How Stackable Cars Will Work

The MIT Smart Cities group designed its City Car to fold and stack against other City Cars when not in use, much like shopping carts. See more small car pictures.
Franco Vairani/MIT Smart Cities group

W­hat will city ­­traffic be like in the future? Will we "drive" through the air? Will we bury our roadways underground? Or, will existing streets simply experience the same old congestion with newer, high-tech automobiles? Many designers insist that we don't just need to improve roads and automobiles, but rather completely rethink why we need cars and reinvent how they serve us.

­For all the convenience of owning a car, consider such negatives as cost, ­air pollution, congestion, limited parking and traffic accidents. While air pollution and high-speed wrecks have only become hazards of private transportation since the 20th century, commuters have dealt with traffic congestion since as early as the rule of Julius Caesar. And while technology has come a long way since then, advancements in overall planning haven't always kept up with the pace.


In many European cities, residents travel the same roads their medieval ancestors used. First, horses and carts clogged the streets, then gas-powered cars -- what difference will clean-running electric, solar-powered, or even flying vehicles make if our basic approach to transportation doesn't change?

­Today, designers and entrepreneurs have turned their attention to creating smaller, environmentally friendly vehicles and car-sharing programs. In 2003, the designers at the Massachusetts Institute of Technology Media Lab's Smart Cities group set out to take these concepts even further by designing an electric vehicle that's not only shareable, but also stackable. When a City Car isn't in use, it would collapse into a smaller size and stack against other City Cars, much like a row of shopping carts. In this manner, eight vehicles could fill the area of a parking space traditionally used for one standard-sized, conventional automobile.

To use this stackable car, all you'll need to do is swipe your credit card at the nearest City Car rack -- but don't go looking for one just yet as the City Car is still in the developmental phase. The concept, however, has already generated quite a bit of interest from automobile manufacturers, city governments and commuters. In this article, we'll look at just how a stackable car would work and the manner in which it might revolutionize the way we think about automobiles.


City Car: Reinventing the Automobile


In designing the City Car, the MIT Media Lab Smart Cities group set out to create a vehicle that wasn't the continuation of old designs, but rather a drastic rethinking of what city residents need from an automobile.

For many, public transportation and private automobile usage have long been mutually exclusive choices. Car-sharing programs that allow multiple individuals to share short-term use of a vehicle have changed this. Say you wanted to travel to a destination a few miles from the nearest train station. A car-sharing program would allow you to rent a vehicle at the station to complete your journey.


Existing car-sharing programs allow members to borrow a vehicle, use it, and then return it to its original location. Bicycle-sharing programs, however, typically allow residents to obtain a bicycle from one location, ride it to their destination, and leave the bike at a nearby drop-off point. The City Car design follows this second model, setting out to create a vehicle that can be checked out at one location (ideally near public transportation stations and major attractions) and returned at another. The vehicles wouldn't replace busses and trains, just fill in their geographic and logistic gaps.

A user would check a City Car out from the front of a rack, travel to his or her destination, and drop the vehicle off at the back of another City Car rack.
Franco Vairani/MIT Smart Cities group

Currently, designs call for the City Car's lightweight body to boast a folding axis and a simple pivoting mechanism to fold the vehicle into a vertical position. The lightweight cars would weigh about 1,000 to 1,200 pounds (454 to 544 kilograms) [source: Lombardi]. The vehicle could then be locked into place directly behind another City Car. These racks would be connected to the city's power grid, allowing stored cars to charge their lithium-ion batteries. Ideally, a vehicle would be fully charged by the time it cycles through to the front of the stack.

The City Car's frame is designed to fold in the middle, effectively cutting its already short length in half.­
Franco Vairani/MIT Smart Cities group

But where does the engine go? Amazingly, the design calls for the car's body to house only computer systems and a power source. The City Car's motor, suspension and even its steering are all located within its high-tech wheels.


Riding on Robot Wheels

City Car's design features special wheel assemblies, each containing its own engine, suspension, brakes and steering. See more small car pictures.
Franco Vairani/MIT Smart Cities group

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.

Small Car Image Gallery


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.
This Swiss motor wheel operated in a similar fashion to modern hubless wheels that are found on various concept vehicles. The outer wheel turns by the movement of a gear set in its interior.
Fox Photos/Hulton Archive/Getty Images
  • 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.


