Snub Braking

Snub Braking Technique Explained

Disclaimer - This article is for informational purposes only and is not an endorsement of any specific braking technique(s).

"Snub braking" is one of many braking techniques used to control the speed of a tow vehicle and trailer when descending relatively long and/or steep grades. Long descents can be particularly dangerous as brake performance begins to fade as the pads or shoes and rotors or drums become heat soaked. It is for this reason that auxiliary braking systems, such as exhaust brakes and engine brakes, are commonly used on vehicles that tow any considerable weight. The snub braking technique aims to reduce brake fade through brief moments of heavy brake input followed by a brief moment of coasting.

Braking Methods for Controlling Speed on Long Grades

Snub braking is a method of controlling a truck and trailer while descending grades, particularly those that are relatively long and somewhat steep. There are two basic methods of braking during long descents, each with inherent advantages and disadvantages. For safety reasons, we can't advise you on which method to use, although "snub braking" is preferred by certain industry experts.

Brake fade - It's important to understand the concept of brake fade. In any process, the coefficient of friction and therefore frictional force between two surfaces is reduced as the temperature of the surfaces increases. In terms of braking, this means that the brake pedal will require greater input to achieve the same vehicle braking force as the brake rotor/drums build heat. This is known as brake fade, and in extreme circumstances it can result in less effective, even insufficient brake performance.

Constant brake input - One braking method is to ride the brakes with light and relatively constant brake pedal input during the descent. The theory is that light brake input, or rather using just enough brake pedal pressure to keep the vehicle at a safe speed, reduces the pressure between the brake pads/shoes and brake rotors/drums. Doing so theoretically reduces the heat generated during the braking process. From a short term prospective, it absolutely does when compared to heavy braking. Braking would therefore begin at or shortly after initiating your descent.

Snub braking - Snub braking is a method of braking that requires frequent heavy brake pedal input followed by periods of coasting. Under this braking method, a safe descent speed is achieved and the brakes are applied somewhat heavily until the speed of the vehicle is reduced by roughly 5 mph. Once the reduction is achieved, the brakes are released until the vehicle once again reaches is maximum perceived safe speed. The process is repeated, cycling between the regarded safe vehicle speed and slightly below said speed. Driver discretion is required to determine a safe descent speed, which is likely to be below the speed limit of the roadway.

While intuition may advise you to stick with the constant braking method over the intermittent brake input outlined by snub braking, snub braking may actually be beneficial with regard to safety and brake component wear - assuming your brakes are in good condition. Continuous, light braking produces less heat at a given moment than applying the brakes heavily. However, the laws of physics dictate the conservation of energy law; energy in is energy out. Therefore if we look at the overall process from the beginning of a descent until the end, the energy produced (in terms of brake heat) is roughly equivalent for both methods. Snub braking produces a significant amount of heat rapidly, but there is not a constant heat input. Meanwhile, riding the brakes produces a lower amount of heat at a given instant but the heat input is constant.

The theory behind snub braking is that, while a relatively high amount of heat is generated during braking periods, the rotors/drums are allowed to cool during brief periods of coasting. On the contrary, constant brake input produces less heat but the rotors are never allowed to cool. Thus, in theory, the brake rotors/drums may be of equal or greater temperature by the end of the descent. Snub braking is believed, in many instances, to reduce brake fade during the descent.

Exhaust Brakes & Engine Brakes

Exhaust and engine brakes are employed to increase the degree of safety during descents. An added benefit is reduced brake system wear, however this comes second to the fact that exhaust/engine brakes provide supplementary braking for loaded vehicles. Imagine a semi-truck traveling at maximum GCWR when the brakes become insufficient for the load and it begins to gain speed; a dangerous situation that engine/exhaust brakes are designed to prevent. Diesel engines lack a throttle blade to restrict airflow to the cylinders during deceleration. Thus, when the accelerator pedal is released and the vehicle coasts the engine breathes freely and the lack of restriction does little to control engine speed.

Engine Brakes (Engine Retarder)

Engine brakes are most often called "Jake Brakes", named after the most prominent manufacturer, Jacobs Vehicle Systems. The technical term for this system is an engine retarder. An engine retarder, when activated, converts an engine into a power absorbing system rather than a power producing system. This is achieved by opening the exhaust valve near the top of the compression stroke, releasing the potential energy created by the engine during this process. This increases the rate at which the engine speed decreases and, since the engine and transmission are statically connected during engine braking, it aids in slowing down and/or controlling the speed of the vehicle. Engine braking produces a distinct, sometimes high volume sound that is unmistakable.

Exhaust Brakes

An exhaust brake is a device ultimately used to increase the load on an engine during the exhaust stroke. By restricting exhaust flow, the engine must work harder to pump exhaust gases from the cylinder. As a result, the engine speed will slow at a much faster rate. When used properly, an exhaust brake can greatly reduce reliance on a vehicle's braking system. In addition, they aid in maintaining vehicle speed during descents.

There are two types, or methods rather, of using the exhaust as a braking mechanism. The original and likely most familiar type of exhaust brake is the standard butterfly valve. This exhaust brake is mounted at the outlet of the turbocharger (turbine outlet) or further downstream in the exhaust system. A butterfly valve, typically with a small bypass orifice built into the valve, is closed to increase exhaust backpressure and provide effective engine braking by increasing the force necessary for the engine to evacuate exhaust gases from the cylinders.

A secondary method of achieving engine braking, which is common on modern diesel engines, is by controlling the position of the vanes in a variable geometry turbocharger. Recall that VGT vanes are closed to reduce the effective size of the turbine housing in order to reduce turbo spool time by increasing drive pressure, and opened to increase the effective size of the turbine housing and prevent an overboost/overspeed condition by reducing drive pressure. When braking is desired, the VGT vanes can be closed in order to increase exhaust backpressure, effectively forcing the engine to work harder to evacuate each cylinder. The result is an engine braking effect, whereby the engine produces negative work on the vehicle. To ensure maximum braking efficiency in vehicle's with automatic transmissions, a specific torque converter lockup schedule is required to hold gears. This shift schedule and VGT operation is activated by a vehicle's "Tow/Haul" setting.

In summary, a combination of the proper techniques and correct equipment will make you tow confidently and safely when it comes to slowing down or maintaining the speed of a fully loaded trailer.