Underhood: Timing The BMW N53 And N54
Adapted from Larry Carley’s article in BRAKE & FRONT END
If you have ever serviced brakes, you understand what brake rotors do. They provide a friction surface for the disc brake pads to rub against when the brakes are applied. The friction created by the pads rubbing against the rotor generates heat and brings the vehicle to a stop.
The underlying scientific principle here is that friction converts motion into heat – a LOT of heat! The amount of heat that’s generated depends on the speed and weight of the vehicle, and how hard the brakes are applied.
A large, heavy vehicle, like a Chevy Suburban, will obviously generate more heat when braking than a Toyota Echo if both vehicles brake from the same speed. But the little Toyota may produce more heat than the big Suburban if the braking speeds are different, say 60 mph for the Toyota and 20 mph for the Suburban. Speed multiplies the effect of weight and creates momentum (also called “inertia” or “kinetic energy”).
How Stopping Started
Over 300 years ago, a guy sitting under an apple tree in England made a startling observation that was to change history. As the story goes, an apple fell out of the tree and bonked him on the head. This got him to thinking about the nature of gravity and motion. Since he was no ordinary bumpkin, he soon formulated some basic rules about the way objects behave when in motion.
One of the “laws of motion” he came up with says that any object in motion stays in motion unless acted upon by an outside force. Though this seems pretty obvious to us today, at the time it was a scientific breakthrough because nobody up to that point in history had figured out a way to express the concept mathematically. He soon developed formulas that could accurately predict the exact amount of force needed to accelerate an object of a given size (mass) to a given speed, or conversely how much force it would take to stop an object of a given mass traveling at a given speed. The guy’s name was Isaac Newton.
Newton’s laws of motion and the formulas he developed became the foundation of modern physics and engineering. The guy even invented a new branch of mathematics called calculus so automotive engineers would someday have the tools to analyze and evaluate braking performance in today’s vehicles.
The Heat Is On
OK, we’ve explained how brakes produce friction, and friction produces heat, and this is what brings a vehicle to a stop. But we haven’t explained how much heat is actually produced.
Think of heat as a form of energy or power. A more familiar term is “horsepower.” We all know what horsepower is, right? It’s the stuff that spins the crankshaft when fuel is burned inside an engine. Combustion produces heat, and heat pushes the pistons that make the crankshaft go around. One horsepower is equal to 33,000 pounds-feet of torque per minute, or 550 pounds-feet per second.
We measure an engine’s horsepower output by hooking it up to a dyno and seeing how much force it can exert against the resistance created by the dyno. In effect, the dyno acts like a giant brake, so the engine’s power output is sometimes called its “brake” horsepower output.
By the same token, we can also measure how much horsepower a vehicle’s brakes must absorb when bringing the vehicle to a stop from a given speed. This is also called “brake” horsepower, but in this case it refers to the brake system, not the engine.
Don’t worry about the math because it depends on the speed and mass of the vehicle and the stopping distance. The important point is the brakes often have to absorb a great deal of heat in a very short period of time.
One thing all brake manufacturers monitor very closely when testing and evaluating pads and rotors is the temperature of the brakes. Every time the brakes are applied, the pads and rotors generate heat that must be absorbed and dissipated. A quick stop from 60 mph can easily push the rotor temperature up 150 or more degrees! Several hard stops in quick succession can push brake temperatures into the 600, 700 or even 800-degree range. Remember, the heavier the vehicle, the more heat it creates when it brakes.
Riding the brakes down a steep mountain road or repeated hard brake applications can produce so much heat the brakes begin to fade.
When brake temperatures get too high, the pads and rotors are no longer able to absorb any more heat and lose their ability to create any additional friction. As the driver presses harder and harder on the brake pedal, he feels less and less response from his overheated brakes. Eventually, he loses his brakes altogether. (Uh-Oh)
All brakes will fade beyond a certain temperature. Semi-metallic linings can usually take more heat than nonasbestos organic or low-met linings. Vented rotors can dissipate heat more rapidly than nonvented solid rotors. Thus, high performance cars and heavier vehicles often have vented rotors and semi-metallic front brake pads to handle high brake temperatures. But if the brakes get hot enough, even the best ones will fade.
The Role of the Rotor
Now that we’ve covered some of the physics of braking and the effects of friction and heat on the brake system, let’s look at the rotors role in all of this. As we said earlier, the rotor’s job is to provide a friction surface, and to absorb and dissipate heat.
Big rotors can obviously handle more heat than small rotors. But many cars today have downsized rotors to reduce weight. Consequently, the brakes run hotter and require better rotor cooling to keep brake temperatures within safe limits.
