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We are seeing more and more tri-drive trucks are on the roads these days. Tri-drive tractors are useful in some situations as the extra axle only adds about $20,000 - $25,000 to the total cost of the truck. A heavy piece of equipment on a lowbed trailer being pulled with a tri-drive tractor does away with having to strip down the hauled machine to prevent being over loaded. The extra weight that the 7 axles allows in all comes close to B-train payloads without the bother of double-trailer configurations. However there are still issues with different provinces and states as whether you can run one of the types of trucks. Some states and provinces do allow them for their resource industries while other areas do not allow them period. Most of these tri-drives then end up in the forest industry and oil patch where off road travel is the norm.
Pretty much every manufacture of trucks today will build a tri-drive. Kenworths seem to be the most common followed by Western Star, Peterbilt and Mack. Hooked to a tridem trailer and these rigs can usually haul an additional 7-8 ton depending on jurisdictions. In North America most of these types of trucks can be found working in British Columbia and Alberta and Saskatchewan. Elsewhere, Australia is noted for heavy haul tridrive trucks.
Tri Drive trucks are used in mostly off road applications such as logging, mining and oil field transport. These are the trucks that haul heavier than normal loads. These load would be special loads like drilling rigs, oil processing equipment and such that can't be broke down to small sections such as seen in the photo below.
Tri Drives are also used a lot with truck mounted equipment such as vacuum tanks, water tanks, pumping equipment, dump boxes and other heavier than normal equipment. Having a Tri Drive allows one to haul more on a truck without having to have a trailer. More wheels on the ground allows for more weight. More drive axles allows for more push and pulling power along with added traction. This is advantageous in off road conditions where drivers of these units are not only hauling heavy loads but are having the added pressures of steep terrain, mud, ice and gravel roads.
In extreme cases you will find some of the tridrive trucks built with a tandem steering axle. The extra axle up front allows for even more weight of the trucks payload. Most of these configurations however are for body mounted equipment like flush by units and other heavy oil patch related work trucks. There are however tandem tridrive type trucks with cranes or pickers that will have a 5th wheel rigging for towing a trailer.