Joined: Mar 27, 2010
Fri Dec 09, 2011 7:51 pm
Those little brakes that stop our car, what's up with those? How do they operate and what is the mechanism of their wear? Let's find out.
Well, first thing first, the brakes don't actually stop the car. They apply a certain force (a torque of a few dozens to a few hundreds of neutons per meter) against the direction in which the rim rotates. This slows down the rotation of the rims and tires and this is what actually slows down the car.
The old type of brakes, still used in times in the back axle of some small cars, are drum brakes. These are drums made of a metal cylinder which rotates with the rim. Inside the cylinder lay one or two "shoes" which hold the brake pads (which are made of an abrasive material) and an application of hydraulic pressure or pulling down a wire (in case of a handbrake) pushes the shoes against the inner wall of the drum, creating friction that slows it down and brakes the car.
The downside of drum brakes is with the more immediate buildup of heat due to it's closed shape. The reason why it is still used in the back wheels of many cars is that the rear tires experience smaller braking loads, and hence the braking force distribution to them is reduced. When you brake, the tires' grip forms a vector of negative acceleration. In other words, the tires are "pulling" back and thus slow down or stop the vehicle.
At the same time that the tires pull back at ground level, the force of inertia is pushing forward at the center of gravity, a good scores of inches above it. This conflict of physical powers creates a torque that seeks to rotate the car forward, which is experessed as a weight transfer from the rear to the front. With the weight transfer, grip is transferred to the front axle and greater braking forces can be applied to it, and the opposite occurs in the rear tires. This is why at least 70% of the braking force comes from the rear tires.
In trucks, the distribution is more equall, as it is also in race-cars and in rigs there are bigger brakes in the rear, which has to retardate the bigger loads of the trailer or loads, as well as keep the car stable and prevent any oversteer or jacknifing that might happen upon rear wheel lock-up.
Modern brakes are "disc brakes" which involve a caliper that can press the brake pad (which is fitted on a metal plate connected to two to six hydraulic pistons) against an open disc. The advantages are that the disc is open for ventilation, and it's size acts as a leverage that increases the braking torque, and even more so with bigger rotors in performance vehicles. To further reduce the problem of over-heating, slits and drills are inserted into performance-intended brakes to increase ventilation.
The link between your right foot and the brake calipers is a closed hydraulic system using an athenol-based brake fluid with a high working temperature. The pressure against the pedal pushes a piston through the the main reservior. Modern systems have two pistons for the front and rear and in the event of brake failure.
Even so, physically pushing with your foot enough to clamp the brakes can be quite laboring, thus the brake booster was invented. The booster does NOT increase the pressure on the system, it only allows the driver to push more lightly against the pedal to operate the brakes. It is a big round membrane that creates a suction force based on a vacuum that arrives from a small servo, or from the vacuum in the intake manifold of petrol engines.
The brake fluid itself wears down, as it is exposed to heat, oxygen and mainly moisture. The fluid is hydrophylic and any exposure to wetness will result in problems, since water are not as compressible as the brake fluid, as well as boil much sooner. The fluid degrades as to require replacement within two years at most. A bad fluid increases the wear inside the system and can seriously damage the main cylinder by causing abrasion in the cylinder's walls that will wear the piston if it is pused under low pressure.
Another important subject here is brake lag. The braking system takes a brief moment to react to your inputs. In order, it takes about 0.2 of a second, but this will increase to 0.3 of a second or more when the fluid even just begins to degrade.
Heavy-duty rig brakes are different. They operate under such loads that cause the brakes to be the weakest link in the system that slows down the car. In a road car, pressuing the brakes hard will cause the wheels to lock-up and slide and this puts a limitation to how quickly one can stop the car. However, in a rig, the loads are much larger, but the rims and brakes cannot be made quite so big to handle the greater loads. Therefore, in higher speeds it becomes impossible to lock up the wheels and instead, the limitation of the braking distance is the force of the brake and it's ability to withstand heat.
Therefore, rig brakes a air brakes with at least two or three channels for the event of failure. These pneumatic brakes have a disadvantage is the form of about twice the brake lag of a normal braking system: about 0.6 of second to react!
Brake pads are made of carbon-based materials that withstand relatively high temperatures. However, for their proper operation they require proper bedding or breaking-in, as well as a sufficient working temperature. Bedding-in the brakes is the manner in which one "introduces" a new set of brake pads after installing them. It takes about 400 miles to get this right.
Why bed-in the brakes? Well, the first reason is to burn the resins that cannot be perfectly removed from the material in the process of manufacturing, or materials that are used to preserve the pads when in long-term storage. Another reason is that when the pads are pushed against the rotor, some of the material remains on the rotor's face and this can bind ever more mightly with the brake pads. So, you need to break into the pads to put some of the pad material on the rotor or to remove the material of an older pad and replace it with the material of the new pad.
