new2rocks Guide to the TRX-4


The much-anticipated Traxxas TRX-4 scale crawler is finally here, and it’s a good one. Traxxas clearly put lots of thought into the design and engineering of this all-new truck. I’ve started this guide to help you sort through the new features and components, and to help plan your inevitable mods and upgrades. Because this platform is so new, I’m intending this guide to be a work in progress as we learn from our collective experiences with the TRX-4.

Here’s what we’ll cover in the guide:
1. Overview of the TRX-4
2. Transmission and driveline
3. Motor and ESC
4. Axles
5. Suspension
6. Electronics
7. Wheels and tires
8. Chassis
9. Body
10. Reserved for future

If you have any questions, suggestions, or feedback, please let me know. Enjoy!

1. Overview of the TRX-4
The TRX-4 combines the familiar c-channel chassis architecture and linked suspension architecture with a host of features not typically found (some not previously found at all) in RTR scale crawlers. Some of the highlights include:
- 2-speed transmission
- Remote locking and unlocking front and rear differentials
- Portal axles for added ground clearance
- 45 degrees of steering
- Option to upgrade radio with Bluetooth programming capability
- Inner fenders included with stock body

The initial release of the TRX-4 includes a licensed Land Rover Defender 110 polycarbonate body, with some great looking scale accessories bolted on (think roof rack, spare tire, jerry can, high lift, and faux winch):

By the standards of RTR Lexan bodies (admittedly a low bar to clear for this crowd), the biggest letdown is the use of stickers for “windows” instead of masking off inside the body before painting to leave the windows clear. From a functional perspective, the body feels very strong and should prove to be very durable out on the trails and rocks.

Getting the TRX-4 running is as simple as adding 4 AA batteries to the remote and a charged pack to the truck, turning it on, and heading off on the trails. But before you head out on any extended runs, we recommend taking a few preventative steps:
1. Check the pinion gear placement and pinion/spur mesh (see transmission section below)
2. Check the diff, tranny and transfer case gears for grease. Greasing from the factory in early production models has been hit or miss, which isn't uncommon with RTRs. Cases shouldn't be packed with grease, but there should be enough grease to give the gears a nice, full coating.
3. Check your steering endpoints. There have been a few early reports of snapped CVDs, most likely due to oversteering the axles.​

So how does it drive? Box stock, the TRX-4 is well balanced with a very smooth drivetrain and highly capable suspension. Even with the diffs unlocked, the TRX-4 is capable of handling most trailing and light crawling duties. With diffs locked, the TRX-4 turns into a highly capable crawler whose only major let down in stock form is the weak stock servo. But that’s an easy fix (for more on that, see below). Here's a short video I put together that shows the TRX-4 in action in a variety of terrain:

And here are a links to a few helpful video reviews:
Harley Designs (good overview followed by other videos with more details):

John Holmes of Holmes Hobbies (excellent technical overview):

Matteo's RC (excellent demonstration of different features):

And here's a link to a good write-up from Big Squid RC...yes the guys who are best known for their love of bashing have taken a keen interest in scaling over the past year or so:"]http://[/URL]

Box stock is all well and good for a little while, but tinkering will be necessary in some cases and, as we all know, a lot more than half the fun. As you’re starting to consider the options, there is one important limiting factor to keep in mind. If you’re planning to keep the stock axles, your body and wheel/tire choices will be limited by the width of the stock axles and size of the portal knuckles. Simply put, the TRX-4 axles wouldn’t look right under most narrow to medium-width bodies, which pretty much eliminates using this as a class 1 truck with the stock axles. But that still leaves tons of options for making the TRX-4 your own as a class 2 truck, so let’s get on with it…


2. Transmission and driveline
The TRX-4 comes standard with a 2-speed High/Low transmission with 32 pitch gears, mated to a 1:1 transfer case also utilizing three 27T 32P gears. The transmission is mated to a 45T spur with twin plate slipper clutch. There are two important things to note about this transmission:
1. The difference in final drive ratios between high gear and low gear is much greater than most (any?) other 2-speeds on the market. With the stock spur and pinion, the final drive ratio in low gear is 64.64:1 compared to 25.85:1 in high gear. That's a biiiiiig difference.

