Читать книгу Muscle Car Brake Upgrades - Bobby Kimbrough - Страница 9

CHAPTER 2

COMPONENTS AND THEIR FUNCTIONS

With the fundamentals of the hydraulic brake system understood, it’s time to review the many components within the automotive braking system. Similar to the vehicle they’re used to slow and stop, there are many variables, accessories, and styles of components used depending on the application.

As the majority of automotive brake systems operate on the same principles of hydraulic pressure and physics, many of these components are common to nearly any car on the street or track. Granted, advanced electronics have stepped into the braking performance world with antilock systems on newer vehicles, but we’ll keep our focus on the braking systems used in the majority of hot rods, muscle cars, and road-course warriors being built and run on the track. Many of these components were lightly covered in chapter 1, but the following sections break them down in greater detail through a systematic approach.

Brake Pedal and Assembly

Since the braking process begins with your foot pressing the brake pedal, it is fitting to start our discussion there. The brake pedal assembly is one component that is probably taken for granted when a brake system upgrade is planned, but it should certainly be considered and reviewed to see how it could be improved.

Like engine parts, brake system components come in a lot of shapes and sizes, depending on your application and braking performance expectations.

The brake system starts with your foot pressing a pedal, so weigh your options for pedal position and movement along with how much pressure you’re comfortable with providing.

The pedal assembly acts as a simple lever. There is another rod connected to the lever that forces a pushrod into the master cylinder chamber to pressurize the brake system. The position and length of the pedal lever and its pivot point affect how much force is supplied to the master cylinder and how much force is required. It is also important to consider the location of the brake pedal in relation to the throttle and clutch pedals. Many older cars or trucks have quite a space between the pedals, which will not bode well in a performance application.

Depending on the goals for your car, a completely new brake pedal assembly may not be necessary, but when you’re getting serious about performance and braking, many of the aftermarket assemblies will provide the strength and adjustments needed. There are assemblies that mount to the floor or the firewall and have adjustments to position each pedal exactly to fit your needs. Having the pedals closer together and on a single plane will provide a much better driving and braking experience.

Also, moving to a new pedal assembly provides more alternatives and solutions for mounting the master cylinder(s). Aftermarket pedals are designed to accept remote master cylinders or even dual units (one for the front, one for the rear) that provide increased adjustments for pedal feel and braking bias between the front and the rear. For those applications, an entire assembly for the throttle, brake, and clutch will be in your future and would be a wise investment.

Brake Fluid and Hydraulics

The brake system is like a mini hydraulic network with plumbing to each wheel running up to a common point: the master cylinder. The brake pedal uses mechanical lever-age to exert force onto the pushrod and piston of the master cylinder, which pressurizes the fluid in the lines and against the pistons of the calipers or wheel cylinders.

Wilwood offers a list of pedal assemblies for street cars through sprint cars. This example is built with a clutch pedal and allows the master cylinders (hydraulic clutch too) to be mounted inside the firewall. (Photo Courtesy Wilwood Engineering Inc.)

The small piston and area within the master cylinder can move a larger piston (like in the caliper) with more force, albeit a shorter distance. Think of it like a floor jack and how big of a stroke is needed with the handle to move the lifting mechanism.

Pedals and pedal pads can dress up the interior and put your own personal touch in your pride and joy. This could be the perfect companion to a disc brake upgrade to your classic muscle car. This set is from Billet Specialties.

Pedal Pressure

Knowing the pedal ratio (mechanical leverage) of the brake lever will allow the amount of force required to activate the brakes to be increased or decreased. By varying the pedal ratio, the brake pressure can be adjusted without changing the amount of pressure applied by foot. The trade-off is that the amount of lever movement necessary will change.

To calculate the pedal ratio, measure the distance from the pivot point of the brake lever to the middle of the pedal push point and divide that by the distance from the pivot point to the pushrod connection (A / B = PR).

A: The length of the pivot point to the center of the pedal

B: The length of the pivot point to the master cylinder pushrod

PR: Pedal Ratio

Example: A is 5 inches, and B is 1 inch, so the ratio is 5:1.

