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Traxxas TRX-4 Bronco 2021 Crawler 92076-4


Trail tech explained, and why the latest TRX-4 Works So Well

Is the new Bronco the best TRX-4 yet? Maybe! Any TRX-4 will be a top performer on the trail, but the Traxxas replica of Ford’s red-hot 4X4 is proving to be an especially big hit with off-roaders everywhere. The 2021 Ford Bronco TRX-4 replica truck unleashes extreme-terrain performance with its proven portal axles, and unbeatable versatility with remote-locking differentials and shift-on-the-fly High-Low transmission. These features are topped with maximum convenience and realism thanks to an all-new clipless body mounting system. Simply put, you get a ton of truck with the 2021 Bronco. Let’s take a look at what makes this new machine an extra-special TRX-4.

Approach, Breakover, and Departure Angles
No matter how well-spec’d a 4X4 is in the suspension, power, and tire departments, it’s not going to get too far off road without appropriately steep approach, departure, and breakover angles. If your truck’s front bumper is drilling into the hillside before the front tires even reach the grade, you’re just going to be parking in front of that hill, not climbing it.


Bronco off-road abilities
An off-roader’s ability to climb and descend grades and traverse obstacles without dragging hard parts is determined by the approach, departure, and breakover angles. 

As illustrated below, drawing straight lines from the tire’s tread to the first intersecting hard points of the chassis reveals the approach and departure angles. Any grade up to that steepness can be climbed or ascended without hanging the truck up on the terrain (assuming you’ve got the power and traction—we’re just talking clearance). The breakover angle illustrates the maximum peak the truck can traverse without “high centering” and getting stuck. Breakover is measured at maximum suspension extension (or “droop”) since gravity will fully extend the shocks if you’re in a maximum-breakover situation.

Bronco angles
Steep approach and departure angles are essential to off-road performance, and breakover clearance can be the difference between making it over an obstacle and getting stuck on it.

Bronco droop

Like the other TRX-4 models, the 2021 Bronco’s breakover angle is measured at maximum suspension extension, or “full droop.”

TRX-4 Guide
Click here to see and compare the full TRX-4 lineup.


Center of Gravity: Lower is Better
Designing a capable off-roader represents an engineering dilemma: you can jack the truck up on a high-lift suspension for maximum ground clearance and steep approach and departure angles, but that raises the center of gravity and compromises stability. If you optimize for a low-CG design, you’ll improve sure-footed handling on less difficult terrain but at the expense of climbing and crawling capability in rugged conditions. Naturally, there’s a sweet spot in the middle, and the TRX-4 models in general and the 2021 Bronco strike that balance while also offering greater ground clearance and a lower center of gravity than other rigs on the trail.

Bronco low-CG
The TRX-4 platform holds the battery and motor as low as possible without compromising the drivetrain or suspension geometry. The stepped battery tray can also hold a smaller battery in an even lower position.

The TRX-4 platform positions the motor behind the front axle rather than over it, and centers the transmission so the motor and drivetrain can sit as low in the chassis as possible without interfering with the suspension links or driveshafts. The battery tray is centered in the chassis for equal distribution of weight along the chassis’ centerline, and the tray features a recessed position that allows you to run the truck on a 2-cell or 3-cell compact 1/16 scale battery. The smaller battery saves weight, and the tray holds that reduced weight even lower in the chassis for an extra-low CG—without compromising ground clearance. The 2021 Bronco replica body also helps lower the truck’s CG, by virtue of its low hood and rooflines.

Bronco profile

Like the full-size 2021 Bronco, the TRX-4 model’s low roofline compared to taller trucks helps lower its CG for a sure-footed feel. 

Bronco droop
Mounting a compact 1/16 scale battery between the fender wells shifts more weight over the front axle for enhanced climbing and crawling ability, as seen here. Use the 8222 tie down strap to keep it secure.


Portal Axle Performance
We can’t talk TRX-4 trucks without mentioning portal axles, and the TRX-4 platform’s signature feature is a big part of what makes the 2021 Bronco such an outstanding off-roader. Portal axles come with two huge benefits: a major boost in ground clearance without raising the chassis and body, and the elimination of the power-wasting, traction-robbing torque twist that straight-axle trucks suffer from. Let’s get into it…

TRX-4 portal axles
The portal axle lifts the differential housing for increased ground clearance. As shown in red, a straight-axle design is much more prone to striking obstacles.

