New generation smart fortwo and forfour offer new drive systems, suspension

4 November 2014

The new generation smart forfour and fortwo. Click to enlarge.

The new generation smart fortwo and forfour city cars (earlier post), developed in cooperation with Renault, offer a new range of drive systems: three three-cylinder engines rated at 45 kW/60 hp, 52 kW/71 hp and 66 kW/90 hp, coupled with a five-speed manual transmission or the twinamic dual clutch transmission.

The two most powerful engines will be available for the launch on the market in Europe on 22 November, and the basic gasoline engine will follow a few months after the market introduction. The smart forfour now also features the rear-mounted engine design of the fortwo.

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The naturally aspirated engine generates 52 kW/71 hp from a capacity of 999 cc, withmaximum torque of 91 N·m (67 lb-ft) at 2850 rpm. Top of the range is the 898 cc turbo engine rated at 66 kW/90 hp, with maximum torque of 135 N·m (100 lb-ft) at 2,500 rpm.

smart fortwo and forfour engine. Click to enlarge.

All engines share an aluminium construction, overhead camshafts and four-valve cylinder heads. Internal friction in the engine is reduced by optimized pistons with graphite-coated piston shafts, a friction-optimized timing chain and coated bucket tappets. Further features include the low height and installation at an angle of 49 degrees.

Both engines have an infinitely variable oil pump, which only brings as much lubricant to the engine as necessary.

Heat management ensures that the engines reach the optimum operating temperature more quickly and therefore contributes to lower emissions and less fuel consumption. An electromagnetic valve in the coolant circuit is a key element. As long as the coolant temperature is below 80 ˚C, the valve stays closed and prevents circulation of the coolant in the engine block and the cylinder block. This therefore heats up more quickly. As soon as a suitable temperature is reached for energy-efficient operation, the valve opens so that the coolant can flow. The oil around the cylinders also heats up more quickly.

The automatic start/stop system, a standard feature, enables greater efficiency especially in city traffic. As soon as the driver brakes, shifts to idle position and releases the clutch as the vehicle comes to a standstill, the engine automatically shuts down. The engine spontaneously restarts in a way that is barely perceptible when the driver operates the clutch to shift into first gear.

The driver can deactivate the start/stop system when required. If certain conditions are not met the system will not shut down the engine—for example if the necessary operating temperature for optimum emission control or the desired interior temperature have yet to be reached.

The turbocharger of the more powerful smart engine features an electronically controlled wastegate which enables increased engine dynamics in conjunction with a sustained reduction in fuel consumption. The wastegate reduces the exhaust gas backpressure at high revs.

In addition to the turbocharger, a range of other technical measures also contributes to the excellent fuel efficiency and performance. These include variable adjustment of the inlet valve timing.

Another trait of the all-aluminium three-cylinder engine is the fully integrated intercooler in the plastic intake module.

Transmissions. The five-speed manual transmission offers sporty and precise gear changing. It has an integrated differential, friction-optimized gear sets, a dry weight of just 35 kg (77 lbs) and is actuated via cables.

The twinamic dual clutch transmission is constructed as a 3-shaft dual clutch manual transmission and has six forward gears and one reverse gear. Both clutch actuation and gear changing are fully automatic and enable particularly smooth shifting without any interruption in tractive power. The transmission either works fully automatically, or it can be shifted manually with a selector lever in a separate gate or via the shift paddles included in the Sports package.

With the two sub-transmissions, changing to the next highest or lowest gear is immediate and without loss of tractive power. If required, the electronic controller skips individual gears instead of changing down through each individual gear. It therefore offers the comfort of an automatic transmission combined with the efficiency of a manual transmission.

Suspension. A new front axle design adopting elements of the previous Mercedes-Benz C-Class, an optimized De Dion rear axle (a rigid drive axle often used in classic sports cars where the differential is fitted separately from the axle), substantially increased spring travel all round, tires with higher side walls and an unprecedentedly small turning circle are the highlights of the new chassis.

smart forfour undercarriage. Click to enlarge.

