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| The smart fortwo cdi: big things come in small packages (August 10, 2004) |
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smart, along with Mercedes-Benz and Maybach, are brands of the Mercedes Car Group of DaimlerChrysler AG, a company that has always been a leader in automotive technology and safety. This heritage has been passed along in the smart fortwo cdi coupé and cabriolet which have been developed since 1994 to provide a fun, comfortable, affordable and safe mode of transportation for the millions of people who seldom find the need for a back seat.
Research has shown that, on average, a car carries only 1.2 occupants to work. By these standards, the smart fortwo cdi is downright generous with comfortable room for two -- including all of their stuff. There are some bonuses to this design since, at just 2.5 metres long, smart owners have no trouble finding a parking spot. In fact, two smart fortwos can fit in a single standard parking spot.
Body structure: the tridion safety cell
The most important consideration for a vehicle of this size is safety and in the development of the smart fortwo, occupant safety was given top priority from day one. In a collision, the tridion safety cell acts like a shield to protect both driver and passenger. The rigid steel frame, reinforced with high-strength solid steel at important stress points, absorbs much of the impact energy. The cell's transverse and lateral struts also transfer some impact energy to the crumple zones of the other vehicle involved in the collision.
Whereas the "hard shell" protects the occupant space, energy-absorbing safety features reduce the force applied on the occupants. These consist of short, but nonetheless extremely efficient deformation zones at the front and rear of the car.
In addition, even with the smart fortwo's compact design, the driver and passenger sit well back in the vehicle, distanced from the effects of a front impact.
smart restraint systems
In a severe impact, Emergency Tensioning Devices (ETD) automatically tighten the seat-belts in just a few milliseconds, ensuring that both the driver and passenger remain firmly secure in their seat. In this way, the occupants "take part" in the deceleration of the vehicle earlier and secondary impact forces are reduced. Then, in milliseconds, a belt force limiter controls the restraining action, reducing the risk of chest and shoulder injuries. In addition, the steering column collapses, absorbing energy and greatly increasing protection for the driver.
Sometimes small is safer
If another car hits a smart forwo cdi from the side, the compactness of the smart fortwo cdi coupé and cabriolet is a great advantage. Thanks to the ultra-short wheelbase, the other car will almost always hit a wheel with the suspension behind it, providing excellent protection against penetrating the occupant area.
Standard air bags
The smart fortwo cdi coupé and cabriolet also come equipped with standard full-size frontal air bags for both the driver and passenger as well as side airbags that provide protection against side impacts.
World class brakes
It goes without saying that the best way to avoid an accident is to stop before contact is made. The smart fortwo cdi has world-class stopping power, using a front disc/rear drum configuration with a standard electronic four channel anti-lock braking system (ABS), a system first developed by Mercedes-Benz in 1978. The braking system also incorporates Electronic Brake Distribution (EBD) and Brake Assist (BAS).
Electronic Stability Program
The DaimlerChrysler AG relationship has yielded many other benefits, not the least of which is the Mercedes-Benz-developed Electronic Stability Program (ESP), which is standard in all smart vehicles and offers dramatic handling advantages. ESP® enhances driver control and helps maintain directional stability in turns as well as when driving straight ahead, including driving over uneven surfaces and even patchy snow, ice or gravel. The straight-line traction control function is integrated into ESP, which uses a steering angle sensor, speed sensors at each wheel, sensors for lateral acceleration (or lateral g-force) and vehicle yaw (the vehicles rotation on a vertical axis), and computer logic to calculate the path being steered versus the car's actual path.
If there is a discernable difference between what the driver asks (through steering) and the vehicle's path, ESP applies selective braking (braking one wheel, front or rear, left or right) to put the car back onto the driver's intended path. ESP measures the tendency toward understeer (when the front resists steering changes causing "plowing") or oversteer (when the rear end "fishtails"). When it senses understeer, ESP increases brake pressure to the inside rear wheel, while in oversteering, it increases brake pressure to the outside front wheel. ESP is effective under acceleration, braking or coasting.
