Citroën 2CV Technical info

Construction

The level of technology in the 1948 2CV was remarkable for a car of any price in that era, let alone one of the cheapest cars on the planet. While colours and detail specifications were modified in the ensuing 42 years, the biggest mechanical change was the addition of front disc brakes in 1981 (from the discontinued Citroën Dyane), for the 1982 model year.

Features of the 1948 2CV

• unusual four-wheel independent suspension, front and rear wheels which connected the front and rear suspension on each side
• leading arm front suspension
• trailing arm rear suspension
• rear fender skirts, but the suspension design allowed wheel change without removing the skirts / rear wings
• front-wheel drive
• inboard front brakes, in order to help lower unsprung weight thus making ride even softer
• Four-wheel hydraulic brakes, (British Austin economy cars of the time only had hydraulic front brakes, the rears were by mechanical linkage)
• small, lightweight, 9HP air-cooled flat twin engine, (with overhead valves when side valves were still common), mounted very low in front of the front wheels for stability
• 4-speed manual transmission, (when three speeds were common) with an unusual dashboard push/pull/twist linkage
• bolt-on detachable front and rear wings/fenders
• detachable doors, bonnet (and boot lid after 1960), by "slide out" P profile sheet metal hinges
• front rear-hinged "suicide doors"
• flap-up windows, as roll up windows were considered too heavy and expensive.
• detachable full length fabric sunroof and boot lid, for almost pickup-truck-like load carrying versatility
• ventilation in addition to the sunroof and front flap windows was provided by an opening flap situated underneath the windscreen.
• rack and pinion steering mounted inside the front suspension cross-tube, well behind the front wheels, away from a frontal impact
• load adjustable headlights.
• a heater (heaters were standardised on British economy cars in the 1960s)

Body

The body was constructed of a dual H-frame platform chassis and aircraft-style tube framework, and a very thin steel shell that was bolted to the chassis. Because the original design brief called for a low speed car, little or no attention was paid to aerodynamics. The result was that the body had a drag coefficient (Cd) of a high 0.51.

Suspension

The suspension of the 2CV was almost comically soft; a person could easily rock the car side to side dramatically (back and forth was quite a bit more resistant). The leading arm / trailing arm swinging arm, fore-aft linked suspension system together with inboard front brakes had a much smaller unsprung weight than existing coil spring or leaf spring designs. It was designed by Marcel Chinon.

• The system comprises two suspension cylinders mounted horizontally on each side of the platform chassis. Inside the cylinders are two springs, one for each wheel, mounted at each end of the cylinder. The springs are connected to the front leading swinging arm and rear trailing swinging arm, that act like bellcranks by pull rods (tie rods). These are connected to spring seating cups in the middle of the cylinder, each spring being compressed independently, against the ends of the cylinder.
• If each cylinder was rigidly mounted to the chassis, it would provide fully independent suspension, but it is not rigidly mounted. It is mounted using an additional set of springs, originally made from steel, called "volute" springs (that are visible on the ends of cylinder in the external linked drawing above), but on later models made from rubber. These springs allow the front and rear suspension to interconnect.

• When the front wheel is deflected up over a bump, the front pull rod compresses the front spring inside the cylinder, against the front of the cylinder. This also compresses the front "volute" spring pulling the whole cylinder forwards. That action pushes the rear wheel down on the same side via the rear spring assembly and pull rod. When the rear wheel meets that bump a moment later, it does the same in reverse, keeping the car level front to rear. When both springs are compressed on one side when travelling around a bend, or front and rear wheels hit bumps simultaneously, the equal and opposite forces applied to the front and rear spring assemblies reduce the interconnection significantly, or even completely. This stiffens the suspension after a certain amount of body roll has been achieved. It allows the 2CV to have very soft "bump mode" absorption, without wallow or uncontrolled float.
• It reduces pitching, which is a particular problem of soft car suspension.
• At high angles of body roll, the swinging arms that are mounted with large bearings to "cross tubes" that run side to side across the chassis; combined with the effects of all-independent soft springing and excellent damping, keeps the road wheels in contact with the road surface and parallel to each other across the axles. A larger than conventional steering castor angle, ensures that the front wheels are closer to vertical than the rears, when cornering hard with a lot of body roll. All this provides excellent road holding, while appearing to look like a softly sprung American car with poor handling and road holding because of poor body control.

