Building the Traxxas TRX-4 Scale Crawler

Traxxas TRX-4 Build PicturesBack in 2009, while the rock crawling segment of the RC car hobby was still in its teething stages, there was quite a bit of speculation that Traxxas would jump into the mix with their very own crawler. Not long after, the Traxxas Summit was released. Based on the Revo platform, the Summit had a long arm independent suspension, a futuristic body, remote locking differentials and a hi/low transmission. It was a very cool truck in its own right but it wasn't quite what most RC crawling enthusiasts had hoped for. It sure seemed like Traxxas would never release a true solid axle RC rock crawler. Fast forward 8 years and the amazing Traxxas TRX-4 was announced. Not only was the TRX-4 the solid axle crawler we all wanted, Traxxas went even further with remote locking diffs, a 2 speed transmission, high clearance portal axles, and an ultra scale Land Rover Defender body. To make things even better, less than a year later Traxxas released a kit version of the TRX-4 for those of us who like to build our own trucks. Now I've gotten a hold of one and will document the build process here for all to enjoy. Let's get started!

Traxxas TRX-4 BoxTraxxas TRX-4 Box Art Photos
The Traxxas TRX-4 comes tightly packed in a deceptively small box. Included in the box is nearly everything needed to build the TRX-4 including wheels, tires, a full set of electronics, a few hex wrenches, a tube of grease and a bottle of shock oil. All that is needed to complete the truck is a 12.3" wheelbase body, paint, tire glue and a battery pack. Traxxas TRX-4 Box ContentsTraxxas TRX-4 Electronics IncludedUnlike any other RC car kit I've come across, the Traxxas TRX-4 includes a full set of electronics. While I'm sure many enthusiasts would like Traxxas to make a version of this truck without electronics, the inclusion of them makes building a kit like this much more inviting to someone new to the hobby. I know that when I first got into this hobby, one of the most confusing parts of building a kit was trying to determine which electronics were compatible with the vehicle I was building. Traxxas reduces that complexity and confusion by including all the electronics needed. Personally, I really like it. After all, if you don't like the electronics they include, you can always throw them on ebay to recoup some of the costs. Traxxas TRX-4 ServosTraxxas TRX-4 XL5 HV ESC
The TRX-4 comes equipped with four servos. Yes, four servos. The largest servo, which is the all metal gear Traxxas 2075X, handles steering duties, while the three micro servos control the truck's remote locking diffs and 2 speed transmission. All four servos are completely waterproof for all weather fun. Traxxas TRX-4 Assembly Instruction Manual
In contrast with any other RC car or truck that has come out of the Traxxas factory for the past twenty years, the TRX-4 comes with a full color assembly manual that makes the build process a breeze. As a kit, you'd expect it to come with a manual like this, but it's still a little weird to see it in person. Traxxas TRX-4 Metal Transmission Gears
The first step of building the TRX-4 kit involves assembling its transmission. The TRX-4 includes a 2-speed transmission which gives you the capability to travel from obstacle to obstacle at a higher rate of speed than ordinary RC crawlers, while retaining the ability to crawl slowly and precisely over that next hurdle. Shifting is accomplished via a transmission mounted micro servo that is actuated by a two position switch on the radio transmitter. A spring loaded servo saver safeguards the fragile gears of the micro shift servo. When it comes to internals, the earliest models of the TRX-4 came equipped with plastic transmission gears that were quickly upgraded to sintered steel metal units. The transmission output shaft goes through an integrated transfer case that is also outfitted with sintered metal gears. Although the metal gears of the TRX-4 transmission should be more than strong enough for extended use, Traxxas added a dual friction disc slipper clutch to help protect the transmission as well as the rest of the driveline. Traxxas TRX-4 24t Transmission Gear ProblemsOne of the only complaints I've heard from multiple people about the TRX-4 kit has to do with its 24 tooth transmission output gear. Apparently, the first production run of this truck included a gear that was machined incorrectly