Export successes and powerhouses

Twenty years after the first Unimog prototypes were built and ten years after the launch of the successful model series 411, in January 1966 the Unimog U 421 heralded the launch of a new model series – later also described as the "light-duty model series". It aimed to close the output and weight gap that existed at the time between the successful 406 model series (available from 70 hp) and the 411 model series (with 34 hp at the time). Originally designed for passenger cars, the OM 621 was initially offered in a 40 hp variant; later increases in output of up to 60 hp followed.

Otherwise, components from the U 411 were largely used for the Unimog U 421, which also explains the visual similarity. That not only brought economic advantages but also meant that bodies and implements could continue to be used in many cases. The clever feature of the new light-duty model series: the cab could be folded forward in just a few simple steps, which made maintenance work on the engine considerably easier.

The 421 model series proved to be enduringly popular and was produced for over two decades until 1988. With almost 19,000 vehicles, it is one of the most successful Unimog model series of all time.

Also in 1966, Daimler-Benz introduced the model series 403, which was later also produced in a long version as the model series 413. A major innovation compared with the visually almost identical model series 406 was the 4-cylinder OM 314 engine, which, in the uprated version with 66 hp, enabled speeds of up to 80 km/h.

In agriculture in particular, demand for fast tractors for agricultural logistics was high in these years. The Unimog positioned itself as an attractive alternative to the truck. Thus, versions of the model series 411, 413, 416 and 421 were also developed with truck registration. They allowed a higher permissible speed on the road and a higher gross vehicle weight.

1973 finally became the birth year of the MB-trac, which, like the Unimog before it, was specially developed for agriculture and forestry. The MB-trac, whose series production ran until 1991 on the same production line as the Unimog in Gaggenau, incorporated many components developed specifically for the Unimog: from the chassis with an all-wheel-drive transmission and four equally sized wheels to the portal axles.

Demand for Unimog continued to grow in its third decade, leading to successive increases in production at the Gaggenau plant. The reason: its versatility meant the all‑rounder long ago offered a convincing concept for year‑round use not only in agriculture. It also won over users in municipalities, in the construction sector, and as a transport vehicle, even for use on the railway.

Added to this was continuous technical development, with safety increasingly coming into focus. Thus the Unimog's open cab was fitted with roll bars even before it became legally required, and more powerful braking systems with disc brakes on all wheels were introduced — a genuine innovation in the truck sector.

In May 1966 the 100,000th Unimog rolled off the production line. Only five years later they were already celebrating the 150,000th Unimog. The model series 416 made a significant contribution to this production success, offering increasingly powerful engines of up to 125 hp. Export successes included the licensed production of the model series 426 and 431, developed specifically for South American needs and assembled in Argentina.

As with earlier Unimog premieres, it was once again a DLG exhibition in 1974 that marked the start of a new era: that of the "heavy series" (SBU). Their angular design remains characteristic of the Unimog to this day and was even awarded the Federal Design Award. But there were also significant changes under the hood: in addition to the 125 hp engine, from the mid-1970s the heavy series adopted a completely new chassis.

The increasingly powerful engines demanded a fundamental overhaul of the drivetrain, which on the Unimog had to provide not only high speeds but also a crawler gear and the ability to drive numerous power take-offs such as PTO shafts. New legal regulations were also introduced to limit noise and vibration in the cab. The result: the engine and transmission were no longer bolted directly together, and a new 8-speed gearbox with double reduction was developed. This enabled speeds between 0.15 and 80 km/h.

Another technical highlight of the heavy series was the cranked U-section frame. This design, which had already been successfully used in the Unimog S, provided excellent torsional flexibility — a major advantage for off-road driving.

Production of the heavy series began in 1975 with the Unimog U 1300, named under the new naming scheme. The long-wheelbase variant launched at the same time, designated by the letter "L", remains popular to this day as a robust and easily convertible base for conversion into travel and expedition vehicles.