Portal axles are a feature sometimes found on full-scale off-road cars and trucks. The basic concept is that gears located near the wheel hubs allow the axle to be offset above the middle of the wheel. This provides increased ground clearance, which is often a handy feature when off-roading.
Until recently, I had never seen an RC vehicle with portal axles. That changed when I found an advertisement for the Traxxas TRX-4. It includes these types of axles as standard equipment. I was interested to see if portal axles are actually useful in 1/10-scale. Traxxas provided a TRX-4 Tactical Unit ($450) for this review.
The TRX-4 is a factory-built model that includes a 4-channel 2.4GHz radio system. You will have to provide four AA alkaline batteries for the transmitter as well as a battery for the vehicle. There is a wide variety of applicable vehicle batteries, so I'll cover that in more detail a little later.
My first impression while unboxing the TRX-4 was that it is quite heavy. I later weighed it and confirmed that this truck is indeed beefier than my other 1/10-scale vehicles. The TRX-4 comes in at 7.2 pounds without a battery. My other 1/10-scale vehicles average about 4 to 5 pounds. As you will see, the TRX-4 is heavier for good reason.
Even the body is notably heavy. Like most RC truck bodies, it is made of polycarbonate (Lexan). This one comes pre-painted in a 3-tone camouflage pattern. Its extra weight comes from bolt-on scale details such as a spare tire, gas can, and fender skirts.
The good news here is that the TRX-4 is a rock crawler, trail rig, and basher, so a little extra weight is usually no big deal. Once you include the "cool factor" of the features that make this truck big-boned, I doubt you'll mind the extra baggage. It certainly hasn't bothered me.
This truck is built around a ladder frame chassis with steel beams. The suspension and steering linkages are also steel. Other than the aluminum-bodied shocks, most of the other components found on this truck are made of molded nylon.
The portal axles are just one aspect of the TRX-4's very unique driveline. It all starts with a front-mounted 21-turn brushed motor that is bolted to a 2-speed transmission with a slipper clutch. A thumb-actuated rocker switch on the transmitter allows you to choose low or high gear on-the-fly. Telescoping driveshafts carry power from the transmission/transfer case to the front and rear axles.
Not only do the axles have portal gearing, but they also feature locking differentials. When a differential is locked, both wheels on that axle always turn in unison. This provides better traction and climbing prowess at the expense of turning agility. A 3-position switch on the transmitter controls whether the front, both, or neither differential is locked.
In addition to the large metal-gear servo that handles steering, there are three mini-servos that operate the drivetrain. Two are used to lock the differentials while one shifts gears. The receiver is housed in a sealed case to help protect it from the elements.
The motor is controlled by an XL-5 HV ESC. You can choose from 5 different profiles that dictate how the ESC and motor respond to your commands. I made my usual wiring modifications by removing the motor connectors and swapping the Traxxas battery connector for a Deans Ultra Plug to match my battery stash.
You can use many different types of batteries in the TRX-4. An offset lip on the built-in retention strap lets the battery tray accommodate packs with different heights. There is also a recessed area meant for securing smaller-sized batteries.
Most of my driving has been with a hard-case 2-cell 5000mAh LiPo. 6 and 7-Cell NiCD/NiMH batteries will fit as well. If you want a turbo boost, the system can handle the extra power of a 3-cell LiPo. My hard-case 3-cell batteries ware too tall to fit, but I was able to utilize some of my 3-cell aircraft batteries.
Driving the TRX-4
At this time of year, driving an off-road RC truck in Western New York means driving in the snow. Luckily for me, the TRX-4 is a natural in this stuff. I initially drove the truck completely stock. It did surprisingly well in different types of snow. I thought that the truck's weightiness would make it bog down somewhat easily, but that was not the case. While I could get it stuck in deep snow, this truck handled most of the frozen terrain without much protest.
I experimented with the locking differentials quite a bit. Obviously, the TRX-4 was more apt to get stuck when neither diff was locked. Things improved dramatically once I moved the switch. However, I could not tell any difference in snow-trampling performance between having both differentials or just the front locked. It seemed to do equally well both ways.
Shifting the transmission from low to high gear makes a significant difference in top speed. Even with a 2S LiPo, the TRX-4 is no slouch in high gear. It is even faster with a 3S battery. Low gear was very effective when trudging through the deep snow. I felt like I had very precise control over the wheel speed.
The TRX-4 is tailor-made for rock crawling. While I haven't scaled any rock piles yet, I did climb a few snow drifts. It's fun and there is definitely a lot of driving technique involved. This is one area where I think the truck's weight may have come into play, or at least its weight distribution. I felt like it was top-heavy and would tip over more easily than my other crawlers. Another symptom of top-heaviness I noticed is that the TRX-4 was prone to traction roll when cornering in high gear.
After experiencing a huge improvement in snow driving when I added tire chains to my Kyosho Outlaw Rampage, I wanted to see if they would make much difference with a truck that was already sure-footed in the snow. All I can say is WOW! With chains on all four tires, the TRX-4 was able to drive right through snow that was above the axles. At times, the snow was as high as the bumper, but the TRX-4 never faltered. You could see a bow wave of snow being pushed ahead of the truck!
Cold weather can make plastic parts brittle and prone to break easily. But I have not experienced any malfunctions or broken parts with the TRX-4. Neither the chassis nor the electronics seem to mind being deluged with snow. This is a fairly complex machine as RC trucks go. So I will be interested to see how all of the servos and mechanics hold up over time. Traxxas gear has a reputation for being tough, so I don't expect much trouble.
As I look back over my experience testing the TRX-4, I only have one persistent squawk. The truck is switched on using a momentary button on the ESC. I think the button is somewhat difficult to operate. Holding it down for too short or too long a time prevents the truck from arming. I rarely get it right on the first try. Also, when I forget to disarm the truck before unplugging the battery, I have to relink the transmitter to the truck's receiver. I'd prefer to have no power switch at all, but I'm slowly adapting to the nuances of the XL-5 HV ESC.
There is a lot of headroom to add scale details to this rig. I think the styling of the body invites variations of military or post-apocalyptic themes. I will be exploring those options. If you prefer a more civil look, the TRX-4 is offered with a Land Rover Defender body. Traxxas has also announced a 1979 Ford Bronco body that will be available for the TRX-4. Of course, the best mods are often the ones that you make yourself.
My initial interest in the TRX-4 was squarely focused on its portal axles. To be honest, I don't really think that all of my meandering through the snow provided much of a targeted analysis for those parts. The offset of the TRX-4's portal axles results in about .5" (13mm) of additional ground clearance. That was never a critical factor in the snow. So I think some serious springtime rock crawling is in order before that jury can come in.
I am confident that I stress-tested the truck's other unique features in challenging conditions. Not only did the locking differentials and 2-speed transmission not break, these parts proved to be worth the additional weight and complexity they introduce. The TRX-4 is certainly a more capable and versatile machine because of these features. I know it performs great in all types of snowy conditions. I also look forward to testing this truck in the mud and dirt that will arrive with warmer weather.
Terry is a freelance writer living in Buffalo, NY. Visit his website at TerryDunn.org and follow him on Twitter and Facebook. You can also hear Terry talk about RC hobbies as one of the hosts of the RC Roundtable podcast.