If you look at a typical automobile, you'll notice that the engine, suspension system, axles and steering column take up a great deal of space. Creating a smaller car means either creating smaller versions of these components or creating new mechanisms that provide the same functions. The team of designers at MIT Media Lab Smart Cities group chose to solve this problem by putting the responsibility of all of these components into what they call robot wheels.
Each of the vehicle's four wheel assemblies is a self-contained mobile unit, supplying its own motor, suspension, brakes and steering. Each robot wheel has only two outside needs: electricity (supplied by the car's battery) and digital data to tell the wheels what to do. In a traditional car, the steering wheel physically turns a steering gear, which in turn directs the front wheels in the chosen direction. The City Car, however, will use a drive-by-wire system. The steering wheel won't physically turn anything -- it'll just send the information to the self-steering wheel assemblies.
The robot wheels implement two particularly innovative technologies.
- Hubless design: If you look at a conventional wheel, you might wonder where City Car designers plan to put all of these components. The answer is inside the wheel itself. Traditional wheels have a hub in the center and spin on a central axis. Hubless wheels are open in the middle -- no spokes, no hubcaps. The inside surface of the wheel is basically an inverted gear, with the teeth pointing in towards the center. A smaller system of gears spins the main wheel -- not unlike a hamster running at the bottom of a hamster wheel. The design most frequently appears in custom motorcycle designs.
- In-wheel suspension: The job of a car's suspension system is to maximize the contact between the tires and the road, provide steering stability and ensure passenger comfort. Normally, designers place shock absorbers between the wheel assembly and the vehicle frame. The City Car design calls for the force to be absorbed between the wheel assembly and the actual wheel. There wouldn't be room for this arrangement on a typical wheel design, but the hubless design allows for the shock absorbers to be mounted inside the center of the wheel. For more information on suspension systems, read How Car Suspensions Work.
The robot wheels would work in unison, allowing the car to rotate 360 degrees [source: Mack]. This provides an incredible degree of maneuverability, such as allowing drivers to parallel park by simply driving sideways.
There's more to the City Car than amazing wheels and a folding axis. On the next page, we'll look at the ways in which its designers hope to protect drivers, allow customization and even enable vehicles to talk to one another.