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Police Brake Pad Tests
Police Fleet Manager magazine recently tested different kinds of aftermarket police brake pads—semi-metallic, ceramic and non-asbestos organic. Police package vehicles came from the factory (Original Equipment, OE) with semimetallic pads.
The test vehicles were all brand new 2010 Dodge Chargers with the 5.7 HEMI® V8. Six Chargers were borrowed from the Lake County, Ind. Sheriff before they went into service. Thanks to Sheriff Roy Dominguez, Chief Marco Kuyachich, Staff Services Commander William Paterson, Police Garage Foreman Steve Baroevich, and Maintenance Technician Bill Laviolette. Two more Chargers were borrowed from Thomas Dodge, Chrysler, Jeep— the dealership in Highland, Ind. Thanks to Fleet Sales Manager Steve Kizaric.
To avoid possible wear and tear of a single vehicle for a multibrand test, we used a dedicated Charger for each make of brake pad. We kept the OE rotors for all the testing, with one exception. (More on that in “PFM Tests Raybestos Police Rotors,” page 60.) Importantly, this was a test of replacement pads intended for all police sedans, not just the Charger.
Testing brakes is all about precise measurements of stopping distances from exact and known speeds. We used a Vericom VC- 2000, the same instrument used by Tire Rack for all of its tire and brake tests.
Our brake tests were conducted at No Limit Raceway in Morocco, Ind. About an hour south of Chicago, this is the former US 41 Dragway. A drag strip offers the ideal flat road, giving us a controlled surface to test on.
Just like the 2009 police tire testing by Police Fleet Manager, these police brake pad tests did not involve any kind of brake wear or pad life evaluation. No valid accelerated wear test exists to predict pad life under the extreme variety of police use—some departments get 8,000 miles on OE pads and other departments get 48,000 miles on the same pads.
Burnish Brake Pads
The “burnish” is a way to break-in the brake pads. This process essentially heats up the pad to drive off the gases from the chemical adhesives used to bind the friction materials together. You have probably heard about brand new police cars dropped off right from the car carrier at the Academy. After the first hard EVOC run, the car is parked and the brakes have a flash fire.
This is caused by the igniting of gases from the binding material. The brakes were not properly burnished to drive off these gases prior to hard use. The goal of any burnishing procedure is to progressively heat up the brake pads to generate and then drive-off the chemical gases.
Brake pad burnishing should always be done, according to both aftermarket brake pad makers and auto makers. In reality, it is almost never done. We get away without doing a brake burnishing because these gases are slowly driven off by routine brake pad heating during normal driving. In fact, the only time we see a problem is with hard use on brand new pads: like a pursuit right after a brake job, EVOC training on brand new cars, or brake testing on brand new pads.
One of the problems with brake burnishing is that no one agrees on the “proper” procedure for semi-metallic pads. One brake manufacturer recommends 30 easy stops from 30 mph. Another calls for 8 moderate stops from 35 mph, then 2 hard stops from 45 mph. Raybestos, for example, specifies 8 moderate stops from 40 mph, 8 harder stops from 60 mph and 5 hard stops from 80 mph.
The “proper” brake burnishing may even vary from year to year on the exact same model of police car. The 2007 Dodge Charger Supplement calls for 20 hard stops from 70 mph. The 2008 Dodge Charger Police Supplement calls for 40 moderate stops from 60 mph. For the record, we followed the Raybestos method of moderate (40 mph), harder (60 mph) and hard (80 mph) stops for all the brake brands. However, we minimized any minor technical differences between all of the burnishing procedures by what we did next.
Season Brake Rotors
Driving off the gases in the friction material during the burnishing process just gets the pads ready for the next step, rotor seasoning. It takes time for the surface of the pad to fully seat, to fully contact, the surface of the brake rotor. More importantly, it takes time for the pad to smooth the micro-surface of the rotor and to transfer tiny bits of friction material to the rotor. A car with brand new brakes and brand new rotors does not stop as well as the same car with 200 patrol miles on the brakes.
Rotor seasoning is a step often missed in brake testing. The pads and rotors must be seasoned together, mated together and kept together. You cannot simply put new pads on a new rotor, burnish the pads and start the tests. You cannot swap pads on a rotor seasoned with a different make of pad, burnish the pads and start the tests.
Burnishing is required to drive off flammable gases. Seasoning is required to fully seat the pads to the rotors. After burnishing, each Charger was driven in stop-and-go, urban and suburban driving for 150 miles to fully season the brake pads and rotors.
The combination of pad burnishing and rotor seasoning also fully scrubbed the brand new tires. After all this pad burnishing and rotor seasoning we were in technical compliance in the strictest sense and ready to start the tests.
