http://www.essexparts.com/learning-cent ... ost/Bed-inNow in case the link gets broken or you would rather practice your reading skills.
Bedding-in Brake Pads and Rotors
When's the last time you bed-in your brakes? If you're an enthusiast who wants to get the most out of your car on the street and track, your answer to that question hopefully isn't, "I can't remember," or even worse, "what the heck is this guy talking about?" As aggressive street and track drivers, we're constantly upgrading our cars, and we've accepted that preparing our new parts for heavy use is not only the best way to get the most out of them, it's also common sense. When we get a new car, we seat our piston rings through a specified engine break-in procedure. When we get a new set of race tires, we heat cycle them so we can get the most wear and grip out of them over the long haul. When it comes to brake pads however, we tend to ignore them. Somewhere along the way, it become a fairly common practice to just throw a new set of pads in and go bombing around the racetrack on the warm-up lap.
There are a whole bunch of reasons why that's a bad idea, and they all add up to costing you a lot of time and money. In this episode of Know Brakes, we're going to take a close look at pad and rotor bed-in: Why we do it, how we do it, and how it's going to make your street and track driving even more enjoyable than it is now.
What is Bed-in?
The goal of bedding-in your brake pads and rotors is to mate them together properly and prepare them for heavy use. Let's begin by first taking a closer look at how exactly pads and rotors interact. When you look at the coupling between pads and rotors, you have two potential friction mechanisms, abrasive or adherent. The abrasive mechanism describes the situation when the pad in your caliper is directly rubbing against the rotor face. When you throw some new pads in the calipers and hit the brakes cold, that's what's happening. What you may not know, is that most pads today are actually designed to work optimally in a different manner. With an adherent pad, the pad material is transferred to the rotor face in a thin layer when heated, which fittingly, we call the transfer layer. Then the pad in your caliper actually rides on that thin layer of pad material that you've put down on the rotor, rather than rubbing directly on the iron rotor face.
Drag racers should be familiar with the concept. On a drag strip, you do your burnout in the box to get the tires spinning. As they spin, they heat up, and start laying rubber down on the track. When you see the pros run, they extend that burnout up to and past the starting line, laying rubber the whole way. When they launch from that rubber-coated surface, their tires dig in and have more grip than if they were trying to launch from the uncoated track. The like material, the rubber on the track and the rubber on the tires, bonds together more solidly than the differing materials of the tires and track. Well, it's the same situation with brake pads. With most pad materials today, some hot pad-on-pad action will give you more friction than a pad-on-rotor interface.
So what does all of that mean? It means that if you don't lay down a nice, even transfer layer of pad material on your rotors, you're leaving performance on the table! A good transfer layer is going to give you superior brake pedal feel, as your pads will bite more evenly on your rotors.
In most cases, you'll also have less noise. When the pad in the caliper runs on a layer of pad material on the rotor, rather than directly on the iron rotor face, you tend to hear a lot less noise and squealing. You're also going to get less pad and rotor wear.
More concerning however, is that if you don't intentionally establish a good transfer layer on your rotors, you may do it accidentally when you really heat the brakes up for the first time through aggressive driving. Then you run the risk of getting uneven pad deposits on your rotors, which means the dreaded thud-thud-thud vibrations when you step on the brakes. In the worst case scenario, your rotors may not even be recoverable, which means you'll need to buy new ones. That's always a fun scene at the dealership, when they tell you need new front and rear pads and rotors, and by the way, that will be $1,000 please.
Finally, during a proper bed-in cycle, the rotors are introduced to heat stress through a progressive temperature increase. The rotors are brought up to temperature over time, rather than being instantly shocked at an extremely high temperature. Ramping up the heat in the rotors prepares them for heavy use as the metal gradually expands. Wild temperature swings from very cold to very hot are the greatest cause of cracked rotors, which is something we're obviously trying to avoid.