so that the bearing it rides on wouldn't fit properly. It appears those issues have since been resolved as our bearing fit perfectly. For those who come across a kit that still has this issue, Traxxas will send you a replacement gear free of charge. Traxxas TRX-4 Transmission CaseTraxxas TRX-4 2 Speed Transmission ShaftsTraxxas TRX-4 Transmission Assembly PicturesUnlike some other RC crawlers, that shall remain nameless, the TRX-4's transmission went together quite easily. The hardest part was trying to keep the shift fork engaged with its corresponding shift dog while putting the tranny cases together. Traxxas TRX-4 Trans AssemblyTraxxas TRX-4 transmission assembly instructionsTraxxas TRX-4 transfer case assembly instructionsAssembled Traxxas TRX-4 TransmissionTraxxas TRX-4 Transmission PicturesTraxxas TRX-4 transmission transfercaseOnce the main transmission assembly is put together all that is required to complete this section of the build is to place the transfer case gears in place and bolt on a cover. All of shafts and gears in the TRX-4 kit ride on high quality rubber sealed bearings. Traxxas TRX-4 slipper clutch partsTraxxas TRX-4 slipper clutch assemblyTraxxas TRX-4 slipper clutchHere's a shot of the aforementioned dual disc slipper clutch along with the truck's 45t spur gear. Though I can't image why anyone would want to, Traxxas offers a cheap slipper clutch eliminator kit (#8249) for those looking to remove this feature from their truck. Traxxas TRX-4 transmission shift servoTraxxas TRX-4 shift servo installTraxxas TRX-4 shift servo
The TRX-4 shift servo is mated to the transmission with a separate molded plastic mount. The servo horn is then connected to the shift shaft via a short linkage. The servo horn comes pre-installed and factory-centered to make this step easy. One of the biggest warnings that Traxxas reiterated in the manual was not to turn the servo output shaft by more than a few degrees to connect the linkage, otherwise gear damage could occur. Judging by my past experience with micro servos, I'd say they aren't being overly cautious. Traxxas TRX-4 differential gears and casesTraxxas TRX-4 steel diff gears
The next step in the build process, and one of the most exciting in my opinion, involves assembling the TRX-4's awesome portal axles. To start, we've got to get those differentials together. The TRX-4 is equipped with sintered steel metal ring and pinion gears. This is one spot where the SCX10 II kit, with its spiral cut machined gears, really outshines the TRX-4. How well these sintered units hold up to extended use and abuse remains to be seen. Optional machined steel ring and pinion gears, as well as underdrive and overdrive gear sets are available from Traxxas if needed. Traxxas TRX-4 diff assembly picturesTraxxas TRX-4 diffTraxxas TRX-4 differentialHere's a sight you won't see in many other RC rock crawlers. The TRX-4 axles come packed with spider gear style open differentials. You won't see any solid spool lockers here. Traxxas TRX-4 locking differentialsTraxxas TRX-4 diff lock partsOne of the things that make the TRX-4 axles so unique is that the differentials can be locked and unlocked remotely from the radio. The driver has the choice to lock or unlock them all, as well as locking just the front. All from a three position switch on the top of the transmitter. Now you can have your cake and eat it too. With them all unlocked, you get better high speed handling and a tighter turning radius. Lock the front diff and you've got some extra climbing power. And when things get really tough, throw that switch over once more and both diffs are locked for maximum climbing power. Traxxas also offers a spool (#8297) that allows you to keep the diffs locked full time if you'd like to reduce the complexity of this system. Traxxas TRX-4 locking diff detailsTraxxas TRX-4 diff lock cables
Traxxas TRX-4 diff installed
Traxxas TRX-4 diff unlocked
Traxxas TRX-4 diff locked
Traxxas TRX-4 differential unlocked
Traxxas TRX-4 hub shafts
Traxxas TRX-4 front c-hubs
Traxxas TRX-4 front axles
Traxxas TRX-4 front axle portal hub
Traxxas TRX-4 portal axle
Traxxas TRX-4 diff cover
