Tamiya Fuel Tank Trailer Project

Page 2: Adding motorized legs

The trailer is completely done and usable at this point, but I still have one more accessory to add: motorized support legs. This will allow me to control the trailer legs from the transmitter and therefore couple and decouple without ever touching the truck. Considering that all the electronics are in the tractor, controlling legs on the trailer is no small feat. The way it is accomplished is quite ingenious.

The first steps are actually performed on the tractor. Step 1 is removing existing parts. At this point I've built three different 5th wheel units. The first, on the left, is the stock unit which uses a manual release. The second, which is shaped differently, came with the MFC and adds a micro-switch so the controller knows when a trailer is coupled. The third comes with the support legs and looks like the first except that it has a slotted link as you can see on the lower right. This is the part that mechanically interacts with the trailer to send commands to the legs. It has little serrations on the end to keep it from slipping against the trailer trigger. There's also a fourth coupler option, the one I actually used. You see, you don't need the MFC to use the motorized legs, you can just control them with a Channel 4 servo. However with the MFC you get extra functions, so that means I need to move the micro-switch from the 2nd coupler to the 3rd. This was not at all obvious from the instructions so I was frustrated for a bit whilst I figured it out.

Here is the mount for the 4th channel servo. This will go right behind the cabin, hidden between the frame rails. If will actuate the slider on the 5th wheel. A small motion back tells the legs to retract. A large motion back tells the legs to extend. The trailer distinguishes the two by having two micro-switches and looking at the relative states of the two. There are no circuit cards involved though, just simple wiring. Movement of the servo forward releases the coupler. There's enough lost motion in the slider that aft movement doesn't affect the coupling hook. The hook itself locks over center so that thrust loads on the kingpin do not tend to release the hook.

Here in Step 5 the coupler and servo have been installed on the tractor. The servo needs to be wired into the MFC. The port you see at the upper right of the sleeper opening is for trailer lights which, sadly, still need to be attached manually. Since this is how real trucks do it though, I can hardly complain.

Step 6 removes the old trailer legs and associated mechanism (a bit of a shame) and Step 7 builds the battery box. That's right, you actually have to build the 4AA battery box including putting in the terminals and attaching the wires. They have a clever solderless system which uses little bits of rubber tubing. Step 8 builds the micro-switch can. I didn't take pictures of any of that. This is Step 9 which is the gearbox. You can see a little motor (370 sized) driving a worm gear. Using a worm gear is important so weight on the legs can't backdrive the motor. There are 3 total stages of gear reduction not counting the rack gear on the legs themselves.

At this point Step 10 tells you how to wire everything together and it isn't trivial. We have two micro-switches, leads to 4 batteries, a 3 position toggle switch for manual operation, and a motor. All these wires need to be carefully routed and hidden within the trailer frame. Wouldn't want anyone to know it's a model!

Here is the battery box and the completed motorized legs from Step 16. Inside each leg is rack and pinion gear along with a spring loaded clutch system which allows the pinion gear to slip if you keep driving after you reach the end of travel, but without dropping the trailer.

This bottom view of the completed trailer shows the wiring, battery box, and motorized support legs. The wires can't be seen when the trailer is upright.

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