• The other key factor in the quality of its road holding is the very low and forward centre of gravity, provided by the position of the engine and transmission.

• The suspension also automatically accommodates differing payloads in the car- with four people and cargo on board the wheelbase increases by around 4 cm (2 in) as the suspension deflects, and the castor angle of the front wheels increases by as much as 8 degrees thus ensuring that ride quality, handling and road holding is almost unaffected by the additional weight.

• On early cars friction dampers (like a dry version of a multi-plate clutch design) were fitted at the mountings of the front and rear swinging arms to the cross-tubes. Because the rear brakes were outboard, they had extra tuned mass dampers to damp wheel bounce from the extra unsprung mass. Later models had tuned mass dampers at the front (because the leading arm had more inertia and "bump/thump" than the trailing arm), with hydraulic telescopic dampers / shock absorbers front and rear. The uprated hydraulic damping obviated the need for the rear inertia dampers. (It should be noted that only dampers designed to be able to work horizontally should be used as replacements. Some that will physically fit do not work properly horizontally.)

• It was designed to be a comfortable ride by matching the frequencies encountered in human bipedal motion.

This sophisticated suspension design ensured the road wheels followed ground contours underneath them closely, while insulating the vehicle from shocks, enabling the 2CV to be driven over a ploughed field without breaking any eggs, as its design brief required. More importantly it could comfortably and safely drive at reasonable speed, along the ill-maintained and war-damaged post-war French Routes Nationales. It was commonly driven "Pied au Plancher"—"foot to the floor" by their peasant owners.

The 2CV suspension and vehicle dynamics was assessed by Alec Issigonis and Alex Moulton in the mid-1950s (according to an interview by Moulton with CAR magazine in the late 1990s); this inspired them to design the Hydrolastic suspension system for the Mini and Austin 1100, to try to keep the benefits of the 2CV system but with added roll stiffness in a simplified design.

Front-wheel drive

Front-wheel drive made the car easy and safe to drive and Citroën had developed expertise with it due to the pioneering Traction Avant, which was the first mass-produced steel monocoque front-wheel-drive car in the world. The 2CV was originally equipped with a sliding splined joint, and twin Hookes type universal joints on its driveshafts; later models used constant velocity joints and a sliding splined joint.

Gearbox

The gearbox was a 4-speed manual transmission, an advanced feature on an inexpensive car at the time. Boulanger had originally insisted on no more than three gears, because he believed that with four ratios the car would be perceived as complex to drive by customers. Thus, the fourth gear was marketed as an overdrive, this is why on the early cars the "4" was replaced by "S" for surmultipliée. The gear shifter came horizontally out of the dashboard with the handle curved upwards. It had a strange shift pattern: the first was back on the left, the second and third were inline, and the fourth (or the S) could be engaged only by turning the lever to the right from the third. Reverse was opposite first. Although this may seem an odd layout, it is in fact logical. The idea is to put most used gears opposite each other: for parking, first and reverse; for normal driving, second and third. This layout was adopted from the H-van's 3-speed gearbox.

The windscreen wipers were powered by a purely mechanical system: a cable connected to the transmission; to reduce cost, this cable also powered the speedometer. The wipers' speed was therefore dependent on car speed. When the car was waiting at a crossroad, the wipers were not powered; thus, a handle under the speedometer allowed them to be operated by hand. Although this system was far from perfect, it was better than some 1950s British Ford economy cars that had wipers powered by inlet manifold vacuum that ran at full speed at engine idle but slowed down to a crawl when cruising at speed. From 1962, the wipers were powered by a single-speed electric motor. The car came with only a speedometer and an ammeter.

