A Complete Guide to Boom Racing BRX PHAT™ Axles

The BRX PHAT™ axle ecosystem is a modular scale performance platform engineered for maximum compatibility across different vehicles, suspension configurations, and driving styles.

The BRX PHAT™ axle ecosystem supports:

• Straight Axle
• Center Pumpkin Axle
• Offset Pumpkin Axle
• Portal Axle
• Pass-Thru Axle

Built Around Interchangeability

The BRX PHAT™ axle platform was designed around one core principle: adaptability.

Instead of separate axle systems for every application, the BRX PHAT™ ecosystem shares a wide range of compatible components across multiple configurations. Builders can mix and match parts depending on the vehicle, terrain, or driving style they want to achieve.

This modular approach allows the platform to evolve alongside your build.

Straight Axle





Straight axles are the foundation of the BRX PHAT™ platform. Designed for scale realism, durability, and predictable handling, they are ideal for traditional trail trucks and scale builds.

Available in BRX70, BRX80, and BRX90 widths, the system supports a wide range of body styles and chassis setups.

Features include:

• AR44-style triangular keyed gears available in standard, overdrive, and underdrive ratios
• Reversed-cut rear gears for counter-rotating driveshafts
• BADASS™ heavy-duty steel universal driveshafts
• High steering angle geometry
• Realistic scale dimensions
• Weighted cast alloy housings for a lower center of gravity
• Durable construction for heavy hardbody trucks
• Compatible with linked and leaf sprung suspensions
• Reversible bevel gear positions
• Detachable truss and link mounts
• Adjustable caster angles
• Removable third member for 6x6 and 8x8 pass-thru configurations
• High pinion angle design for improved clearance

Specifications





List of the widths and weight information with part numbers for each axle.

Note that all of the above axles come with standard rotational gears, while BRX01 and BRX02 replacement rear axles (part numbers not listed above) come with reversed gears.

Offset Pumpkin Configurations



The offset BRX PHAT™ axle system expands the platform further with offset differential housings for improved scale realism and driveline alignment on specific vehicle platforms.

The offset system includes:

• Reversible offset differential housings
• Dedicated BADASS™ offset universal driveshafts
• Aluminum upper link truss system
• Compatibility across BRX70, BRX80, and BRX90 straight axles
• Conversion parts for existing center pumpkin axles

Offset pumpkins are especially useful for builds requiring more realistic driveline placement and scale accuracy.

Portal Axle Configurations



For builders looking for increased ground clearance and enhanced crawling capability, full portal axles and portal conversions are currently available for straight, center pumpkin axles.

Portal axles provide:

• 15mm or 0.59" additional axle clearance
• Reduced drivetrain torque load
• Improved obstacle capability
• Enhanced technical crawling performance
• Hardened steel gears
• Four machined heavy-duty steel gears for strength and durability
• Reverse gear compatibility for rear axle setups
• Anti-twist steel brackets for 0° and 10° caster configurations

Pass Thru Axle Configurations


(Part #BRX026623)

The BRX PHAT™ pass-thru axle system was engineered specifically for multi axle trucks and specialty scale builds where drivetrain layout, driveshaft direction, and axle rotation become far more complex than a standard single axle setup.

Unlike traditional axle designs, the Boom Racing pass-thru axle uses a heavy-duty helical gear with center-pinion outputs on both ends of the axle. This center-pinion design gives builders significantly more flexibility when configuring custom drivetrains for 6x6, 8x8, and other multi axle vehicles.


(Part #BRX026615KIT)

Depending on whether the build uses counter-rotating or standard transfer cases, the center-pinion helical gear can be flipped inside the axle housing to reverse driveline rotation without affecting the pinion output position for the connecting axles. This is one of the biggest advantages of the BRX PHAT™ pass-thru axle system, allowing builders to configure multi axle drivetrains with far greater flexibility while maintaining proper driveshaft alignment and consistent axle geometry.

For Boom Racing BRX drivetrains, which use counter-rotating transfer cases, the pass-thru axle works perfectly out of the box, including the third (last) rear axle. For custom builds, the same axle can also be configured for standard rotating drivetrains, giving builders maximum compatibility across different chassis and drivetrain layouts.

The center-pinion design also eliminates a common compromise found on many traditional multi axle setups where one side of the axle operates as a low-pinion output. Low-pinion outputs can create steeper driveshaft angles, reduced ground clearance, increased driveline stress, and less efficient power delivery, especially on the last axle of a multi axle truck.



By keeping both outputs centered, the BRX PHAT™ pass-thru axle maintains improved driveline alignment throughout the vehicle for smoother operation, cleaner drivetrain geometry, and more efficient power transfer.