For this purpose, it is customary to lightly brake for the first few hundreds of miles, and that begin to break-in the pads more aggressively by a series of moderate to mild braking efforts from moderatly-high speeds (four to five relatively strong brakings from 50 to 10mph will do).
The whole concept of breaking-in a pad, as well as the pad's working temperatures, are the main reasons why ceramic or carbonfyber pads are not used in road-cars, as these materials, as abrasive as they might be, require high working temperatures, a labored breaking-in procedure, and involve higher wear levels to the pads themselves and the rotors. Furthermore, as we established, this will NOT reduce stopping distances at all, unless the tires and suspension are improved to match this!
When a pad overheats some of the material will be melted and possibly reformed on the pad surface as a hard, blue-ish surface. This mainly happens when the brakes have been working hard and than held against the rotor in one spot by stopping. This is why track drivers have a cool down lap (or two...) and why taking a visit to the nurburgring will help you see a range of methods of cooling down the brakes on the move by rocking the car back and forth (when holding back in a queue) or crawling slowly along the queue, etc.
If you stall the car and apply the foot or hand brakes for a good deal of time, the problem will be aggrevated since the air flow around the pads will be slower. This will make the heat disperse from the pads and rotors to the rims themselves, brakes lines and even unto the master cylinder, wrapping the rotors alltogether (i.e. demolishing the brakes) and causing further damage to other systems.
This is also why braking somewhat intermittently is preferable to a continuous braking effort. If you brake lightly across a long period of time (say, when coming down from the highway) you should pause your braking effort for a second after each, say, five seconds of braking or so.
This is also the reason for going down the gears on long downhill slopes. If you ride the brakes down, you will overheat them (although you might be surprised by how much modern brakes could take! ). The brakes are meant to slow you down, not to keep the car at a certain speed. This is what the gears are for.
In normal braking situations, you do not need to do this in a manual transmission, not to mention in an automatic. The brakes are more than capable of enduring the loads. It's "brakes to slow, gears to go." Slow down as best as possible while in gear, and than declutch into the desired gear for stopping (first gear) or for the slower desired speed.
Emergency braking and safety systems
The shortest braking distance is achieved by "threshold braking" which is braking as hard as possible, on the threshold of the tires losing grip and locking. This is achieved by applying the brakes rapidly, but not instantly to just the right amount of pressure and than modulating that pressure to keep the car on that threshold. Any additional braking effort or any attempt to further slow down by downshifting will only cause the car to exceed the limit and slip. This is also why performance drivers resort to clever methods of downshifting (mainly heel and toe) to preven the additional braking effect of the downshift.
The main thing here is that this operation is highly counter-intuitve. When you brake, your speed is obviously fastest when you just began braking, isn't it? Well, here's the catch: the faster the car is moving, the faster the wheels are rotating and the greater braking force is required to slow them down, thus the brakes should be applied faster and the brake pressure be bigger. However, the higher the speed, the bigger is the human pressure of nailing the pedal.
This also dictates that as you brake, and the speed is reduced, the brakes will actually stop you harder and harder, so to counter this and remain just on the threshold, you need to modulate the braking pressure by progressivelly letting back the pressure on the pedal as the speed is reduced.
This is true for the track, where you know where you start braking, where you finish braking, and where the environment, car and other drivers are mostly predictable. This is under no circumstances achievable in the circumstances of emergency braking on the road. This is exactly why ABS systmes were made to mimic this kind of braking.
Old ABS systems were a sensor with a valve that turned the braking system on and off repeadetly when the wheels locked-up. The result was highly ineffective, except on ice where it failed to operate alltogether. The system in most modern cars, operates much more rapidly and accurately, slightly adding and removing a bit of pressure in intrevals of 0.005 of a second. These systems reduce braking distances on all but very abrasive dry roads (in which case there is a slight, neglegible longer distance) and allow to steer while braking.
In newer cars, the system became ever more sophisticated, preventing lock-up from even occuring, and keeping the tires around the threshold as best as possible (while still taking longer to stop than in proper threshold braking). These systems have four channels, each monitors each of the car's wheels and keeping it unlocked. The systems are calibrated in a manner that can acheive shorter braking distances or better steerability or stability, exchanging these advantages slightly. The noises of the system arise from the super-fast operation of the valves under the high loads inside the system.
The system is aided by an Electronic brake force distribution system that governs the braking force distribution to all four tires and changing it depending on how load (posed by luggage and passengers) is distributed inside the car. Other auxiliary systems involve a system that compensate for brake fade of moist on the pads, or an brake assistant (BA or EBA) which recognises panic braking and increases the force to full braking. This is important because people tend to hesitate and not brake as hard at the start, or because some people have a hard time pressing the pedal as hard as it goes, like short people, feeble women or old people.