Rob Gray :: ontheroad :: wothahellizat :: wot1 :: diaries :: issue-026


28 May 2000

As I write this it's snowing outside. Big fluffy crystals of frozen water are swirling, driven by the wind, in a sky that was supposed to be blue and far too warm for snow.

You see last winter was supposed to be just that, our "last winter" however the truck is nowhere near finished. Apart from moving house and a couple of weeks of total motivation atrophy I have done the following...

  • Finished the cladding (well almost)
  • Installed the stair raising/lowering buttons and relays
  • Mounted the hot water system (not connected)
  • Installed the slide rails for the TV
  • Built the main frame for the deck

That's something I suppose but in general I've been a while getting back up to speed.

15 June 2000

Brrrrrr, minus 6 last night and with all the hype about GST I've thought of some expansions of the acronym more appropriate for me.

  • Although it's not as fast as I'd like the motor home project is Getting Slowly There.
  • I'm really Getting Stuckin Toit now though.
  • As soon as it's finished I'll Get Seriously Tanked.
  • Then hit the road and Goto Sunny Townsville.

For the past week or so I've been working on the deck. "A deck!" you say. Yep, after a very wet Roma rally where we saw most motor homers with sodden under-awning areas and one with a lovely dry deck we decided that's for us.

Fig 1. The basic configuration.

Figure 1 shows the basic configuration where the deck stores vertically at the rear of the rig, is hinged at the bottom and folds down. Naturally this has to be lowered somehow and my first impulse was to use gas struts in the usual manner something like this...

Fig 2. Using a gas strut in the usual way.

The problem here is that the strut would hit the body when the deck is raised so an extension would be required.

Fig 3. Adding an extension to clear the body.

Trouble is this creates a protrusion that people will bash their head on and that may be damaged when exiting a steep creek crossing.

My next thought was to use a winch, this would probably have been the easiest method but they're expensive and really noisy. Similarly hydraulics are expensive, noisy and a little hard to install.

So I returned to gas struts.

What was needed was a way to use the struts on the top of the deck but this would mean they have to pull up, and that's not the way they're designed. So how about using a cable to pull on the struts as follows?

Fig 4. Using two struts and a cable.

In this configuration two struts are ganged, attached to a cable which runs over a sheave (pulley) to an attachment point on the deck. This provides a lifting affect at a point about 200mm from the deck's pivot point. As the deck in about 1800mm long the force required is about nine times the deck's weight. As it happens this is about 500kgs so I duplicated this setup on the other side of the rig giving me a total of four struts, each of 130kgs force.

So far so good, but the trouble is that the force applied by the struts is more-or-less linear but the force required is not. As the deck is raised less force is needed to hold its weight.

Fig 5. Less force need as the deck is raised.

As the deck approaches the point shown in Fig 5 you are pulling against 500kgs of gas strut (the deck is raised and lowered from the ground below the rear of the truck). Naturally you would loose and the whole lot would slam against the rear of the truck when closing and be impossible to open in the first place.

One answer is to design the cable to be of a length that allows the struts to be fully extended when the deck is at around the position shown above. At this point you don't really need any help so it's a simple matter to go inside the rig and pull the deck closed.

Fig 6. Photo of the struts.

So we have a lowering deck but what's going to keep it level and strong enough to hold people?

My first thought was legs. A couple of adjustable legs positioned near the outer corners would do the trick but I didn't want any obstruction under the deck and there's also the fact that the legs would have to be stored somewhere. I wanted the deck to be self-supporting. Enter the handrails.

As you have to have handrails anyway why not make them structural?

Fig 7. The handrail becomes a structural member.

Here we see the handrail in place. It is hinged to the deck at the bottom (blue line) and attached to the motor home body (red arrow). This creates an effective steel beam 700mm in height, more than strong enough to support a couple of people.

There is a rear handrail as well but this has no real structural value. It just ties the two side rails together and stops us from falling off.

To stop the deck at the correct level I have added to high-tensile chains as follows.

Fig 8. High tensile chains added.

This deck is two metres from the ground, not far to jump on purpose but a long way to fall by accident. As mentioned elsewhere I'm a firm believer in redundant systems where there is a backup in place for the primary device. With the deck this translates to,

  • The strut/cable arrangement holds the weight as the deck is being lowered or raised. There are four struts organised in two assemblies each with a steel cable. If any part of either assembly fails the other will not hold entirely but will slow the rate of fall to an acceptable rate.
  • When the deck is deployed the main stress is taken by the handrails. There are two rails and each one can easily hold if the other fails.
  • There are two high-tensile chains that backup the handrails. As before each would hold the entire weight if both handrails and one chain fails (well you never know). The chains are welded to different locations than the handrails on both the body and the deck, no point having a backup assembly dependant on the same thing as the primary.

Fig 9. Top shot of the deck

Fig 10. Underneath shot of the deck