In this lesson, students begin making a different type of car. There are two options: the belt-drive car and the propeller-drive. Teachers may decide to do one or the other, provide both options to the students, or extend the unit by two lessons and do both.
Two new kinds of car:
The class discusses other ways to get a motor to drive a car besides direct-drive. These include a propeller, gears (as most Lego Technic™ constructions), pulleys & belt (found inside a washing machine), and sprockets & chain (as in a bicycle). Students have the option of making cars based on either of two drive systems: the belt-drive or the propeller-drive. Review each one briefly:
Propeller-drive: A propeller is attached directly to the motor. The air blown by the propeller makes the car go, like a propeller-driven plane or boat.
Belt-drive: The motor turns a pulley, which is connected to another pulley by a rubber belt. The second pulley is what makes the car go. Although the parts are different, the idea is similar to the sprockets and chain of a bicycle.
Design a propeller-drive car:
Show students a propeller and ask:
How could this be mounted on a motor?
How could the motor be mounted on a car?
As they come up with idea, suggest they make diagrams on the blackboard or chart paper, for everyone to discuss. Additional questions might be:
How could you use the simple car from Lesson 4?
How could you predict the direction the car would go?
What kinds of things would you have to look out for?
Students will probably suggest attaching the propeller to the motor shaft, and letting the air flow push the car (See this video). Since the motor and propeller will be doing all the work, the car could be the simple one from Lesson 4 with the propeller raised by a block (see this video) or a triangular structure (see this video). Provide worksheets for students to record their ideas.
Design a belt-drive car:
Mount a bushing on a motor shaft, and use a pencil to support a spool. The spool should be able to rotate. Show students how a rubber band can be looped over both the shaft and the spool. Click here for a diagram. If the rubber band is stretched slightly, turning the motor shaft will also turn the spool. Have a student help you show this by rotating the bushing by hand while you hold the motor and spool. Ask:
Suppose the motor was on and turning the spool, like I just showed you. How could you use this spool to drive a car?
A simple way is to make the spool do double duty as a wheel. That way it can drive the car without having a tight connection to an axle and wheels (See a video). Another way is to make the axle fit snugly in the spool, and attach the axle to wheels snugly at either end. In this design, the spool turns the axle, which will make the wheels turn too. Here is a video of a 4 - wheel belt drive car.
As students are working, ask them to record issues as they come up. The Worksheet provides space for recording them. If time permits, conduct a class meeting to discuss some of the issues for either or both types of car. See the download of Issues, Causes and Fixes below.
Students did troubleshooting when they put a switch in a circuit.(“Make a Switch”), when they made a simple car (“Let’s Roll”), and when they made a direct-drive car. Many of the issues presented by propeller-drive and direct-drive cars are the same issues. The Troubleshooting sections of these earlier lessons apply to this lesson as well. What follow are issues specific to propeller-drive cars and direct-drive cars.
Troubleshooting a propeller-drive car:
Sometimes the propeller spins but the car does not move. Usually the problem is one of those discussed in “Let’s Roll”. There are two other possibilities:
One other problem encountered with propeller-drive cars is that the Car Tips Over.
Troubleshooting a belt-drive car
Here are four issues confronted with direct-drive cars:
Motor does not turn:
Motor and spool turn but car does not go:
Rubber band comes off motor pulley
Car does not run well