2. The 25.85:1 final drive ratio in high gear is very tall for a scaler/crawler, which explains why Traxxas recommends using a 550-can motor instead of a 540-can motor if you are going with a brushed motor (see motor and ESC below).​

The first thing you should do before running your TRX-4 is check your spur/pinion mesh and pinion gear alignment. Proper gear mesh is important for prolonging the life of your spur and pinion gears and keeping temperatures down. The TRX-4 motor plate uses fixed motor mounting positions designed to take the guess-work out of adjusting gear mesh. The holes on one side of the plate are labeled with the letters A-H:

The manual includes a table specifying which holes to use for different pinion and spur combinations:
Spur pinion mesh chart cropped.jpg

This is a nice idea for newcomers, but there is a problem in the execution. The recommended motor placement for the stock gearing (hole "C" in the motor plate) leaves spur/pinion mesh too tight, which can cause premature wear of the spur gear and undue strain on the motor (which can in turn cause overheating). Moving the motor one hole further out to hole "D" provides much better mesh for stock gearing:

My testing of several other pinion sizes indicates a similar issue with the recommended motor placement for other spur/pinion combinations. It thus appears that the proper motor placement should be one hole further out than indicated in the table for most, if not all spur/pinion combinations. Given these issues, I suspect it won't take long for the aftermarket to provide a motor plate with more adjustability for experienced builders.

While you have your spur gear cover off, you should also check your pinion/spur alignment. On some early production models (including mine), the pinion gear is installed too far out on the motor shaft, causing the pinion and spur teeth to be offset:

This in turn can cause premature spur gear wear. The fix is simple...move the pinion closer to the motor so that the teeth align better with the spur gear:

With respect to choosing a gear ratio for your setup, if you use the stock motor and ESC, Traxxas recommends no larger than a 12T pinion with the stock 45T spur, and that?s sound advice in my book. The stock Titan 21T 550-can motor running on 3S has plenty of juice in high gear, while low gear provides gobs of usable torque and excellent low-speed resolution.

If you need to service your transmission or transfer case, Harley posted an excellent video showing how to disassemble the tranny and t-case:

Stock driveshafts use hardened steel U-joints connected by plastic male and female shafts. If the durability of the Traxxas Maxx shafts is any indication, these driveshafts should hold up well.

3. Motor and ESC
The TRX-4 comes standard with the Traxxas XL-5 HV ESC and Titan 550 21T reverse direction motor. The XL5 supports up to 3S Lipo, and I definitely prefer 3S to 2S with the stock setup. Although a 21T motor on 3S might seem a bit fast and perhaps lacking in torque for a scaler, the combined gear reduction of the drivetrain and axles more than compensates. Low-speed resolution and torque are very good, particularly on 3S in low gear, and there is plenty of wheel speed in top gear when needed or for giggles when the trail flattens out.

Traxxas and motor guru John Holmes of Holmes Hobbies don't recommend 540-sized brushed motors in the TRX-4 due to the tall 25.85:1 final drive ratio in high gear. So when choosing a motor/ESC combo, keep in mind that you will want to stick with a 550-can brushed motor or a lower kv brushless setup than you would normally choose in other rigs to keep wear and motor temps under control. Holmes Hobbies offers the TorqueMaster Pro 550 in 21T and a limited edition 30T. They also offer the Crawlmaster Pro 550 (which is a 5-slot design) in 10T and 12T flavors, which would be comparable to a 21T and 27T 3-slot motor, respectively.

If you plan to go brushless, a 2200 kv 540-sized sensored brushless 4-pole motor should be a good baseline to get performance similar to the stock 21T motor. My current favorite 4-pole sensored brushless motors are the Puller Pro line (stubby or standard 540 size) and the new Castle 4-pole crawler motors. Matched with a good ESC like the Holmes Hobbies Trailmaster BLE or the Castle Mamba X, both offer very smooth startup and low speed resolution combined with lots of torque and plenty of top end (relative to the kv rating). My word of caution on brushless motors with the TRX-4 is not to overdo it either with kv selection or driving style this early in the game. The TRX-4 is a very new platform, too new for us to know its true weak points, much less for the aftermarket to have addressed them. The last two major releases (the SCX10 II and the Ascender) had weak points at initial launch (transfer case gears and pinion/driveshafts) that seemed fine with a stock RTR setup but didn't hold up so well to brushless power, especially when pushed hard. Eventually, the manufacturers and/or aftermarket developed more durable alternatives, but it took a little while.

One neat feature of the stock ESC when used with the stock radio is the ability to program it through the optional Traxxas Link module. But as noted in the electronics section below, it would be nice to see Traxxas add more programming options through Traxxas Link.


4. Axles
The TRX-4 comes with trick new portal axles that feature steel gears and remote locking/unlocking differentials on the front and rear. The differentials use a 34T ring gear and an 11T pinion, producing gear reduction of 3.1:1. The portal gears use a 10T input gear with a 23T output gear to provide an additional 2.3:1 gear reduction, for a combined 7.1:1 gear reduction at the axle, which is over twice as much reduction as most of the existing scale axles on the market. With 45 degrees of steering on tap (which has become the new standard thanks to the Vaterra Ascender and Axial SCX10 II), steering is excellent with the diffs locked, and it gets even better with the diffs open.

