Tow Truck: 12 Pounds of Lego Technic

Cyrus Tabrizi, 7/10/12

      In total, the model used 4 XL motors, 6 M motors, and 1 RC motor. All this power-hungry machinery was powered by three battery boxes and controlled by 5 PF receivers -“5 receivers?” you might ask. Yes, when a model is more than two feet in length, one must use extension cords and when, in my case, more than two extension cords are needed, then it’s time to get creative. I determined logically that if a second receiver were to be connected to the output of another receiver, then it could serve the same purpose as an extension cord under the following parameters:


Conditions for Using a Second Power Functions Receiver as an Extension Cord Replacement:

      1. The first and second receivers must be set to the same channel as the remote and the second receiver must be connected to one of the first receiver’s outputs (for the purpose of this explanation, the second receiver will be attached to the red output of the first receiver). Additionally, the first receiver must be connected to an appropriate power supply, and there may not exist interference or disruption that would cause the two receivers to receive different IR signals from each other.

      2. The red output of the first receiver (or whichever of the first receiver’s outputs the second receiver is attached to) must be turned on. If this and Condition 1 are met, then the second receiver will be turned on and the red output of the second receiver will respond in place of the first receiver’s red output – the second receiver will have served the same purpose as having an extension cord be connected to the first receiver’s red output.

      3. The red output of the second receiver must be on (Conditions 1 and 2 have been met) for the  blue output of the second receiver to correspond with that of the first receiver. Neither of the second receiver’s outputs will work (the second receiver will be off) if the first receiver’s signal is blocked or if its red output is not on, but, conversely, the first receiver’s blue output (or whichever output is not being used by the second receiver) will work regardless of whether the second receiver’s signal is blocked or not.


      It is important to note that using this fifth receiver as an extension for another does not mean that all the outputs of these two receivers can or should be used to power a motor –each receiver is still limited in the number of motors it can power at one time. Subsequently, only one output on the first receiver was used to power a motor - the second output was used to power the extended receiver- and only one output of this extended receiver was used to power a motor (as explained previously). Additionally, this arrangement (that of using a fifth receiver) does not increase the number of channels or functions the system can handle (a total of eight) – it only means that two receivers are running on the same channel and receiving the same signal. To exceed eight RC functions in this model, I decided to use three gearboxes -two in the truck and one in the crane (this will be discussed in depth later on).


8Wheeler 8258

      While the platform is loosely based on the platform of my Modular 8 Wheeler (shown above on the left), my inspiration for this model came from my purchasing the 8258 Crane Truck (shown modified above on the right). At the point in this hobby at which I bought this set, I had not built anything nearly as complex as that truck. It was interesting to have so many functions controlled by a single M Motor and, despite the, in my opinion, relatively poor performance of some of these functions, the set was somewhat fun to play with and roll around –this, in fact, was more fun for me than the rest of the motorized functions simply because the decently large truck moved so smoothly. I immediately thought to motorize the steering and the drive train with an M and an XL motor, respectively, and I succeeded to some extent in my quest. I didn’t want to tear the model apart so I replaced the engine block with the XL and put an M motor at the rear of the cabin (my efforts are documented on Brickshelf) – it worked but I wasn’t content with the result. After playing with the crane truck for a few days, I destroyed it -the reason I bought it in the first place was because I thought it would give me the best bang for the buck considering the number of linear actuators, 11L gear racks, and specialized gear train pieces it had. After building and taking it apart, I had this picture in my head of the crane truck rolling on these enormous Power Puller tires with huge travel in the suspension, all-wheel steering, large stabilizers, and some sort of legit manipulator – i.e. tow arm or crane – I couldn’t get it out. One of the first things I did was to draw a picture that gave a crude representation of the truck after making these modifications.
Inspiration

      That is what I had planned for the Tow Truck. Unfortunately, Power Puller tires were (and still are) out of the question for their weight and cost – the latter more importantly, but their weight and their size becomes an issue when trying to motorize a huge truck with 8 of them – it may be possible now with the new portal hubs, but this piece wasn’t even in the drawing board – I would think- when I was working on this model. So instead of those tires, I went with the newer 94.8 x 44 balloon tires from the 8297 Off Roader. I had wanted to build an RC 8x8 vehicle since before this project and the first step towards doing so was building the Modular 8 Wheeler that I mentioned earlier – I had ordered four more tires and the pieces required to build the suspension and steering assemblies (universal joints, steering links, CV joints, steering arms etc) from Lego Direct to complement the four assemblies and tires that I had from the Off Roader. With this, I built the Modular 8 Wheeler as a test bed, destroyed it, and began construction on the Tow Truck.