custom Zoid: Megasuchus

Open the Box

In order to keep the front and rear legs in synch, I planned to modify a standard Gojulas Giga motor by exchanging the motor shaft with a longer one of similar diameter, then use the modified motor to power the rear legs and join it mechanically to the motor for the front legs. Of course, this meant disassembly of the gear box, then disassembly of the motor itelf. 

Here is what greeted me when I opened the gear box and removed the left side cover. I discovered that Tomy uses a single circuit board to generate sounds electronically, which are voiced via a small speaker. The sounds are controlled by three microswitches: one "roar" switch triggered by a nub on the neck roll gear, one "stomp" switch triggered by two nubs on the output shaft for the legs, and one "mode" switch controlled by the right leg position. The left leg position moves a lever to engage or disengage a dog clutch which activates the neck roll movement.

Motor Mod

Here is a picture of the rear motor after modification. I sacrificed an old motor with a similar but longer shaft. I opened the Giggles motor, removed the armature from the shaft, installed the armature on the longer shaft, then reassembled the motor. (The OEM shaft is at the bottom of the picture.) A bench test reveals the motor still runs, but at substantially reduced RPMs. Most likely it got damaged as a result of the modification. I proceeded anyway, and hoped the modified motor was good enough.

Spacing

At this point, I assembled the front and rear halves of Megasuchus, complete with head, neck, legs, and tail, in order to get an idea of how much space was needed between the front and rear legs. With that spacing determined, I disassembled and used CDCL (Compact Disk Case Liner) as bracing between the front and rear gear boxes. I used slightly modified Giggles footpads to fill in the space between the front and rear body shells, then filled the gaps with more CDCL. I did the right body shell first, then the left. The stock spine cap F3 is used to cover the first 2/5 of the spine, and a modified F3 spine cap (along with the discarded AAA battery box from the rear gear box) covers the remainder. Here is a pic of the work in progress.

Motor Union and First Dry Run

I disassembled again in order to mechanically couple the front and rear motors. This picture shows the front motor being connected to the rear motor via the connecting shaft. The rear motor is powered from the same leads as the front motor, and so they are controlled by the same switch. I ran an addition pair of wires to optionally power the sound chip from the rear AA battery box.

I made a bench test of the joined gear boxes and all seemed well, so I did a complete reassemble: legs, head, neck, tail, and spine caps. I carefully set Megasuchus down on the floor, hit the switch and...he groaned to life, walking painfully and ever so slowly forward. Weight distribution is a problem: he needed more weight up front so as not to walk awkwardly. Replacing the rear AA batteries with AAA batteries in the OEM position up front helped. Still, the walk was very slow. After about 4 more feet of progress, he ground to a halt.

Hmmm. A disassembly and test confirmed my suspicions: the rear motor did not turn under load, and the front motor is doing all the work. Arrgh! Back to the drawing board...

Mid Motor

Once again, I opened up the gearboxes. Then I gutted the rear motor by removing the armature and magnets, so that what remains is little more than a bearing. I carefully altered the shaft position of the new motor (in order to have shaft on both sides of the motor), then mounted it between the gearboxes. This time, I used drilled-out sprue as shaft connectors. The mid motor position also slightly improves the weight balance. Here is a picture of the work in progress. The front gear box with OEM motor and intact innards is on the left, then the new motor, and the rear gear box is at the far right. 

Since I was in here anyway, I removed the nubs that activate the stomp switch, since the new stance changes the timing of the foot fall. (During the first dry run, it seemed odd to hear "THOOM!" a second after the foot hit the ground.) I also added a bit of paper clip forward of the dog clutch (removed in this picture) to cure the Giggles "floppy neck" syndrome.

Weighty Issues

The final full-up test run gave a slow but successful walk forward, however weight balance was still somewhat of an issue. At times, the tail swing would be incomplete, which resulted in a stagger. At first, I added modified fishing weights in the nose (see pic at right) but the resulting load was too great and Tomy's overload mechanism kicked in (...click-click-click...). So I moved the weights to below the front of the spine cap, at the base of the neck, which gave proper walk behavior.