Development and body design The car's predecessors, the four door
W 111 (1959–65) and
W 112 (1961–65), had helped Mercedes-Benz develop better sales and achieve
economy of scale production by unifying the entire Mercedes-Benz range onto a single
automobile platform and body, reducing both manufacture time and cost. The external styling with American
"fintails", on the other hand have quickly gone out of fashion. Taking over the design school at Mercedes-Benz in 1957,
Paul Bracq began steering the styling away from the flamboyance with sharp angles to a more contoured silhouette, with strong emphasis on function. The two door W111 and W112 models, introduce in 1961 were the first to show this approach. They was followed by the much venerated
W113 Pagoda and the regal
W100 Größer, showing the new corporate style. Work on the replacement of the Fintails began in parallel and by 1963 the design was finalised, as the vehicle would retain the platform and underpinnings. The development was governed by a strict hierarchy of functional considerations over stylistic or fashionable trends, a decades-long practice at Daimler-Benz. The primary goal was to increase the passenger compartment while maintaining the favourable exterior dimensions of the previous six-cylinder models, simultaneously achieving particularly high levels of driving safety, ride comfort, and performance. While the external dimensions were only marginally increased relative to the preceding Type 220 S—specifically, an increase in length of , an increase in width of , and a reduction in height of —the utilization of interior space was radically improved. This was achieved primarily through the adoption of curved side windows and a lowered beltline. The interior width consequently increased by in the rear seating area and by in the front seating area. Furthermore, the floor in front of the rear seats was lowered by , resulting in increased footwell space. The elongated, low silhouette served the primary engineering purpose of lowering the vehicle's centre of gravity, a factor cited as being of particular importance for good road holding. Although the fins' departure was the most visible change, the lowered beltline facilitated larger window areas; the windscreen alone was 17 percent larger than that of the W 111, with total glass area increasing by 12 percent, prominent enough to be referred to as a "greenhouse." The result was a visibly newer-looking, sleeker car with an open and spacious interior. The flat roof and smooth rear end treatment were noted as being stylistically reminiscent of the Mercedes-Benz coupés, whose sporty and elegant design had garnered worldwide acclaim. The front aspect featured the traditional Mercedes-Benz radiator grille with a wide opening and lowered hood, flanked by vertically arranged lighting units that combined the functions of high beam, low beam, parking lights, fog lights, and turn signals into a single, integrated assembly.
Body construction and paintwork The body was welded to the chassis floor plate to form a single torsionally resistant unit. Vulnerable areas received a seven-layer paint finish consisting of zinc phosphate anti-rust treatment, chromic acid neutralisation, two primer coats, a stone-chip protection coat, undercoat, and topcoat, baked at . The underside, inner fenders, door sills, and front valance were given a thick protective coating of
polyvinyl chloride (PVC) weighing approximately 14 kg, which remained elastic to prevent stone damage and provide sound insulation. The engine compartment and underside components were sprayed with a wax coating. Chrome-plated exterior trim comprised five layers over steel: matte copper, polished copper, semi-matte nickel, polished nickel, and a double heavy chrome layer.
Interior design and comfort The seats were developed in collaboration with medical doctors, using firm cushioning to allow small posture changes and prevent the body from bouncing. The backrest followed the S-shaped curve of the spine, and the
bucket-shaped design provided lateral support. The driver's seat offered three-way adjustment: fore-aft, vertical height, and infinitely adjustable backrest angle down to horizontal. Multi-layer insulation separated the engine compartment from the cabin: 18 mm at the bulkhead and over 20 mm on sections of the floor. Ventilation air passed through a dust filter; a three-speed blower operated even when stationary. Used air was extracted through slots under the rear window, keeping it free of condensation. Heating was independently adjustable for left and right sides. Luggage capacity was 610 litres. Optional extras included an electrically operated
sunroof, electric window lifts, DB
power steering and air conditioning. Twin high-pitched horns were standard; a third horn was fitted to the later 300 SEL 3.5 and 300 SEL 6.3. From the 1968 model year the interior received new fabric patterns, revised colour schemes, and nylon carpet on the floor and sills.