The differentials in the TRX-4 axles are actuated by two chassis mounted micro servos via a sleeved micro cable. This design allows the servos to be mounted away from the axles for increased scale realism without the need for complex linkages. Also, since the servos aren't on the axle, they are better protected from dirt and debris, and are less likely to interfere with suspension linkages during full articulation. Traxxas TRX-4 axle portalsTraxxas TRX-4 axle portal assembly instructionsTraxxas TRX-4 portal assemblyTraxxas TRX-4 portal axleTraxxas TRX-4 portal housingTraxxas TRX-4 portal gearboxesTraxxas TRX-4 axle gearboxes
The following step concerns building the TRX-4's portal gearboxes. This is probably my favorite part of this truck. The portal gearboxes, which are bolted to each end of the axle housings, accomplish multiple things. First off, they add a ton of ground clearance. Since the portal gearboxes raise the axle centerline relative to the output shafts, the axle tubes and diff pumpkin are also lifted, giving you tons of additional ground clearance. Also, since much of the truck's gear reduction is accomplished in the portals, torque twist is greatly reduced. Oh and they just look darn cool too! Traxxas TRX-4 Axle vs Axial SCX10 AxleTraxxas TRX-4 Assembled Rear AxleTraxxas TRX-4 Rear Axle
To accommodate those locking differentials Traxxas had to make the TRX-4's pumpkins quite a bit larger than the more scale SCX10 II AR44 axles. The trade off is well worth it in my opinion. Traxxas TRX-4 Axle Portal GearboxWith the rear axle built, it's time to move on the front. Building the front axle is much like the rear with the addition of a couple of c-hubs and CV shafts that allow the front portal gearboxes to pivot for steering. All of the TRX-4's axle shafts and CV joints are made from a heavy duty hardened steel. Traxxas TRX-4 AxlesHere are both TRX-4 portal axles completely assembled and ready to be bolted to the truck. Check out how beefy everything is. Not only do the axle tubes look super durable, Traxxas even made the suspension mounts nice and thick. Traxxas TRX-4 Pre-Applied ThreadlockAnother thing that separates the TRX-4 from other RC crawlers is the way Traxxas has pre-applied thread locker to any metal to metal screw contact. This may not sound like a big deal but it really speeds things up and eliminates the mess of using a liquid threadlock. It's the little things that count.
Traxxas TRX-4 steel chassis rails
The Traxxas TRX-4 is built around a 1.5mm thick steel ladder frame chassis with injection molded plastic cross members. While the original TRX-4 featured a 12.8" wheelbase, the kit version is designed with a more common 12.3" wheelbase for use with standard crawler bodies. Traxxas also sells additional suspension link kits that let you adjust the truck's wheelbase from 11.8" to 13.2".
Traxxas TRX-4 chassis assembly
Traxxas TRX-4
Traxxas TRX-4 chassis front
Traxxas TRX-4 chassis rear
Assembling the chassis is as straight forward as could be. Just bolt the right side fenders and floorboard, along with the cross members to the right chassis rail. Then bolt the left fenders, floorboard and chassis rail in place and you've got yourself a chassis.
Traxxas TRX-4 front electronics tray
In this picture you can see the front battery plate. Although the TRX-4 also has a larger battery tray that accepts your average lipo battery packs, this plate gives you the choice of using a smaller battery for added forward weight bias. It can also be used to mount other electronic accessories like light or sound modules.
Traxxas TRX-4 front fenderwell
Traxxas TRX-4 rear fenderwell
Another feature that you don't see on many other RC crawler kits are these sweet nylon composite injection molded inner fenders and floorboards. Not only do they add to the scale realism of the truck, but they also help protect your equipment from dirt and debris. The only down side of these fenders is that since they were designed to fit tightly against the original TRX-4's Land Rover body, they usually need a good bit of trimming to fit other bodies. Also, since the shock towers are integrated into the fenders, you can't just remove them. On the up side, Traxxas does offer stand alone shock towers (#8216) that let you delete the fenders completely. There are also quite a few aftermarket manufacturers that make aluminum shock towers as well.
Traxxas TRX-4 rock light mounts
Another cool feature of the TRX-4's fenders is that they've got little mounts molded into them for use with the optional Traxxas rock light kit. Traxxas also includes little plastic covers for those not planning to use the light kit. Oddly, our kit was missing one of the covers and the screw that holds it in place.