Just because a replacement pad is available in a size that “can” be used on your police vehicle doesn’t mean it “should” be used on your police vehicle. In fact, some pads should not be used on any police vehicle.
The NAPA Web site, for example, lists five different police pads for the Dodge Charger. Three of these police pads are ceramic. One of these police pads is non-asbestos organic (NAO). One of the pads is a semi-metallic. Two of the ceramics are more expensive than the semi-met, while the NAO and one of the ceramic pads are less expensive. All are claimed to be OE-equivalent and identified as such.
Not knowing any different, the lowest bid OE-equivalent police brake pad is the organic, so we tested that one. The ceramic is also less expensive than the semi-met, and offers longer pad life, makes less dust and less noise, so we tested that one too. The OE factory pad uses semi-metallic friction material pad, so we tested a variety of them.
Why buy ceramic pads? Why not just buy semi-met pads? Semi-met pads are widely available from local auto parts stores. They are what the factory built the police package around. They are reasonably priced. No one complains about stopping power with semi-met pads. So why get something different? Four reasons.
First, semi-met pads can be a bit noisy—they squeal, squeak, screech and grind, grunt, groan. Some semi-met pads are quieter, some makes are known for their noise. Proper pad installation, i.e., the use of lube on the backplate can reduce the noise. Some makes of semi-met pads come with rubberized attachment hardware, which reduces vibration (noise).
However, no semi-met pad is as quiet throughout the entire operating use as a ceramic pad. Some fleet managers are tempted to select ceramic pads to keep patrol officers from complaining about noise.
Second, semi-met pads can be a bit dusty and dirty. This isn’t as obvious on police cars with black wheels and small center caps. However, the brake dust from some makes of semi-met pads can turn chrome, full wheel covers a shade of black. Ceramic pads, on the other hand, are absolutely clean. No one with ceramic pads complains of brake dust. Some fleet managers are tempted to select ceramic pads for just this reason.
Third, semi-met pads wear at a higher rate than ceramic pads. Brake pads are the same kind of compromise that tires are. You trade off performance when you opt for longer life. An 80,000 mile guarantee tire has much less wet traction than a police-spec tire. In the same way, you give up certain aspects of brake performance when you select a longer-life ceramic pad instead of the semimetallic pad.
Finally, semi-met pads may be more expensive than ceramic pads. Some premium ceramic pads are very high-priced, but other ceramic pads are less expensive than semi-met pads. From the same supplier, a set of front ceramic pads listed for $59.99; while a set of front semi-metallic pads were $86.99. From another chain, it was $46.99 for the ceramic pads and $65.99 for the semi-met pads. (Organic front pads were $31.99—the price being the only reason anyone would be tempted to buy organic pads.)
Ceramic pads have a definite appeal. Perhaps someday, a brake manufacturer will develop a police-specific ceramic pad. Until that day, semi-met pads remain the best choice for police patrol vehicles.
“When looking for performance, it is a semi-metallic pad,” said Terry Heffelfinger, director of product engineering with Affinia Global Brake & Chassis. Affinia (Raybestos) makes both semi-met and ceramic brake pads for passenger vehicles. The Raybestos Police Patrol / Pursuit pads, of course, use semi-metallic friction materials. Only a semi-met pad can be truly OE-equivalent.
The Cold 60 mph ABS Stop
All brakes work the best when they are warmed up. This is equally true for all the common friction materials used on passenger vehicle brakes—semi-metallic, ceramic and organic. Nothing has warmed up and expanded to the right clearance, i.e., the steel piston and the cast aluminum caliper expand at different rates.
In normal patrol and admin use, the pads are “warm,” i.e., they operate around 350 degrees F. However, the police vehicle must be able to stop under all conditions, and that includes cold brakes. Cold brakes does not mean cold ambient weather, because the brakes heat up even in winter conditions. Instead, cold brakes mean temperatures for pads and rotors well under 100 degrees F.
This is not the acid test for brake performance. That would be a long series of high-speed tests. Instead, this is a time when almost everything in the brake system is at its worst. Emergency stops with cold, or at least cool, brakes, i.e., brake temps under 350 degrees F, are a reality of police work. It can be argued that the police car makes more cold-cool brake stops than ultra-hot pursuit stops.
With the cold 60 mph stops, we found little differences among semi-met, ceramic or organic pads. Urban legend has it that semi-met brakes have to be warmed up to work well. Not so. The ceramic and organic pads did well in this test, as expected, but so did the semi-met pads. During this test, the Chargers with all of the different pads stopped with in 6 feet of one another.
The cold 60 mph stop did prove to be difficult for all three kinds of pads. With every single pad tested, the cold 60 mph stop was longer than the second stop from 60 mph. In some cases, the cold 60 mph stop was 4 feet longer than the next few stops.
