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3-Speed Transmission with Clutch
Building: Expert
Program: Advanced

Building Instructions






Important: After completing this step, check to make sure that all of the axles spin very easily.  Adjust the position of the bushings, axle extenders, and other parts slightly as necessary to allow everything to spin and coast down easily.



Note: It is normal for the gears to get stuck when trying to shift with the shift lever when the gears are not turning.  See the next step for testing and adjusting the gears.


Important: Now check to make sure that all of the gears are aligned properly as shown in the three pictures below.  It is normal for the gears to get stuck when trying to shift with the shift lever when the gears are not turning, so grab the middle shaft as shown and twist back and forth slightly as you push in and out to find the three gear positions.





Note: After completing this step, you should check again to make sure that all of the parts spin easily.  You should be able to spin the set of three tires, and they should coast easily by themselves in all three different gear positions.  You should also now be able to shift gears using the shift lever if the tires are turning.










Note: The axle will need to bend slightly for this part to fit as shown below.  This creates the spring force that keeps the clutch engaged when the clutch lever is not squeezed.




Note: The blue peg on the diagonal cross beam will normally not go into any hole.  Inserting the peg into the hole in the bottom beam will lock the gas pedal down by keeping the touch sensor pressed.  This is the "cruise control" feature.


Use the two longest wires to connect the motor to port A and the touch sensor to port 1 on the NXT. 
Keep the wires out of the way of the spinning tires and the clutch parts.

3-Speed Transmission with Clutch Programming

Use the Transmission program for your transmission.  This program will make the motor respond to the gas pedal and display the motor speed (RPM) on the NXT's display.  You could also use the Spin_A program if you want to simply control the motor power using the NXT buttons instead of the gas pedal.


Using the Transmission
Using all of the Controls

If you use the shift lever with your right hand, then you will squeeze the two gray angled beams of the clutch lever together with your left hand, and you can use one of your feet to press the gas pedal, as shown here.

This is not exactly like a car or a motorcycle, so it may take a little practice, even for adults!


Shifting Gears Like a Real Car

To practice shifting like a real car, try the following example sequence:

  1. Start in 1st gear (gear level pulled toward you), and with the clutch squeezed.
  2. Step on the gas pedal to rev the engine freely (no gears will turn).
  3. Slowly release the clutch to get a smooth start in 1st gear.
  4. Drive in 1st gear until the motor RPM stops increasing.
  5. Squeeze the clutch and lift off the gas at the same time.
  6. Push the shift lever into 2nd gear (middle position) then release the clutch.
  7. Drive in 2nd gear until the motor RPM stops increasing.
  8. Squeeze the clutch and lift off the gas at the same time.
  9. Push the shift lever into 3rd gear (all the way toward the spinning tires) then release the clutch.
  10. Drive in 3rd gear.

Using the Cruise Control Feature

If you don't want to bother with the gas pedal, then you can keep it permanently pressed by engaging the "cruise control" feature to hold the touch sensor down, so you can operate with transmission with just two hands.  Just insert the blue peg on the diagonal cross beam into the bottom beam.

Normal setting: Gas pedal must be pressed Cruise Control Engaged: Sensor is held down

Storing the Transmission

When you are not using this model, it is a good ideas to disconnect the clutch axle "spring".  This will help keep the axle from bending over time.

Clutch spring connected Clutch spring disconnected for storage


  • Practice your driving skills by shifting gears up and down smoothly using the clutch.

  • This is a "manual" transmission because you have to shift it by hand.  Can you use one of the other motors to make an automatic transmission and have the NXT shift gears for you?

  • The Transmission program uses the rotation sensor of the motor to calculate the RPM (motor speed).  It would be really cool to have the NXT also display the speed of the output shaft (the spinning tires), so you could see the effect of the different gears.  You could try hooking up one of the other motors to the output shaft to use as a rotation sensor, but the NXT motors have a lot of resistance, so it would keep the output shaft from coasting easily.  Gearing it down would help, if you have some extra gears.  But a better solution would be to use one of the older RCX-style rotation sensors if you have one (and the adapter cable for use with the NXT):

RCX-style rotation sensor

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