Traxxas TRX-4 transmission install
Traxxas TRX-4 installing the transmission
Traxxas TRX-4 spur gear
Once the chassis is assembled you get to start bolting everything to it, starting with the transmission. It is held to the chassis with just three screws which makes for easy repair and maintenance later on.
Traxxas TRX-4 gear mesh
Powering the TRX-4 is a Traxxas Titan 21t 550 brushed motor. I've never had much luck with Traxxas brushed motors in their other vehicles but I believe that is mostly due to over gearing from the factory. That shouldn't be a problem here but I guess only time will tell how well this one holds up.
Traxxas TRX-4 motor
The motor is mounted to the transmission with a fix position style mount. Unlike motor mounts that require you to carefully adjust the pinion to spur gear mesh, fixed position mounts determine gear mesh based on the position the motor is screwed to the mounting plate. This virtually eliminates the possibility of setting the mesh incorrectly. The downside to this type of system is that it usually provides less gear ratio choices. According to the assembly manual, this mount gives you the choice of 16 different gear ratios which should be more than enough for a vehicle of this type.
Traxxas TRX-4 gear cover
Once you've selected the proper motor position for the pinion/spur gears you are using, all you've gotta do is bolt on the gear cover. This holds everything in place and protects the gears from damage.
Traxxas TRX-4 electronics installation
Included in the Traxxas TRX-4 kit is the XL-5 HV electronic speed controller. This may look like the XL-5 ESC of old but it features a few important updates. Those include the ability to run 2S and 3S battery packs, as well the inclusion of additional profiles. The XL-5 HV comes with the standard Race, Sport and Training profiles of the original XL-5, plus the new Trail profile which has a little drag brake, and the Crawl profile which has max hill holding drag brake and instant reverse. As with the servos, the XL5 HV is waterproof so that neither rain, nor snow, nor creek crossings will slow this crawler down.
Traxxas TRX-4 wiring mess
As you can see, hooking up all those servos and an ESC to the TRX-4's 5-channel receiver can be quite the wiring mess. Just take it one step at a time and it isn't too bad. Speaking of the receiver, the TRX-4 comes equipped with a TQi radio system which features a unique multi-function knob that, in combination with a couple of buttons, allows you to changes all sorts of settings including steering and throttle exponential, steering dual rate, and braking percentages. An added feature of this system is Traxxas' exclusive Cruise Control system which allows you to use the mutli-function knob to control the rate of speed of the truck for longer runs such as hiking on trails.
Traxxas TRX-4 diff lock servos
Traxxas TRX-4 differential locking servos
Traxxas TRX-4 battery tray
Traxxas TRX-4 underside battery tray
Along with the front battery plate I showed you earlier, the TRX-4 also has a larger battery tray that allows you to run standard battery packs. More specifically, Traxxas says that this tray will hold 7,600 mah 2S or 5000 mah 3S packs. To secure the battery in place, the tray features an integrated hinged quick release strap that requires no clips. Now you don't have to worry about losing battery hold down clips during pack changes on the trail. The battery strap can even be flipped over for added adjustability.
Traxxas TRX-4 aluminum shock bodies
When it comes to damping, Traxxas has you covered with these gorgeous aluminum GTS shock absorbers. They feature threaded bodies for quick ride height adjustments, a silver anodized finish for added durability and improved looks, and dual x-ring shaft seals for no leak reliability. That may sound like a marketing gimmick but from everything I've read these seals are really great at keeping the shock oil where it should be. Even a lot of the Axial guys use these seals in their shocks because of how little they leak.
Traxxas TRX-4 shock oil
The Traxxas TRX-4 GTS shock bodies come pre-assembled from the factory. All you have to do to complete them is to fill them with the included shock oil and install the springs. When it comes to filling them up, there are a few things to watch out for. Since these shocks don't have bladders or bleed screws in the caps you'll need to make sure not to overfill them. I suggest filling them about 3/4 of the way and slowly work the piston up and down a little (near the bottom of their stroke) to work out any air bubbles. Then push the piston up and fill them to about a 1/4" from the top. Once you screw the cap on, work the piston up and down. If you feel any major resistance toward the top of the stroke, you have too much oil in the shock. Drain a tiny amount and try again.
