Key concepts
Engineering
Physics
Design
Vibration

Introduction
Have you ever ridden in a car over a pothole or a speed bump? You might feel the bumps and get tossed up and down in your seat a little bit, but not nearly as much as you would if the car did not have suspension. Try this engineering project to learn how suspension can help give you a smoother ride!

Background
You might not give it much thought if you are riding down a smooth, nicely paved road, but a car’s suspension is very important when driving on a bumpy road or over obstacles. It helps protect the car (and the passengers) from damage when the car goes over a big bump. A car’s frame is not rigidly connected to the wheels. If it were, every single bump or vibration experienced by the wheels would be transferred directly to the car and the people inside—making for quite an uncomfortable ride. Instead, the wheels are connected to the rest of the car with a combination of springs (metal parts that bounce back to their original positions when stretched or compressed) and dampers (parts that use friction to slow down motion, usually in the form of a piston filled with viscous oil), also called shock absorbers. Combined, these parts help absorb and decrease road vibrations. In this project you will build your own simple model car and design a suspension system to help prevent the cargo (packing peanuts) from being tossed out of the car when it drives over bumps!

Materials
This is an engineering design project, meaning there is not an exact list of materials that you must use. The following list is a suggestion, but feel free to substitute other materials.

  • Office and craft materials to build a simple car. (Alternatively, if you do not want to build your own car from scratch, you could use a medium-size toy car.) Search online for “balloon-powered car” or “rubber band car” and you will find many design ideas. In general you will need:
  • Something to use as the car’s wheels, such as old CDs or plastic bottle caps
  • Something to use as axles, such as wooden skewers or pencils
  • Something to use as the car’s frame, such as a plastic bottle, cardboard tube or straws
  • Glue or tape to attach the parts together
  • Small cardboard box that will fit on your car
  • Packing peanuts
  • String
  • Assorted rubber bands and/or small springs (You can get springs by disassembling ballpoint pens—after asking for permission, of course.)
  • Straws or pencils
  • Tape
  • Scissors


Preparation

  • Assemble the body of your car. (Skip this step if you are using a toy car.)
  • Assemble the axles so they can rotate—for example, thread wooden skewers through straws.
  • Attach wheels to the axles. For example, carefully poke holes in the center of plastic bottle caps, then poke wooden skewers through the holes.
  • Build a frame for the car and attach the wheels and axles. Make sure the axles can rotate freely and without too much resistance.
  • Tie a string to the front of your car so you can easily pull it along.
  • If your small cardboard box has flaps, carefully cut them off so its top is open.
  • Use tape to attach the box to the top of your car and fill it to the brim with packing peanuts.


Procedure

  • Set up “speed bumps” for your car to drive over. For example, tape several straws or pencils to a tabletop, parallel to one another and a few inches apart.
  • Pull the car quickly across the speed bumps. What happens?
  • Now try to design a suspension for your car to prevent the packing peanuts from falling out of the box. For example, you could suspend the cardboard box from rubber bands. This is where the engineering design process really comes in to play!
  • Once you have built your suspension, try pulling your car over the speed bumps again. Try to do it at the same speed. What happens this time? Do fewer packing peanuts fall out?
  • Don’t get discouraged if your suspension doesn’t work on the first try! The engineering design process is iterative, meaning engineers often go back and redesign, rebuild and retest their devices to improve them. There are many different things you can tweak about your design. For example:
  • Try changing the type of rubber bands. What happens if you use thicker or thinner rubber bands, or longer or shorter ones?
  • Try changing the tightness of the rubber bands. What happens if the box is hanging from them loosely or if the rubber bands are pulled tight?
  • Try changing the weight of different parts of the car. (Weight has a big impact on vibrations.) For example, what happens if you tape a bunch of coins to the cardboard box to make it heavier? What happens if you tape the coins to the frame of the car instead?
  • Keep iterating and tweaking your design. Can you get to a point where no packing peanuts fall out of the box?


Observations and results
You should find that when you attach the cardboard box directly to the frame of your car, many of the packing peanuts are tossed out of the box when you pull it over the speed bumps. This occurs because the vibrations from the wheels are transferred directly to the cardboard box. This would be like riding in a car with no suspension—quite an uncomfortable ride! When you add a suspension, it helps absorb some of the vibrations, so the cardboard box does not bounce up and down as much and doesn’t toss out as many of the packing peanuts. It might take some tweaking, however, to get your suspension working well. The stiffness (thickness/length/tightness of the rubber bands or springs) and weight of an object (for example, adding coins) have a big impact on how it reacts to vibrations. Real engineers carefully design a car’s suspension to optimally absorb vibrations on the road, just like you did in this project!

This project was inspired by the Trash Sliders activity, by Larry Richards, via eGFI.

More to explore
Balloon-Powered Car Challenge, by Science Buddies
Suspended Science: How Does a Hovercraft Hover?, from Scientific American
Fight Slippage with Friction, from Scientific American
Science Activities for All Ages!, from Science Buddies

This activity brought to you in partnership with Science Buddies

Science Buddies