Chassis Details

The process for ordering a new class 8 truck is quite involved. Unlike buying a car, where you might have two or five separate option packages to select from, a heavy truck has literally hundreds of options that need to be considered. Each truck is built to order for every customer. The only "off the shelf" models that a heavy truck dealer might have on their lot are ones that the dealer has ordered based on what their customers are most likely to buy. So when someone comes in that doesn't want a freight tractor, dump truck or bin hauler, they have to sit down in front of a computer and go through the huge long list of build parameters to spec out the truck. This process took a number of visits to the dealer because of build specifications we needed to develop, and the dealer had to get clarification about some of their specs.

The reason that this process had to be done on a computer is that the available build parameters are vast, and not all items are compatible with each other. So as we went down the list and selected something, a red flag might pop up with a note that this item was not available with something selected earlier on the list. So we'd have to go back and change it until both were compatible. The process took many hours. If you look at our order document linked below, it provides a real eye-opener as to just how involved the process is.

Volvo Truck Brochure                  Our Volvo Order Spec

Chassis Planning

As mentioned, our truck of choice was a Volvo VHD. It's the vocational model that's generally used for rugged use situations. It comes in two cab configurations. A day cab, for use where the operator is not staying overnight in the cab, and an overnight sleeper, which is a much scaled down, flat roof version of a highway tractor sleeper unit. Essentially, the cab is made longer by about 800mm and a bunk is installed. Under the bunk are two large storage bins which can be accessed from outside by two lift up doors behind the cab entry doors.

Using a day cab has the advantage of a shorter length. A desirable feature if one is trying to make the shortest truck possible. But the problem with them is that there is very little room in the cab. So little in fact that it makes it difficult to move between the camper section and the truck cab through a connecting tunnel. And there is precious little room for any gear that you might like to have quick at hand when travelling, like camera equipment and such.

Since we knew we'd likely never be attempting some winding goat track in the Himalaya or Andes mountains, absolute minimum length was not an imperative for us. So we elected to go with the sleeper version of the truck. It would provide a number of advantages for us that would make the long hours spent in the cab more comfortable.

Frame Length

Since we already knew the length of our box, it was easy to determine how long the frame of the truck had to be. We simply took the box length of 22 feet (6700mm), added 5 inches to that for the distance the box would sit away from the back of the truck cab to allow for the connecting tunnel, and then subtracted an inch so the frame would end just forward of the back of the box. We then took that dimension and used it to select the appropriate frame length. Volvo makes their frames in increasing length increments of 2 inches. We just chose the closest one that kept the frame end hidden.

One Axle or Two

Of all the parameters that needed consideration, the rear axles are among the most important. Not only did we have to decide on whether we wanted one axle or two, we also had to determine where they would sit along the length of the frame. In other words, we had to determine the wheelbase of the truck. We even had to decide on how far apart the two rear axles would sit from each other, if we decided we needed two axles.

What determined the quantity of rear axles was the weight of the overall build. Each axle has a maximum load capacity, but so do the wheels and tires. Compounding this is whether we'd be running the more common dual wheels, or going to a single tire on each end of the axle.

For the purposes of off-road travel, single wheels are more desirable because it allows the rear wheels to travel in the track made by the front wheels. Dual wheels don't allow for this because of their increased width. Since we were planning on a lot of off pavement travel, we wanted single rear wheels. This meant that the total weight carried by the tires and wheels would be divided between a total of four wheels instead of eight. A significant increase per wheel/tire.

So we now had to look at the weight capacity of each wheel/tire assembly. We knew we wanted to run the Michelin XZL 395/85R20 military tire because of the tread pattern and toughness of the design. So checking the specs on those tires we found that their maximum load is 5,600kg or 12,345 lbs. So two tires on an axle means it would support 11,200kg or 24,690 lbs.

Calculating the weight of our build as best we could, and adding to that the weight of the back portion of the truck frame and axles, we figured we'd be pretty close to 24,000 pounds. This would essentially give us no safety margin and would be taxing our wheels and tires to their maximum at all times. A proven recipe for disaster. So we had to have two axles in order to build in a healthy safety margin.

The rear tandem axle location along the frame was really determined by the amount of overhang we were willing to have. More overhang means a smaller departure angle for the back of the truck when off-road. And conversely, a smaller overhang increases the wheelbase of the truck which makes its turning radius greater, and reduces the break over angle under the truck. So it's a compromise. By playing with the dimensions on a computer drawing, the axles were positioned so there was about 48" from the back of the tire to the end of the truck frame.

The Remaining Decisions

All that remained after we had done the frame length and axles were the operational details. Size of engine (13 litres), horsepower (405hp) and torque amount (1450lb./ft.), transmission type (Allison 4500 Heavy Duty Offroad 6 speed Transmission), differential and inter-axle lockers (Meritor MT-40-14X 40,000lb.), and then all the options for hot and cold climates for the engine, transmission and fuel systems. The wheels and tires we selected when ordering the chassis from Volvo were the cheapest combination we could get. They were only required to get the truck chassis to the body builder, and then they would be replaced with the Hutchinson military wheels and Michelin XZL tires.

We placed the order with Volvo in October of 2014 and the truck was ready for pickup at the dealer at the end of February, 2015. It was designated as a 2016 model year.