Brake Bias (brak bi-es) noun [origin: Latin] The distribution of braking force between a vehicle's rear and front wheels. Also known as Brake Balance, Brake Proportion, Fore/Aft Brake Set.
We all want our cars to stop on the proverbial dime. A well-designed and tuned braking system is not only safer, but it makes for a more enjoyable driving experience. While there are many upgrades on the market for improving your vehicle's braking system, one of the most important (and overlooked) is simply taking the time to dial-in the vehicle's brake bias correctly.
Why Brakes Don't Stop The CarWhen the brakes are applied on a moving car, the rotational energy of the wheel, tire and brake rotor at each corner is converted into heat at the rotor-pad and/or drum-shoe interface. Physicists refer to this as a process in which kinetic energy is transferred into heat energy. This energy transport is what stops the rotation of the tire and wheel, but it's not what actually slows the car. That task is performed by the tires pushing forward against the road. Yes, tires are what slow and stop your car, not the brakes. Don't worry; we'll come back to this idea in a moment. But before we do, let's look at what happens to a car when we stomp on the brake pedal.
When brakes are applied and a car starts slowing, the forward momentum of the vehicle is transferred into vertical loads at the front tires. Take a look at the picture to see how this works. When a car is sitting still or traveling along at a constant speed, the vertical load on each of the four tires is roughly equal at all four corners. (Yes, most street-driven cars have a front weight bias, but that just complicates this discussion. For now, let's pretend that we have a Miata with perfect 50/50 front/rear weight distribution). As the brakes are applied and the car begins to decelerate, the front tires pick up more load and the rear tires actually lose some of their load. Engineers call this phenomenon dynamic load transfer and it's dependent on a number of vehicle parameters, such as wheelbase and where the center of gravity (CG) is located. For instance, the higher the car CG is above the ground, the more load is transferred forward under braking. Conversely, the lower the CG, the less forward shift of load. In a similar vein, longer wheelbase cars have less dynamic load transfer than shorter ones.
As we said earlier, tires are actually what stop a car's forward motion under braking. They do this by pushing against the roadway via friction. Physicists like to talk about material coefficients of friction and normal forces when they discuss friction. Tires are actually a bit more complicated than this simplistic laboratory explanation, but as a rough starting point, we can use the basic lab rat model. The friction force between two sliding bodies (a block of rubber being pushed across asphalt, for instance) can be calculated by multiplying a constant (called the coefficient of friction) by the vertical load pressing the bodies together (called the normal force). Or, with Greek letters, the formula looks like this:In this case, "" is the coefficient of friction and "Fn" is the normal force.
Coefficient Of Friction And Normal ForcesThe coefficient of friction is largely dependent upon the type of materials involved. For example, a block of rubber on a smooth steel table has a higher coefficient of friction then, say, a smooth block of wood sliding on that same steel table. Childhood experience teaches us that the rubber block is harder to push laterally than the smooth wooden one. This is because the rubber block has a higher coefficient of friction than then wooden one.
Now let's look at the other variable in the equation, the Normal Force. Set up two rubber blocks of equal size on the steel table. Apply a vertical load (Fn) of 10-lbs on one block and 50-lbs on the other. As the equation predicts, the block with the bigger Fn is harder to push across the table.
As we mentioned earlier, reality is a little messier than the laboratory. The coefficient of friction () for a rubber tire actually depends on many number of things, including operating temperature, vertical loading, the chemical make-up of the rubber, road conditions, camber settings and slip angles just for starters. In this discussion, however, we'll keep it simple and pretend that the coefficient of friction for a tire is constant under our braking test. This isn't exact, but it's close enough.
Holding the coefficient of friction constant, the friction formula simply reduces to one in which the more vertical load that is on a tire, the more friction, or braking force that can be developed. As we've already noted, as a car starts slowing, the front tires pick up more vertical load and the rear tires lose some of their load. This means that the front tires are actually capable of generating more friction, or stopping force, than the rears. To slow a car most efficiently, we clearly need to apply a braking force to each wheel in proportion to the weight on that wheel. In other words, when decelerating (slowing), the forward weight shift due to dynamic load transfer means proportionately more braking force should be applied to the front wheels than the rear.
