Basic Braking 101
The most fundamental principle relating to stopping a vehicle safely is that of converting the moving energy of the vehicle into heat and dissipating it into the atmosphere efficiently. Anything that affects the system's ability to accomplish this principle will result in poor braking performance and a dissatisifed customer.
For instance, it is impossible to stop a vehicle on a gravel road as effectively as on a paved road due to the difference in traction. The conversion of energy into heat is affected by the weight or mass of the vehicle, the traction between the vehicle and the road, and the ability of the brake design to convert the energy into heat. Worn tires, sudden maneuvers and an overloaded vehicle will dramatically affect the performance of the system.
Another factor in braking is the heat during braking is generated very quickly and the time to dissipate that same heat takes much longer. This heat can be 300-500 degrees on lighter stops and as high as 1,000 degrees on harder/faster stops. This means the friction material must handle constant high temperatures and facilitate the transfer of that heat into the rotor/drums for dissipation without warping. Keep in mind that most of today's vehicles run smaller and lighter braking components to increase fuel economy. Quality of components now becomes a larger issue and adds to the expense of a "proper" brake repair.
The choices a technician is faced with when choosing friction materials today is mind boggling. If you look at a typical brake catalog you will be faced with choosing one of five or more possibilities. While there may be a compelling reason for brake manufacturers to offer these choices, the engineers designed the brake systems for each vehicle with one and only one material in mind. This does not mean they chose the best option but it is the proper choice to start with.
Friction is not just a matter of how rough something is. The amount of friction developed by two objects in contact is _greatly_ affected by the force applied and the size of the area that the force is applied over. For example, in winter it is popular to put sand bags in the bed of a pickup for traction. The extra force counteracts that the COF (coefficient of friction) of the surfaces in contact (the road) has gone down. This is the same reason snow tires tend to be more narrow. Reducing the surface area increases the total friction.
The force required to produce enough friction between the brake linings and the disc/drum will need to be multiplied. This is performed by several different components.
- Mechanical leverage (brake pedal length/pivot, braking hardware, etc.)
- Boosters (hydraulic, vacuum and electric)
- Hydraulic multiplication (master cylinder, caliper, wheel cylinder and brake line diameters)
So in the big picture every single component plays a crucial role in the entire braking experience.
Brake Fluids - Why Flush the Brake System?
For all fluids that meet DOT 3 specification, the minimum "dry" (contains no water) boiling temperature is 401 degrees F and the minimum "wet" (saturated with water) boiling temperature is 284 degrees F. For fluids that meet the DOT 4 specificaiton, the minimum dry boiling point is 446 degrees F and the wet boiling temperature is 311 degrees F.
To transfer hydraulic force without pressure loss, the liquid cannot compress. Silicone brake fluid is slightly compressible, which is the reason it is not to be used in ABS systems. Rapid cycling of the ABS modulator tends to aerate silicone fluid, which in turn, causes a loss of braking. Fluid must have hot and cold viscosity to ensure flow without fluid restriction. Fluid must also contain enough lubricating qualities to avoid wear of metal and rubber system components. Along with all of this, brake fluid must remain chemically stable and dry.
To understand why moisture enters the brake system, you need to know something about the chemistry of brake fluid. Fluids containing Polyalkylene Glycol Ether are regarded as DOT 3, DOT 4 and DOT 5.1. They are "hygroscopic" by nature, which means they attract water. Under normal driving conditions, brake fluid will absorb moisture from the air at a rate of 2% to 3% per year of service life.
This process cannot be stopped, even in a closed system. Imagine how much water must be in some vehicles that are six, eight, or 10 years old and have never had the fluid changed.
So why do the car manufacturer's use glycol-based brake fluids? Because it is hygroscopic, it disperses the moisture that enters the system throuhgout the fluid to dilute the contamination. This prevents moisture from forming puddles in calipers or wheel cylinders that could boil and cause pedal fade if the brakes get too hot. The downside is that moissture lowers the boiling temperature of the fluid, increases viscosity and promotes internal corrosion. That's why various chemical additives are put into the fluid to help it fight corrosion and oxidation.
Brake fluid contains Amines which neutralize acids that develop in the hydraulic system. These bases lose their effectiveness over time, from both heat and chemical depletion. Brake fluid also contains lubricating and coating agents that are designed to slow surface corrosion of components.
Flushing is required at time/mileage intervals to reduce corrosion and bring the boiling point back up so it won's overheat/boil during harder stops. Keep in mind, brakes get very hot.
Under the Car, Suspension Inspections
Steering and stopping are by far the most important aspects of driving today. The simple truth is this: your vehicle cannot stop properly without a sound suspension system.
- Test all shocks and struts for proper performance. Listen for any noises when bouncing the vehicle. Remember the vehicle may not bounce excessively during this test, but may still have weak shocks or struts because of age.
- Check air pressure in all tires including the spare. Make sure the tires are the same size. If the tires are oversized the brake performance could be compromised. Next, measure the curb ride height of the vehicle.
- The weight of the vehicle must be on the tires to ensure an accurate inspection of the steering system and an accurate ride height evaluation. Marginal inner tie rods and other steering and suspension components can be missed of this step is not followed. While the weight of the vehicle is on the tires, have a helper turn the steering wheel side-to-side, putting alternating tension and compression on each steering joint. Put your hand on each steering joint and feel for any play or popping action.
- Raise the vehicle and double check all steering components as well.
- Alignment an tire wear are also factors.
There is no room for error on brake or suspension work, all systems must work properly or the results can be very dangerous.