The reliability of the car was increased by the fact that, being air-cooled (with an oil cooler), it had no coolant, radiator, water pump or thermostat. It had no distributor either, just a contact breaker system. Except for the all hydraulic brakes, there were no hydraulic parts on original models as damping was by tuned mass dampers and friction dampers. On later models the mass dampers and friction dampers were replaced by conventional shock absorbers.

Early models used a combination of steel pipes and flexible rubber hoses in the braking system. Later 2CV used only steel pipe in the hydraulic braking system; no flexible rubber hoses were used. The front inboard brakes were fixed to the gearbox and did not move with the wheels, while the rear brake pipe was coiled multiple times around the rear trailing-arm mounting tube to absorb suspension movement. This allowed cheaper and lighter assembly, greater reliability and a solid feel at the brake pedal.

Engines

The engine was designed by Walter Becchia and Lucien Gerard, with a nod to the classic "boxer" BMW motorcycle engine (it is reported that Becchia dismantled the engine of the BMW motorcycle of Flaminio Bertoni before designing the 2CV engine). It was an air-cooled, flat-twin, four-stroke, 375 cc engine with pushrod operated overhead valves and a hemispherical combustion chamber. The notoriously underpowered earliest model developed only 9 bhp DIN (6.5 kW). A 425 cc engine was introduced in 1955, followed in 1968 by a 602 cc one giving 28 bhp (21 kW) at 7,000 rpm. With the 602 cc engine, the tax classification of the car changed so that it became in fact a 3CV, but the commercial name remained unchanged. A 435 cc engine was introduced at the same time in replacement of the 425 cc; the 435 cc engine car was christened 2CV 4 while the 602 cc took the name 2CV 6 (although a variant did take the name 3CV in Argentina). The 602 cc engine evolved to the M28 33 bhp (25 kW) in 1970; this was the most powerful engine fitted to the 2CV. A new 602 cc giving only 29 bhp (22 kW) at a slower 5,750 rpm was introduced in 1979. Despite being less powerful, this engine was more efficient, allowing lower fuel consumption and better top speed, at the price of decreased acceleration. All 2CVs with the M28 engine can run on unleaded petrol, but attention is needed to ensure that valve clearances are maintained. Although there were not any more powerful engines for this model, Citroen used the same engine design on other cars, like the AMI, the LN, the Dyane. The slightly increased capacity 652cc mapped electronic ignition version in the Visa was significantly different. They had a bit more power, and many 2CV owners installed those engines in their car for more flexibility. The final development of the engine was within the PSA Peugeot-Citroën / Renault / French government ECO 2000 project, first prototype SA103 of March 1982. It was watercooled and increased to 704cc. Later prototypes used a three cylinder Fiat FIRE engine instead, which was then being developed in conjunction with PSA.

The 2CV used the wasted spark ignition system for both simplicity and reliability and had only speed-controlled ignition timing, no vacuum advance taking account of engine load.

Unlike other air-cooled cars (such as the Volkswagen Beetle and the Fiat 500) the 2CV's engine had (for simplicity and reliability) no thermostat valve fitted to its oil system to allow the oil to reach normal operating temperature quickly in cold weather. All the oil in the system passed through an oil cooler mounted behind the fan and received the full cooling effect regardless of the ambient temperature. This removes the risk of overheating from a jammed thermostat that can afflict water- and air-cooled engines and the engine can withstand many hours of running under heavy load at high engine speeds even in hot weather. To prevent the engine running cool in cold weather (and to improve the output of the cabin heater) all 2CVs were supplied with a grille blinds (canvas on early cars and a clip-on plastic item called a 'muff' in the owner's handbook, on later ones) which blocked around half the grille aperture to reduce the flow of cool air to the engine.