Of course, the biggest benefit to portal axles is ground clearance, and the new TRX-4 axles don?t disappoint. Based on my bench measurements, the TRX-4 with stock tires has over 52mm of ground clearance at the pumpkin, which is approximately 12mm more than you get with an SCX10 II on stock tires. Here's the TRX-4:

Here's an SCX10 II with stock tires:

It makes enough of a difference that the pumpkin rarely comes into play out on the rocks. The center skid becomes the one to look out for.

It should come as little surprise that, between the remote lockable diffs and portals, servicing the axles is a fair bit more involved that servicing a typical locked diff axle. Harley put together an excellent video showing how to disassemble the axles:

As you can see from Harley's video, the axles are a fairly complex design, and with complexity comes opportunities for parts to fail, particularly when pushed to the limits. Thus far, the axles seem to be holding up just fine to typical trailing and crawling. But if you are one of the early adopters, I would recommend some discretion when setting up and running your TRX-4 until we have a better idea of where the points are and, perhaps more importantly, the aftermarket has kicked in with heavier duty upgrade options.

One other design feature to note is the diameter of the axle stubs. The stock wheel hexes leave a portion of the 6mm stub shaft exposed before the stub tapers to the 4mm threads:

With this setup, certain 1.9? wheels will not fit all the way onto the axle without modification. Options include adding a spacer between the wheel hex (this works if the hex slot on your wheel is deep enough to bite with a spacer between the wheel and hub), drilling out the inner hole of your wheels so they fit over the 6mm portion of the shaft, using wheels that take SLW hubs (which fit over the shaft with no problems), or using a wider wheel hex to cover the 6mm portion of the axle stub.

5. Suspension
The TRX-4 utilizes fully linked suspension with 3 links and panhard in front and 4 links in the rear. Links are all metal and use a new rod end design. The TRX-4 also includes newly designed 90mm aluminum GTS coil-over shocks. These new shocks come pre-filled with 30W oil with staggered spring rates (.45 in front, .54 in the rear). The stock setup is silky smooth and provides excellent damping with the stock body and a typical sized battery pack. The shocks are also easily tunable to suit your preferences. I have a sneaking suspension these may become the go-to 90mm shocks for scalers. They are that good (at least so far).

That said, the stock shocks leave the truck riding fairly high. Together with the top-heavy body, that makes for a pretty tippy ride. Switching over to 80mm shocks lowers the CG enough to make the truck much more stable and capable over a wider variety of terrain. Here are a couple of pix, starting with the stock shocks:


After installing 80mm shocks:

If you want to keep the stock shocks, another good option for lowering the ride height is to insert a spacer around the shock shaft below the piston. Either way, lowering the ride height is one of the best performance mods you can do, and it also helps in the looks department. Win win!


6. Electronics
Making all of the gadgetry work requires more electronics than you typically find in an RTR scale off-roader, and time will tell whether this ends up being the Achilles heel of the TRX-4. Here’s a breakdown of what’s included:

  • Steering servo: The steering servo is an updated version of the 2075, called the 2075X. On the plus side, the 2075X is waterproof and uses all metal gears. The downside is that, with only 125 oz-in of torque on tap and a .17 second transit time, it is underpowered and slow for this application. When driving, it’s workable at least for a little while to get you out on the trails and for light crawling, but it didn’t take long for the initial wave of buyers to start seeing failures. So the servo should be at or near the top of your upgrade list. I recommend a waterproof servo with at least 275 oz-in of torque paired with an external BEC. But regardless of whether you are running the stock servo or an aftermarket upgrade, it's very important to set your endpoints so that the servo doesn't oversteer the front axle and put unnecessary stress on the CVDs.
  • Shift and locker servos: Shifting and locker duty is handled by three Traxxas 2065 servos, mated to servo savers. These servos are not known for being the most durable, so it will be interesting to see how well they hold up in the TRX-4.
  • The radio is a 4-channel, Traxxas Link-enabled TQi radio with a 2-position 3rd channel switch for the shift servo and a 3-position 4th channel switch for the lockers (both open, front locked with rear open, or both locked). Although very basic in appearance, the radio allows you to make a variety of adjustments, including exponential throttle curve, steering end points, etc. Without an LED display, you’ll need the manual to figure out how to make it all work. The optional Traxxas Link module allows you to adjust certain settings through the Traxxas smartphone app and a BT connection between your phone and the receiver. The app does make it easier to fine-tune some of your settings, but some basic adjustments (like putting the ESC into crawler reverse or other throttle modes) still require using the transmitter or ESC. All in all, the system is quite workable, but hard core enthusiasts are likely to upgrade their radios at some point to gain more control over their setups.

All in all, with the notable exception of the steering servo, the stock electronics provide a very workable starting point to enjoy the TRX-4. So my recommendation is to start by replacing the steering servo (adding a BEC unless your servo will run directly off 2S or 3S power from the battery) and then replace other components as needed when things break or you’re looking for feature upgrades.

