Tamiya Monster Beetle Trail Project

Page 1: Assembly

The Monster Beetle Trail comes in a surprisingly large box for what I thought would be a small model. It turned out to be much bigger than I expected. The contents of the box are divided into three sections with the body on the left and the tires on the right.

Here are the parts trees arranged on my building table. Apart from those, there are three labelled hardware bags, a torque tuned motor, an assortment of gears, and a mixture of ball bearings, metal bushings, and plastic bushings. My plan was to upgrade to full ball bearings throughout, but I later found that my "complete" bearing kit was not complete. More info on that later.

The build begins with the huge dual differentials. Each differential housing has a spur gear on the outside diameter and a set of three metal bevel spider gears on the inside. Outputs are internally splined shafts. Front and rear differentials are identical. These are unsealed units so they would normally get some grease but not be filled with oil. In this case though, the differentials are locked by putting a small ball of stiff putty inside each. The white "tape" shown on the left contains the putty. This results in an effectively locked diff, but still with enough compliance to absorb some impact and protect upstream gearing and axles.

This chassis is just one giant gearbox filled with spur gears. The differentials sit at the ends as shown. There are then three sets of gear reduction, identical front and rear, until we get to a gap in the center. This is where the motor pinion will sit, engaging both front and rear gear trains in parallel. 16 ball bearings are required to support all of this. The results is a solid chassis with incredible bending stiffness due to its vertical height, and great torsional stiffness due to its large cross section.

The left hand image shows two sets of mounting holes for the motor which allow either an 18T or 20T pinion. The kits comes with the lower speed aluminum 18T pinion which I replaced with a steel variant. The pinion cannot be adjusted in the usual manner by moving radially away from the mating spur, so instead the second set of holes move the motor slightly down. The installed Torque Tuned motor is shown on the right.

These portal axle hubs have a LOT of parts. The input and output gears are steel while the pair of parallel idler gears are brass. Brass is certainly less strong, but the use of double gears largely mitigates that issue. I assume brass was chosen so that there wouldn't be a steel on steel gear mesh which is subject to galling (and noisy). The softer brass should be where the wear occurs. My bearing kit did not come with ball bearings for the tiny 6x3mm metal bushings which support the brass gears. I considered buying some separately but decided against it. The balls in bearings this small would be so tiny that they would not support much load. In addition, spinning the brass axle on a bronze bushing inside a greased housing should actually be pretty smooth. This is also the lowest speed portion of the gear train where friction is least detrimental. The far right image shows a metal shielded ball bearing on the input, bronze bushings on the idlers, and a rubber shielded ball bearing on the output. The rubber shield on the output will help keep water away from the balls.

Here are the completed front and rear portal gearboxes. They differ only in the provision for steering on the front. The left and right steered parts are also identical, but use a mirrored steering horn as a separate part. There are 6 screws and nuts locking each housing. This is actually important for strength since threading directly into the ABS would not hold as well. The model uses drive cups and dog bones for torque transmission to the hubs.

Here the portal hubs have been installed to the independent suspension. The lower control arms are wishbones (A-arms) and the upper arms are fixed length links.

Regular readers will know that I detest friction dampers. The pure pogo sticks are the worst, but this model at least has some rubber tubing inside the shocks to provide some damping. Oddly, I don't hate them. Something about the combination of this model's speed and the compliance of the balloon tires makes for acceptable performance. It is theoretically possible that I won't replace them.

I figured that with large tires and the potential to drive in some semi-serious off road conditions, I should use a moderately powerful steering servo. I chose a waterproof 110T from Eco Power with 280oz-in of torque. The steering system is strange. The servo horn attaches so it is at 45° when the servo is at neutral. Another funny little chicken foot crank, shown on the left, will transfer the motion to the front knuckles.

The bottom view on the left shows how the steering works. Since the chassis is a solid block of gears, the servo can't be installed in the center. It is hanging off one side and inverted as shown. A diagonal link connects to the chicken foot which sits in a slot in the chassis. It looks strange but works rather well. The top view on the right shows all the electronics installed. The ESC and receiver sit on top of the battery cover. The battery compartment fits a standard size rectangular hard pack.

The tires use a chevron pattern with round spikes just like the tires from the regular Monster Beetle, but they are not the same size. They are smaller in diameter, but wider. These tires are not common to any of my other models.

First I sprayed the body with Italian Red, then I moved on to the detail work. The pictures above show the masking for the front window trim followed by the result. I've also applied the stickers and a couple coats of clear.

This Quick Drive Monster Beetle body includes a scale rear engine and roll cage which protrude from the back window as shown. The chrome engine and exhaust actually look pretty good. I also painted black rings around the tail lights. The left picture is a test fit before anything was painted; the right picture shows the final result.

Here is the completed body with installed headlights and fog lights. On the left the body has been installed on the chassis, completing the model. I can't say I've ever seen a Beetle quite like this before.

Let's look at some comparisons with the original Monster Beetle. I used a different red for the original which makes it look a bit more pale. The wheels are obviously different, but from a 3/4 view they look pretty similar. From above, however, you can see how much longer and wider the original body is.

Although the original Monster Beetle body is wider, the chassis of the Trail makes up for it and ends up with a similar track width. The original is also significantly taller with more ground clearance. From the front you can see that these are different versions of the Beetle. The OG uses headlights mounted to the nose while the Trail mounts them to the front fenders. The fenders are also more curved on the Trail.

The GF-01 CB is obviously a heavily modified version of another chassis. All of the parts shown here would have been used on a regular version, but are just extra here. That's a lot of spare parts, though none of them are usable for this model.

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