Traxxas TRX-4 shocks
Traxxas TRX-4 shock build
Once you've got the shocks filled with the proper amount of oil, just slide the springs on and pop the lower spring retainer in place.
Traxxas TRX-4 steel links
Traxxas TRX-4 rod ends
Traxxas TRX-4 link assembly
The next step in the build process deals with installing the axles onto the chassis. Usually at this point you'd have to kill your fingers by screwing tiny rod ends onto the suspension and steering links but thankfully Traxxas pre-assembled them all at the factory. The TRX-4 kit comes equipped with beefy 5mm steel suspension links with heavy duty 4mm end links and full y captured steel hollow balls. They aren't quite as "bling" as the polished aluminum links that came with the SCX10, but there is no doubt that these solid steel links will take anything you can throw at them.
Traxxas TRX-4 front axle install
Traxxas TRX-4 front axle build installation
Traxxas TRX-4 front axle pictures
It isn't just the center of the axle that benefits from the additional ground clearance of the Traxxas TRX-4's portal axles. This design allowed Traxxas to also raise the link mounts, as well as the steering tie rods, much higher than they'd normally be with your average axle. Not only does that keep the links out of the way of those pesky rocks on the trail, but it also reduces suspension link angle for less stress on the links and mounts. The TRX-4 features a 3 link with panhard bar design up front...
Traxxas TRX-4 rear axle install
Traxxas TRX-4 rear axle installation
Traxxas TRX-4 rear axles pictures
Traxxas TRX-4 rear axle
Traxxas TRX-4 driveshafts
Traxxas TRX-4 front driveshaft
Traxxas TRX-4 rear driveshaft
Traxxas TRX-4 axles installed onto chassis
Transferring power from the transmission to the axles is a set of molded plastic shafts with hardened steel joints. They appear eerily close to the sliding center shaft design seen on the SCX10 II. The TRX-4's driveshafts went together quickly, and thanks to the pre-applied thread locking compound we mentioned early, attaching them to the truck was as easy as can be.
Traxxas TRX-4 wheels and tires
Traxxas TRX-4 wheels
Traxxas TRX-4 tire details
Traxxas TRX-4 tire tread
You know the build is quickly coming to an end once the wheels and tires come out. The TRX-4 includes a set of black split spoke wheels that have cool looking faux beadlock rings for that trail ready vibe. Wrapped around those are Traxxas' very own 1.9" Canyon Trail crawler tires in the flexible S1 compound.
Traxxas TRX-4 assembled chassis
Traxxas TRX-4 chassis rear
Traxxas TRX-4 chassis rear
Traxxas TRX-4 chassis
Traxxas TRX-4 chassis side
Traxxas TRX-4 chassis front
Traxxas TRX-4 chassis
Traxxas TRX-4 chassis top
Other than a few odds and ends, our Traxxas TRX-4 is nearly completed! Those scale fenders and floorboards definitely give this truck a unique appearance, and the integrated wire clips sure keep things neat and tidy.
Traxxas TRX-4 chassis front bumper winch shackles
Traxxas TRX-4 chassis rear bumper
Traxxas TRX-4 chassis tow shackles hooks
To finish the TRX-4 chassis build I bolted on the included front and rear bumpers. Something that bugged me about the SCX10 II kit was that Axial didn't include a winch for its front bumper. The bumper has a nice spot for a winch, and it even has screw holes for one. Yet there is no winch in the box. Traxxas, on the other hand, does indeed include a faux winch with the TRX-4. Sure, it isn't the prettiest thing in the world, but at least there isn't a giant hole left on the front bumper. Also included with the TRX-4 are these sweet scale tow hooks for both bumpers. Nice! Now on to that body...
