Tamiya TRF 420X Project

Page 2: Assembling the Chassis

While writing about this build, I am going to concentrate on the differences between this and the last chassis in the TRF line I built (the TRF 419).

There are a lot of new parts in this kit. While the chassis plate and aluminum parts almost always change between model versions, most of the plastic parts stay the same and some have been used for decades. Let's explore some of the new parts and see where they came from.

By far the most obvious thing you'll notice when you open the box is that there are two lower chassis decks included. The 2.25mm thick carbon plate is what you would expect to find in a TRF kit, but the 2mm thick optional aluminum chassis is a nice bonus. Previously there was an optional aluminum chassis (42350) available for the TRF 420, but this is not the same part. It is actually unique to the TRF 420X. On the right you can see the suspension components (C-parts [knuckles], D-parts [lower arms], E-parts [uprights], and F-parts [C-hubs]) which are all carbon filled Nylon and stamped with 54886 (2019) which marks them as coming from the TRF 419 Suspension Upgrade Set. Even though these parts were sold as an upgrade for the TRF 419, they came out 2 years after the last version (TRF 419XR) but the same year as the TRF 420. Since both versions of the TRF 420 came with these parts out of the box, they were probably designed for the 420 but made available for the TRF 419 for backwards compatibility. Note that because they use 3mm hinge pins instead of the previous 2.6mm, they must be used as a matched set and cannot be mixed with older parts.

On the left you can see the B-parts (plain) and K-parts (glass filled) which are also also Nylon and marked with 42345 (2019) which means they were new for the original TRF 420. The body posts had been the same all the way back to the TRF 414, so changing them now is really a milestone. The K-parts include the differential bearing retainers which have been updated and also some shields for the center pulleys. On the right you can see the N-parts and V2-parts which were also new in 2019 but were available in places other than the TRF 420 kit. The N-parts are the inserts for the new suspension mounts and are marked with 54880 which is a separate aluminum adjustable suspension mount kit compatible with many chassis but standard on the TRF 420. Much more on that later. The V2-parts are updated TRF damper parts and are marked with 54871 because they were available as a retrofit for older dampers. They include strengthened rod ends for both the original 5mm balls and the updated 5.8mm balls and spring perches for two different spring sizes.

The Y-parts are axle caps and diff nuts. Axle caps were previously made from Delrin but these have changed to glass filled Nylon. It is quite odd that this sprue is marked with three different set numbers and apparently each set came with only a portion of the overall sprue. The original kit number is 49381 which was the TRF 415MSX from way back in 2005 and included 4 copies of Part 1 (the axle caps) and 1 copy of Part 2 (the diff nut). The first update came with kit number 42240 which was the TRF 417V5. It still included 1 copy of Part 2 (the diff nut) but swapped to 4 copies of Part 3 (a modified axle cap). The latest update is marked with 54879 which is a front spool upgrade for the TRF 419. This was apparently when the material change happened. The Part 2 (diff nut) stayed the same, but the axle caps changed again this time with 2 copies of Part 4 and 2 copies of Part 5. These caps seem to have different thicknesses. The thinner parts are used on the rear axle and the thicker parts on the front axle of the TRF 420.

The 5mm ball adjusters had been the same forever, but these new parts (54869) are stronger and open on both sides. The odd looking parts on the right are the 54870 sway bar 5mm ball adjusters. They seem needlessly complex to me.

These parts are all new for the TRF 420 series, and the front bulkheads have been altered even further for the 420X by changing the way the sway bar bearings attach. More on that later. A close look at the image on the left will show that I incorrectly used one front and one rear bulkhead here because I didn't notice that they were different. The one marked "B" on the far left is actually for the rear and has a wider end. More on that later as well. Since I have a whole pile of TRF chassis on display and they all use a carbon deck, I decided to use the optional aluminum deck for my build. It has a lot of cutouts as you can see.

These new suspension blocks are probably the most significant change introduced with the TRF 420 series. The corner split blocks seem at first glance similar to those from the TRF 419, but they now have plastic inserts (shown at right) which allow a wide range of tuning options. While changing toe angle, track width, roll center, skid angle, and sweep angle is still possible by changing suspension blocks and adding or removing spacers, even more adjustment is now available by changing inserts. The inserts have a centered hinge pin hole by default, but also include optional offsets of 0.375mm or 0.75mm either vertically or laterally meaning that there are 7 different insert pairs with 25 different position options at each corner. That's almost 400,000 ways to combine the options at all four corners of a suspension setup if you don't require that only matched sets be used, or more practically 625 combinations by matching inserts left to right. That's with a given set of suspension blocks, but those are also replaceable. Seems like an adequate number of options. I used the manual defaults.

