Welcome to G/N's, The major downside is sudden breakage of suspension cords, when raised ensure you have a secure locking prop. - Also ensure any counterweights are enclosed in a tube (PVC tubing great) to ensure no-one (or pooch) gets clocked on the scone when lowering bed. It is a great idea that I have seen in several vans, a real space saver that allows bed to remain made-up.

Mazza Bee wrote:

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Cheers! Have you seen any with, say, a motorised or hydraulic system?

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 Have seen a few with geared chain and sprocket, once with pneumatic piston, never an hydraulic - Keep it simple  with single sheave pulley and rope with tie down points weight is your enemy with over engineered systems. Also less to malfunction at the wrong-time

ie night-time in the dark. 

Remember you need to synchronise all four corners on lift - Geared chains are easily put out of synch, difficult to realign.