7. Wheels and tires
The TRX-4 includes all-new 1.9 Canyon Trail tires glued to plastic wheels with 12mm hexes. The Canyon Trail tires have a 4.64” OD, and I must say they work very well with the stock foams for an RTR setup. Grip is good in a wide range of conditions, and the sidewalls hold up well to sidehilling loads. If you’re just looking to have fun out on the trails and rocks and aren’t planning on comping, these tires will be more than up to the task. But we know that most folks will want to try their own, so here are some things to keep in mind when choosing your wheel/tire combo.

Let’s start with wheels. As noted in the axle section above, you’ll need to take into account the stock axle stub design and size of the portal knuckles when choosing your wheel/tire combo. For wheels that use SLW hubs, you’ll want to take into account the inner diameter of the beadlock ring when choosing hubs. If the inner beadlock ring has an ID or 40mm or greater, you should be able to clear the portal knuckles with positive offset that tucks the wheels over the knuckles. If not, you’ll need to choose a negative offset that keeps the inner beadlock ring outside of the knuckle. svt_923 put together an excellent thread on RCC showing fitment of different wheel/hub combinations:

One last word on wheels. I don’t recommend 1.55” wheels on these axles. The combination of already wide axles and bulky portal knuckles would require pushing 1.55 wheels way out on the stubs, which would look a bit silly. Stick with 1.9s (or even 2.2s with 1.9 tires stretched onto them).

Choosing a tire for your truck involves lots of variables, including what body you plan to use and how you plan to drive the truck. If you are keeping the stock body, you’ll be able to fit 4.75” OD tires without rubbing or modification, which opens the door to Pit Bull’s 1.9 Rock Beast XL and 1.9 Growler, RC4WD Baja Claws, Pro-Line Super Swamper XLs, and Voodoo KLRs. Anything less than 4.5” will look a bit small inside the large wheel openings on the stock body, but that obviously will be different with other bodies.


8. Chassis
The TRX-4 chassis is designed to support multiple wheelbases ranging from 11.8? (300mm) to 13.2? (336mm). Out of the box, the TRX-4 chassis is set to a 12.75? (324mm) wheelbase to fit the stock Defender 110 body. The stock configuration places the motor and transmission in front of the truck and battery tray in the middle between the chassis rails.

More info on different WB setups coming soon...

9. Body
As I mentioned in the intro, the stock D110 body is one of the nicest RTR bodies on the market. Although it seems quite large at first glance, it turns out to be dimensionally almost identical (excluding the bolt-on fender flares) compared to the RC4WD D110 and Team Raffee D110 bodies.

Here are a few comparison pix with an RC4WD Gelande 2 D110:

And here are a few comparison pix with a Team Raffee D110 pickup sitting on top of a GCM CMAX chassis:



The interior dimensions are also nearly identical to the RC4WD D110 body, which means that it should be fairly easy to install an RC4WD interior. It would take a little bit of cutting to make room for some of the extra stuff underneath the TRX-4, plus ditching the rear body post tower and modding (or replacing) the rear fender wells, but all of that would need to be done to run any interior.

More body-related info to come...


New member
Nice right up. The only thing I questioned is the radio. I know in previous radios, the three position switch actually controlled channels 4 and 5. Both channels would only swing to one side of center.

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Nice right up. The only thing I questioned is the radio. I know in previous radios, the three position switch actually controlled channels 4 and 5. Both channels would only swing to one side of center.

This looks like it uses the same setup. Traxxas calls the radio 4-channel, but the front diff lock servo uses the fourth channel on the receiver and the rear diff lock servo uses the fifth channel on the receiver.


Latest round of updates includes links to some good video reviews, recommended maintenance/vehicle checks before driving, motor recommendations, and links to Harley's excellent videos showing tranny and axle disassembly.


Updated the spur/pinion mesh section based on further testing and monitoring forums/FB. The stock motor position (hole "C") leaves mesh too tight with the stock pinion (11T). Placing the motor at hole "D" with the stock pinion provides much better mesh. Based on bench testing other pinion sizes, the manual's recommended motor positions for other spur/pinion combos also appears to be off (too tight) by one hole, so recommendation is to use one hole further out that recommendation in the manual.

Also, pinion alignment with the spur gear appears to be off on many of the first production examples. Recommendation is to check alignment and adjust as needed before running the truck.


have you mentioned that know traxxas has Over-Drive and Under-Drive gear options for the axles. and spools are also out for those having micro servo issues, and a Single speed as an option.

SSD has a Portal Delete kit which may open up your wheel and tire options. but on flip side affects your gearing greatly.

dont know if you covered these points but just thought i would mention it incase.