Here are the rear bulkheads which are shown not matching because I hadn't figured out my error yet. The one marked "A" is the correct part for the rear. The only difference is the thicker tab on the right of the left image. Out of the box that tab doesn't do anything, but the longer screw hole is there to accommodate an extra link attached from the bottom is the optional rear toe control kit is installed. It took me a long time to figure out what it was for because it only appears in use in the options section of the manual. That T-shaped carbon part is a chassis stiffener. It is secured with 3 slotted aluminum inserts as shown on the right. By default the inner holes are used, but the attachment can be changed to the outer holes for higher stiffness. This is new for the TRF 420X and did not exist on the 420. In the picture on the right, you can also see the little white lines on the suspension mounts that help you see which insert you have installed.

Everything here is also new for the TRF 420X. The TRF 420 used a 3-piece motor mount and center bulkhead. The 420X changes to a 2-piece mount which only supports the center shaft on one side. I'll complain more about that later. The TRF 420 had an optional 52g center ballast weight, but the 420X changes this to a 20g weight which doubles as a support for the carbon electronics deck. This floating deck is only attached to the chassis on the center line so it doesn't add any stiffness. If the ESC and receiver are attached to this deck, they can be easily replaced without scraping off the servo tape every time. Since the ballast supports the electronics deck, it cannot be omitted without also omitting the deck. If using the optional aluminum lower chassis like I am, there is really nothing to attach the electronics to if you don't use the carbon deck because there are so many cutouts.

The center bulkhead and motor mount are shown at the left. That screw you see at the right of the image supports some bearings that route the belt very carefully away from the motor (different diameter bearings are used based on rear differential attachment height). This did not exist on the TRF 420 and the reason it is necessary here is that the center pulley has moved from aft of the motor to forward of the motor. The motor position itself has not changed, but the position of the center pulley has which means the rear belt is now much longer and has to clear the motor and the pinion. More on this later.

This sealed gear differential is carried forward from the TRF 419, but the TRF 420 uses much more viscous #3000 oil instead of #900. The gears and housing have remained the same.

This image of the completed rear differential assembly shows the new strengthened K parts (the cam shaped bearing supports that adjust belt tension and differential height).

While the TRF 419 used one-piece upper bulkheads, the 419X changed to separate upper arm mounts. By changing the spacers between the upper bulkhead and upper arm mounts, the camber link position can be modified. The same design is carried forward to the TRF 420 (although they don't share the same part numbers). 0.8mm spacers are used here. Installation of the upper bulkheads locks in the rear differential. (I've also fixed the incorrect lower bulkhead here.)

Here's a different type of support than we've ever seen before. The same 116T spur gear is used as on the TRF 419, but the 20T pulleys have separate plastic belt guards introduced on the TRF 420. The even bigger change to the 420X is the steel "battery stopper" shown here.

That angled link doesn't provide any vertical support at all to the center shaft which means that the whole thing is effectively cantilevered from the center bulkhead. My engineering brain shudders at how terrible this idea is. Thankfully, having belt tension on either side of the spur does at least a reasonable job of balancing the forces. Tamiya correctly called it a "battery stopper" because it's only real function is to keep the battery away from the gears and belts.

The plastic front spool is new for the TRF 420. All previous TRF models used an aluminum insert for either the one-way or locked spool to attach to the front pulley. The TRF 420 is the first model to incorporate the spool directly into the pulley. The aluminum drive cups slot into the spool and are secured with screws. This seems much less robust than previous chassis to me, but I suppose it is lightly and has less rotating inertia. The pulley itself uses two parts with a separate belt retainer, but unlike some previous versions this part snaps into place and does not require any glue.

The front pulley is installed using the same type of upper bulkheads as those used on the rear. With the cams adjusted to tension the belts, the drivetrain installation is complete. In this image you can see how much further forward the spur gear is than on previous chassis.

This picture shows the critical new belt routing. Because the center pulley is now ahead of the motor, the rear belt has to pass by the motor. Hasn't one of the belts always had to pass by the motor? Yes, but the front belt was always on the other side on the spur so it was outboard of the motor pinion. The rear belt, on the other hand, is on the same side of the chassis as the motor so now comes very close to the motor and pinion. The ball bearing guide shown here keeps the rear belt away from the motor mount. We'll take a closer look at the proximity to the pinion once the motor is installed.

