Spare Tire Carrier
We wanted to carry two spare tires. Our off road experiences with the Land Rover Defender taught us why it's a good idea to do so when travelling in remote areas. More than once have we, or someone we were travelling with, suffered more than a single flat tire during a lengthy off road trip. Sure, with the smaller tires, you can carry a repair kit that can plug most holed tires. But many times, a tire can be damaged by having the side wall sliced open from a sharp rock poking from a bank, and nothing can fix those.
The problem with heavy truck off road tires is that the tread material thickness makes plugging them difficult out in the field. The other issue is climate. If we were travelling in the winter time in a northern region, field repairing a tire in snow and ice could be virtually impossible.
Which Tire Size
Planning on carrying two spare tires somewhat dictated what size of tire we could use. The Michelin XZL tires come in eight different sizes, all made to fit a 20" rim. But the outer diameter of these different tires varies a lot. The smallest is 40.7", and the largest is 53" in diameter. The general goal for off-roading is to go with bigger diameter tires. This helps both in ride comfort and increased clearance under the differential. The problem for us was the width of our box. If we wanted to carry two spares fitted in-line across the width of the truck, they would have to sit inside the 98" width of the box. The two larger diameter tires exceed this amount by a significant degree. They also would be too large to fit inside the front wheel wells.
In the end, the largest tire we could fit was the 395/85R20. They have an outside diameter of 46.8". So they just fit nicely side by side, leaving an inch of room between them to allow the lift cable to run up to the hoist.
Tire Carrier Design
The carrier design had to work in conjunction with the lift mechanism. Each wheel and tire assembly weighed about 300 pounds, so moving them about had to be easy and efficient.
We also planned to have our Hutchinson rims nicely finished with a metallic grey powder coat. So we didn't want to damage them by struggling to get the wheels on or off the wheel studs, up on the tire carrier.
A lot of spare tire carriers built for these large overland trucks have a fairly simple carrier which uses an overhead arm and a winch to raise and lower the tire. It's a fairly difficult task with a risk that something could go wrong, and the wheel could come crashing down. We didn't want that. Instead we designed a properly engineered lift platform that can carry many times the weight of a single wheel assembly, and it's set up so the hoist operator can be on the lift with the tire when its coming down. That way if something goes wrong, and the tire gets away from the operator, it will fall away from them, instead of onto them.
The tire carrier will also be an integral part of what secures the lift to the back of the truck. This way all the forces are dispersed over a greater area. Since the box is essentially made out of 1/8" fibreglass sheet with a styrofoam core, one has to think carefully about the forces exerted on it. By including the tire carrier as an integral part of the lift, we increase the number of attachment points, and the distance between them, which all tie into structural members placed inside the wall surfaces before they were glued together.
To increase the strength even further, our design also ties into the roof protection rails at the back. The forces pulling backward on the tire carrier and lift mechanism are distributed along the front to back corner angle that glues the roof panel to the side walls. In order to break off that 1/4" thick 4" x 4" fibreglass angle, it would have to tear apart a 22 foot long joint. That's not likely to happen.
We also incorporated a roof into the tire carrier so the hoist motor and tires will be shaded from the UV rays of the sun, and driving rain. This helps with longevity of the hoist electrics and tire rubber. The roof also increases the overall strength of all the connections between the lift mechanism and the tire carrier. It allows gusset brackets to be installed which help support the roof structure, and therefore better supports the weight when the lift is used.