The BRX PHAT™ pass-thru axle system is also highly modular. Builders can configure the axles as steering or non-steering versions and choose from all available BRX PHAT™ widths including BRX70, BRX80, and BRX90 configurations. This gives builders exceptional flexibility when creating anything from compact trail trucks to full-size 6x6 and 8x8 heavy-duty rigs.

Advantages of the BRX PHAT™ Pass-Thru Axle System:

• Compatible with both standard and counter-rotating drivetrains
• Reversible center-pinion helical gear design
• Eliminates low-pinion output compromises
• Improved driveshaft alignment and driveline geometry
• Better ground clearance and reduced driveline angles
• Smoother and more efficient power transfer
• Reduced driveline stress and binding
• Available in BRX70, BRX80, and BRX90 widths
• Configurable as steering or non-steering axles
• Ideal for 6x6, 8x8, and specialty scale builds

(Part #BRX026624)

Pass-thru axles allow drivetrain power to continue through the axle assembly, making them ideal for:

• 6x6 and 8x8 trucks
• Heavy utility builds
• Military style platforms
• Custom multi axle projects

The rear axle on a multi axle truck requires a unique third member design. Unlike a standard high-pinion third member, it uses a center-pinion output with a proprietary AR44-style helical-cut axle gear set (#BRX026615KIT).

With the center pass-thru diff unit #BRX026624, building a pass-thru steering axle is identical to building a standard steering axle. Simply use the appropriate knuckles, matching C-hubs, and universal driveshafts for the selected axle width.

For non-steering configurations, use lockouts with the matching rear driveshafts for the selected axle width.

Linked and Leaf Sprung Compatibility



The BRX PHAT™ ecosystem was designed to work across multiple suspension architectures.

Compatible configurations include:

• 4-link suspension
• Leaf sprung setups
• Panhard configurations
• Servo-on-axle
• High clearance link systems
• Hybrid linked and leaf sprung suspensions
• Custom chassis configurations

Builders can tailor the axle setup to match the realism and performance goals of their project. Leaf spring retainers and shackles are available in different lengths to suit your custom builds.

Optional BRX PHAT™ Axle Accessories



A wide range of optional tuning and upgrade parts improve durability, weight bias, driveline setup, steering response, and scale realism.

Available upgrades and accessories include:

Weight Tuning

• brass knuckles
• brass lockouts
• additional adjustable knuckle brass weights
• brass portal weights

Drivetrain Tuning

• OD/UD gears
• offset conversion

Scale Realism & Performance

• brake disc hexes
• trusses (centered/offset)
• link mounts
• XT diff cover
• stainless steel skid plate

These optional components allow builders to fine tune weight distribution, steering response, driveline configuration, clearance, and overall performance while maintaining compatibility across the BRX PHAT™ ecosystem.



Front or Rear Steering Link Setup

Different knuckle arm options are available to suit both front and behind-the-axle steering link setups, allowing you to configure the axle for your preferred steering geometry and chassis layout. Compatible with both front and behind-the-axle applications for added versatility in custom builds.






Shared Components Across the Ecosystem

One of the biggest advantages of the BRX PHAT™ ecosystem is shared component compatibility.

Many parts are interchangeable across multiple axle configurations, including:

• Ring and pinion gears
• BADASS™ universal driveshafts
• Steering components
• Wheel hubs and hexes
• Brass upgrades
• Bearings and hardware
• Suspension mounting components

This allows builders to upgrade or reconfigure their rigs without replacing complete axle assemblies.

Built to Evolve

The BRX PHAT™ ecosystem continues to expand with new configurations, upgrade parts, and tuning options.

Whether building a scale expedition truck, heavy hardbody crawler, competition style rig, or multi axle utility vehicle, the platform was engineered to be future-proof and compatible across the BRX lineup.

The BRX PHAT™ axle platform is more than just an axle. It is a modular scale performance system built to adapt, evolve, and grow with your build.

Which PHAT axle is right for your build?

• BRX70 → narrow scale builds
• BRX80 → traditional scale width
• BRX90 → wider heavy hardbody builds
• Straight → realism and lower CG
• Portal → maximum clearance
• Offset → realistic driveline layouts
• Pass-thru → multi axle trucks

FAQ

What are the differences between the red colored pumpkin and the black one?

The red pumpkin complete axles feature CNC machined C-hubs, knuckles, and lockouts, while the black pumpkin axles use cast components instead. Black pumpkin axles come standard on most BRX kits.

Machined parts offer higher strength and durability, while cast parts provide additional weight down low for improved center of gravity. Both setups have their own advantages depending on the build and driving style.

Below is one of the earlier videos when the BRX70 axles first released. Enjoy!

To browse the BRX PHAT Axle catalogue, visit:
https://www.boomracing.com/all-phat-axle-parts?sort=p.date_added&order=DESC