Tamiya TRF 414X Project

Page 2: Assembling the Chassis

There are no traditional "steps" in the manual, but the first exploded view shown is of the ball differential. The ball diff uses ten 3mm hard tungsten balls arranged in an aluminum hub which doubles as a pulley. To avoid having the metal balls ride directly on the metal hub, each ball sits in a cylindrical Delrin insert. The pressure plates are hardened steel supported by mild steel drive cups. The thrust bearing is a swaged unit that doesn't need to be assembled. The manual says nothing about grease so you just have to be smart enough to know you need some.

The regular TRF 414X kit came with a front one-way, but also included parts for an optional front ball differential identical to that used in the rear. Since my version came built, I do not have the second optional ball differential. The pictures above show the parts for the front one way which is built into the front aluminum pulley. If you zoom in on the photo on the right you can see that the one-way bearing part itself is from INA in Germany, uses a 6mm bore, and is pressed into an aluminum hub. There are actually two of these bearings, one pressed in from either side. This allows the left and right drive cups to spin at different rates in corners, simulating a differential.

Ever wonder how a one-way bearing actually works? You'll need to zoom in on the full size photo to follow me on this one. The cylindrical needle rollers sit in an asymmetric slot. In one direction they can roll freely, but in the other direction they are forced into a hardened tang which bites into the rollers and causes them to jam against the mating shaft. Please ignore the hand model I hired for this photo.

Here are photos of the completed ball differential (left) and front one-way (right). They look pretty similar and both use 32T pulleys but you don't want to get them mixed up. It's also really important not to install the front pulley backward or the front wheels won't be driven.

The dual bellcrank steering is designed for precision. The steering bridge is a carbon fiber plate with flanged bearing inserts The use of 0.7mm spacers makes the aluminum cranks clamp very slightly (0.2mm) proud of the bridge surface so they don't jam against it. The cranks ride on dual ball bearings.

The model uses a high torque servo saver with a white Delrin servo horn. I don't plan to drive this model, but I did want to put a simple servo in place to at least hold the wheels in the right position.

Time to assemble the rear bulkhead. The side panels are machined aluminum. The manual shows that these come with bearings already pressed into them, but I didn't notice that until I'd already knocked the bearings out to take this photo. Luckily I did not damage anything in the process. The rear suspension mount is a carbon fiber plate while the forward mount appears to be machined Nylon and will add lateral support to the whole assembly. The short rear belt has to be installed over the pulley now because it won't be accessible later. The motor mount plate is integral with the rear bulkhead.

These photos compare the forward suspension mount from each side. Note that one side has an indentation in the middle. This feature is there to identify which direction faces forward. Why should it matter? If you look very closely, you'll see that the holes are very slightly closer to the top of the part on one side than the other. That's because the holes have been drilled at an angle that is not perpendicular to the face.

The picture on the left shows the results of those angled holes. The inner suspension pivot is not parallel to the chassis plate, but angles down toward the back which results in a positive skid (or anti-squat) angle. Note that the carbon rear suspension mount does not have a hole drilled at an angle, so the hinge pin needs to be wedged into it pretty tightly. I've used the default lower hole, but the roll center can be changed by using the upper hole on the front suspension mount and flipping the rear mount upside down. On the right you can see the bulkhead installed into the chassis as viewed from below. It really bothers me that two of the screws are steel instead of titanium. As far as I can tell from the manual, these ought to be all the same material. Since my version was built in Japan a year after the original release, perhaps they ran out of stock. I don't have another explanation for the swap. I was eventually able to find some replacement Tamiya 3x18mm titanium JIS flat head cap screws to replace them, but it wasn't easy (or cheap).

The front bulkhead assembly proceeds much like the rear except that we're using a one-way here and a longer belt. The assembled result is shown on the right.

Here is the chassis with both bulkheads installed. I'm using a tool as a temporary holder for the belts until the center drive pulley goes in. This is also the first full view of the chassis plate I've shown which reveals how the batteries are configured. This chassis is set up for saddle packs with 3 sub-C cells on either side of the belt (lithium batteries were not a mainstream thing yet in 1999).