While the steering cranks are unchanged from the TRF 419, the bridge has been updated to include optional inserts (the manual calls them "steering pivots") which alter the distance between the ball joints which connect to the steering links. These parts, shown in the upper right of the left hand image, have either a 8.0mm or 8.5mm distance between holes. I can't imagine this makes any relevant difference to anyone, but I think that about a lot of minor adjustment options. This was introduced with the original TRF 420.

Although the turnbuckles are unchanged from the TRF 419, the ball connectors (the manual calls them "adjusters") have been changed after long use all the way back to the TRF 414M. These new parts are now open on both sides, although a single side has a mark which indicates the direction from which the ball should be inserted. As far as I can tell, the only difference on that side is a slight chamfer on the hole making it easier to pop the ball in. Another less obvious impact of this change is that the outside of the adjuster no longer has flats which means you cannot use the Tamiya shock tool to hold the adjuster while inserting the turnbuckle. I was pretty surprised by that downgrade in utility.

These beefy front suspension arms looks nothing like those from the TRF 419. They were released as part of the TRF 419 Suspension Upgrade set the same year as the release of the TRF 420 on which they were standard. Apart from the solid webbing making them very stiff, the other new feature is the "suspension arm bushings". These aluminum inserts have the hole for the damper attachment ball, and use of different inserts changes the position of the hole. This allows for finer adjustments than would be possible by just having multiple holes in the arm. The TRF 420X comes only with Bushing B which centers the hole in the slot. Optional Bushings A and C (the same part flipped over) move the hole inboard or outboard. The ball joint which attaches to the damper has also increased to a diameter of 5.8mm.

The rear suspension arms have also been greatly reinforced as shown and use the same Suspension Arm Bushing B as the front. On the right you can see how they look installed along with the rear suspension mount (Type E).

When viewed from directly above you can see the new stronger suspension arms and the white line on the bushings which identifies them as Type B with the hole centered. You can also see the front sweep angle rear toe angle of the arms indicated in red. The front uses both suspension mounts E and inserts N2 with the holes shifted outboard. The use of symmetric mounts and inserts results in no front sweep angle and using inserts with the hinge pin holes shifted outboard slightly increases the track width. The rear uses forward suspension mounts A and rear suspension mount E and inserts N2 with the holes shifted inboard. The use of asymmetric mounts with symmetric inserts results in a rear toe angle of 2°. Adjustments in 0.5° increments can be made by changing the rear inserts only.

These pictures of the suspension hinge pins shown from the side illustrate the skid angle (or "dive angle" in the front and "squat angle" in the rear). Since the N2 inserts are centered vertically there is no skid angle, but using asymmetric inserts would alter that angle. Using symmetric inserts with higher or lower holes would alter the roll center. Apart from changing the inserts, these options can also be adjusted by adding shims under the suspensions mounts either differentially or in matched sets.

These CVD style axles are a lot different than what was used on the TRF 419. While the dogbone end is still aluminum, the blue anodize has gone away and the length has reduced from 44mm to 43mm. The steel cup end has been heavily pocketed for weight, and the pin retention has reverted to a set screw instead of an axle ring.

The only noticeable difference in the uprights between these new versions and those from the TRF 419 is that the holes for the hinge pins have increased from 2.6mm to 3.0mm in diameter. Obviously the hinge pins themselves have also gotten bigger, but the retention flat in the middle has also become a groove that goes the all the way around. This makes it much easier to install the retention set screw because nothing needs to be aligned. The uprights are identical left and right meaning there is no built-in toe angle. The new adjusters are also on display in the camber links.

The front double cardan axles have also been updated. The center joint pipe has changed from steel to aluminum and the dogbone axle has lost it's black oxide finish. The steering knuckle has also changed considerably. More on that in a bit.

The C-hubs have a nice big square opening to make room for the double cardan pipe, and the upper and lower pivot bushings are now both the same size which makes assembly much less prone to error. There is a 4° caster angle built into the hubs. A completely new feature is that the steering arm is now a separate part instead of being built into the knuckle. The carbon fiber part is used by default, but an optional plastic part is also available (both shown at far right). The carbon part has is slightly longer in the longitudinal direction so while the Ackerman is the same, the carbon part produces more steering angle for a given servo input. I decided to use the carbon parts.

Did you notice the difference between those two ball studs? I didn't. One of them is 8mm long and the other is 9mm long. Tamiya made them different colors to make it more obvious, but I still didn't notice until I'd already installed them wrong. The longer one is the upper hub pivot and camber link attachment. The shorter one is the steering link attachment. On the right you can see the installation of the front hubs onto the chassis using the larger 3mm hinge pins again.

