Guide: Mercedes-Benz W198 300 SL Roadster
Background
By the mid 1950s, two-seat roadsters were so popular that Mercedes-Benz set about creating an open top version of the 300 SL as soon as the original Gullwing coupe had gone into production.
Such was his confidence the Roadster would be a hit, Mercedes’ US importer, Max Hoffman, placed another large advance order.
Mercedes unveiled the 300 SL Roadster at the Geneva Motor Show in March 1957. This latest iteration cost around 20% more than its predecessor.
Production began in May 1957, when the Roadster took over the Gullwing’s line at Mercedes’ Sindelfingen factory.
By this time, the extraordinary 300 SL Gullwing had become arguably the most glamorous Gran Turismo of its day.
An impressive 1400 were ultimately built as the model quickly acquired icon status.
The Gullwing’s commercial success came despite some obvious drawbacks.
In reality, the doors were impractical; they were difficult to get in and out of and had windows that could not be wound down. There was also the concern of how occupants would exit should the car end up on its roof.
The new W198 300 SL Roadster would address these issues and more.
Not only did it have proper doors, it was more powerful and more refined with improved handling and extra luggage space.
The engine, transmission, radiator, front suspension and many other small parts were interchangeable between Gullwing and Roadster. However, the Roadster’s chassis, rear end and bodywork came in for major attention.
Chassis
The original W198 tubular steel chassis derived much of its torsional rigidity from the high sills that dictated its unusual ‘Gullwing’ door configuration.
To accommodate conventional doors with wind up windows, the Roadster’s sill tubes were lowered by 50%. A series of supplementary tubes were added elsewhere to regain the lost rigidity.
Thanks to modifications at the rear and a redesigned 120-litre fuel tank (25% smaller than before), there was finally space for a proper boot. Mercedes fitted the spare wheel under the new boot floor.
High speed handling was dramatically improved as a result of a new rear axle assembly. The Gullwing’s original swing axle was joined only at the differential and not the wheels which meant extreme changes in camber could be encountered at high speeds or on poor surfaces.
By contrast, the Roadster incorporated a new single joint axle with lowered pivot points. A compensating spring was added to reduce roll stiffness.
Handling became more neutral and the original W198’s tendency to oversteer was reduced significantly.
Coil sprung telescopic dampers were fitted all round along with servo-assisted Alfin drum brakes.
A torsion bar stabiliser was installed at the front.
The original 5 x 15-inch bolt-on steel wheels were widened to 5.5-inches all round. Michelin tyres were standard factory equipment.
Steering went from two to three turns lock-to-lock.
Engine / Gearbox
Mercedes had offered the W198 Gullwing with an optional Sonderteile engine for customers who wanted the fastest, most powerful car available.
For the new 300 SL Roadster, the high compression Sonderteile motor with its competition camshaft became standard equipment.
The SL’s single overhead camshaft straight six engine comprised a cast-iron block and big valve aluminium-alloy head. It employed race-proven dry-sump lubrication and Bosch direct mechanical fuel-injection; the W198 was the first series production car to use fuel-injection.
The valve cover and cooling fan were both magnesium.
The crankshaft was forged steel and came with a vibration damper.
Ignition was via a single coil and one spark plug per cylinder.
Like the rest of the 300 range, displacement was 2996cc thanks to a bore and stroke of 85mm and 88mm respectively.
With its 9.5:1 compression ratio, the W198 Roadster had a quoted output of 240bhp at 6100rpm and 202lb-ft at 4600rpm.
Transmission was via a four-speed all-synchromesh gearbox and single dry-plate Fichtel & Sachs clutch.
The Roadster’s gearbox was the same as before, but the Gullwing’s straight-cut gears were replaced with helical gears to provide a more refined driving experience.
Bodywork
From the waist down, the Roadster’s body retained the same proportions as its Gullwing predecessor.
Most obviously new were the combined Pillar-type headlights. These elegant assemblies housed the stacked head and side light behind a vertical glass cover.
The slim horizontal tail light clusters were made bigger and came mounted in a new chrome shroud.
The all new cockpit featured a wraparound windscreen, bigger front-hinged doors and wind up windows.
When lowered, the handsome canvas roof stowed out of sight below a retractable body coloured panel.
Otherwise, the Roadster retained classic Gullwing features such as the wide front intake, full width bumpers, wheelarch eyebrows and elaborate engine cooling vent carved out from each front fender.
Body panels were mostly welded steel. Aluminium was used for the doors, bonnet, bootlid, sills, bulkhead and floors.
Interior
Numerous discrete changes were made inside too.
The Roadster came with an entirely new dash that was larger than before.
Directly behind the steering wheel was a redesigned instrument binnacle that housed a rev counter and speedometer.
Between the rev counter and speedo was another feature unique to the Roadster: a rectangular read out that gave information on fuel level, oil pressure, oil temperature and water temperature.
Leather was used to cover practically every surface bar the floor.
Between the dash top and knee roll was a body coloured fascia that housed the redesigned ventilation controls and glove box (a new addition).
Bucket seats were more comfortably padded than before and a wing mirror was fitted as standard.
Weight / Performance
The better quality interior combined with the reinforced chassis accounted for much of the Roadster’s 95kg weight gain (now 1390kg).
It had a top speed of 145mph and 0-62mph time of 7.1 seconds.
Options
Optional extras included Rudge centre-lock wheels, a passenger wing mirror, a Blaupunkt or Becker radio, a fitted leather luggage set and seat belts.
The standard rear axle ratio was 3.64:1. Mercedes also offered the following optional ratios: 4.11, 3.89, 3.42 and 3.25:1.
USA Version
US market W198 300 SL Roadsters came with a 3.89:1 rear axle ratio as standard.
American legislation also prohibited covered headlights, so a new two-piece chrome headlight shroud was fitted to cars bound for the USA.
Production Changes
Roadster production started in May 1957.
The first updates arrived in 1958 when single-piece armrests were introduced. Earlier examples came with opening two-piece armrests.
Around the same time, an electromagnetic valve was added to the mixture control to prevent overrun.
Late in 1958, Mercedes started to offer an optional steel hardtop.
For 1960, they brought in plastic-lined fuel tanks and front brakes with dual floating shoes.
A particularly significant development was the arrival of four-wheel Dunlop disc brakes in March 1961 (from chassis 2780).
Another key update followed a year later when, in March 1962, Mercedes switched to aluminium-alloy engine blocks instead of cast-iron (from chassis 3049).
End of Production
Production ended in February 1963.
By this time, 1858 examples of the 300 SL Roadster had been completed (all of which were left-hand drive).
Text copyright: Supercar Nostalgia
Photo copyright: Mercedes-Benz - https://www.mercedes-benz.com
