A towering 4×2 cabover tractor unit, towing a semi-trailer with three axles and six super singles. Travel anywhere in Europe and you’ll notice that this is the most common big rig configuration on the continent. And in the UK and Ireland, for that matter. Usually the tractor has a 450 to 550 hp engine and is built by DAF, Scania, Volvo, Iveco, Mercedes-Benz, MAN or Renault.
Pictured above is a 2017 DAF XF with a Super Space Cab and a 450 hp DAF-Paccar MX-11 engine (as in 10.8 liter displacement). The semi-trailer is a so called curtainsider, the usable bed length of this widely used type of semi-trailer is around 13.60 m (44’7”).
During the seventies, super singles started to replace the dual wheels more and more. Instead of a semi-trailer with a widespread tandem and dual wheels, the norm became a chassis with a tridem and super singles. The main advantages of these wide, single tires are less wear and tear and a better fuel economy for the tractor. The closer a tire is to the center of the axle, the more it will wear and tear when cornering. On a dual wheels setup, the inner tire will always wear faster than the outer tire.
Regarding the tractor’s fuel economy, a wide super single has less rolling and air resistance than a set of standard dual tires.
A very similar rig to the one in the first picture, yet this superbly 2015 DAF XF tractor has more grunt, as it’s equipped with a 460 hp MX-13 engine (12.9 liter displacement).
In multiple EU-countries, these 12-wheelers are rated at a maximum GVM of 44 metric tons (97,000 lbs). The gross weight is limited to 40 metric tons (88,185 lbs) though when the rig is crossing borders, so when transporting goods internationally.
A close-up of a tridem I shot last year, BPW axles with Fulda super singles.
Many types of transport follow the 12-wheeler concept, like this Scania R-series V8 with a LAG dry bulk tipping tanker. LAG (Lambert & Arnold Geusens) is a renowned Belgian manufacturer, specialized in tankers. The company used to build buses and coaches too, but in 1991 that division was taken over by Van Hool, also from Belgium.
An Italian O.ME.P.S. dry bulk tanker, towed by a 2017 Scania S450 tractor (450 hp – 12.7 liter). The drive axle of a 4×2 tractor in this segment is rated at a legal maximum axle load of 11.5 metric tons (25,350 lbs).
A 2018 Volvo FH (460 hp – 12.8 liter) with a dry bulk tipping tanker, actually tipping. Modern trucks and tractors have a big mouth. Especially when said mouth -aka the grille- is completely blacked out and stands out even more, like on this Volvo. Currently, engines compliant with the latest emission standards need a lot of cooling capacity, so there’s the answer.
Since we just walked by some tankers, another important advantage of the super single tires worth mentioning, is the fabrication of a wider (semi-) trailer frame so that the tank can be mounted lower, which means more stability while on the road. Of course the same applies to the self-supporting type of tankers. Dual wheels on tanker trailers and semi-trailers have become extinct almost entirely.
The last one, a reefer, towed by a splendid and brand new DAF XF with a 480 hp MX-13 engine. These days many top segment tractors are fully air suspended: front axle, drive axle, cab and seats.
Dual wheels on big drawbar trailers and semi-trailers are still prevalent in logging and heavy- and special haulage. In the earth moving and brick hauling business they also hold their own quite well. For the rest the duallies are long gone and won’t come back.