Last week we took a deep dive into the TRX-4® drivetrain and installed all the Traxxas aluminum upgrades and heavy-duty gears. For part three, we’re going to swap the stock gray-anodized GTS shocks for blue, and install the new High-Torque 400 Red Brushless Digital servo along with the required High-Output BEC. The blue shocks won’t alter suspension performance, but they’ll sure look nice with all the other blue parts. The servo is the real performance booster—it’s got three times the muscle of the stock unit to keep our project TRX-4 pointed in the right direction no matter how gnarly the trail gets.
Shocks and servoBlue GTS Shocks and High-Torque 400 Red Brushless Servo
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Part 3 Build List
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  • Blue Aluminum GTS Shocks x 2 (8260A)
  • High-Torque 400 Red Brushless Servo (2255)
  • High-Output BEC kit (2262)
  • Metal Servo Horn (8247X)
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GTS Blue-Anodized Shocks
The GTS shocks are available in red, green, silver and blue, and are sold in pairs, so you’ll need two sets to fully outfit your truck. Shock oil is included, but springs are not—we’ll re-use the stock springs. The GTS shocks are sealed by dual X-rings. It’s not required, but you can unscrew the lower cap and put a drop of oil on the seal to help it slide more easily for the filling process.

GTS shocksBlue GTS Aluminum Shocks
Silicone shock oilSilicone shock oil
Oil the X-ringOil the X-ring

Put a few drops of oil into the shock and cycle the shaft to lube the top seal, then fill the shocks per the instructions. After filling the shocks, all you have to do is transfer the springs over from the stock dampers. Note that the rear springs are marked with a spot of green paint. They’re “.54 rate,” which means they’re stiffer than the .45 springs used on the front shocks. We’ll set the shocks aside until it’s time to reinstall the axles we removed and upgraded last week.


Lubricate the shocksLubricate the shocks
Gray and blue shocksGray and blue shocks
Green springGreen spring

What’s an Ounce-Inch?
Servo torque is most often listed in ounce-inches (oz.-in.), or you may see it as kilogram-centimeters (kg-cm). Either way, the principle is the same: the unit refers to a specific weight acting on a lever of a specific length. Here’s a good example: you’re changing a flat tire, and a lug nut is stuck, so you stand on the lug wrench to loosen it. If that lug wrench is a foot long, and you weigh 180 pounds, then you are applying 180 foot-pounds of torque to the lug nut. If we install a servo arm that is one-inch long on a servo rated for 400 oz.-in., it will be able to lift 400 ounces (25 pounds). More ounce-inches = more torque = more tire-turning power.


High-Torque 400 Servo Install
Here’s the stock 2075X metal-gear servo, as viewed from the underside of the truck with the front axle removed. It’s a 125 oz.-in. servo and no slouch, but the 400 servo is more than three times more powerful.

Stock servo2075X stock servo installed

Working from the top of the chassis, remove the four screws that hold the servo in place. The steering link must also be removed—we already disconnected it when we removed the axles. Next, remove the main cover and the wire clamp from the receiver box so you can unplug the steering servo and extract its wire harness. We’ll install the 400 servo with the metal servo horn. We don’t want to waste any of the servo’s torque on servo-horn flex. Before installing the horn, plug the servo into the receiver, power up your transmitter and your truck, and center the steering trim knob on the transmitter. Operating the servo directly from the receiver with no load on the servo won’t hurt anything, and we want to be sure the servo’s output shaft is centered before installing the horn.

Remove the servoRemove the stock servo
Unplug the steering servoUnplug the servo from the receiver
Metal servo hornMetal servo horn


Servo cutaway
Everybody likes a cutaway. The 400 servo is a serious piece of equipment, with an aluminum mid-case, full-metal gears, and dual ball bearings. The motor is brushless, which not only helps the servo deliver its high torque capability, but also gives it faster transit time than a brushed-motor servo. And since it’s Traxxas, it’s waterproof. Not “water resistant,” waterproof.

Before installing the metal servo horn, apply a little grease to the servo’s splines to prevent corrosion. This is optional, but it will make the horn much easier to remove down the road. An ounce of prevention... Secure the servo horn with the screw from original servo. Install the horn as shown in the photo below.

Grease the splinesApply a little grease to the splines
New servo installedThis is what the servo looks like installed

Here’s the installed servo and metal horn viewed from under the chassis. Don’t plug it into the receiver just yet, we need to install the High-Output BEC.

High-Output BEC Installation
What’s a BEC, you ask? Quick history lesson: when electric RC cars really started to take off in the late 70s and early 80s, receivers were designed to operate on 6 volts via a receiver battery (typically four AA “dry cells” or a 5-cell NiCd battery). The 6-cell, 7.2 volt battery that powered your car could damage the receiver and servos if it powered them directly. And so, it was common for RC cars of the era to carry a separate 6-volt receiver battery along with the 7.2-volt “main battery.” The “battery eliminator circuit” (BEC) solved this problem by stepping down the voltage of the main battery to 6 volts. True to its name, the BEC eliminated the need for a separate receiver battery. Today, the BEC is incorporated into the speed control, to assure the receiver gets the same voltage no matter what battery you use in your vehicle.


High-Output BEC2262 High-Output BEC

OK, so why do we need a separate BEC to operate the 400 servo? Because it requires a lot of power when called upon to deliver its full torque capability. Without the High-Output BEC, the 400 servo may draw more power than the speed control is designed to deliver to the receiver, causing the receiver to glitch or “brown out.” The High-Output BEC makes certain the servo and receiver always have full power, even when the servo is maxxed out. The 2262 High-Output BEC kit includes everything you need to install the 2260 BEC in a TRX-4. The BEC mounts on top of the receiver with a foam pad. Install it as shown in the instruction sheet. Place the BEC on the receiver as shown in the instructions, so it will fit under the new receiver box lid properly.

Apply the BEC padApply the adhesive BEC pad
Mount the BECMount the BEC onto the receiver
Receiver box coversReceiver box covers

The receiver-box cover included with the BEC is taller than the stock part. Plug the BEC in per the instructions (the diagram shown here will also help). The speed control’s plug goes into the port on the top of the BEC (be sure to press it in all the way), and the BEC’s plug goes into the receiver’s channel-2 port. Thread the red and black wire harness through the box cover, then install the cover and wire clamp to seal it. A fresh o-ring and silicone grease are supplied in the kit.

Wiring diagramWiring diagram
Y harnessY harness

The BEC’s male red plug goes into the speed control’s female plug—which in our case, is already being used by the LED lighting system. No problem—the BEC includes a Y-harness so both accessories can share the plug.

See Part 1: custom Defender body, beadlock 1.9″ tires and wheels, and LED lighting.

See Part 2: machined diff gears, blue-anodized aluminum accessories, and more.

We’re Not Done Yet - Jump to Part 4 where we put it all back together and hit the trail!

Get your wrench on, and show us how your Trail Truck build turned out! Post your pics to Facebook, Instagram or Twitter with #MyTRX4Build, and we’ll be sure to share some of our favorites.