Project Build: TRX-4 Ford Bronco Ultimate Crawler

There’s no hotter off-road-rated SUV in the full-scale universe than the new Ford^® Bronco, and Traxxas’ high-performance TRX-4^® model of Ford’s massively popular 4X4 is just as big a hit. Like all TRX-4’s, it’s super-detailed and officially licensed for display-worthy realism, and extremely well equipped with premium features to tackle adventure on the trail just like the “real thing.” For our latest Project build, we’ve dialed in a TRX-4 2021 Bronco for competition-ready crawling performance while making it even more realistic with functional Pro Scale^® accessories. Our Bronco build delivers jaw-dropping terrain-crawling capability without sacrificing the authentic looks and versatility that makes the TRX-4 platform the choice of so many trail truckers. Let’s get into it!

Chromoly, Machined-Steel, and Going Slipperless
The Bronco rolls out with a full set of metal gears in the axles and transmission, but we took durability up even higher by installing the optional hardened-steel ring and pinion gears and axle shafts. The extra-strong gears are particularly good for our build because we also removed the factory slipper clutch in favor of a direct-drive setup using the 8249 Slipper Eliminator. As the name says, the slipper clutch will slip when overloaded to protect the drivetrain. Even when tightened down fully, the clutch will slip if enough torque (a lot of torque) is applied. For virtually all off-road scenarios, it’s a great feature. But for comp-style driving, we want our Bronco to keep putting torque into the tires no matter how much resistance an obstacle is giving. The Slipper Eliminator replaces the slip mechanism with keyed steel plates that lock the spur gear to the input shaft for total torque transfer.

Understanding Overdrive
Back to those machined-steel ring and pinion gears: instead of using gears with the same tooth counts as the stock gears, we chose the Overdrive and Underdrive gear sets so front axle will spin its wheels faster than the rear axle, even though both axles are receiving the same input RPM from the transmission. Why do this? When an obstacle or climb requires a steep chassis angle, much of the truck’s weight is transferred to the rear tires, reducing the front tires’ grip. Applying the throttle transfers even more weight to the rear tires, leaving very little grip for the fronts. It’s almost entirely up to the rear wheels to push the truck up over obstacles. Overdriving the front axle helps the front wheels pull the truck up slopes and over obstacles, instead of being pushed from the rear axle. That’s the short version—take a look at How To Maximize Extreme-Terrain Crawling Ability with Overdrive for a deep dive.

The TRX-4 platform’s high-strength composite parts are engineered to deliver all the durability you need for any off-road adventure, including comp driving. Traxxas offers a lineup of aluminum axle components that are even tougher than composite, but the benefit you’ll actually notice is the extra traction the heavier aluminum parts help provide. That, and the anodized parts simply look fantastic. In the case of our comp build, adding the aluminum outer portal covers also let us install brass portal weights on the front axle. More weight up front is a plus no matter what the chassis’ attitude is, but it’s especially helpful when a steep approach is required and weight transfer to the rear of the truck robs front grip. Adding weight to the front axle helps keep precious grams over the front wheels.

All TRX-4s include aluminum-body GTS shocks equipped with dual X-ring seals and plated shafts for smooth damping action and leak-free performance. To give our Bronco the ultimate in slick shock action, we upgraded to the hard-anodized versions with PTFE-coated bores and titanium-nitride coated shafts. These coatings reduce the amount of friction that must be overcome to get the pistons and shafts sliding through the seals and along the shock body. This is particularly important for a comp rig where you’re crawling slowly over obstacles and low-velocity weight shifting is doing nearly all the shock activation, as opposed to the sudden powerful jolts that easily overcome “stiction” when off-roading at speed. While we were swapping shocks, we also traded the plated steel hollow balls for PTFE-coated aluminum balls to achiever even smoother suspension action.

Most TRX-4s come fitted with a 125 oz-in metal-gear servo and metal servo arm, so there’s no shortage of steering strength. But for comp duty, there’s no such thing as too much servo torque. For this build we outfitted the Bronco with the High Torque Red 400 servo. This high-tech powerhouse uses a brushless motor to aim the front wheels with 400 oz-in of torque. That’s enough power to pull 25 pounds, and the Red 400 does it with digital precision and the strength of full-metal gears.

The Canyon Trail's aggressive tread design and soft S1 compound match or even outperform all the comparable aftermarket treads we’ve tried. So, no upgrade required in the tire department. Where we did upgrade is the wheels. Beadlock wheels clamp the tires in place mechanically, so it’s easy to reuse the wheels when it’s time for new tires or you want to try out a new tread. The Method 105 replica 1.9″ beadlock wheels are sold in pairs in two finishes: charcoal gray (our choice), and black chrome. The beadlock rings are sold separately in different colors, so we went with red for a bright high-tech look. The rings include mounting screws with a black-oxide finish, but you can also swap them out for the optional stainless steel screws.

The 2021 Ford^® Bronco and other officially-licensed Ford, Chevrolet^®, Land Rover^® and Mercedes-Benz^® TRX-4 and TRX-6 trucks are authentically proportioned and loaded with scale detailing. Pro Scale accessories bring even more life to these TRX machines, and the lineup includes bright, custom-configurable LED light sets for all models as well as a high-torque electric winch.

We equipped our Bronco with the winch and complete Pro Scale Advanced Lighting Control System, and added the optional Rock Light kit for maximum realism and trail-brightening capability. We also installed the Traxxas Link^™ Wireless Module in the TQi^™ transmitter to enable full control of the lighting system right from the transmitter. For all the details on Pro Scale Lighting, check out Pro Scale Advanced Lighting Control System Basics, How to Add Lighting Accessories to Your Pro Scale Lighting Kit, and the TRX-4 Pro Scale Lighting Guide.