Time to install the center pulley shaft and spur gear. The 93013 kit came with a 15T pulley locked to the shaft to drive the rear axle, and either another of the same or a one-way to drive the front axle. My built version only included the one-way. I've never quite understood why you need a one-way both on the center shaft and also on the axle, but this chassis has both. The carbon reinforcing plate you see here for the spur gear was not part of the 93013 kit but did come with the later 49132 TRF414 from 2000. Since my built version of the 414X came from 2000, it appears to have included this "upgrade". Note that the model that came out of the Stealth Box as built did not have the spur gear installed at all.

The 120T spur gear is actually not a Tamiya part. Since racers tended to use 64p pitch rather than the metric 0.4 mod equivalent, Tamiya chose to include a racing spur from Kimbrough. The center shaft is installed as shown on the right, so now the belts can be rotated and tested. Note that there is no belt tension adjustment in the hubs.

Now we can install the steering bellcrank assembly onto the chassis plate using the posts and spacers shown. In order to get the steering bridge to sit at the right height to pass between the top and bottom of the belt, all 5 spacers need to go on top of each post.

Now the asymmetric upper chassis plate can be installed with a receiver antenna mount. This plate also ties the front bulkhead to the tops of the steering posts. Everything is massively stiffened after this is installed. Why is it asymmetric? I can't find any particular reason for the extra space allotted to the right side by this design.

The rear shock tower comes with a lot of options. There are 9 holes on each side (arrayed in a grid) for connection of the upper suspension link, and also 9 holes on each side (arrayed in an arc) for connection of the upper shock mount. I used the holes indicated as default in the manual because I don't know any better.

The front shock tower offers slightly fewer options for link attachments than the rear (6 or 7 at each location instead of 9), but it is still a very customizable configuration. It is shown installed on the right. Both shock towers are reused on the TRF 414 production model, but simplified for the TRF 414M with the same overall shape but fewer holes.

Here is the chassis with both shock towers installed onto the bulkheads. Time to build the suspension.

These very early TRF shocks are a thing of beauty. The outside of the cylinders and caps are anodized in gold. All four shocks use 2-hole Delrin pistons, but front and rear versions use different spring rates and preload spacers as shown. A third spring rate which is between the other options is available but unused in the base build.

The rear suspension comes first. The uprights are machined aluminum and the axles have a dogbone at one end and a CVD universal at the other. The axles come assembled with pressed pins so cannot be taken apart. The hexes are very thin and the cross pin is unusually retained with an O-ring. Once built, the hub assembly attaches to the machined suspension arms with smooth hinge pins retained by E-clips. A set screw is used to contact the chassis plate and set the lower limit of suspension travel which controls ride height.

Now the previously completed rear shocks can be attached and the whole rear suspension assembly connected to the chassis with hinge pins. The fully assembled rear suspension is shown on the right.

Now we can do the same thing for the front suspension. The C-hubs and knuckles are both machined aluminum. The king pins are single piece step screws with a smooth portion to act as a steering pivot. Everything else goes together very much like the rear.

The exploded view on the left shows the components for the front suspension installation, and the photo on the right shows everything complete. Note that the upper arms attach to the sides of the hubs rather than the top. This keeps the arms low, but also introduces side forces into the hub.

At this point the chassis has been returned to the assembly condition in which I received it, but it is not quite done. Neither the bumper nor the body posts were installed in the hard case so I'll put them on now. There's nothing out of the ordinary going on here, although these body posts are unique to this chassis.

This chassis did not come with any wheels or tires since those would usually be chosen by the driver for a specific track. Instead this model came with setting dishes, a type of part I'd heard of but never seen. These solid aluminum disks are installed in place of wheels and, having the same outer diameter, provide a platform on which to perform adjustments and measurements of such parameters as camber, caster, roll center, ride height, and toe. The dishes have a larger outer O-ring just to provide a but of friction to keep the chassis on the table. The pictures above show the chassis from the side and from above. From the side you can see how much open space there is, though much of this would be filled by batteries, motor, servo, ESC, and receiver in a completed read-to-run configuration.

On the left is a final overhead isometric view. On the right I've installed some basic temporary electronics to "complete" the chassis for display. I've installed a standard silver can motor despite the fact the no one in their right mind would ever use such a motor in this chassis. Paired with the standard 35T pinion, this at least keeps the chassis from rolling and allows a quick battery connection to see the wheels spin. The steering servo has no hard mounting which is a weakness of this chassis. It has to be installed with servo tape.

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