The chassis and suspension of the new smart generation have evolved on the basis of the experience acquired with previous smart models and have been designed for markedly enhanced ride comfort.

The new smarts have essentially been configured for gently understeering handling characteristics. The different tire sizes at front and rear have thus been retained, whereby the narrower front tires in conjunction with a high outside wheel turn angle contribute to the small turning circle. ABS, ASR and ESP are ready to intervene at the critical limits of road-holding. A hill-hold function and brake-assist are integrated in the brake system. With crosswind assist as a standard feature, the latest generation ESP assists the driver even when there are sudden gusts of wind from the left or the right.

For the front axle, the engineers designed a new McPherson suspension; each wheel is controlled by a wishbone under the wheel center, the McPherson strut and a tie rod. Comfort and guide bearings for the wishbones provide for added comfort, along with the markedly increased overall spring travel (160 mm/plus 30 mm). During the compression phase in particular, the new smart runs noticeably more smoothly over uneven surfaces than previously.

Like mid-size cars, the supporting bearing of the McPherson strut now has three paths: there is separate application of force from shock absorbers, springs and auxiliary springs. The shock absorber is supported on the inner elastomer bearing that is perfectly configured to the response of the shock absorber. The auxiliary spring designed for comfort with a progressive characteristic is supported by the bearing housing on the body, while the force from the coil springs is transferred to the body via the ball bearing. Elastomer shims for the springs provide for noise decoupling.

A further new feature is a front axle integral support designed as a third crash level.
Further features of the strut are the weight-optimized design with a hollow piston rod and a single-shell head bearing (opposite contour through bodyshell). The tubular stabiliser bar is connected to the struts with a kinematically optimized torsion bar. The tubular torsion bar is fastened to the body with vulcanized rubber bearings without any bearing friction. This improves the response of the suspension and reliably prevents disturbing noise.

The new smart models feature a De Dion rear axle, which has been modified to substantially increase comfort. The advantage of this construction principle is the high instantaneous center (roll center) required for driving stability coupled with the space available for the drive system. The center bearing connecting the engine and differential to the body is a new development.

New elements are the twin-tube shock absorbers and separate barrel-shaped helical springs. Two elastomer shims provide for effective noise decoupling of the helical springs between body and rear axle. A pin bearing connects the shock absorber to the body. Its soft gimbal mounting contributes to minimizing friction in the shock absorber and thus improves responsiveness.

The position and the ratios of the shock absorbers offer a good compromise between roll damping and pitch damping for the suspension setup. In addition, the angled position of the shock absorbers achieves longitudinal damping of the axle. As on the front axle, the damping characteristic is degressive.

All smart models feature the Direct-Steer system with variable ratio and electric power assistance as standard. With electric power assistance, shorter steering ratios are possible at a medium steering angle and the driver’s steering commands are thus realized more directly. Further advantages of the electric power steering are active backward movement to improve the steering alignment, damping control to prevent steering wheel overshoot, and the greater efficiency compared with hydraulic power assistance because the steering assist only needs energy when the driver is actually steering.

Because there is no engine in the way between the front wheels thanks to the smart’s rear engine concept, at 51 degrees it was possible to make the steering angle of the two smart models exceptionally large. With its extremely compact exterior dimensions the smart fortwo therefore has an unparalleled small turning circle of 6.95 meters from curb to curb—more than a meter less than the previous fortwo. The forfour is also extremely maneuverable and its turning circle of 8.65 meters from curb to curb is only slightly larger than that of the previous fortwo.

The new smart is equipped with a hydraulic dual circuit brake system with X‑arrangement. One of each of the diagonally opposite wheels on the front and rear axle is supplied by a circuit. The main brake cylinder has an aluminium construction and a classic position on the bulkhead.