Even in the UK, I prefer the idea of being self sufficient, but with plans to drive far into Central America, I do not like the prospect of having to wait for days by the side of the road for a suitably equipped tyre fitter to come past. It wads therefore fairly important that I got a spare tyre, and found somewhere to put it, before the truck gets dropped at Southampton docks next week.
The most common place to fit a spare tyre on a heavy rigid, is either under the chassis just in front of the back wheels, or mounted centrally behind the back wheels. Jim's wheelbase is fairly short, and so once I had fitted the generator and auxiliary fuel tank there was no space on the chassis rails for a spare. In addition the overhang at the back is very short, sand so there is no space behind the back wheels either.
This left me with a few options, mostly ideas pilfered from large expedition vehicles. The simplest option seemed to be to mount the tyre on the roof of the cab. I would only have had to mount a platform on the roof of the cab using the threaded mounting points provided by Mercedes from the factory, and fitted a simple swinging arm and chain block to get the wheel up and down. I quickly decided against this option due to the additional weight on the cab mountings. The cab is already far heavier than intended due to the all the armour plating from it's life as a Brinks truck, and so I wanted to avoid putting more stress on the cab hinges and the hydraulic tilt system. In addition, the front axle is rated at 5 tonnes less than the rear, and so putting the weight so far forward isn't preferable. For the same reason I discounted the option of mounting the tyre on a bull bar type bumper.
The roof of the box body is almost completely covered by hatches and solar panels and so mounting the tyre on top of the living accommodation was impossible. In addition, at 2.5m wide, Jim is already a handful on narrow roads and so mounting the tyre on the side of the body would have been absurd. This left me with finding a way to mount the tyre at the rear of the truck.
I had originally intended to mount the tyre on a swing out carrier fixed to the box; the tyre would swing out of the way to lower or raise the tail lift, and the platform of the tail lift could be used to take the weight of the tyre if I ever needed to use the spare. The problems with this option where finding suitably strong mounting points on the body to fit an 150kg mounted tyre on a 100cm lever arm, and also the practicalities of swinging the tyre around when it was nearly 2m off the ground. I took Jim to a specialist vehicle fabricator in Kent, who came up with the great idea of mounting the hinges to the column of the tail lift. The tyre goes up and down with the tail lift, and weight is taken on a platform specified to carry 1.5 tonnes.
The carrier works like those fitted to the rear doors of land rovers, except that the tyre moves up and down hydraulically, and the unit is fabricated from 10mm steel plate. I feel fortunate to have found a company, who are not only familiar with working on heavy commercial vehicles, but also equipped with CNC laser cutters, water jet cutters and the biggest press I've seen.