The smart fortwo cdi is the only micro-compact car to offer ESP or a similar and equally effective sytem as a standard feature.
Outstanding results in all crash tests
It sounds like a bold statement to say that smart's unique crash management system makes the smart fortwo one of the safest cars in the micro-compact car segment -- but it is true. The smart fortwo's superior passive safety has been confirmed by more than 50 crash tests carried out in accordance with the same standards employed by Mercedes-Benz, as well as several independent accident simulations. The crash tests included the following:
- frontal crash at 65 km/h against a deformable barrier;
- rear-end crash at 55 km/h;
- side impact at 50 km/h;
- car-to-car crash where a smart fortwo and a Mercedes-Benz E-Class (which is twice as heavy) collided at a speed of 50 km/h in an offset frontal collision.
In every test, the strength and rigidity designed into the tridion safety cell provided greater occupant safety than in other small cars and the deformation values measured were below all internationally recognized standards for all vehicles and in some cases the results for smart were significantly below these standards. This is particularly true in the case of a collision between the smart and a larger and heavier vehicle -- for example a collision with a Mercedes-Benz E-Class sedan.
The smart fortwo proved what it is capable of withstanding in the following tests:
In 1998, the German motoring magazine AutoBild carried out a crash test on five small cars using the Euro NCAP method travelling into a deformable barrier at 64 km/h with a maximum vehicle deceleration of 32.6 g, concluding that passengers travelling in a smart fortwo would be by far the safest. The passenger cell was virtually unscathed with the A-pillar being displaced by only 20 mm. By comparison, other cars included in the same test had A-pillar displacement of between 25 and 61 mm -- up to three times that of the smart. The smart fortwo achieved the best results in all relevant areas of the car and showed a clear overall advantage over the competitive cars involved, as indicated in the following chart.
Reduction in footwell size (%)
smart fortwo: 21
other cars: 26 to 39
Instrument panel penetration (mm)
smart fortwo: 13
other cars: 87 to 140
Pedal penetration (mm)
smart fortwo: 15
other cars: 85 to 186
Driver door opening force (Newtons)
smart fortwo: 226
other cars: >500 to *
Steering column displacement(mm)**
smart fortwo: -84 / 18
other cars: 59-91 / 45-118
* driver's door opened on impact
** horizontal / vertical
In this comparative test, even the windshield of the smart remained intact which might seem surprising given the short deformation zone of the car. This is due to the extraordinary strength of the tridion safety cell.
Other crash tests provide further testimony
In 1999, the Auto, Motor und Sport motoring magazine (edition 19/99) carried out a head-on crash test between a Mercedes-Benz S-Class and a smart fortwo, with both vehicles travelling at 50 km/h. They concluded that "Despite its small size, the smart offers its occupants great safety potential."
In June, 2003, the experts from DEKRA put a smart fortwo up against another small car in a head-on crash test, with both vehicles travelling at 50 km/h. The result was that while the passenger cell of the other car collapsed completely, the smart fortwo's rigid tridion safety cell remained intact. The smart fortwo driver, they concluded, would have survived with only minor injuries.
Crash tests also show low application of force on occupants
In a frontal impact, the extraordinary strength of the tridion safety cell inevitably leads to high vehicle deceleration, however, tests have shown that the forces transferred to the occupants is very much reduced. Dangerous load peaks are avoided by the integrated timing and interaction of the seat belts, airbags and deformable steering column. Low head injury criteria values (driver 306, passenger 387, versus limit value of 1000), which measure the strain on the brain, and totally uncritical head acceleration values of 51g for the driver and 50 g for the passenger (versus limit value of 80 g) clearly demonstrate this.
