Tamiya Mammoth Dump Truck Project

Page 2: Building the Chassis

The lift actuator needs to be built before we get started on the chassis because it sits between the chassis rails. As you can see, all the gears for the lift system are metal. The motor is 370 sized with a worm gear attached. You can see that the gear second from the left has a lobed driver which can theoretically act as a clutch if too much torque is applied, but I've never slipped it.

Here is the arrangement of gears inside the gearbox. The motor will insert from the right. The protruding shaft is output. There are 3 total stages of reduction. The number of teeth per gear is not listed in the instructions and there are no plan views of gears B and D to count so I can't calculate the total reduction, but it is considerable. There are metal bushings here, but I thought they were adequate for this application so did not replace them with ball bearings. The completed gearbox is shown on the right.

The lift actuator uses a dual rack and pinion system. The output shaft of the gearbox will drive the small pinions shown which translate the metal lift racks. This layout is only possible because the Mammoth is so tall. When retracted, these racks hang far below the chassis.

The chassis is built using a TVP (Twin Vertical Plate) support system. The black chassis rails shown on the right are plastic, but the huge supports under then are aluminum. The long vertical aluminum plates are supports for the suspension links. You can see "4x4x4" etched into the plates from the Juggernaut, but it is not correct for the Mammoth since we don't have rear steering here.

We can now connect the left and right chassis rails with a front and rear plastic cross member. The majority of the lateral strength actually comes from the lift actuator and the gearbox though. On the right you can see the lift actuator installed between the chassis rails, and you can see how far down the rack gears protrude. The slot in the rear cross member is where the battery will go.

The steering servo is chassis mounted and faces down as shown. I chose a Futaba metal gear servo with a torque of about 125 in-oz. This is not a huge torque compared with a crawler servo, but I'm not going to be doing heavy off roading with this vehicle. Note the metal bracket which helps support the servo output horn. On the right you can see how the gearbox rigidly ties the left and right chassis rails together.

We can finally install those huge axles to the chassis using the 8 identical links we made earlier. Installation consists simply of popping on the 16 ball joints and attaching the drive shafts.

This model uses 8 shocks. These are friction dampers with no oil, and in fact they don't really have any friction either. As it turns out, the suspension is so stiff that it barely moves so the level of damping is irrelevant. At least they look pretty good. Golden oil dampers were available as a hopup, but are worth their weight in real gold now so I don't have any.

Wiring a 2-channel RC is usually a pretty trivial matter, but not so here. We have servos for both steering and throttle because of the Mechatronic Speed Control. The MSC uses a servo for input, but it is not mechanical. It has an actual FET and proportional control with a built-in BEC and no external resistor. This means the battery power has to pass through the MSC before going to the radio and to the main motor. But that's not all, we also have the Plus 1ch Control Unit which drives the lift actuator without an additional channel. This unit is commanded by holding full left or right steering for more than 1 second, but only when stopped (no throttle). It therefore needs power input, power output, and pass-through for both radio channels. It also uses a pair of limit switches to shut off the lift actuator at full up and full down to prevent stalling the lift motor. Add all of this together, and you get the huge mess of wires shown. I had originally planned to replace the whole system with a modern ESC and put the lift actuator on a 3rd channel, but the system works well and is part of the charm of this historic model so I have left it stock.

The model includes a good system for routing and restraining the wires and making them reasonably neat. This is pretty important because there are some big moving parts in back that could destroy the wiring if it was not carefully protected. The modern receiver on the far left looks out of place for being so tiny. The Mechatronic Speed Control shown on the right is a convoluted idea if ever there was one. First we use a potentiometer on the transmitter to convert a rotation to a signal. Then we sent that signal to a receiver which outputs to a throttle servo. That throttle servo uses another pot to detect its own output position. Then, finally, the MSC takes the servo output through another pot to command motor output. A modern system eliminates two of these steps.

Here you can see the wiring path back to the battery. The cross bar between the lift racks is what will lock into the bed once it is installed. On the right you can see the polycarbonate tubs which cover the electronics pods.

The rolling chassis is completed by installing the huge wheels and tires which are unique to this model. A real haul truck would have dual rear wheels, but that wouldn't be possible on the model without significantly narrowing the rear axle. This chassis is one heavy beast, but even so there is virtually no suspension compression driving off road. This makes sense since the suspension would be sized based on carrying a full heavy load in the bed.

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