The most obvious portal-axle benefit is the additional ground clearance it offers by lifting the differential housing. The diff housing is the lowest part of a 4X4’s underside, and the clearance between it and the terrain is often the difference between going over an obstacle or around it. Steel driveshafts pass through the axle tubes as in a straight axle, but instead of exiting the axle tubes to meet the wheels, each shaft turns a pair of gears to transfer the driveshafts’ rotation to the wheels. These gears lower the output shafts and raise the differential relative to the wheels’ centers. The portal gears also increase the amount of gear reduction built into the axle. This additional gear reduction comes with its own benefits, as we’ll discuss next.

Portal reduction
The TRX-series portal axle has a total gear reduction of 7.90:1, so less gear reduction is required at the transmission.

A straight axle’s gear reduction is determined solely by the ring and pinion gear and the gear ratio is usually around 3:1. This ratio is much too “tall” for off-roading, so most of the total gear reduction in a straight-axle truck comes from the transmission. That means most of the truck’s torque is also being generated by the transmission. When that torque is delivered to the axles via the driveshafts, it rotates the chassis in the opposite direction, resulting in “torque twist.” Watch a straight-axle truck as the driver leans into the throttle to climb or accelerate, and you’ll see the chassis lean over. This compromises traction and stability, and doesn’t look very scale.

Torque twist
Torque-twist occurs when the torque applied to the axles also affects the chassis, causing it to lean under power.


All these forces are also at play in a TRX-4, but much more torque is generated by the axles’ gearing instead of relying on the transmission to provide all the gear reduction and make most of the torque. Since the transmission doesn’t need to send nearly as much torque through its driveshafts to the axles, the amount of torque that must be counteracted by the chassis and suspension is very low. When the 2021 Bronco (and any other TRX-4) encounters a tough obstacle or steep climb, you get straight, stable forward motion—not torque twist.


Axle Articulation: Just Right
All the TRX-4 models use a 3-link front suspension with a Panhard bar and a 4-link rear suspension to allow plenty of bump- and obstacle-absorbing axle swing and articulation while keeping the axles centered under the chassis. It’s common to see RC 4X4s modified to allow articulation to the point that the axles can be rotated a full ninety degrees to each other. That’s great for striking impressive poses, but not a good setup when the goal is impressive off-road capability. Axle articulation is a good thing that a truck can have too much of—and that’s a bad thing.

2021 Bronco articulationFull-scale axle articulation is often demonstrated and measured using an articulation ramp with a 20-degree incline.

You may have encountered “RTI” as a measurement of articulation, which stands for Ramp Travel Index. By measuring how far a truck can drive up a 20-degree ramp before the trailing wheel lifts, then dividing that measurement by the wheelbase and multiplying by 1000, you get a Ramp Travel Index or “RTI score.” The full-size 2021 Bronco has an RTI score of about 700 (with its swaybars detached), and the Traxxas replica goes even higher (in scale, that is), scoring a 950. Even more articulation is possible, but not helpful. A truck with too much articulation will prevent the chassis from weighting the wheels properly to generate maximum grip, and super-articulated axles will offer little resistance to the chassis flopping to the side when turning or side-hilling.

The 2021 Bronco and all the TRX-4 models have been carefully engineered and tested to offer the perfect amount of articulation and roll resistance for high performance in extreme terrain while offering stability and secure scale-like handling in smoother conditions. You can fine-tune your suspension setup on the trail using the GTS shocks’ threaded spring collars. Decreasing spring preload from the factory settings will make the axle articulation feel more “active” in technical terrain.

GTS shock adjustment
All TRX-4 models feature threaded-body GTS shocks for tool-free preload adjustments.


Trail-Taming Tires
Even the best-built comp truck will struggle if it’s not wearing a set of versatile high-performance tires that can deliver the gift of grab. Thankfully, the 2021 Bronco’s Canyon Trail 1.9” tires are some of the best trail treads available, besting even popular aftermarket tire choices. The tires are molded in soft S1 compound, a proprietary rubber formula that offers superior wet and dry traction on all types of surfaces. The aggressive, realistic Canyon Trail tread design features widely spaced and siped lugs that wrap onto the sidewalls to keep the Bronco gripping and grabbing even at extreme angles. Truly trail-worthy tires are an expensive add-on for some other trucks, but not the 2021 Bronco or any other Traxxas TRX-4 model—you get premium performance tires right out of the box.