How to make an Ikea Beddinge lay flat

We purchased an IKEA Beddinge futon a couple years ago for use as a bed in our office/guest room. Many reviews noted that while comfortable as a couch, it didn't quite lay flat in bed mode. To be precise, there is a small valley that remains where the mattress folds to accommodate being a couch. I noticed this when I first assembled the Beddinge but thought it was negligible. In addition, my sister in-law and her husband slept on it while staying with us for about a month without complaint. I didn't realize how bad it was until we started night weening or second child, which meant I was sleeping on the thing with the toddler for a couple months.

Let me say that that little valley was just enough to make rolling over difficult, resulting in
  • daddy sleeping only in "the crack"
  • baby choosing to sleep perpendicular to "the crack" (and kicking dad in the face)
  • one sore back in the morning
After much googling the only fix I could find was a this video ...

shot at low resolution in portrait on someone's cellphone in some Germanic European country Czech with no additional information.

Today, I post my detailed fix.

First, the problem:
Here is the Beddinge frame assembled according to Ikea's instructions and using the provided hardware.  In the photo below, I'm using my framing square to show just how non-flat the two pieces of the frame lie when in bed mode.

Not that much - perhaps at most a 5-10 degree angle from horizontal, but certainly enough to be uncomfortable after many nights. (If I was a princess, this would be my pea).

So why is there an angle?  Well, it appears that Ikea welded the frame support tabs on the leg posts about an inch too high.  In the photo below, where I'm using a broom handle for reference, you can see that the frame hinge at its highest height doesn't quite align, and so has to bend a bit when frame is mounted.

Here's the fix - new mounting screw assemblies that hold the frame to the hinge:
  • 3" long 1/4-20 machine screw
  • 0.75" long nylon spacer
  • 1/4-20 nut
  • 1/4-20 lock nut
For all the assemblies I needed (8 total), it only cost me about $10 at my local hardware store.

Here are a couple assemblies (sans lock nuts) on one of the frames:

Here it is from below after mounting to the hinge:

The result is that now the frame now sits with a slight peak:

which is actually cancelled out by a dip in the mattress along that line.  Both my wife and I have slept on the Beddinge after this fix and it is a significant improvement.

If you really want the frame to be truly flat, you can remove the nuts under the nylon spacers:
but doing so makes assembly a bit trickier.

Either way, I hope this fix will help to make Beddinges the world over more comfortable to sleep on.