Mercedes 500-540K
The Mercedes 500K and 540K are famous mainly for their luxurious and elegant bodies created in the Sindelfingen plant. The most outstanding of these models is surely the cabriolet “Spezial-Roadster”, considered by many to be the most beautiful car of the classical era.
The use of the Roots supercharger was another important characteristic of these cars. In fact, the “K” in their name stands for “Kompressor”.
Gallery
540K
The front indipendent suspensions 
540K Spezial Roadster
Luxury and speed
Thanks to the use of supercharger, these Mercedes models were part of a small set of passenger cars of the 30’s that were able to reach and exceed 100 mph (160 km/h). All the passenger cars of this period that overcame this speed barrier became part of the elite, but the formula one models of Mercedes could already go much faster then 300 km/h...
The formula for luxury
The 500K presented in 1934 had an engine of 5000 cc., equipped by a supercharger. This 8-cylinder in-line engine was derived from not very strong and short-lived model 380.
The model 540K, with the displacement raised to 5400 cc., was presented on the Berlin Motor Show in 1936. It had the usual system of compressor supercharging.
The use of supercharging has earned Mercedes a legendary reputation in the racing car circles already in the 20’s. The S, SS, SSK and SSKL models were reaping victories more or less everywhere. These cars were very Spartan and difficult to drive, but in these years, pilots often used for racing the same model in which they left their home to arrive to the racing circuit. Passenger versions of these cars rarely had more luxurious appearance then their corresponding racing “brothers” (Trossi’s famous SSK is one of the most eminent exceptions).
The situation changed in the 30’s. Compared to their ancestors, the 500K and 540K looked much more like the passenger cars. They were more elegant and luxurious but still had the supercharger type Roots and could reach high performances. But they were also very heavy and their engine was less sophisticated as it was equipped by stem-based valve timing and equalizers.
The supercharging
The declared maximum power of 180 Hp looks nice but people sometimes forget to mention that it was possible to reach such performance just with the supercharger turned on.
And that is the point. The Roots supercharger of these cars would be engaged only by pressing the accelerator pedal to the bottom. Practically speaking, this system was very similar to the more recent “overboost” that provides the maximum power only when it is asked for it and can be very useful, for example, in the overtaking phase (but the overboost operates on top of already active supercharging system). It is also important to keep in mind that the supercharger, once it has been engaged, could not be kept active for more then about twenty seconds. If you exceeded this time limit, the situation quickly became dangerous and there was a high risk of explosion...
Lets have a look at this system. The supercharger is moved by the drive shaft and is frontly connected to it by a few pinions and more importantly by a multidisc friction, which is used to progressively engage the supercharger itself, protecting it from sudden shocks that could damage it.
Once engaged, the supercharger intakes the air from the filter and sends it under pressure towards the carburettor. It is placed on the top of the carburettor and not after it as on the Alfa Romeo 8C or Bugatti 57C. All the competitors of the Mercedes had constantly active superchargers.
The system of the engagement of the supercharger is ingenious and quite complex. There is a system of compound levers, connected to the accelerator pedal that operates at the end of stroke on the actuating rod. The rod does simultaneously three things: 1) it influences the carburettor by activating an additional jet that works only in the phase of superchargment; 2) it influences one of the throttle valves that carries the intake air directly towards the compressor; 3) it releases the clutch that engages and sets to rotation the supercharger.
The weakest point of the system is in the engaging clutch, that wears out fast and it sometimes makes the start of the supercharger uncertain.
This imperfection was already well known to car owners of the era. The device would sometimes misfire and cause an embarassing slowdown instead of an immediate acceleration. This behaviour wasn’t surely very pleasant especially in the overtaking phase.
Independent suspensions
The chassis is one of the strong points of this car. In the front there is a system of two independent suspensions with A-shape arms, which was a real rarity in this period and only Alfa Romeo 8C 2.9 and Lagonda V12 had something similar, while the rear train is build of swinging axes with big helical springs. There are no leaf springs then, either in the front or in the rear.
The 500K and then the 540K were supplied with two different sizes of chassis: a long one of 329 cm and a short one of 298 cm.
One detail should not be overlooked; the inside of the radiator of these cars is made of thousands of small pipes welded together and what you see from the outside is not a covering chassis that hides it, but it is the real radiator containing water. Just imagine how many working hours were needed to produce such piece...
Design masterpieces
The bodies of the nicest exemplars of the 500 and 540 are the result of Mercedes internal work and not, as it was common in these times, of a cooperation with external body-makers. They were produced in the Sindelfingen plant that had 1500 employees and a production capacity of 500 bodies a month. The person in charge of the design was Hermann Ahrens. The most luxurious bodies were treated with a special care by a sort of "special department". There were numerous typologies of offered bodies, from coupè with stiff roof to 7-seat sedan.
Take only one of the typologies, the Cabriolets; there were no less then 6 different subcategories of them!
the "Spezial Roadster" was the most luxurious version of a cabriolet provided with some special characteristics. It is important to know that at that time the term "Roadster" was in germany used for a convertible with a hood that was really just an emergency sheet without any real supporting chassis, which is normaly used on cabriolets. The hood could have been folded and made all dissapear in the body. Additionally, the side windows of these roadsters were often fixed in order to preserve a more Spartan racing spirit, free of any heavy accessories.
The “Spezial Roadster”
Only some of the 500/540K roadsters were called “Spezial” and it seems that the determining difference was in small details depending on client’s order and his will.
From what Ahrens sais, it seems that all the “Spezial” had the V-shaped windscreen with two windowpanels separated by a central rib, while some of the normal roadsters were equipped by a single windowpanel (we recommend Griffith Borgeson’s article in Automobile Quarterly Vol. 26 – No. 4 for more information).
The first version of Spezial Roadster that appeared on 500K chassis was recognizable by its shoulder-line that would bend down near the doors just to climb up again onto the rear mudguards. In 1936, the shapes of the Spezial Roadster were modified and the new version was presented on the Paris and London motor shows in autumn 1935. The new shoulder-line went almost straight from the radiator to the rear mudguards and the model had much longer and slimmer tail. This second version was used on 500K and 540K chassis.
When you examine models made in 1936, the year of transition between 500 and 540K, it is quite impossible to tell from the outside appearance if the car has an engine of 5000 or 5400 cc. One not very precise method to find out is to examine the air intakes on the side of the bonnet that are vertical on models 500 and horizontal on 540. But this rule may be misleading on many models, as shows the example of the car owned by Hermann Goering marked as 500K but really equipped by a 5.4-liter engine. There are also models of 540 that have the bodies made in the style of the first version of the roadster.
One of the most interesing accessories of the “Spezial” was the lid covering the space behind the spare wheel. The cover makes the slim shape of the tail even more exciting. The lid connects perfectly to the bending of the body and even has the hint of a fin as a terminal point to the chromed stripe on the centre line. what a luxury!
The cover could be used with only one wheel whereas if you decided to carry the second spare wheel, this would inevitably stick out of the body mass (as you can see on a photo on the side). At the time, there were very few asphalted roads and it was almost obligatory to have two spare wheels, especially on long extraurban journeys.