What Are Motion Control Units (MCUs)?

SuperSteer MCUs are compact, maintenance-free suspension devices that regulate airflow in and out of your motorhome’s suspension air bags.

Here’s what they do:

• Control bounce and sway by slowing rapid airflow into and out of the air bags.
• Reduce rocking and porpoising after hitting bumps/dips, or when you hit the brakes hard.
• Improve steering stability in crosswinds or when being passed.
• Enhance comfort and safety by reducing driver fatigue.

They’re not a replacement for sway bars, shocks, or track bars — instead, they complement those upgrades by targeting a different part of your suspension system.

Selecting MCUs is all about getting the right fit and valving for your specific chassis and axle.

1. Air Line Size

MCUs are chosen based on the outer diameter of the air line that feeds each suspension air bag.

• SSE40xx → 1/4" air line
• SSE50xx → 3/8" air line
• SSE60xx → 1/2" air line

👉 Note: These are the most common sizes, but they are not the only ones. Some coaches may have larger or less typical line sizes — always measure to be certain.

Pro tip for measuring: Grab a 1/4", 3/8", and 1/2" open-end wrench and use them like a quick caliper. Slide the wrench over the air line — whichever one fits snugly is the outer diameter (OD) of your line.

2. Valving

Each MCU also comes with a specific valving restriction:

• “Under 30k” (less restrictive) → typically installed on the front/steer axle, as well as the tag axle (if equipped).
• “Over 30k” (tighter valving) → typically installed on the rear/drive axle.

Examples:

• SSE4045 → 1/4" line, front axle.
• SSE4055 → 1/4" line, rear axle.
• SSE5055 → 3/8" line, front axle.
• SSE5065 → 3/8" line, rear axle.
• SSE6065 → 1/2" line, front axle.
• SSE6075 → 1/2" line, rear axle.

👉 Note: One kit includes two units, enough to do two air bags. If you have a coach with 4 air bags per axle, such as the Roadmaster RR8/RR10 chassis, you'll need two kits each for the steer and drive axles.

3. Chassis Patterns (General Guidelines)

While the best way to confirm the required size is by measuring your air line outer diameter directly, many chassis follow predictable patterns:

• Freightliner XC: Usually 1/2" front, 1/4" on the drive axle.
• Exception: early 2000s may also be 1/2" for the drive axle.
• Many newer coaches use a tee on one rear air bag (see Tee Fittings below).
• Tiffin PowerGlide: Typically 3/8" front and rear.
• Roadmaster: Typically 1/4" front and rear, unless ping tanks have been installed.
• However, we have also occasionally seen 3/8" lines even without ping tanks.
• Freightliner M2 & S2RV: 3/8" rear (no front air bags).

One of the best things about MCUs is that they’re straightforward to install — many owners handle it themselves in a couple of hours. If you prefer to watch a video on how to do this, you can view it at the bottom of this blog.

Tools & Prep

• Brake Parts Cleaner or similar solvent with rags, for cleaning dirt/paint/undercoat from air line
• Tubing cutter
• Tubing wrenches (only needed for "T" MCUs)
• Soapy water (for checking for leaks)
• Adequately rated ramps or jack stands for access
• Safety glasses and gloves

Step-by-Step Process

1. Drive onto ramps for the axle you are working on, or place properly rated frame stands.
2. Chock wheels
3. Depressurize the suspension. Note - the coach will drop. Stay clear to avoid injury.
4. Clean the air line where you plan to place the MCU, using rags with cleaning solvent.
5. Double-check that the airbags are flat. Then, for standard push-connect MCUs, use a tubing cutter to make a clean, straight cut. If installing a "T" style, first pull the lines out of the existing tee and then unscrew the tee fitting from the bag.

Before you buy and install MCUs, keep these things in mind:

• Slower air dump times: Since MCUs restrict airflow, the process of dumping air from the bags (such as when leveling) may take a bit longer. For example, if it took 15 seconds before, it may take an extra 30 seconds afterward.
• Potential interference with electronic leveling systems: Some systems, such as the Valid Trueline, use electronic ride height sensors. They are programmed to expect a specific change in ride height within a given time. The added restriction from MCUs can confuse the system, causing it to “hunt” for the correct ride height when driving.

👉 Check before you buy:

• Look under your coach — if your suspension uses mechanical ride height valves with the air lines plumbed directly into them, MCUs are generally compatible.
• If you see electronic ride height sensors with a wire harness, you likely have a system such as Valid Trueline.
• If your dash panel says “Valid,” that’s another clue. Be aware that some earlier Valid systems were only used for parked leveling and still used mechanical valves for driving — so it’s best to verify by checking for ride height valves vs. ride height sensors.

We strongly recommend confirming whether your coach uses mechanical ride height valves or electronic sensors before purchasing MCUs.

If your coach is compatible, the difference is usually noticeable right away:

• Less sway and rocking over uneven roads or when exiting steep driveways.
• More stable steering in crosswinds or when passing.
• Reduced porpoising on uneven roads.
• Flatter braking with less nosedive.
• Greater driver confidence and less fatigue on long drives.

For many owners, it feels like they’re driving a different motorhome.

SuperSteer MCUs are a simple, affordable, and long-lasting upgrade that smooths out your ride and improves safety. By selecting the correct size and valving for your chassis and confirming compatibility with your suspension system, you’ll unlock the handling potential your motorhome was designed for.

✅ Check your air line size (use the wrench trick).

✅ Choose the correct MCU model.

✅ Confirm you don’t have a Valid Trueline system.

✅ Install them yourself or have a shop handle it.

Ready to improve your ride? Check out our Chassis Solution Center or contact us at 888-898-3281 with your chassis information, and we’ll help you get the right fit.