Sunday, July 15, 2012

Motorizing a Lego 8081-B Model Hot Rod

I want to get into motorizing some of Lego's Technic cars. One present that I got for my 23 birthday was an 8081 set from Jessica. This set can build two specified models. The A model is a truck. The B model is a hot rod. I decided to build the hot rod because I can't bear the thought of a just two wheel drive truck. However, a two wheel drive hot rod seems perfectly natural. After building the stock hot rod, I wanted more. I then set out motorizing this model to warm up for combining Mindstorms with an 8070 model supercar that I also got for my birthday. Thanks Mom!

One of the first things that I did was  changing the wheels. I substituted the balloon style tires for the street style tires. I also made the rear wheels larger to give more of a hot rod look. I wanted to maintain the stock functions as much as possible. This meant keeping the working model engine block, dimensions, and body. I pretty much resided myself to taking the driver's area apart to cram in the necessarily parts. I removed the seats, and placed a Power Functions battery box in the driver's area. I wasn't quite able to get the geometry compact enough to get the battery box fully inside the area. This meant that I had to remove the roof.

I then used the existing steering system including the front steering assemblies and the shaft which is used to move the steering by hand. I added a Power Functions medium motor with a worm-gear assembly that uses the white 24 tooth slip gear to actuate the steering. This not only allows for the steering to be precise and controllable, but also lets the motor try overrun the steering boundaries without causing any damage. This is important because kids do not seem to observe common sense with playing with other people's stuff.

While I was at it, I also put the Power Functions IR Receiver in the rear of the model. It fits quite nicely back there and helps the steering motor stay snug against the frame. I had to do a little bit of mechanical acrobatics to actually connect the receiver to the vehicle because the Technic holes ended up being spaced half off.

Given that I want to preserve the engine block reciprocation, I had to also preserve the shaft that connected the crankshaft to the wheels. Other than redesigning huge chunks of the model, preserving the shaft was the only real option. I searched for ways to finagle motors in at right angles, but nothing was quite fitting as well as I really wanted. Finally, I saw the KISS solution which was mounting another medium motor directly to the back of the frame which held the differential. It was much easier than doing any other solution and also preserved the frame and body. I am pretty happy with how this turned out:

Overall View

Right Side View
Left Side View

Underside View


  1. The headlights fall off to easily.

  2. You could put the battery box as the roof.
    It basically has the same roof dimensions besides height.
    And you get to more stock parts (seats,etc)
    So it is a win-win!