As the number of high-speed stops continued, the average semi-met pad had, by far, the most consistent braking performance. In other words, in all the high-speed stops that followed the cold 60 mph stop, the cars with semi-met pads stopped more like they did during the first ABS stop than the ceramic and the organic pad. The extreme spread for the semi-metallic pads was just 2.4 feet, compared to 5.4 feet for the ceramic pads and 8.8 feet for the organic pads.
High-speed, 60 mph Series of ABS Stops
The purpose of the High-speed Test was to simulate brake performance under pursuit conditions. Unlike most other higher speed brake tests, this evaluation did not involve any braking from very high speeds to “warm-up” the brakes. Nor did the protocol involve any heat-soak or cool-down period in the middle of the evaluation.
This makes perfect patrol sense for many reasons. First, the police car does not start a pursuit with red-hot brakes. It starts with patrol temperature brakes, and perhaps even with cold brakes. Second, there is no stopping until the pursuit (the scoring) is over.
Finally, and most importantly, it allows us to see how the braking system reacts as it heats up from patrol-warm to racing-hot. For that reason, none of the data, the so-called flyers, were thrown out. You can’t ignore a flyer (unusual brake performance, good or bad) during a pursuit.
The test was a series of 12 ABS-activated, 60-mph stops. After each stop, it took about 30 seconds to record the data, then about 6 seconds in the HEMI Charger to reach 60 mph again and the whole process was repeated. According to a 2008 study by the IACP and George Mason University, the average maximum pursuit speed is 66 mph. The average length of time for a pursuit is 5.5 minutes. That is just about how long it took for this 12-stop sequence.
This was the acid test as all the pads transitioned from cold to warm to hot to very hot. The semi-met pads had better braking performance, as evidenced by slightly shorter stopping distances, as they got hotter. That is why police cars come from the factory with semi-met pads. They also had a more consistent pedal feel and more consistent stopping distances than the ceramic and the organic pads. The semi-met pads took this brutal test in stride. No brake fade. No smoke rolling out of the fender wells. Nothing overheated. No drama. The semi-met pads actually worked better for stops six through 12 than they did for stops one through five.
The ceramic pad had good stopping power, but it was very inconsistent as it heated up. After the first 60-mph stop, the ceramic pads had improved stopping distances for the next three stops, and then got noticeably worse for the next two stops. The ceramic pads then improved again for the next five stops, but spiked again for the last stop. After the 12th stop, smoke from the fender wells indicated the system was too hot. Ceramic brakes do not transfer or dissipate heat as well as semi-met brakes.
It would have been interesting to continue the series of stops, perhaps another six or eight. The ceramics were very, very hot and it appeared that boiled brake fluid and brake fade could not have been too far in the future. That said, contrary to conventional wisdom, the ceramic pads gave acceptable stopping distances during this series of tests. They were not very consistent, but they did not “fail.”
The organic pads were a different story. Organic pads are what passenger cars used (and some still use) until they were replaced by semi-met and ceramic pads. Organic pads and brake fade sort of go together. Not because the brake fluid boils, but because the organic
The organic pad did very well for the cold 60-mph stop. The next two stops were a bit shorter than the cold 60-mph stop. Then the organic pads began a steady climb off the chart. Each of the next stops was many feet longer than the previous stop.
The organic pads were still able to stop the Charger at the end of the test series, but the pedal feel deteriorated and the brake odor was significant. Making a prediction, the organic pads would double the stopping distance from 60 mph within the next couple of stops. This car would not be able to stop after a longer pursuit.
Spencer Geswein has a Bachelor’s Degree in Mechanical Engineering from Rose-Hulman Institute of Technology and a Master’s Degree from Clemson University. For nine years, he worked for Michelin Tire Corp. as a Development Engineer/On-Vehicle Testing and Evaluation. He conducted tire testing and subjective evaluations for the Ford Motor Co. account and did the driving for the Dodge Viper tire development program.
Geswein also developed engineering and driving programs for specialty high performance tire lines such as the Michelin Pilot Sport and the BF Goodrich g-Force T/A KD. He was the leader of the worldwide wet handling development team and coordinated the Michigan-New Zealand snow testing campaigns.
Just as importantly, Geswein is an accomplished race driver. He has raced on two dozen different road courses across America. He finished well in his rookie year at the 1998 SCCA Solo II National championships, and scored wins in the 2000 SCCA Endurance Championship Series. Geswein’s crowning achievement is back-to-back wins (2000, 2001) in Car and Driver’s One Lap of America. Driving a Dodge Viper, he was the Overall Winner twice in a row. Geswein is way over-qualified for our request to just step on the brakes a couple of times in a HEMI Charger. We were lucky to get him. Spencer, Thank You!
Published in Police Fleet Manager, Sep/Oct 2010
Rating : 9.9
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