Model differentiation: W 108 and W 109 Originally, the W 108 was seen as a combined successor to the
W 111 and the short-wheelbase sedans in the
W 112 line lacking air suspension. The W 111 had been a successor to the six-cylinder
Ponton models of the mid 1950s; the W 112 derived from it was a stopgap to replace the hand-built body-on-frame
300 "Adenauer" (W 189) state car pending the 1963 introduction of the
600 "Grand Mercedes" (W 100; ). In addition to being lavishly decorated with additional chrome trim, the W 112 inherited the alloy block
M 189 engine of the W 189, and featured a
self-levelling air suspension. The dividing line between the W 108 and W 109 ran between luxury, performance, and air suspension. The W 109 300 SEL kept the self-levelling air suspension, while the W 108 models from the 300 SEb down retained steel
coil springs and a hydropneumatic compensating spring on the rear axle. A double-acting hydraulic device that used the movements of the
swing axle halves to maintain a nearly constant ride height regardless of load. The 300 series were more luxuriously appointed than the 250 series, featuring burled walnut dashboards and fine wood trim. Externally all W 109 models could be distinguished by their door window frames and A-pillars finished with polished metal bright trim. Many optional appointments for the W108 such as power windows, power steering and automatic transmission were standard on the W109, though a fully synchronised manual transmission discount option was available. Later 3.5 and 6.3 a vacuum-controlled
central locking system was standard.
Chassis and steering The front suspension employed a U-profile pressed front axle carrier with
double wishbones on each side, separating wheel guidance from springing. Coil springs, gas-filled telescopic shock absorbers, and progressive bump stops were used. A front
anti-roll bar was standard; the later 300 SEL 3.5 and 300 SEL 6.3 received an additional rear anti-roll bar. The rear suspension was a single-joint
swing axle with a low pivot point; gas-filled shock absorbers prevented oil foaming. The entire suspension was rubber-isolated from the body, and an additional shock absorber on engine mounts damped engine vibrations. The
power steering was a hydraulically assisted
recirculating ball design. Below 600 grams of steering wheel force, steering was purely mechanical; above that, a control slide admitted pressurised oil. Road feel was retained, and if assistance failed the car remained fully steerable. The steering system featured a telescopic column, a deformable bushing under the steering wheel, and the steering housing mounted behind the front axle carrier for frontal collision safety.
Air suspension (W 109) The W 109 300 SEL models were equipped as standard with Daimler-Benz air suspension, developed with Phoenix for the rubber rolling bellows and
Bosch for the regulating valves. A compressor drew air through a frost-protection device and delivered it at 16 bar to a reservoir. Three level-control valves (two front, one rear) operated mechanically via rods from the suspension. The driver could select a "high" position via a dashboard switch, raising the body by approximately . The system maintained constant ride height regardless of load, allowing a softer spring rate and optimal toe and camber adjustment.
Safety architecture In the design of the new body by
Béla Barényi the latest findings regarding internal and external occupant protection, based on intensive accident research conducted by
Daimler-Benz AG, were incorporated. The structure adhered to the principle of a robust passenger safety cell combined with impact-absorbing
crumple zones. The passenger compartment itself was engineered to be exceptionally robust and rigid, while the front and rear sections were designed to be relatively easily deformable to mitigate impact energy. Internally, a comprehensive suite of passive safety measures was standardised across all models. The instrument panel was embedded within a shock-absorbing frame, and the steering wheel was equipped with a safety hub featuring a large, padded plate. The dashboard was effectively padded along both its top and bottom edges. All pull knobs and door handles were fabricated from elastic materials, and the doors were fitted with special safety locks designed to prevent them from springing open under any circumstances. For exterior protection against minor parking damage, rubber protective strips were integrated into the
bumpers, and plastic strips were incorporated into the side trim mouldings. All models had a dual-circuit
power braking system with
disc brakes on all four wheels. The parking brake was a small drum inside the rear disc hubs. A
brake force regulating valve on the rear wheels allowed larger rear wheel cylinders for stability during light braking while preventing lock-up under heavy braking. The brake booster supplied approximately three-quarters of pedal pressure. From approximately 1969, the regulating valve was eliminated on the sedans in favour of a constant brake pressure distribution, research having shown it equally reliable for the sedan's weight distribution. On the W109 models the rear brake calipers were connected to the chassis via reaction arms to prevent rear-end lift during braking. == First Series (1965–1967) ==