The Physics of Deceleration: Integrating Auxiliary Braking Systems in Towed Vehicles

When a 30,000-pound motorhome tows a 4,000-pound SUV, the combined system becomes a complex problem in Newtonian physics. The challenge is not getting it moving; it is stopping it.
In a panic stop, the unbraked mass of the towed vehicle (the “dinghy”) exerts a massive vector force on the motorhome’s hitch. Without a supplemental braking system, this force extends stopping distances exponentially and can induce catastrophic Jackknifing.

The Roadmaster 8700 InvisiBrake represents a permanent, integrated solution to this dynamic problem. Unlike portable “robot foot” boxes that sit on the floorboard, the InvisiBrake is a distributed system installed deep within the vehicle’s chassis.

This article deconstructs the physics of auxiliary braking. We will analyze the Kinetic Energy Management of towing, the Pneumatic Actuation logic of the InvisiBrake, and the critical engineering challenge of braking a “dead” vehicle—one without engine vacuum or power assistance.

The Vacuum Problem: Braking Without the Engine

The most significant engineering hurdle in towing a modern car is the Vacuum Booster. * Normal Operation: When you drive, the engine creates a vacuum. This vacuum is stored in a booster canister. When you press the brake pedal, the vacuum assists you, multiplying your foot pressure by a factor of 3 to 5. * Towing Operation: The engine is off. The vacuum source is gone. After one or two brake applications, the booster is depleted. The pedal becomes rock hard (“dead pedal”).

The Force Multiplier Requirement

To brake a dead car effectively, you cannot simply press the pedal lightly. You need immense mechanical force to overcome the lack of vacuum assist. * Pneumatic Actuation: The InvisiBrake uses an onboard Air Compressor to generate this force. It creates approximately 80 psi of pressure. * The Cylinder: This pressure drives a pneumatic cylinder, which pulls a cable clamped to the brake pedal. This system bypasses the need for the car’s own power assist. It creates its own “artificial muscle” to depress the pedal with the necessary force (often 50-100 lbs of pressure) to engage the hydraulic calipers at the wheels.

System Architecture: Pneumatics vs. Electronics

Why use air? Many competitors use electric linear actuators. * Hysteresis and Speed: Pneumatic cylinders have low inertia. They can snap the brakes on instantly when the valve opens. This rapid response time is critical to matching the braking curve of the motorhome. * Force Density: A small air cylinder can generate massive force compared to an electric motor of the same size. This allows the InvisiBrake’s actuator to be hidden under a seat or behind a panel, preserving the driver’s legroom when the car is being driven normally.

The “Hidden” Integration

The “Invisi” in InvisiBrake refers to its installation topology. * Distributed Components: The compressor control unit is separate from the actuator cylinder. They are connected by air lines and a Bowden cable (pulley system). * The Physics of Installation: This separation allows for flexible packaging. However, it introduces the complexity of routing. The cable geometry must be precise. If the angle of the pull is wrong, the cable binds (friction loss), or the pedal is pulled sideways, potentially damaging the pedal arm. This explains the “10-hour installation” feedback—it is not just bolting a box; it is engineering a mechanical linkage.

Energy Management: Parasitic Drain and Charging Logic

An electronic braking system is a Parasitic Load. It consumes electricity from the towed vehicle’s battery. * The Drain: The compressor draws significant current (up to 20A) when active. The control logic draws a small standby current. * The Risk: On a long tow (6-8 hours), this drain can flatten the car’s battery. A dead battery means the braking system stops working (safety hazard) and the car won’t start upon arrival.

The Trickle Charge Solution

The InvisiBrake integrates a Battery Maintainer (Trickle Charger). * The Physics: It siphons power from the motorhome’s 12V auxiliary line (via the umbilical cord) and steps it down/regulates it to charge the car’s battery (approx. 2 Amps). * Galvanic Implications: This creates a shared ground between two distinct DC systems (the RV and the Car). Proper grounding is essential to prevent ground loops or voltage floating, which can confuse the logic boards of modern cars.

Control Logic: Proportionality vs. Binary Braking

How does the system know how hard to brake?
Some systems use Inertia Sensors (accelerometers) to detect deceleration and brake proportionally. The InvisiBrake, primarily, uses the Brake Light Signal from the RV. * The Mechanism: When the RV brake lights turn on, the InvisiBrake activates. * Pressure Adjustment: The user sets the desired braking pressure (psi) via a regulator knob on the controller unit. This is a “Set and Forget” fixed pressure.
* Critique: It is not truly proportional. Whether you tap the brakes gently or slam them in a panic, the InvisiBrake applies the preset pressure.
* Refinement: The system ramps up pressure to smooth the engagement, preventing a jerky “tug” on the tow bar. But fundamentally, it relies on the user tuning the pressure to match the weight ratio of the two vehicles.

Conclusion: The Mechanical Co-Pilot

The Roadmaster InvisiBrake transforms the towed vehicle from a dead weight into an active partner in deceleration. It solves the “Vacuum Problem” with brute pneumatic force and solves the “Dead Battery Problem” with integrated charging.

For the RV owner, it is a significant engineering commitment. It requires drilling, wiring, and plumbing. But the result is a system that is invisible to the eye but essential to the laws of motion. It ensures that when mass times acceleration ($F=ma$) demands a stop, the physics are on your side.