Getting The Balance RightVarious devices can be used to provide this different braking force at the front and the rear. Different size rotors and calipers, inline proportioning valves, and separate front and rear master cylinders are just a few of the options available. Regardless, if the brake designer does his or her job correctly, the front brakes will be stronger that the rears, allowing the brakes to get the most out of the available friction at each tire. If the proportion of stopping power is balanced correctly between the front and the rear of the car, the maximum possible friction is developed at all four corners of the car. In a perfect, idealized world, we would want this brake bias to exactly match the traction limits at each end of the car. But we don't live in a perfect world, do we? Most street-driven cars actually need slightly more front bias than rear. Specialized racecars, on the other hand, may require a completely different bias, depending on the type of racing and the track and car conditions. To understand this, we have to take a look at what happens when a tire exceeds its traction limit.
As a tire begins to lock up and skid, the rubber that is in contact with the road starts to heat up. Now, if it gets too hot, the rubber will actually start to liquefy. We've all seen brake skid marks on the roadway; these are just remnants of tire rubber that skidded and consequently changed phase from a solid to a liquid. Can you say Flat Spot? When this happens, the coefficient of friction falls off rapidly, and the tire begins to slide even more. More sliding means even more heat, which in turn means even less friction. This is one of the major reasons why locking up your brakes is about the worst thing you can do in a panic stop situation. Your vehicle's braking ability falls off just as rapidly as the coefficient of friction and you end up hitting that cow or telephone pole you were trying so hard to avoid.
As a side note, this is the basic operating principle behind anti-lock braking systems (ABS). Just prior to lock-up, a tire is generating close to its maximum stopping power. Just after lock-up, however, a tire's braking ability drops off. ABS systems simply keep track of each individual tire's rotation; if an immanent skid (lock-up) at any of the wheels is sensed, the ABS computer momentarily relieves brake pressure at that particular wheel to avoid lock-up.
In order to maximize braking at all four corners, we want the front and rear to lose traction at exactly the same time, right? Well, no, not really. We actually want a slight bias toward the front brakes. Because of variations in dynamic load transfer, road conditions, tire wear and the like, it's virtually impossible to ensure that all four tires of a car will behave identically and reach their traction limits at the same time, under all braking conditions. In simpler terms, this means that one end of the car is going to lose traction before the other during panic braking. With that in mind, brake designers try to build a bit of front bias into most street-driven vehicles. This is because it's usually safer to have the fronts lock before the rears. Why? Because locking the rear brakes is all-too often followed by the back of the car overtaking and passing the front! If the front brakes lock up first, steering control is lost, but the car usually just continues in a straight line. While not ideal, skidding at the front is generally preferred to swapping ends at speed.
Now that we've said that, however, there are instances when you don't want the fronts to lock up first. For instance, if you're setting up a car for autocross racing on a tight cone-course, you might want to have the brakes slightly biased toward the rear. Having a loose rear on a tight circuit is sometimes the fastest way around the cones. A good example of this is the wickedly fast, Ford Cosworth-powered Lotus Elan campaigned by Chris O'Donnell of Laguna Beach. O'Donnell owns 11 national autocross championships (yes, you read that correctly-11!). This car/driver combination is so fast around autocross courses that his nearest competitors are often 3 to 4 seconds behind him-and that's on a nationally competitive basis. One of O'Donnell's tuning secrets for very tight autocross courses is his occasional penchant for a rearward brake bias. With this type of a brake balance, O'Donnell is able to initiate car rotation (literally toss the rear of the Elan out in tight corners) with judicious use of the middle pedal. For more open courses, O'Donnell tends to prefer a more balanced front-to-rear brake bias, but in all cases, he avoids forward bias like the plague. Locking the fronts before the rears on an autocross course generally results in flying cones-and no first place trophies. O'Donnell would be the first to admit, however, that when the Elan is set up with rear bias, the car doesn't stop worth a damn on the straights. In fact, it is essentially an unstable car, with the rear end popping out under hard braking. This is clearly not a good trait to have in a street car, but it can be very useful on a tight autocross course with the right driver behind the wheel.
Putting The Brakes On This DiscussionOK, so now that we know what brake bias is, what does it all mean? In a word: performance. You can install the best possible brakes money can buy, but if the bias isn't set correctly, you may not be stopping any better than you did with the stock system. In fact, you might even be worse off then you were before spending all that hard-earned dough for big rotors and light calipers. A perfect example is the AEM brake rotor upgrade we installed on Project Civic Si back in the April, 2000 issue. Since the Si already has a substantial front brake bias, adding larger front rotors actually made things worse, increasing our 60 to 0 stopping distance from 111 feet to 113 feet. (This was with Hoosier race tires providing a substantially higher than the stock tires). Adding a larger AEM rear rotor to match the front improved brake bias and reduced our stopping distance to 104 feet.
