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Trailer Pull
Building: Expert
Program: Easy

Building Instructions






Note: You will need to rotate the orange rotors of the motors until the axle holes line up so that you can insert the axles from the side motors. 

Important: Make sure that the size 6 axles (with the small gears on them) that go through the motors are pushed all the way in and don't stick out the sides.




Attach the short wire to the center motor and the two medium wires to the side motors.


Important: Make sure all three wires are coming out the top as shown below.



Attach the NXT to the two blue pegs on the base, then plug the center motor wire into port A on the NXT.

Then plug the two side motor wires into ports B and C on the NXT (doesn't matter which is which).

Then attach the wheels and secure them with bushings.



Important: Make sure that the gears and bushings are not pushed too tightly onto the axles.  The axles should be able to spin freely.








This step builds the optional rear trailer fence (useful for holding loads on a slope) and the optional flexible trailer hitch.

Note: Long (3M) pegs would be better for the four outside pegs on the 13-beam here, but I ran out of them in the standard part count.  If you have any extras you can use them instead.  This will make the fence easier to attach and remove because the pegs will stay with the fence instead of sticking in the trailer.

Trailer Tow Programming

The Trailer_Pull program is a simple program to make the tow vehicle pull forward with all three motors and full power.  It will run until you abort it with the dark gray button on the NXT.  The Trailer_Push is the same thing except that the vehicle will drive in reverse, in case you want to try pushing the trailer.


Towing Options

Flexible Hitch Rigid Hitch
Use the Rear Fence for loads that will fit. Remove the Rear Fence for very large loads.



  1. The tow vehicle is not designed to be able to turn, because all three motors are attached together to turn in the same direction at the same speed, so do not attempt to program the vehicle to turn.

  2. Although the trailer is designed to hold heavy loads for a LEGO robot, if you overload it, something will eventually break (maybe your 12-axles, which are difficult to replace).  So use common sense, and be careful with the balance of the load. 

  3. Unlike the trailer, the axles on the tow vehicle are not supported on the outsides, so they will bend significantly under load.  Make sure not too much weight is placed on the tow vehicle. 


  • Experiment with carrying different loads using the two different hitch methods.  You will notice that the rigid hitch is usually better at towing more weight.

  • Try towing some loads up a slope and see how much weight you can carry up how steep of a slope.  As a starting point, I have tested the following:

Load Slope (vertical as % of horizontal)
30 lbs (13.6 kg) 0% (flat)
21 lbs (9.5 kg) 10%
Vehicle with Empty Trailer 95% (almost 45 degrees)
4 lbs (1.8 kg) pushing not pulling 60%
  • If towing a load doesn't work, and the problem is that the tires turn but slip on the surface, then traction is the problem, not power (torque), which means that not enough of the load is over the drive wheels.  The NXT motors have a lot of torque, and using three motors all geared down 3:1 gives you a lot to work with.  In all of my tests that failed, traction was ultimately the problem, not torque.  Getting enough weight over the tires to stall the motors instead of slipping the tires would probably require redesigning the tow vehicle to brace the drive axles on both sides of the wheels (like the trailer), so that you can put more weight on it without the axles just bending. 

  • Real very heavy trailers perform better if they use the "fifth wheel" configuration, where the load is placed ahead of the rear axle on the tow vehicle, not behind it.  This distributes some of the load to the front wheels of the tow vehicle to keep it from "popping a wheelie".  Care to give this a try?   If the load was centered on the tow vehicle and if the tow vehicle had 4-wheel drive then you would increase traction and cut the amount of axle bend and tire compression by half...  Then if you have the parts, you could also add more axles and wheels like a semi-truck, to further reduce axle bending.

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