Suspension: A focus on comfort

Nov 4, 2014
A new front axle design adopting elements of the previous Mercedes-Benz C-Class, an optimised De Dion rear axle, substantially increased spring travel all round, tyres with higher side walls and an unprecedentedly small turning circle are the highlights of the new chassis. 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 tyre sizes at front and rear have thus been retained, whereby the narrower front tyres in conjunction with a high outside wheel turn angle contribute to the unprecedentedly 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. Thanks to 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.
Front axle: McPherson strut with three load paths
The engineers have designed a new McPherson suspension for the front axle. Each wheel is controlled by a wishbone under the wheel centre, 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, whilst 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-optimised 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 optimised torsion bar. The tubular torsion bar is fastened to the body with vulcanised rubber bearings without any bearing friction. This improves the response of the suspension and reliably prevents disturbing noise.
The reduction of friction in the shock absorber seal-guide unit also serves to increase comfort. To this end the coil springs have a geometric design as "side-load" springs which compensate the effect of the contact-patch force on the transverse force in the seal-guide unit. This results in optimum shock absorber response.
The tyres also roll more softly thanks to their larger sidewall height (+14 mm). The standard size on the front axle is now 165/65 R15 (previously: 155/60 R15).
Rear axle: sophisticated construction in line with the De-Dion principle
The new smart models feature a sophisticated De-Dion rear axle, which has been modified to substantially increase comfort. The advantage of this construction principle is the high instantaneous centre (roll centre) required for driving stability coupled with the space available for the drive system (see "Under the microscope"). The centre 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 minimising 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.
The tyres at the rear also contribute to ride comfort. The new tyres measuring 185/60 R15 (previously: 175/55 R15) cushion road bumps more effectively thanks to the larger sidewall height (+5 mm). All tyre sizes available for smart come with an automatic tyre air pressure check as a standard feature.
Optionally available: sports suspension
The Sports suspension package is optionally available to those seeking sportier handling from their smart. The springs are firmer here and the vehicle is ten millimetres lower. The sports suspension is always combined with 16-inch wheels (otherwise available as an option) with tyre size 185/50 R16 at the front and 205/45 R16 at the rear. The sports suspension is a standard feature of the "proxy" equipment line.
Steering: smallest turning circle of any car
All smart models feature the Direct-Steer system with variable ratio and electric power assistance as standard, offering considerably enhanced driving enjoyment and even better handling. Thanks to the electric power assistance, shorter steering ratios are possible at a medium steering angle and the driver's steering commands are thus realised 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 metres from kerb to kerb – more than a metre less than the previous fortwo. The forfour is also extremely manoeuvrable and its turning circle of 8.65 metres from kerb to kerb is only slightly larger than that of the previous fortwo.
Even the basic version of the new smart models features a three-spoke steering wheel that is covered with plastic, leather or perforated leather (sports steering wheel) depending on the equipment. Cruise control is operated with buttons on the steering wheel as standard. As an option, buttons are available for operating the head unit and the telephone, as are shift paddles for the dual clutch transmission. The steering column is now optionally height-adjustable by +/-30 mm.
Brakes: adapted to the concept and engine
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.
The floating calliper disc brakes on the front axle are solid for the lighter vehicle variants (dimensions 259 x 12 mm); on the heavier vehicle variants they are internally ventilated (dimensions 258 x 22 mm). Drum brakes with vehicle-specific dimensions are mounted on the rear axle. Whilst the smart fortwo is equipped with 8-inch drum brakes, large 9-inch brake drums are fitted on the smart forfour. As before, a lever-type hand brake is used for reasons of comfort and space. The lever-type hand brake acts on the drum brakes on the rear axle.
The new smart fortwo 2014: The safety bodyshell of the smart fortwo
The new smart fortwo 2014: smart fortwo with highlighted tridion cell
The new smart fortwo 2014: smart fortwo tridion cell
The new smart fortwo 2014: smart fortwo bodyshell
The new smart fortwo 2014: smart fortwo undercarriage
The new smart forfour 2014: graph bodyshell
The new smart forfour 2014: smart forfour tridion cell
The new smart forfour 2014: undercarriage
The new smart forfour 2014 with highlighted tridion cell
The new smart forfour 2014: bodyshell