Anybody who works on brakes for a living knows that rotors can cause a lot of brake problems. Uneven rotor wear (which may be due to excessive rotor runout or rotor distortion) often produces variations in thickness that can be felt as pedal pulsations when the brakes are applied. The condition usually worsens as the rotors continue to wear, eventually requiring the rotors to be resurfaced or replaced.
Rotors can also develop hard spots that contribute to pedal pulsations and variations in thickness. Hard spots may be the result of poor quality castings or from excessive heat that causes changes in the metallurgy of the rotors. A sticky caliper or dragging brake may make the rotor run hot and increase the risk of hard spots forming. Hard spots can often be seen as discolored patches on the face of the rotor. Resurfacing the rotor is only a temporary fix because the hard spot usually extends well below the surface and usually returns as a pedal pulsation within a few thousand miles. That’s why most brake experts replace rotors that have developed hard spots.
Cracks are another concern with rotors. Cracks can form as a result of poor metallurgy in the rotor (too hard and too brittle because the rotor was allowed to cool too quickly during the casting process), and from excessive heat. Some minor surface cracking is tolerable and can often be removed by resurfacing, but large cracks or deep cracks weaken the rotor and increase the risk of catastrophic failure. So cracked rotors should always be replaced.
The Science of Metallurgy
The metallurgical properties of a rotor determines its strength, noise, wear and braking characteristics. The casting process must be carefully controlled to produce a high quality rotor. You can’t just dump molten iron into a mold and hope for the best. The rate at which the iron cools in the mold must be closely monitored to achieve the correct tensile strength, hardness and microstructure.
When iron cools, the carbon atoms that are mixed in with it form small flakes of graphite which help dampen and quiet noise. If the iron cools too quickly, the particles of graphite don’t have as much time to form and are much smaller in size, which makes for a noisy rotor.
The rate of cooling also affects the hardness of a rotor. If a rotor is too hard, it will increase pad wear and noise. Hard rotors are also more likely to crack from thermal stress. If a rotor is too soft, it will wear too quickly and may wear unevenly increasing the risk of pedal pulsation and runout problems.
The composition of the iron must also be closely controlled during the casting process to keep out impurities that may form “inclusions” and hard spots. One rotor manufacturer says they sample the molten iron every 15 seconds to make sure the composition is correct. The molten metal is also poured through ceramic filters that trap contaminants. Even the sand that’s used to make the molds is specially treated to control moisture content. This helps keep the sand in place and prevents core shifts that can affect porosity, dimensional accuracy and balance.
Reasons for Ribbing
Vehicle manufacturers use a wide variety of different cooling rib configurations in their rotors. They do this to optimize cooling for different vehicle applications. So even though the brakes may appear to be identical on two different models, one may require increased cooling because the vehicle is heavier, has a more powerful engine, has less airflow around the brakes, etc.
Some aftermarket rotor manufacturers use the same rib design and configuration as the OEM rotors, while others do not. Some change the rib design to simplify the casting process or to reduce the number of different rotor SKUs in their product lines.
The OEMs currently use almost 70 different rib configurations in their rotors. Some ribs are straight, some are curved and some are even segmented. Some rotors are directional and some are not. Some rotors have evenly spaced ribs while others do not. Some ribs radiate outward from the center and others go every which way.
One reason why they use so many different rib patterns is to maximize cooling and to reduce harmonics that contribute to brake squeal. Changing the rib design changes the airflow, cooling and noise characteristics of the rotor – which may make things better or worse depending on the application. That’s why some aftermarket rotor manufacturers use the same basic design as the original, while others stick with more traditional venting.
A heat dam is often machined into the area between the friction surface and hat on most rotors. The dam is a thinner section of metal that reduces heat transfer from the rotor surface to the hat. This protects the wheel hub and bearings from the heat and also allows the rotor to flex when it gets hot to reduce the risk of warping and cracking.
If a rotor manufacturer cuts corners and eliminates the heat dam, heat can travel more easily to the hub and wheel bearings and increase the risk of bearing failure. The rotor may also be more prone to cracking under high heat conditions.
Keep it Smooth
Before we come to a stop, we’ll finish up with a few comments about surface finish. Smoother is always better because it affects the coefficient of friction, noise, pad seating, pad break-in and wear. As a rule, most new OEM and quality aftermarket rotors have a finish somewhere between 30 and 60 inches RA (roughness average) with many falling in the 40 to 50 RA range. It’s unlikely you’re going to improve this any by “cleaning up the rotors” on a bench lathe prior to installing them. In fact, you may make the finish worse if you cut the rotors too quickly or use bits that are dull.
New rotors should always be installed “as is” – and indexed on the vehicle with a dial indicator to minimize runout. Few technicians take the time to do this, but if they did they’d probably see fewer comebacks because of pedal pulsation complaints.