This completely new way of mounting the sway bars was introduced with the TRF 419XR. For many years the sway bars had been guided on the chassis with nothing more than a couple of loose plastic slots and set screws which resulted in most of one side's motion being lost in free play before it was transferred to the other side. This new system uses ball bearings guided on the bulkheads to support the sway bars instead. This results in vastly more effective sway bars. The plastic ends of the sway bars are also new. The set screw which retains them has moved from an aluminum collar into the plastic end itself which I suppose is cheaper and lighter. Uglier too though. Remember those 8mm and 9mm ball studs I talked about earlier? These are 10mm just to shake things up a bit.

Here you can see how the sway bar bearings are supported by the bulkheads. This makes the bulkheads much more complex to machine, but it does wonders for the utility of the sway bars. The bearings must be slid outboard into the bulkheads to install them, and then the blue aluminum collars lock them in place with set screws. Compared to the TRF 420, the 420X changed the geometry of the bearing slots in the bulkheads. The 420 has the slots open toward the top which made vertical forces on the sway bars want to pop out the bearings. Now the slots are open toward the front and back instead. Strictly speaking they wouldn't have to be slotted at all, but doing so means a less tight tolerance is needed on the counterbore to grip the bearing with no play.

These shock towers are designed for the super short dampers. They no longer use countersunk screws which makes them slightly stronger since less material is removed by the hole. They have less material connecting the right and left halves than TRF 419 making them somewhat more flexible.

The super short big bore dampers were introduced with the TRF 419X, but the TRF 420 debuted the new V-parts (shown at left). The most obvious difference is the availability of head ends that accommodate 5.8mm balls instead of 5mm balls. The new parts also have the advantage of being reinforced for more strength, and parts for the old ball sizes are also included for backwards compatibility. Short and long rod ends are also included in both diameters. Finally, spring perches are included in two diameters to accommodate both big bore and standard shocks. It's a bit disappointing that we lost the nice aluminum spring perches from the TRF 419. When building these shocks, the manual calls for a 1mm hole to be drilled into the head end cap. This relieves pressure behind the bladder and eliminates rebound.

Installation of the shocks completes the suspension assemblies and the rolling chassis. With the aluminum lower chassis plate, this version is noticeably different than my prior builds.

The cantilevered servo mount is the same idea as the TRF 419 but uses an updated part with a nice TRF logo on it. The bracket is now located forward of the servo mounting tabs instead of behind which moves the bracket quite a bit forward so that it actually mounts between the steering cranks. Only a very low profile steering servo can be used or it interferes with the new carbon electronics tray.

TRF kits had not included a pinion in years, but the TRF 420X comes with a 30T aluminum pinion gear. I'll be using a cheaper brushless motor for display, but for now I've installed a Tamiya Transpeed motor. The picture on the right shows the important impact of moving the center shaft forward of the motor. The front belt is always on the right side of the spur so when the motor was ahead of the spur there was plenty of room for the belt. Now that the motor is behind the spur, it is much closer to the rear belt which is always on the left side of the motor. You can see how the belt actually passes between the pinion and the motor housing and how it just clears the center bulkhead. The pinion position has to be adjusted just right so nothing contacts the belt.

This upper deck is a departure from the design used on any previous model. With a long slot running along almost the entire front and rear length, it offers very little stiffness. However, if optional screws are installed in the slot (circled in red on the right), then the stiffness is increased.

We haven't seen a new bumper and support for a long time either, but these are both new for the TRF 420. The foam bumper is more curved than prior versions. The carbon bumper support is also new but very similar to older versions.

The body posts are also new and are easily distinguishable from the prior version by their squared off top. Once trimmed to length though, this difference will be eliminated. The transponder stay you see between the front posts is an included option.

These battery supports have been continuously evolving. The TRF 419 used pathetic tiny plastic bumpers which required glass tape for battery retention. The 419X added some sliding metal supports which attached directly to the glass tape instead of having to thread it through the chassis plate. The XR added steel L-brackets which were much more secure. The TRF 420 improved further upon these and made them aluminum, but still required glass tape. Finally the 420X includes the little carbon tabs shown which eliminate the need for tape entirely.

These images show some optional extras included with the TRF 420X. The left hand image shows one of the 5g forward ballast weights. The right hand image includes a cooling fan for the motor attached to an optional mount which differs from that available for the TRF 420.

As usual, I added my own dish wheels and standard slick tires to complete the chassis. I also installed a 2s hard case battery pack just for the photo. I ended up removing the electronics tray for display after this photo was taken.

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