As is true with most things automotive, you have to invest as much effort tuning a system as you did installing it-and brakes are no different. To really maximize your brake hardware investment, you will need to do some well thought-out brake bias testing and tuning in a safe location. Start by making sure your tire pressures, alignment and test environment are consistent and recorded. Bring along an assistant, stopwatch, tape measure, video camera and maybe a G-Tech to measure your results. (Also don't forget that several hard panic stops will lead to brake fade, so let those brakes cool between test sessions).
If your car isn't already equipped with a brake-bias adjustment device, you will probably have to add an inline variable pressure biasing control, or look carefully at the various options offered in caliper and master cylinder sizing. The other important thing to note is the fact that good tires are an equally important part of the brake equation and can make a really big difference in stopping ability. Why? Recall our discussion about coefficients of friction: high-performance tires generally have a higher coefficient of friction han cheaper tires. Hey, getting the brake bias set correctly doesn't mean a thing if your tires can't grip the road well enough. Remember that your brakes don't stop the car-your tires do. And that's the, uh, unbiased truth.
Blueprinting, Engine (bloo'print'ing, en'jin ) verb [origin: Latin] An engine that has been machined and assembled with the most favorable combination of internal dimensions, clearances, tolerances and specifications within an allowable range.
You hear it occasionally. A friend rolls up in his low-slung import, the engine fresh from the engine builder. The paint on the new headers hasn't even completely burned off and the owner is already claiming huge horsepower numbers. "It's been ported and polished, dude, and I had it blueprinted, too."
OK, you think to yourself, I know what porting and polishing is, but what the heck is blueprinting? Isn't that what architects use for drawing house plans? And what does it have to do with building a high-performance engine? Well, don't feel bad for not being certain. Our guess is that your friend probably doesn't really know what it means either. Often, blueprinting an engine is mistaken with simply verifying all the bearing and piston fitments or tightening all the fasteners to the correct torque specifications. Sometimes, builders claim that an engine has been blueprinted if they measured all the parts to be within spec and supplied a detailed "build" sheet with the motor, which lists the various internal dimensions and clearances. This is all part of blueprinting an engine, of course, but it's not the whole story. In fact, the real secret to blueprinting an engine lies in a rather painstaking selection of the optimal combination of parts and sizes for maximum power and reliability.
Specifications, Tolerances, & BlueprintingThe term "blueprint" refers to the process in which an engine builder measures and machines all the critical dimensions during engine assembly, with the goal being that (1), they are all within factory specs; and (2), they're all working together to most efficiently produce the maximum possible power. In the good/bad old days before Computer Aided Design (CAD) and laser printers, these dimensions and specifications were sometimes recorded on draftsmen's "blue" line copies, or blueprints, and hence the name. Nowadays, when a knowledgeable builder "blueprints" an engine, he carefully goes through all the specifications in the rulebooks and engine drawings, and he judiciously selects those numbers and values that best suit the owner's particular needs. For a street-driven car, there obviously isn't a rulebook to adhere to, but the basic idea is the same. The builder concentrates on maximizing power, reliability and longevity by paying careful attention to the manufacturing and assembly details. And the key to this entire process can be summed up in a single word: tolerances.
Every single part of an automotive engine, from the most complex cylinder head and intricate crankshaft forging, down to the simplest nut, screw or washer, was manufactured from an engineering specification sheet. These "spec" sheets invariably include a drawing, or set of drawings, which specify exactly what the form, fit and function of the part should be. More importantly, every number and dimension on these drawings has a tolerance associated with it. For example, the diameter of a piston may be called out as 92.50 0.05mm on the OEM manufacturing sheets. When blueprinting an engine, it's the "plus and minus" tolerance value that a builder spends most of his time worrying about. This is done in an effort to optimize the final dimensions of the part so that it provides the best possible performance characteristics. For example, due to friction effects or maybe ring-to-cylinder interactions or perhaps the optimal bore-to-stroke ratio, the maximum horsepower output of an engine may actually be generated with the piston at its smallest value in the allowable range. In this particular case, that value would be 92.50 - 0.05 = 92.45mm. In other cases, the largest, or perhaps nominal mid-value, is the ideal number to use. Regardless, the engine builder would then strive to select, or machine if necessary, a complete set of pistons that all had this same exact diameter. The problems start increasing for the builder, however, when he also attempts to get all the pistons to weigh the same optimal amount and machine identical ring-land geometries that best seal the combustion chamber and have identically matching dish or dome shapes, etc.