If you think I'm overstating the importance of bed-in, take a guess at how many of the NASCAR teams we deal with today are having their rotors prepared before they ever even see them. ALL of them! One of the core components of our business at Essex is to perform pad and rotor bed-in for a long list of professional race teams, and each year we bed-in thousands of rotors. Let's take a look at some of the work we do for the teams, and it should give you a more tangible idea of what we're trying to achieve with our cars.
Burnished Rotors
So far we've been using the term bed-in to describe the act of preparing a set of pads and rotors for heavy use. Another term that means the same thing is burnishing. Burnishing is the more common term used in racing, whereas bed-in seems to be more popular in the aftermarket. We're going to use them interchangeably moving forward.
Take a look at the photos attached to this article of a bed-in rotor, side-by-side with an untouched rotor straight out of the box. They give us a great visual reference for what we're trying to accomplish during our bed-in cycle.
The difference is striking. The blue-grey color on the burnished disc is the pad material bonded to the rotor face. You'll notice how evenly the layer is laid across the face, from top to bottom, and all the way around the entire rotor. There aren't any visible splotches of material, deep grooves, or irregularities.
Yes, the transfer layer will look slightly different depending on the pad compound, so bonus points to those of you thinking about that. The color can vary a bit, and some pads are also notoriously tough to bed-in, so the layer may not be as evenly distributed as this one. Overall though, this gives us a great example of the way your rotors would ideally look after you've finished bedding them.
Burnishing Machine
The burnishing machine is actually a fairly simple device. It spins a rotor up to set speed, and then the pads are pressed against the rotor at a given pressure for a specified period of time. We can then run any sequence of stops that we want in order to get the results we're after, and most of the race teams we work with have their own proprietary procedure for their rotors. We could show you a specific team's burnishing cycle, but then we'd have to kill you.
The goal here again is to gradually bring the rotor up to temperature, and initiate the transfer of pad material onto the rotor face in a controlled manner. How exactly we do that will depend on both the type of rotor and the pad compound. For example, if we're using a large, thick, rotor, such as a short track disc, with a pad that can handle a ton of heat, it will take more effort to increase the temperature of the pad and rotor to create the transfer layer we want. Likewise, if we're using a thin, lightweight super speedway disc, we don't have put as much heat into the pad and disc to get the pad transfer to occur. That's why it takes a lot less effort to bed in street pads on small thin, OE rotors than it does to bed-in a 15" big brake kit with race pads.
So how do we increase the pad and rotor temperature to the level we need? Well, we have a few ways. First, we can increase the speed. As speed increases, the amount of energy in the stop goes up dramatically. That means that a stop from 100mph down to 20mph turns a lot more of a disc's rotating energy into heat than one from 60mph down to 0mph. Take a second and think about how many more times the disc is spinning per second while decelerating at 100mph, down through 90mph and 80mph... and then think about how fast it will be spinning at 60mph, 50mph, down to 0 mph. The dramatic difference in energy transferred to heat should make sense.
The next factor we can control is pressure. On the burnishing machine or dyno we can increase how hard the pads squeeze the rotor. On your street car this isn't an adjustable option, so we won't worry about that one for now. The final factor we can manipulate is time. Now take it easy there hotrod, that doesn't mean we're going to break out the DeLorean. What we can do though is control the duration of the stop, and the time between stops. On the burnishing machine we can hold a certain speed at a certain pressure to generate more heat. We could basically do that until the rig caught fire and burned down. As cool as that would look on video, it's probably not something we should try (well, maybe we will at some point). You could do something similar in your car by dragging your brakes while accelerating, but it's going to be really tough to control that situation. So what does that leave us with? That means that for any given pad and disc combo, the speed from which you'll stop, and the length of time between stops will be primary factors in determining how much heat we can pour into the pad and disc.
What this also shows us is that a one-size fits all approach to bed-in doesn't really cut it. The procedure will vary with the specific rotor and pad that we're trying to prepare. It's something that you'll need to figure out for your particular setup. Hopefully with a good understanding of the theory, along with a basic procedure to get you started, you'll be able to get the most out of your brake package.
