RC4WD OVER/UNDER DRIVE TRANSFER CASE Z-U0039

Overview

The RC4WD Over/Under Drive Transfer Case (Z-U0039) is an advanced drivetrain component designed for serious RC scale crawler enthusiasts in the UK and beyond. This precision-engineered transfer case represents the pinnacle of tunable four-wheel drive technology, allowing you to optimise the front and rear drive ratios independently to suit your specific terrain and driving style. Whether you're tackling technical rock crawling sections, navigating through mud, or executing precision cornering manoeuvres, this transfer case gives you the control to sharpen steering response and maximise traction exactly when you need it most.

Built by RC4WD, a recognised leader in scale RC crawlers and 4WD vehicles, the Over/Under Drive Transfer Case is engineered for demanding RC enthusiasts who refuse to compromise on performance. This is not a basic fixed-ratio component—it's a tunable system that lets you dial in six different gear combinations, making it the ideal choice for anyone running a Trail Finder 2 or Gelande II platform who wants competitive advantage and precision control. The cast aluminium casing houses hardened steel gears that have been proven in competition and expedition-style RC crawling, making this a genuine upgrade that will transform how your scale crawler performs.

For RC car enthusiasts in the UK looking to elevate their crawling game, this transfer case delivers the engineering quality and tunability that separates weekend bashers from serious competitors. The same robust mounting pattern as RC4WD's popular Hammer Transfer Case ensures easy installation on compatible platforms, whilst the extensive gearing options mean you'll always have the perfect ratio for your next challenge.

Key Features

• Six Selectable Gear Ratio Options — The transfer case includes multiple interchangeable gear combinations that allow you to adjust the front-to-rear drive ratio from 1:1.27 all the way to 1:1.67, giving you precise control over steering sharpness and traction characteristics without modifying any other drivetrain components.
• Hardened .8 Modulus Steel Gears — The internal gearing uses hardened steel with a .8 mod specification, which provides exceptional durability and load-carrying capacity even under extreme scale crawling conditions where massive forces are transmitted through the drivetrain.
• Cast Aluminium Housing — The robust cast aluminium casing protects the internal mechanisms whilst remaining lightweight enough to be practical for RC crawler installations where every gram matters.
• Wide-Face Gear Design — The gears feature a wide face profile that distributes load across a larger contact area, resulting in smoother engagement, quieter operation, and extended service life compared to standard narrow gears.
• Hammer Transfer Case Mounting Compatibility — The Z-U0039 uses the same mounting pattern as RC4WD's Hammer Transfer Case, meaning it will bolt directly into existing chassis designs that were engineered for that platform, simplifying installation and parts sourcing.
• Heavy-Duty 5.0mm Output Shafts — The output shafts measure 5.0mm in outer diameter, providing substantial material for engaging with axle components and resisting the torsional forces generated during aggressive crawling and off-road driving.
• Includes Multiple Gear Combinations — Every Z-U0039 comes supplied with 16T, 17T, 19T, and 20T gears, plus the standard 15T and 22T gears (fitted as stock), allowing you to experiment with all six available ratios without purchasing additional components.
• Lightweight at 176 Grams — The entire assembly weighs just 176 grams, making it a practical addition to any scale crawler without creating excessive unsprung or rotating mass that would negatively affect suspension performance.

Technical Performance

The RC4WD Over/Under Drive Transfer Case represents a significant leap forward in RC crawler drivetrain engineering. At its heart are hardened .8 mod steel gears that have been specifically selected for their ability to handle the extreme loads generated during scale crawling. The .8 modulus specification refers to the tooth pitch—this particular sizing strikes an optimal balance between gear strength and precision, allowing for smooth meshing whilst maintaining durability through countless hours of demanding operation. The gears arrive pre-configured in the 15T/22T combination, which delivers a 1:1.47 gear ratio, meaning the front axle will rotate at the same speed as the rear axle. However, the true power of this component emerges when you explore alternative gear combinations. By removing the cover plate on the side with the two output shafts, you can swap the gears to achieve any of five additional ratios. The 17T/19T combination yields a 1:1.27 ratio where the front rotates 27% faster than the rear—ideal for tightening steering response in technical sections. Moving to 16T/20T gives 1:1.33 with 33% faster front rotation. The 19T/17T option provides 1:1.42 with 42% faster front speed, whilst the 20T/17T combination achieves 1:1.57 for 57% increased front speed. At the extreme end, the 20T/16T pairing delivers 1:1.67 with the front rotating 67% faster than the rear, giving maximum steering aggression for confined crawling spaces. This extraordinary range of tunability means you can optimise performance for everything from high-speed desert terrain to tight technical rock gardens without touching any other drivetrain component. The cast aluminium housing provides robust protection whilst maintaining minimal weight—at just 176 grams, the complete assembly won't adversely affect your crawler's weight distribution or suspension dynamics. The 5.0mm output shafts are substantial enough to reliably transmit the power generated by modern brushless motors into your axle systems, and the wide-face gear design ensures smooth, quiet engagement with minimal backlash. For serious RC crawler operators in the UK, this is genuine competition-grade engineering.

