Tutorials & Tuning Guides

Why Proper Filling and Bleeding Matters

Properly filled shocks are critical for consistent suspension performance. Air bubbles trapped inside a shock cause inconsistent damping, unpredictable handling, and uneven response between shocks. Taking the time to fill and bleed correctly helps ensure your shocks perform as intended.

What you need:

• JT's Shock Oil (your chosen viscosity)
• Shock rebuild stand or a clean flat surface
• Shock shaft and piston assembly
• Shock body and cap
• Clean paper towels or shop rags

Step 1 — Disassemble the Shock

Remove the shock from the vehicle following your manufacturer's instructions. Disassemble the shock body, removing the cap, bladder or o-ring retainer, and shock shaft assembly. Clean all parts thoroughly before reassembly to avoid contaminating the new oil.

Important: Refer to your vehicle's manual for the correct disassembly procedure. Shock designs vary between manufacturers and models.

Step 2 — Fill the Shock Body

Hold the shock body upright and fill it slowly with JT's Shock Oil to approximately 80–90% full. Filling slowly helps minimize air bubbles introduced during the pour. Do not overfill at this stage — you need room for the piston and shaft assembly.

JT's Tuning Tip: Pour oil along the inside wall of the shock body rather than directly into the center. This reduces turbulence and helps prevent air from being trapped in the oil.

Step 3 — Install the Piston and Shaft

Slowly insert the shock shaft and piston assembly into the oil-filled shock body. Push the piston down gently and repeatedly to work any trapped air up through the oil and out the top of the shock. You may see small bubbles rise to the surface — this is normal and expected.

Continue slowly cycling the piston until no more bubbles appear at the surface. This process may take several cycles depending on the piston design and oil viscosity.

Step 4 — Top Off and Bleed

After cycling the piston, top off the shock body with additional oil as needed. The goal is to have the shock body filled to the correct level for your specific shock design — consult your vehicle's manual for the recommended fill level, as this varies by shock type and manufacturer.

With the piston held near the bottom of its travel, place your finger over the top of the shock body and gently tilt and rotate the shock to help any remaining air bubbles migrate to the top. Remove your finger briefly to allow bubbles to escape, then repeat until the oil surface is clear.

Important: Fill level affects shock performance. Too much oil can cause hydraulic lock; too little can allow air to re-enter. Follow your manufacturer's recommended fill level.

Step 5 — Install the Cap

Once the shock is properly bled and filled to the correct level, carefully install the shock cap or bladder retainer according to your manufacturer's instructions. Avoid trapping air during this step — keep the piston compressed slightly as you seat the cap.

Wipe away any excess oil from the outside of the shock body and shaft.

Step 6 — Check for Leaks and Reassemble

Before reinstalling on the vehicle, slowly cycle the shock shaft by hand and check for smooth, consistent resistance throughout the travel. Inspect all seals and the shock cap for any signs of leakage.

Reinstall the shocks on the vehicle following your manufacturer's torque and mounting specifications.

JT's Tuning Tip

JT's Tuning Tip: If you are rebuilding multiple shocks, fill and bleed them one at a time and compare resistance by hand before installing. Shocks that feel noticeably different from each other may have inconsistent fill levels or trapped air — address those before putting the vehicle back on the track.

The Basics of RC Shock Setup

RC shock absorbers do more than cushion impacts — they control how your suspension compresses and rebounds, how weight transfers during acceleration and braking, and how the chassis responds to terrain. Understanding the three key tuning variables below gives you a foundation for making informed adjustments. Keep in mind that spring rate, oil viscosity, and ride height are important starting points, but they are not the only factors affecting shock performance. Piston design, shock mounting position, suspension geometry, vehicle weight, and driving conditions all play a role in how your suspension behaves.

Spring Rate

Controls compression resistance and weight support

Spring rate determines how much force is required to compress the suspension and helps support the vehicle's weight. Stiffer springs generally resist compression and body movement more, while softer springs allow the suspension to move more easily.

Neither is automatically better. The right spring rate depends on vehicle weight, suspension geometry, terrain, driving style, and how the damping is set up. Spring rate and shock oil viscosity work together — changing one affects how the other performs.

Consider adjusting spring rate when:

• The chassis feels like it bottoms out or compresses too easily under load
• The suspension feels too stiff and doesn't absorb terrain well
• You've changed vehicle weight (battery, body, or added components) and the handling has shifted

Oil Viscosity

Controls damping — how quickly the shock moves

Shock oil viscosity controls damping by affecting how quickly oil can move through the shock piston. Higher-viscosity oil generally slows compression and rebound, while lower-viscosity oil allows the shock to move more freely.

The actual result also depends heavily on piston hole size and count, piston design, temperature, and the rest of the suspension setup. Two vehicles running the same viscosity can behave very differently depending on those factors.

Consider adjusting oil viscosity when:

• The suspension rebounds too quickly or too slowly after a hit
• You want to fine-tune damping behavior after establishing a spring rate baseline
• You've changed terrain or driving conditions significantly

Ride Height

Affects clearance, weight transfer, and suspension travel

Ride height is the distance between the chassis and the ground with the vehicle at rest. It affects ground clearance, weight transfer, available suspension travel, and center of gravity.

Lower ride height can reduce the center of gravity and body movement. Higher ride height can provide additional ground clearance and usable suspension travel. The correct setting depends on the vehicle, terrain, and how the rest of the suspension is configured.

Consider adjusting ride height when:

• The vehicle is bottoming out the chassis on terrain
• You need more or less suspension travel for a specific track or surface
• You've changed spring rate and need to re-establish a neutral baseline

Starting Point

Begin with your RC manufacturer's recommended shock oil, springs, and ride-height settings as your baseline. Those recommendations are designed around your vehicle's specific geometry, weight, and shock piston configuration.

Make one adjustment at a time and observe how the vehicle responds before making another change. When changing oil viscosity, move in controlled increments rather than assuming one viscosity range works for every RC vehicle.

Important: Document what you change and how the vehicle responds. Small, deliberate adjustments are easier to evaluate and reverse than multiple simultaneous changes.

JT's Tuning Tip

JT's Tuning Tip: Spring rate and shock-oil viscosity work together. If you make a significant change to one, pay attention to how it affects the other and evaluate the complete suspension response — not just the variable you changed. A stiffer spring with unchanged oil may feel very different than the same spring with adjusted damping.