The Design of City Car

The City Car may feature such high-tech features as customizable paint jobs at the push of a button.
Franco Vairani/MIT Smart Cities group

You've just arrived at a train station and need to drive several blocks to a job interview in the city's financial district. You swipe your credit card at a City Car rack, wait for the front car to unfold, climb aboard the vehicle and enter your destination in the car's computer. A warning message flashes across the screen to tell you that other City Cars have encountered delays in that part of town due to road work. The vehicle runs a few computations and suggests an alternate route.

This is just one example of how the City Car's onboard computer, driver interface and inter-vehicle communication system may one day help make drivers' lives a little easier. With access to the latest maps and traffic information (similar to a GPS system) and constantly updated information from other City Cars, the vehicle's computer system could be the equivalent of riding with a veteran taxi driver.


This technology could also play a role in protecting the driver from harm. The vehicle could keep track of other City Cars and prevent accidents by warning if one vehicle's coordinates come too close to another's. The City Car design team also hopes to implement other groundbreaking safety features, such as soft, robotic "fingers" that fold up from the sides of seats to clutch drivers in the event of an accident. Revolutionary "liquid" materials may also play a crucial role. These materials would allow the cabin to change from a hard structure to a soft, shock-absorbent consistency upon impact during an accident. For more information on this technology, read How Liquid Body Armor Works.

From clever bumper stickers to custom vinyl, vehicle owners put a great deal of personality into their cars. While the idea of shared cars may seem like the complete opposite of individuality on the road, City Car designers hope to allow drivers to express themselves in a very unique way. The City Car design may implement a technology that allows you to change the vehicle's color scheme at the push of a button. Thin, programmable displays would cover both the exterior and interior of the vehicle, making many of the car's aesthetic qualities as customizable as a computer desktop.

And this means more than just decking out a City Car in your favorite sports team's colors. All the gauges and dials in the vehicle's cab would run on the same technology, allowing users to adjust dashboard displays to meet their needs and tastes. Additionally, City Car's various traffic signals would no longer be limited by bulb placements, but could incorporate the entire vehicle. Imagine if hitting your emergency lights caused your whole vehicle to flash red, or if a right-hand turn signal lit up the entire passenger side.

The City Car represents a possible shift in the relationship between cars and humans, from a commodity you own to a resource you share with the rest of the population. To explore even more concept car designs, follow the links on the next page.


Lots More Information

Related HowStuffWorks Articles

More Great Links

  • "The City Car." MIT World. June 10, 2008. (June 20, 2008)
  • "City Car." MIT Media Lab Smart Cities group. (June 20, 2008)
  • "City Car: Reinventing Personal Urban Transportation." MIT Media Lab Smart Cities group. (June 20, 2008)
  • Fisher-Ilan, Allyn. "Tackling urban gridlock with foldable car." March 10, 2008. (June 20, 2008)
  • Harris, William. "How Car Suspensions Work." May 11, 2005. (June 20, 2008)
  • Irvine, Dean. "Reinventing the wheel." Feb. 1, 2007. (June 20, 2008)
  • Jha, Alok. "Robot car: streets ahead in cities of the future." The Guardian. Dec. 29, 2005.,9830,1674780,00.html
  • Kahn, Alexandra. "MIT group presents research on city car of the future." MIT News Office. Sept. 1, 2004.
  • Lombardi, Candace. "Is the City Car the solution to 'the last mile' problem?" CNET News. March 11, 2008. (June 20, 2008)
  • Mack, Michael. "Transportation Becomes Architecture in MIT's City Car." MIT Center for Real Estate Professional Development Institute. Oct. 9, 2007. (June 20, 2008)
  • Mitchell, William J. "MIT Media Lab's Concept Car: with GM and Frank O. Gehry." MIT Media Lab Smart Cities group. Oct. 22, 2004. (June 20, 2008)
  • "Traffic Control." Britannica Online Encyclopedia. 2008. (June 20, 2008)
  • Vairani, Franco. "Bit Car." Massachusetts Institute of Technology. 2007. (June 20, 2008)
  • Wright, Sarah H. "'Future of the Car' talk highlights MISTI week." MIT News Office. Sept. 26, 2005.
  • Zarracina, Javier and Aaron Atenico. "The City Car." The Boston Globe. Feb. 18, 2007. (June 20, 2008)