The load on the thorax is also comparatively low. The corresponding values in the smart fortwo are just 41 g / 35 g leading to chest compression of 19 mm for the driver and 30 mm for the passenger. In other cars, values of up to 44 mm were measured on the crash test dummy driver.
Energy absorbing padding in the foot area, knee protection on the cockpit cross member, special safety seats with integral seat belts with optimum belt guide, and the standard fast-triggering side airbags further reduce the risk of occupant injury.
Brake Assist
Another safety feature of the smart fortwo is the Brake Assist function (BAS). Research has shown that in an emergency situation, drivers tend not to brake hard enough or soon enough. Brake Assist senses panic stopping by the speed at which the driver depresses the brake pedal and automatically applies all available braking boost immediately. By helping to eliminate the delay caused by human nature, Brake Assist can potentially reduce stopping distances when it matters most.
Cornering Brake Control
The ability of ESP to act on individual wheel brakes is the basis for a further safety function of ABS - Cornering Brake Control. During extreme cornering, the engine load on the wheel which is located on the inside during cornering is reduced and the cornering brake control function ensures that this wheel does not lock. If the driver steps on the brake in such a situation, ESP senses that more braking power can be transferred from the other wheels than from the wheel with a reduced load. In this way, unnecessary activation of ABS and the attendant pulsing of the brake pedal are avoided.
Hill Start Assist
To make starting on a hill as comfortable as in cars equipped with an automatic transmission and a torque converter, the smart fortwo cdi coupé and cabriolet have a Hill Start Assist function. After releasing the brake pedal when stationary, Hill Start Assist keeps the wheels braked for 0.7 seconds. This leaves the driver enough time to move his or her foot from the brake to the accelerator pedal without the car rolling forwards or backwards. If the driver accelerates, the brake pressure is immediately reduced. The fact that the car is prevented from rolling backwards additionally protects the clutch and makes it possible to drive off without the assistance of the hand brake, even on steep slopes. However, if the driver prefers to drive off in the conventional way with the help of the hand brake, the switch on the hand brake deactivates the Hill Start Assist.
The cdi turbodiesel
For motivation, the smart fortwo comes with a common-rail direct injection three-cylinder cdi turbocharged diesel engine that features low fuel consumption (just 4.2 litres combined over 100 km) and low emissions (90 g of carbon dioxide (CO2) per kilometre). The technology of the smart cdi engine, which is also used in the Mercedes-Benz E320's 6-cylinder diesel engine, provides ample power and torque for the smart fortwo cdi coupé and cabriolet.
Connecting the power to the wheels is a softip transmission which allows the driver to change gears without using the clutch. This sequential 6-speed transmission changes up or down with a gentle tap on the shift lever. First gear is automatically engaged when the car is stationary.
Attention to detail
While designing and building a vehicle that is one of the smallest cars available in the western world, it was important not to forget the little things. For example, the passenger and driver seats in the smart fortwo cdi are slightly staggered, ensuring that the driver has the same clear view out the passenger side as that available out the driver's side.
The interior is spacious, providing lots of elbow room, and the doors open wide making entry and exit as easy as can be. And for the transport of larger items, the back of the passenger seat folds down flat.
Talk about flexibility, the smart fortwo cdi comes with body panels made of scratch proof polycarbonate plastic that are available in a wide range of molded-in colours.
The panels are exchangeable, do not rust and withstand light bumps without denting. Changing panels is so easy, if a change in colour scheme is desired, one simply makes a trip to the smart centre and there, for a modest cost, the panels can be exchanged -- and the colours.
Air conditioning is, of course available, and if it is real fresh air that is wanted, the cabriolet offers an electric tritop convertible roof which allows the driver to decide just how "al fresco" he or she wants to be -- from just a sliver to a wide-open top.
If seeing the sky is fun but spoiling your hair do is not, a glass top is also available on the coupé.
The unmistakeable smart signature is visible in every detail. It is, after all, the small things in life that give the most pleasure.
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