The engine's design concentrated on the reduction of moving parts. The cooling fan and dynamo were built integrally with the one-piece crankshaft, removing the need for drive belts. (Late models (shown in photo) used an alternator mounted high above the engine, to keep it dry, run with a drive belt). The crankshaft was a "built-up" design similar to that used in many motorcycle engines. In place of the split big ends and two-piece big-end bearings commonly used in car engines, the 2CV engine used connecting rods with one-piece big-ends and bearings, which were fitted to the crankpins before the crank was assembled. The crankpins were then hydraulically pressed into the webs after being chilled with liquid nitrogen to cause them to contract. The entire unit (crank, big-end bearings and connecting rods was then fitted to the engine. The camshaft drive gears incorporate a spring-loaded split gear, to reduce the effects of gear wear and backlash on valve timing and ignition timing. With the contact breaker in a housing on the end of the crankshaft there was no separate jackshaft to be affected by chain or gear wear and associated backlash.The use of gaskets, seen as another potential weak point for failure and leaks, was also kept to a minimum. The cylinder heads are mated to the cylinder barrels by a lapped joints with extremely fine tolerances as are the two halves of the crankcase and other surface-to-surface joints.

As well as the close tolerances between parts the engine's lack of gaskets was made possible by a unique crankcase ventilation system. On any 2-cylinder boxer engine such as the 2CV's, the volume of the crankcase reduces by the cubic capacity of the engine (375 to 602cc in the Citroen's case) when the pistons move together. This, combined with the inevitable small amount of 'leakage' of combustion gases past the pistons leads to a positive pressure in the crankcase which must be removed in the interests of engine efficiency and to prevent oil and gas leaks as the pressure tries to escape. The 2CV's engine has a combined engine 'breather' and oil filler assembly which contains a series of rubber reed valves. These allow positive pressure to escape the crankcase (to the engine air intake to be recirculated) but which close when the pressure in the crankcase drops as the pistons move apart. Because gases are expelled but not admitted this creates a slight vacuum in the crankcase so that any weak joint or failed seal causes air to be sucked in rather than allowing oil to leak out. Since the oil serves both as the engine's lubricant and forms a vital part of the cooling system this 'anti leak' system was especially important.

These design features made the 2CV engine highly reliable; test engines were run at full speed for 1000 hours at a time, equivalent to driving 80,000 km (50,000 mi) at full throttle. They also meant that the engine was very much "sealed for life"—for example, replacing the big-end bearings required specialised equipment to dismantle and reassemble the built-up crankshaft, and as this was often not available the entire crankshaft had to be replaced. However, the engine is very under-stressed and long-lived, so this is not a major issue. Until the 1960s it was common for other car manufacturers' engines to need full strip downs and rebuilds at as little as 80,000 km (50,000 mi) intervals; un-rebuilt 2CV engines are still running that are passing 400,000 km (250,000 mi).

If the starter motor or battery failed, the 2CV had the option of hand-cranking, the jack handle serving as starting handle through dogs on the front of the crankshaft at the centre of the fan. This feature, once universal on cars and still common in 1948 when the 2CV was introduced, was kept until the end of production in 1990. The jack handle also served as the wheelbrace (lug wrench) and could be used to remove the nuts that held the front wings (fenders) on—part of the car's design to facilitate easy maintenance.

When asked about the 2CVs performance and acceleration, many owners said it went "from 0–60 in one day". Others jokingly said they "had to make an appointment to merge onto an interstate highway system".Performance

The original 1948 model that produced only 9 hp had a 0-40 time of 42.4 seconds and a top speed of just 64 km/h (40 mph), far below the speeds necessary for North American highways or the German Autobahns of the day. The top speed increased with engine size to 80 km/h (49 mph) in 1955, 84 km/h (52 mph) in 1962, 100 km/h (63 mph) in 1970, but was finally not capable of US freeway speeds of 115 km/h (71 mph) until 1981.

The last evolution of the 2CV engine was the Citroën Visa flat-2, a 652 cc featuring electronic ignition. Citroën never sold this engine in the 2CV, but some enthusiasts have converted their 2CVs to 652 engines, or even transplanted Citroën GS or GSA flat 4 engines and gearboxes. Cars with the flat-4 engines and subtle bodywork changes so they appear to be standard are known as "Sidewinders" in the UK.

In the mid-1980s CAR magazine editor Steve Cropley ran and reported on a turbocharged 602 cc 2CV that was developed by engineer Richard Wilsher.