It's apparent that working through an entire engine in this manner can result in literally hundreds of part dimensions and specs to worry about, and thousands of part combinations that have to be optimized together. Everything from matching the piston and ring diameters to their respective cylinder bores, to sizing bearing clearances, to tightening bolts and studs to equal values needs to be considered. The idea is to ensure that each cylinder of the engine is producing maximum and identical horsepower to each of its neighbors. All kinds of important dimensions and tolerances have to be considered in this process, including such things as matching individual cylinder compression ratios and volumes, and ensuring that all the intake and exhaust valves are exactly the same size and verifying that the camshaft lobe profiles are all perfectly shaped and identical to each other, and matching the intake porting and wall finishes between each cylinder, etc. CC'ing the cylinder heads, selecting connecting rod lengths and sizes, adjusting deck heights and matching valve spring stiffnesses can also be part of blueprinting an engine, as can a hundred other machining and assembly processes. Even relatively simple-minded-yet beneficial-tasks need to be addressed for optimal performance. A good example of this is the typical blueprint practice of adjusting each spark plug seat washer thickness so that all the spark plugs, after being tightened into their threaded holes to the correct torque settings, end up facing the same and optimal directions!
The list doesn't even stop there. For instance, there are several crankcase geometry issues that have to be addressed, such as ensuring that the top of the crankcase is parallel with the crankshaft, that main bearings are all in-line, or that cylinders are at right angles with respect to the crankshaft axis. These things are routinely checked in a normal engine rebuild, but not to the level of precision that they are during a true blueprinting process. Each of these steps can take many long hours of assembly checking and re-machining to get perfect. For the optimal engine, this is important because if any of these dimensions are out of spec, extra friction or heat can show up and rob small amounts of potential horsepower from the engine. Admittedly, the horsepower gains and losses for each of these blueprint steps are usually very small, but when added up, they can be significant, amounting to anywhere from five to 15 percent in total crankshaft output. Better yet, a properly blueprinted engine will last longer than one that was more conventionally built.
Counting The CostThe downside to all of this, of course, is time and money. It takes a tremendous amount of labor and skill to fully blueprint an engine. The builder is faced with spending long hours just measuring and selecting components that are the optimal sizes and weights. Worse, it isn't uncommon for the builder to have to buy two or three times the required quantity of some of the parts, just to ensure that he or she gets enough with the proper dimensions. The builder then has to assemble everything together, checking and rechecking the assembly tolerances at every step. Mixed in with this process is the inevitable tearing down and reassembling to tweak and adjust all the minor things that are discovered during the process. This all means that true blueprinting can get very expensive-very fast. A correctly blueprinted four-cylinder engine can cost two or three thousand dollars more than a standard rebuild. V6s and V8s can cost even more. It can be so expensive, in fact, that some local racing organizations are actually now discouraging the practice of blueprinting-all in an effort to keep the costs practical for local competitors. Once you get to the national levels of competition, however, or simply want to extract the absolute maximum horsepower from your street engine, blueprinting may well provide you with the ultimate competitive edge...for a price. That's the way it has always been and, for better or worse, always will be.
Fortunately (and even though it's expensive), engine blueprinting has become a fairly standard service offered at first-rate performance engine shops. The key is to ask questions of potential engine builders so that you know what they're going to, or not going to provide during the build process. True blueprinting is very labor-intensive, time-consuming and therefore expensive to do. If a builder says he'll throw in a complete balance and blueprint for an extra couple of hundred bucks, just say thanks and walk away. Real blueprinting essentially means hand-building an engine with perfectly fit components, ideal clearances, and exact volumes and weights. This is a painstaking and time-consuming process, if done right-and it will cost accordingly. A blueprinted engine is the utmost in performance and durability possible and its success or failure can be considered a real testimony to the expertise of the engine builder. It's not for everyone, certainly, but if you want to win races, it may very well be for you. Just remember the old racer's adage: Speed costs money, son. How fast do you really want to go?