Braking - Friction at Work

This article will examine the various types and features of brakes used in emergency and personal vehicles.

Driveline retarders work by slowing the rotation of the driveshaft. They are generally electro-magnetic devices that can be installed directly to the front of the rear axle or at a mid driveline mounting. Hydraulic driveline retarders are also available.

Engine brakes work by altering valve timing under deceleration by opening the exhaust valves at Top Dead Center of the stroke for an instant, releasing cylinder compression and eliminating the expansion stroke. In essence, the engine is converted to an air pump as each cylinder compresses and releases. The noise produced is simply that of the released compression pressure. Although there are other manufacturers, these are commonly referred to as "Jake Brakes."

Exhaust brakes consist of a butterfly valve that closes, restricting the flow of exhaust gasses and creating a back pressure on the engine. This slows the engine revolutions per minute (rpms). Some brands are not effective at speeds (rpms) less than 20 miles per hour. Pacbrake* has an exhaust brake that works by a variable orifice size, closing up as the rpms decrease, resulting in a more effective retarder than the fixed orifice design. Exhaust brakes are lighter and less expensive than engine brakes, but not as effective.

Transmission retarders are an integral part of the automatic transmission. Transmission retarders may be of two types, the input type that operates at the input section of the transmission (between the torque converter housing and the main housing), or the output retarder that operates at the tail shaft of the transmission. The output retarder is the most prevalent on emergency vehicles. These systems work by using the transmission fluid to power an opposing direction wheel on the drive shaft within the transmission.

Secondary braking systems can increase the brake lining life by as much as three to five times the normal service period, depending upon the specific design application and use. This can result in considerable savings that can pay for the expense of installing the system. In addition to increasing brake life, these systems reduce other wear factors such as heat damage to brake drums and discs and tire wear. Automatic transmission and driveline retarders are especially effective. Some auxiliary braking devices should be shut off when the apparatus is responding on slippery surfaces. Follow the manufacturer's recommendations for proper operation.


  • NFPA 1901, Standard on Automotive Fire Apparatus
  • Bruce Sebring, Pacbrake Co., Surrey, B.C.
  • Chuck Wittenberg, Merritor-Wabbco, Troy, MI

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