By Christian P. Koop
One very important area that does not get written about is brake testing. In this case, I am referring to measuring brake drum and rotor temperatures after road testing and stopping distances. Measuring these two important factors helps ensure emergency response vehicles meet minimum required stopping distances according to National Fire Protection Association (NFPA) 1911, Standard for the Inspection, Maintenance, Testing, and Retirement of In-Service Automotive Fire Apparatus, and stop safely. Performing these tests when diagnosing brake problems or after completing a brake job will also go a long way to help ensure the job does not come back to the shop for rework. I am sure everyone reading this is acutely aware of how important it is for brakes to function correctly and the liabilities involved if there is an accident because of brake failure, particularly if someone is injured or killed.
First, I will address measuring brake temperatures to diagnose brake problems and ensure the brakes are working correctly and are well balanced. Equal temperatures when comparing wheel end temperatures between wheels on the same axle and between axles is a good indicator that the brakes are working evenly if the temperatures are not excessively hot or cold. There are several companies that have test instruments available to record lug nut temperature that will automatically record and compare each wheel. The instruments provide you with a report indicating whether you have a problem or not. These test instruments work fine but are on the expensive side. I have found that using an infrared digital thermometer works just fine and is faster-and the cost is much easier on the pocketbook. The price of these digital thermometers (ours is manufactured by Mastercool) can range anywhere from $60 to several hundred dollars.
|(1) It is a good practice to hold the infrared meter at the same distance from every component being measured. If not, you may not get accurate or consistent readings.
(Photos by author.)
In our shop, we use digital thermometers to troubleshoot complaints of poor brake performance or after a brake job has been performed. We also use the instruments for quality control purposes. After completing a brake job, manufacturers recommend break-in procedures or burnishing the brake linings. After these procedures, the technician pulls back into the shop and records all brake drum or rotor surface temperatures. We use a standard test sheet we developed that a technician fills out and keeps as part of the repair record. This test sheet can also be used for future reference and placed into a database to refer to in case brake performance drops off in a particular unit or even for comparing sister trucks.
Note that the temperature should be taken in the same area of every drum or rotor. For example, pointing the infrared meter toward the outer edge of the brake rotor’s friction surface vs. the middle or even closer to the “hat” area can produce different temperature readings. It is also a good practice to ensure that the infrared meter be held at the same distance from every component being measured. If not, you may not get accurate or consistent readings.
The key here is to ensure temperatures are very close. Any measurement hotter or colder than the others by 50 degrees or so indicates a problem. For example, you may have a slack adjuster that was not adjusted properly or is malfunctioning. A slack adjuster that has not been adjusted to specifications can be the cause of the problem. If it is too tight, the pads can drag and make the brakes overheat. If the stroke is too long, it will not apply the brakes with enough torque to properly engage them. An incorrect automatic slack adjuster (ASA) may adjust at a different rate than the other ones and cause a brake imbalance that can show up when comparing brake drum and rotor temperatures. Brake calipers that are hanging up or binding on the slide rails or simply worn components that don’t allow the brakes to work properly could also contribute to these types of problems.
We have found this method of comparing brake temperatures very useful in quickly determining where we have a problem, whether the complaint comes from a driver or a technician who feels the braking performance is not where it should be after overhauling the brakes. Recording temperature information could also prove advantageous in a tort case claiming brake failure as the cause.
|(2) The Vericom unit, shown here, is accurate and has an easy-to-read digital display that indicates actual
Measuring Stopping Distances
The other main component for testing brakes to ensure they perform properly is to measure stopping distances. NFPA 1911 requires that fire apparatus weighing in excess of 10,000 pounds gross vehicle weight rating (GVWR) be able to stop at a distance of 35 feet while traveling 20 miles per hour. Refer to section 16.3 for procedures to test brakes and a table that lists the required stopping distances based on GVWR. It is also important to understand that NFPA 1901, Standard for Automotive Fire Apparatus, requires fire apparatus weighing in excess of 36,000 pounds GVWR to be equipped with an auxiliary braking device. Remember, when testing brakes and checking stopping distances, make sure to turn off the auxiliary braking devices. This will help to get a true indication of how well the brakes are actually stopping the vehicle being tested.
There are two basic ways to measure stopping distances: physically measure the distance it takes the vehicle to stop at a specific speed or use an instrument that tells you the stopping distance as a digital readout, calculated by converting deceleration forces more commonly known as “G” forces into a stopping distance.
For many years, our shop used an Ammco liquid decelerometer to test stopping distances. This is a liquid-filled gauge that has suction cups to mount it on the inside of the windshield. It takes two people to get the job done properly and safely-one technician to drive the truck and the other to read the meter. After recording the reading, which is deceleration in feet per second, technicians would refer to a chart, find the speed they were traveling when the brakes were applied, and find the stopping distance. For consistent accuracy, the gauge needed to be zeroed on a flat level surface and technicians had to ensure their eyes were as close as possible to 90 degrees from the zero line of the gauge or they would not be able to read it with any real accuracy. It took a little training to make sure the technicians reading the gauge were taking accurate and consistent readings.
Unfortunately the glass tubes that held the red fluid would break easily if they dropped or fell from the windshield. A few years ago, these decelerometers became hard to find, and we found an electronic unit (Vericom 4000) that is more accurate and has an easy-to-read digital display that indicates the actual stopping distance. This unit still mounts on the windshield like the liquid gauge but gives a direct digital reading of both acceleration and deceleration rates. It also stores the information, which can be downloaded to a PC or laptop. It is simple to use and easily provides accurate data that can be stored digitally or just written on the test sheet and kept with the repair order.
Producing High-Quality Work
We find that the digital infrared thermometer is a quick, accurate, inexpensive tool that is useful in determining brake temperatures and is a good aid in troubleshooting and determining general brake performance. Measuring stopping distances while testing the brakes helps us ensure not only that the brakes are functioning correctly but that they are also stopping the truck safely and according to standards. Adopting these two methods of testing brake performance will also help ensure your shop produces work of the highest quality possible in an area where mistakes can cost lives and liabilities are great.
CHRISTIAN P. KOOP is the fleet manager for the Miami-Dade (FL) Fire Department. He has been involved in the repair and maintenance of autos, heavy equipment, and emergency response vehicles for the past 35 years. He has an associate degree from Central Texas College and a bachelor’s degree in public administration from Barry University and has taken course work in basic and digital electronics. He is an ASE-certified master auto/heavy truck technician and master EVT apparatus and ambulance technician. He is a member of the board of directors of EVTCC and FFMA and a technical committee member for NFPA 1071, Standard for Emergency Vehicle Technician Professional Qualifications.