# How can I compute the force with a fulcrum on a Murphy bed?

I've built the following murphy bunk bed:

It rotates around the circles (the fulcrum) into a closed position:

I decided to buy gas springs (the retracting kind) to assist in closing the bed. Using the measurements from the picture,

from what I understand, the calculation would be:

force = 110*(35.8125)/11.4375 = 344.43

At this point, I was feeling pretty savvy and bought two springs that had 150 lbs of force, thinking this would be perfect. But I had a problem--instead of naturally falling open as I expected, it required all 200 pounds of me at the edge of the bed to get it to budge.

So where am I going wrong?

• I'm confused by your description. Is it stuck in the up position? In order for me to answer your question, you'll need to provide the total weight of each bunk, with mattress included and the weight of each support leg for the top bunk. Also provide the height the gas pistons are attached to the frame above the bed. This geometry is needed since it affects how much of the force is directed vertically and horizontally. – Doresoom Jun 18 '14 at 14:26
• @Doresoom--Thanks for the questions. It is stuck in the up position. The gas springs have an extended length of 30.11 in, and 18.11 in when compressed. The top of the gas spring is attached 23.5 inches above the bed. As for the weight of each bunk/leg support--I'm not sure I can answer that without taking the whole bed apart and weighing each separately. Is that required? I just assumed the standard force calculation would work (which only requires one knowing the force required to lift it to the closed position). – dfife Jun 18 '14 at 14:32
• That's not the issue--the top of the spring is actually set back behind a bit (toward the wall) so when it's closed, it is 18.11 inches. – dfife Jun 18 '14 at 14:56
• A bit late now, but for what it's worth Rockler (among others) have kits with gas springs preselected for the purpose. – keshlam Jun 18 '14 at 15:54
• @keshlam--I did look into a Rockler kit a while ago and decided against it so I could do my own design. (But after having such a headache with it, it may have been better that way). I didn't think about just buying gas springs from them. Oh well. – dfife Jun 18 '14 at 16:13

It's going to take 71.5 pounds of force pulling straight out from the wall in order to begin moving the bed downwards. The required force will actually increase a bit as the door begins to rotate and the tension from the gas springs becomes closer to perpendicular to the bed frame, and then the force required will begin to let off as you reach closer to the bed's horizontal position.

So even if you're putting all 200 pounds of your own weight into getting it started, a good portion of that force is going down through the bed frame to the hinge, which will just add more rotational friction and cause it to be more difficult to open.

If I have time later, I can try to recommend a different mounting point for the gas springs that will alleviate some of this problem. I can tell you right now that the more acute the angle the gas spring makes with the bed when stowed, the easier it will be to get it moving downward. Then you'll have more of your weight working for you as the bed passes through perpendicular with the gas spring.

Here's the force profile for various angles and bed weights. The point where it dips negative is when the force exerted by the gas springs is not enough for the bed to be lifted, and you'll need to assist it that much to stow it upright. (All you other engineers out there on DIY.SE, feel free to make sure I didn't make any stupid mistakes. The angle graph looks right to me, but then again, there's some pretty tedious trig to get that monster formula.)

The weights listed in the graph are for actual bed weights - if you took the whole assembly off the wall and threw it on a freight scale, not the force you experienced while lifting up one side of it. The forces (colors) are the perpendicular forces required to pull the bedframe down into a useable position. Where they go negative is the area you'd have to lift the bed up to get it back in the stowed position.

Since you've already bought the gas springs, the best option may be to change the geometry of how they're mounted instead of looking for a different load capacity. Here are some graphs that give the required pull down (rather sideways) force to open the bed and then the lift force required to stow the bed. Note that it's for an assumed 220 lb bed, since I ran out of dimensions to graph things on. Hopefully these plots will allow you to make a decision on what tradeoffs you want to accept if you rearrange the gas springs mounting point on the frame. (You'll also need to check if the gas springs have the range of motion required beforehand too.) The vertical offset and horizontal offset are measured from the bed's pivot point on the frame (distances h and a in the second sketch, respectively).

• Thanks for the answer, but I'm a bit confused. If it takes 71.5 lbs of force to get it going, why does my 200 lb body not make it move? – dfife Jun 18 '14 at 16:16
• That amount of force has to be perpendicular to the wall, not straight down. Since you can't brace very easily against the bed itself (which you're trying to move), you'll have trouble even if you've got 3X that amount of force working for you in the form of your weight. Plus, the upper bunk needs the force applied at a height of 6', which makes it awkward for even a tall person, and downright impossible for someone short. – Doresoom Jun 18 '14 at 16:30
• Makes sense. So...back to the original question--how hefty should my springs be? – dfife Jun 18 '14 at 16:35
• One other piece of information--the MAX force required to lift the bed from the middle is 110 lbs. My scale seemed to max out at around 45 degrees. – dfife Jun 18 '14 at 16:47
• Really good answer and right. Buy a strap for the bed to pull it out easier. – DMoore Jun 18 '14 at 19:26

You can use the software VÃ©rinPlus 3.00 It automatically calculates the pressure and fixation. You can download the software has gas spring on this site: http://creation.net23.net/index.php?affichage=accueil You can also make a simulation of your gas spring has online free:

I just skimmed this...so sorry if there are misunderstandings. I have found my own solution that doesn't require math. The gas piston is an assist. It just takes the load off. Even if it's not perfect...it's only job is to make it easier. Anyway...you mounted the piston on the wrong side of the fulcrum. It's resisting you. It wants to expand...its pushing the bed down. If you had a metal helix spring that is contracting, pulling the bed up, your design would be fine. I looked at all the other piston-assist murphy beds and this is how they did it. In the image ive provided the red-tube is the axis of the bed. the green is the mounting point modeling the condensed piston (@ 300mm radius according to product info), and blue tube is expanded piston (500mm radius). All those circles is just me using some visual deduction to find the sweet spot for mounting all the axisisisis. Am I right or am i right? To clarify: the image shows the up and down state of the bed overlapped...like a big wooden 'L'...but it's not actually a big wooden L.

I don't have this product in hand..and haven't tested this solution yet...so I could be wrong.

https://www.amazon.com/Suspa-C16-08054-C1608054-Quantity-Supports/dp/B0041EBSX4/ref=sr_1_3?ie=UTF8&qid=1481643822&sr=8-3&keywords=gas+spring

This says it's rated for 200lbs...but yeah, principals of leverage...since the piston mount is so close to the axis, and the end of the bed is like...10x that distance...that assist will be diminished. You guys can calculate that stuff pretty well.

• Had I designed my bed like that, yes you would be correct. But, I didn't know until after the bed was build that gas springs push (not pull). However, there are specialty gas springs (that are very expensive) that actually pull rather than push. – dfife Dec 14 '16 at 20:10