Procedure Overview
Okay, so we have the basic theory out of the way, let's take a look at the actual procedure. First, make sure you have a safe location to perform a proper bed-in. You need a stretch of asphalt with long straights, good visibility, and no potential obstructions. Obstructions are other cars, walls, trees, your neighbor's cat, or any other living or inanimate object with which you probably shouldn't be colliding. With a street pad, you typically won't have to go over 60mph, but with race pads, you probably will. Make sure you are in a position to safely, legally, and repeatedly hit the necessary speeds. Essex in no way suggests or condones speeding or doing anything illegal in your car.
Now we're going to go to walk through the actual bed-in procedure. In this scenario we'll be using an OEM-sized rotor and a common street pad. As described earlier, it will be much easier to generate the heat we need at lower speeds than if we were using a race pad and a larger rotor that flowed more air.
The first couple of stops will gradually bring the rotors and pads up to temperature. After a few stops the pads will start to smell...the resins holding the pad together will start to burn. By the fourth or fifth stops, we'll start to see smoke in most cases. This is where most people hit the panic button and quit the procedure. They think, "I'm hurting my car!" This is actually the critical point where you need to keep pushing through, since this is where pad transfer typically begins. After you've done the procedure enough times, you'll probably be able to feel the pad transferring to the rotor through your foot on the brake pedal. The next couple of stops are where the bulk of the pad material transfer will occur. Then we'll do a cool down, and take a look at what we have. Something critical to notice during the procedure...I never, ever come to a complete stop with my foot on the brake pedal! That's a big no-no, but happens all too easily when you're distracted or don't pay close attention.
In-Car procedure
Okay, here we go. If you have brake ducts on your car, in conjunction with a big brake kit, you may want to block them off to allow the system to heat up more easily. I'm going to accelerate up to roughly 60mph and then decelerate down to a slow roll...maybe 5 or 10 mph. It's not a crisis if ABS intervenes, but you should try to hold the brakes at a point just before ABS intervenes. That will ensure a smooth application of the pads against each rotor, rather than a series of brake 'hits' from each caliper. Once I get down to 5 or 10mph, I'm going to immediately accelerate back up to about 60, so the brakes don't have much time to cool off.
With the cold pads and rotors, the pads won't have a ton of bite, and they may feel a little wooden. After a couple of stops though, they'll begin to 'come in' a bit.
Okay, we're on stop #2 now, and I can already smell the resins and lubricants cooking a bit. We should start to see some smoke coming off the front corners on the next couple of stops.
This is stop #3, and I'm seeing a good deal of smoke. This is where most people get freaked out, which is why they don't typically get a good bed-in completed. Your brakes aren't going to spontaneously combust, so no need to panic, particularly if your pedal still feels solid. There is a chance you'll experience pad fade at this point. You'll know that's happening if your brake pedal is firm when you press it, but the car still isn't slowing down. That means you need to be extremely cautious when performing your next couple of stops...you need to make sure you have plenty of room, because the car will require a greater distance to stop.
Okay, now I can feel the pads transferring to the rotor. The feeling is hard to describe...the pedal feels a little bit different under my foot...the pads feel like they're compressing somewhat, and they also feel like they're sticking to the rotor a tiny bit when I release the brake pedal. We're seeing more smoke now as we pour more heat into the system. Now is when we really need to be careful not to come to a complete stop. If the police show up right now, just wave and yell out the window that you can't stop because you're performing a scientific experiment. I'm kidding, but seriously try not to come to a complete stop at this point.
Now the pads are really starting to fade quite a bit on the final stops, the smoke is heavy, and they smell. Let's go do a cool down and see what we have. For your cool down, you want to get air flowing across and through your rotors and pads. That means it's better to go cruise down the highway for five minutes than it is to putt around town at 30mph. You also don't want to be in a stop-and-go scenario where you have to come to a complete stop. For all of you country bumpkins, it should be easy. For the city folk, it can be a little tougher. When I lived in LA I used do my bed-ins in an old industrial park, then I'd hop on the 405 freeway and just cruise for five or ten minutes. There's just nothin' quite like the smog-filled air ruffling your hair after a good bed-in run. When I went out to Buttonwillow or Streets, I'd sometimes do my race pad bed-in on the long, straight roads that lead out into the desert past the track. Just make sure you have your water bottle with you, and watch out for tumbleweeds and rodents.