Traxxas TRX-4
Traxxas TRX-4 Pictures
Traxxas TRX-4 zombie hunter
Traxxas TRX-4 custom paint job
Traxxas TRX-4 custom paint
Traxxas TRX-4 bloody grill
Traxxas TRX-4 bloody hand prints
Traxxas TRX-4 rust
Traxxas TRX-4 zombie
Traxxas TRX-4 details
Traxxas TRX-4 rusty rat rod paint job
Traxxas TRX-4 rusty paint
Traxxas TRX-4 rusty paint job
Traxxas TRX-4
Traxxas TRX-4 zombie hunter pictures
Traxxas TRX-4 Photos
Traxxas TRX-4 crawler pictures
Traxxas TRX-4 rc crawler
Project Zombie Hunter lives!!! Since I had a Jeep Cherokee XJ body left over from my SCX10 II build, I decided to give it a try on the TRX-4. Overall I'm very please at how nice this body looks on the TRX-4. To get the body to fit I did have to trim about 1/4" of plastic from each fender, as well as a little from the front of the front fenders. As for the paint, I chose to go with a post apocalyptic rusted out look with a ton of zombie blood all over the hood. I can almost see Rick and Daryl mowing down zombies in this sweet rig right as I type this. To get the rust effect I used Tamiya camo brown and Createx red oxide which were applied with a sponge and airbrush. The zombie blood on the hood was made with a custom mixture of Createx red, blue and red oxide which was sponged and brushed on. The zombie hand prints and streaks were hand painted with a small brush and the same bloody paint mixture.
Traxxas TRX-4 rc rock crawler
Traxxas TRX-4 custom paint details
Traxxas TRX-4 paint weathering
SCX10 vs TRX-4
Traxxas TRX-4 taillights
Traxxas TRX-4 rea
Traxxas TRX-4 hood paint
Traxxas TRX-4 bloody paint
Now that my Traxxas TRX-4 is finally completed, the only thing left to do is to take it out for a run on the trail. Since it's the middle of the winter, and there is quite a bit of snow on the ground, I wasn't able to test the TRX-4's true rock crawling capabilities, but I did get some time behind the transmitter. The first thing that became abundantly clear was that those sweet portal axles make a huge difference compared to ordinary axles. Rocks, sticks, snow and other debris that would usually stop a 1/10 crawler in its tracks were no match for the TRX-4. The added ground clearance allowed it to crawl right over anything standing in its way. Another cool aspect of the TRX-4's axles are its remote locking differentials. At first this feature may seem a little gimmicky but just being able to see the difference between locked and unlocked diffs was worth it to me. I don't think I'll ever take locked diffs for granted ever again.
Traxxas TRX-4 snow
Traxxas TRX-4 snow crawling
Traxxas TRX-4 rock crawling
Traxxas TRX-4 bloody snow
Traxxas TRX-4 snow crawling
Traxxas TRX-4 snow day fun
As mentioned earlier, the TRX-4 comes equipped with a two speed transmission. I really didn't think I'd make much use of this feature but it really comes in handy for those times when you're trying to get from one set of rocks to another. Don't get me wrong, you won't be breaking an speed records even in high gear but it still comes in handy at times. Once you get to your next obstacle, just drop it in low gear and the TRX-4 slows down to just the right speed for precision crawling.
Traxxas TRX-4 rc rock crawling
Traxxas TRX-4 building pictures
Traxxas TRX-4 ice
Traxxas TRX-4 winter crawling
Traxxas TRX-4 rock crawling pictures
building the Traxxas TRX-4
Traxxas TRX-4
Traxxas TRX-4 photos
Although I couldn't get much rock crawling done on the snow and ice covered rocks in my yard, I did get a chance to test the TRX-4's crawling prowess on a mixture of steep and slippery inclines. Overall I was very impressed at how well the truck performed in these situations. It felt planted and I didn't have any rollovers on the steeper stuff. I hope to test this truck out in a more typical rock crawling environment in the coming weeks, or whenever this darn snow melts.
Traxxas TRX-4 custom
custom Traxxas TRX-4 pictures
Traxxas TRX-4 bulldog
Traxxas TRX-4 custom build pictures
When it comes to reliability, I have yet to find any flaws. That may be a little boring but it's true. Between the build and its first few runs, the TRX-4 hasn't given me even the slightest hint of trouble. Traxxas really did their homework on this one.
Traxxas TRX-4 zombies
If you'd like to build a Traxxas TRX-4 yourself, while also supporting this blog, please purchase your kit from Amazon. We get a small commission from the sale which helps us fund future projects. Thanks for following along!
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