Now that it has been closed. I think it would be no problem without the foam. It was just that FIRST time that was a bear!
Just make sure you evenly put all your weight back on before you try and close it!
I have not done this mod yet but the one thing I have seen from the people who have and posted about it was that most of them beefed up the wood structure under the beds by adding a 2"x4" to the frame rails where the gas shocks mounted. Seems like adding this extra support allowed them to use more substantial mounting hardware for these much stronger gas shocks. Then it's all about getting the right mounting angle to make the bed just more than neutral and where it will stay up or down properly.
Easy for me to say as I have not done it yet but plan on it soon. I just thought I'd offer what I have gleamed from the numerous posts I've seen on this in hopes it helped.
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My supports work great with the OEM mattress and a 31lb topper. However, the wood, let me re-phrase, the particle board, supporting the matress is bowed up at the side of the bed from the pressure of the shocks. The gas shocks don't seem to be the best way of doing this. I don't think anyone would be opposed to a factory manual method like those whose use a fold down support. I think I will replace the bowed particle board with some thicker plywood, and either still use the gas shocks since they work well with what I have, or do the manual modification.
I think I would hurt my back if I tried to lift the mattress up without those gas struts to assist. It is heavier with the extra foam I added, and the mattress won't stay up by itself with the original struts. But with the leg I added to keep it up when needed, all is well.
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Put the old struts back on the bed. then move either the top or bottom mount a little bit to make them stand more vertical. This will give them more lifting force. Make sure that they still have enough clearance to allow the bed to fully close.
Is there a 100 pound strut you could try?
This is exactly what I did to mine. I moved the top about 1.5 inches and it was plenty to hold the mattress in the upright position. and it was free. just a little time. also like was said. make sure you can put bed all the way down when finished.
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