Post bed-in inspection
We're back in the shop now, and we need to take a look at our results. Here's the front rotor we just bed-in, and here's the shot of it as it came out of the box. While it doesn't look quite as good as what we were able to accomplish on the burnishing machine, you can still see a clean transfer layer. It's smooth, evenly distributed, and dark in color...not bad for the first try! That's what we're after, and it was obvious before I pulled the rotors that the bed-in was successful. The brakes felt great after the cool down...I had good bite, no noise, and the pedal felt solid.
These are the pads I just bed-in. You can see the edges are a grey ash color, and it's obvious that they got cooked a bit. That's how they will look, and no you haven't hurt or ruined them.
Race pads and BBK's
If your rotors and pads don't look like this when you're done, then you just have to go back out and do another cycle. A cycle is the full series of stops. Chances are, you probably didn't get your setup hot enough, and it will take some practice to learn what it takes for the procedure to be effective on your specific car. This can become particularly challenging with race pads on a big brake setup. The rotors typically flow more air than the stockers, they are a larger heat sink, and race pads need a lot of heat before they start transferring material to the rotors. If after each stop you are immediately accelerating back up to speed, that means you have to perform your stops from a higher speed...80mph or 100mph are nice starting points. Keep in mind though, that the energy in those stops is much higher, so you may not need to do as many of them. Instead of 10 stops from 60, you may only need to do 6 from 100. Or, maybe it's 20 stops from 60, instead of 10. Again, it's just going to depend on your particular pad and rotor combo. Please keep your local posted speed limits in mind when working with race pads.
Rear pads and rotors
If you're driving a front-engine car and you take a look at your rear pads and rotors after the bed-in, the transfer layer will likely not be as extreme, and your rear pads probably won't look nearly as cooked. That's fine, and it's not a problem. That's because your car is using substantially less rear brake than a rear-engine or mid-engine car. If you're driving a 911 with your engine dangling out back, you're using a lot more rear brake, and you'll likely see a more balanced bed-in on the front and rear.
An exception would be if you're driving a front engine car with severely undersized rear brakes. A good example would be a non-Track model 350Z. The rear pad on that car is the size of a postage stamp, and the rotors aren't particularly large either. That means it's not going to be terribly difficult to get some serious heat into the rear brake system on that car.
What happens over time?
Okay, so you did your bed-in and your brakes have been flawless for a couple of weeks driving back and forth to work, and carting your girlfriend to the mall. You've had good bite, the pedal feels great, and you haven't heard any noise. Out of nowhere, they start to squeal a little bit one morning on the way to work. By the time you get home, they're making all sorts of noise. When you inspect them, the transfer layer is gone, the rotors are silver, and they look almost like they did when they came out of the box new.
That's completely normal. Just like the other parts of your car, your brakes and their transfer layer needs to be maintained for optimal performance. When you're operating your car in day-to-day driving, they're almost always cold. When they're cold, they slip into the abrasive friction mechanism we talked about at the beginning of this video. That means that on your way to the store, they're constantly scraping away at the transfer layer you laid down on the rotor. The only time they might get hot enough to replenish that transfer layer would be on a canyon run or a trip to the track. Your only solution is to go back out and do another bed-in cycle to get them back in proper form.
Conclusion
Well, that about wraps it up for this episode of "Know Brakes." If you're conscious about properly preparing your pads and rotors for heavy use, it will pay big dividends in the long run. The harder you run the car, the more apparent those benefits will become. Your brake pedal will feel better, you'll have less brake noise, less pad and rotor wear, fewer judders and vibrations, and a much lower chance of trashing your rotors. You'll be getting the maximum performance out of whatever brake setup you're running at the time, you'll save money, and you'll enjoy your car a lot more.
