December 9, 2022
How does a fully steerable chassis work?

The conquest of a significant part of the market by all-wheel drive crossovers happened quite recently, but we all managed to learn that all-wheel drive cars are good. Versatile, reliable, passable, safe. It would seem, what else can you think of with the chassis, which was not there before?

It turns out that they came up with – not only all-wheel drive, but also a fully controlled chassis. That is, the car turns not only the front, but also the rear wheels. How does it work and what is it for?

In fairness, we note that the “new product” is almost a century old – the rear wheels turned even on some pre-war tractors and army all-terrain vehicles. Of course, this was done, as now, to improve maneuverability. And today the task of reducing the turning radius of long cars is just as relevant, although this is not the only thing. The Japanese were the first in the mass segment. In the 1980s, their systems gave controversial results, as they used conventional front wheel steering racks, but mounted back. The main problem was to link the work of the two rails, since it was impossible to connect them mechanically. Complex and extremely unreliable electro-hydraulics were used. Everything changed with the advent of electrically controlled steering mechanisms. Wired to the “brain” of the car, which is responsible for the rotation of the wheels, stabilization and handling, they were able to work selectively and independently of the steering wheel, which, in fact, was required.

Examples are the Audi Q7 and Mercedes-Benz GL. Of the exotic, let’s remember the giant Toyota Mega Cruiser – that’s who exactly needs it! Such a chassis works for most models according to the same principle. At low speeds, up to about 15-20 km / h, the rear wheels turn in the direction opposite to the front ones. This gives a serious (about a meter) reduction in the turning radius and additionally saves wheel drives, since each of them can now work with a smaller angle. The maximum amount of rotation of the rear wheels is small – only about 5 degrees, but this is enough for, for example, the bulky Audi Q7 to easily perform transverse parking.

At high speeds, the rear wheels turn in the same direction as the front wheels. Here the task is different – to save the car from wobbling during rebuilding. However, there is another plus – difficult turns. Everything is clear with rebuilding, when changing a row, the machine does not receive rotation about the vertical axis, but only shifts to the side. No spin – less skidding and no difference in front and rear traction. How about in corners?

The fact is that both the front and rear wheels in a turn receive a pull – a slight side slip with tire breaking. And if the withdrawal of the front wheels can be compensated by the steering wheel, then for the rear wheels it remains only to calculate in advance the speed and grip limit. But if the rear wheels are able to steer, the mechanism is able to compensate for this. The result is a noticeable improvement in unassisted traction, reduced ESP impact, and less rear tire wear for those accustomed to cornering at high speeds.

Of course, in such situations, the stability of the machine is entirely in the hands of the electronic program. It must recognize until when to keep the rear wheels turned in the direction of the turn (for example, when driving on a ramp), when to take a neutral position, and at what point to start helping the car turn in an arc. The intricacies of the programs are handled by test engineers, and today not every driver, in principle, is able to recognize that his car has taxied the rear wheels.

Technically, the rear thruster resembles an electric steering rack, only more powerful and short-stroke. Instead of the usual steering rods, transverse levers are attached to the ends of the mechanism (in fact, they also work as rods). Since the offset is small, the mechanism is not as bulky as a rack and fits behind a geared differential on the rear subframe. Electrical systems have another plus: in the event of their failure, the car simply becomes ordinary – not fully controlled.

Rear steered wheels have become such a mature technology that they are already found not only on premium cars, but also go down a couple of classes below. An example is the latest Renault Astral hybrid crossover.