What's in the Box

• 1x RC4WD Over/Under Drive Transfer Case (assembled with 15T/22T gears installed)
• 1x 16T Gear
• 1x 17T Gear
• 1x 19T Gear
• 1x 20T Gear
• 1x M3 x 6mm Steel Button Head Cap Screw (Black)
• 1x M3 x 10mm Steel Button Head Cap Screw (Black)

What You Need to Complete

• Compatible RC crawler platform: RC4WD Trail Finder 2 LWB, Trail Finder 2 Truck Kit LWB, or RC4WD Gelande II (RTR or Kit versions)
• For Gelande II installations, the O/D TC Lower 4 Link Mount with Battery Tray (Z-S1899) is required
• Replacement fasteners are available separately: Steel Button Head Cap Screws M3 x 6mm (Z-S0651) and M3 x 10mm (Z-S0653)

Frequently Asked Questions

• What is an over/under drive transfer case and why would I need one for my RC crawler? — An over/under drive transfer case allows you to independently adjust the gear ratio between your front and rear axles. In RC crawling, this is crucial because the front wheels often need to turn at a different speed than the rear wheels to maintain control and traction. Over-drive ratios (where the front turns faster) improve steering response and help navigate tight technical sections, whilst under-drive ratios (where the front turns slower) can improve traction and stability on certain terrain. The RC4WD Z-U0039 gives you six selectable options, making it the most versatile solution available for Trail Finder 2 and Gelande II platforms.
• Which RC4WD models is this transfer case compatible with? — The Z-U0039 is specifically designed for RC4WD Trail Finder 2 models (including LWB and Truck Kit versions with Mojave II bodies) and RC4WD Gelande II models (including RTR versions, standard kits, LWB variants, and D110 Truck Kits with hard bodies). For Gelande II installations, you will also need the O/D TC Lower 4 Link Mount with Battery Tray (Z-S1899) to ensure proper fitment and clearance.
• How do I change the gear ratio on this transfer case? — Changing the gear ratio is straightforward. Remove the cover plate on the side of the transfer case that has the two output shafts (you may need to unbolt some components to access this). Once the cover is off, you can swap the gears to your desired combination. The Z-U0039 comes with 16T, 17T, 19T, and 20T gears, and the stock 15T and 22T gears are already installed, giving you access to six different ratios without purchasing additional components.
• What is the stock gear ratio and what does it do? — The stock configuration uses 15T and 22T gears to create a 1:1.47 ratio, which means the front axle rotates at approximately the same speed as the rear axle. This is a good all-around setting that works well for mixed terrain and general crawling. However, if you find you need sharper steering response, you can change to faster front-end ratios; if you need more traction and stability, you can select slower front-end ratios.
• What is the difference between a 1:1.27 ratio and a 1:1.67 ratio? — These numbers represent how much faster the front axle turns compared to the rear. In a 1:1.27 ratio (17T/19T), the front rotates 27% faster than the rear, giving you more aggressive steering but potentially less traction. In a 1:1.67 ratio (20T/16T), the front rotates 67% faster than the rear, giving maximum steering sharpness for extremely tight technical sections. The choice depends on your terrain and driving style—higher ratios sharpen steering, lower ratios improve traction.
• Is this transfer case suitable for beginners, or is it only for experienced RC drivers? — Whilst the tuning capability of this transfer case is certainly appreciated by experienced competitors, it is accessible to all skill levels. Beginners can simply install it with the stock 1:1.47 ratio and enjoy improved performance on their Trail Finder 2 or Gelande II. As your skills develop and your understanding of crawling dynamics improves, you can experiment with different gear combinations to optimise performance. The real advantage emerges as you gain experience and learn how different ratios affect handling characteristics.
• What materials are used in the construction of this transfer case? — The Z-U0039 features a cast aluminium housing that provides durable protection of the internal components whilst keeping weight to a minimum. The internal gears are manufactured from hardened steel with a .8 modulus specification, which provides exceptional strength and load-carrying capacity under the extreme forces generated during RC crawling. The output shafts are solid steel, measuring 5.0mm in outer diameter, and the wide-face gear design ensures smooth, reliable engagement.
• How much does the complete transfer case weigh? — The entire assembled unit, including the cast aluminium housing, all internal gears, and output shafts, weighs just 176 grams. This lightweight design means installation will not negatively impact your crawler's weight distribution, suspension dynamics, or overall performance characteristics.
• Does this transfer case require any special maintenance? — The RC4WD Over/Under Drive Transfer Case is a sealed unit that requires minimal maintenance beyond standard RC care. Keep the external surfaces clean, and ensure all fasteners remain tight before each use. If you frequently change gear ratios, inspect the gears periodically for wear, particularly after intense crawling sessions or competition events.

Why Buy from RC Model Shop?

RC Model Shop is a trusted UK RC retailer and authorised dealer for all the brands we stock, including Traxxas, ARRMA, Losi, FTX, Hobbywing, Spektrum and more. Based in Walsall, we have a physical store where customers are welcome to come in, view products and purchase in store. Whether you are a seasoned competitor, a weekend basher, or just discovering the world of RC for the first time, our team of experienced RC enthusiasts is here to help you find the perfect setup and provide ongoing support. Order the RC4WD OVER/UNDER DRIVE TRANSFER CASE Z-U0039 today with confidence, knowing you are buying from an authorised retailer with genuine products, expert knowledge and a commitment to customer satisfaction. Shop online or visit us in Walsall — we look forward to helping you get the most out of your RC experience.