By adjusting the lengths, the brake force can be increased/decreased without increasing the pedal feel/effort. The diagram illustrates the measurements and effects on the brake pressure. The catch here is that the amount of movement on the pedal will increase.

If you have a pedal ratio of 5:1 with 100 pounds of force acting on a master cylinder with a 1-inch stroke, the pedal pressure is 5 × 100 = 500 pounds, while the stroke is 5 inches. Stepping up to a 6-inch stroke will give you 600 pounds of pressure, but the stroke would be longer at 6 inches (6 × 1 = 6 inches).

Mark Chichester with Master Power Brakes explained the brake pedal ratio as a mechanical lever advantage. “If the overall length of the brake pedal is 12 inches and the distance between the pivot point and where the pushrod connects is 3 inches, the brake pedal ratio is 4:1. Anything between 4:1 and 5:1 is a perfect ratio for a power brake system. A manual brake system is better suited for a pedal ratio between 5:1 and 6:1. In the example above, the distance between the pivot point and where the pushrod connects would need to be changed to 2 inches for a 6:1 ratio.”

The force multiplication of the pedal ratio can be calculated by multiplying the initial force by the ratio. For example, let’s assume the average force of 70 pounds is applied to the brake pedal. Multiply that by 4 to get the total force for a 4:1 brake pedal ratio (70 × 4 = 280 pounds of output force). Likewise, a 6:1 ratio would result in 420 pounds of output force (70 × 6 = 420). The pivot point placement and master cylinder pushrod location in higher-ratio brake pedals tend to have longer pedal travel.

Chichester also mentioned some general rules that most designers adhere to: “Whether your vehicle has power or manual brakes, pedal ratio is important. If you are experiencing a hard pedal, you should check your pedal ratio if you have converted from the vehicle’s original setup. As a general rule, your pedal ratio should not exceed 6:1 for manual brakes with a 1-inch bore master cylinder and 4:1 for power brakes with a ⅛-inch bore master cylinder.” ■

A = Distance from pi vol point to middle ut puah / pull puinl

B = Distance ham pi vol lo paini of push cm master cylinder

P = pivot point

F = Fouce or push

By performing a few quick measurements combined with a little math, the pedal ratio can be calculated, which is helpful to determine if you have enough pressure acting upon the master cylinder piston or if you have the proper amount of travel required to effectively work the master cylinder. (Photo Courtesy Wilwood Engineering Inc.)

The fluid used in a brake system undergoes tremendous pressure and heat cycles. Like engine oils, there are several different grades of brake fluid to choose from to match your goals and driving conditions. The most common fluids are specified by the Department of Transportation: DOT3, DOT4, and DOT5.

DOT3 is the base fluid and is not as capable of performing in higher-performance applications compared to the DOT4 blend.

DOT4 brake fluid is considered a higher-performance brake fluid because of the addition of borate esters, which improve the dry and wet points of the fluid. While DOT4 brake fluids are more stable and have a higher boiling point initially, once the fluid begins to absorb water, its boiling point will fall off more rapidly than most DOT3 brake fluids.

Use care so the brake fluid is not exposed to open atmosphere where it can pull water molecules from the air. DOT4 brake fluids must have a minimum dry boiling point of 446°F (230°C) and a minimum wet boiling point of 311°F (155°C) by US Federal Motor Vehicle Safety Standards (FMVSS).

The dry boiling point of brake fluid is described as the boiling temperature of brake fluid from an unopened container. The wet boiling point refers to the brake fluid boiling point after it has absorbed 3.7 percent water by volume. Most experts estimate that brake fluids reach this point just past the two-year mark. This is why the experts recommend changing brake fluid every two years.

DOT5.1 brake fluids also have a blend of ethyl glycol and borate ester, but this blend meets the standards of the silicone-based DOT5 brake fluid. In simple terms, the DOT5.1 brake fluid is a DOT4 fluid that meets the DOT5 standards. Because the blend is essentially the same, DOT5.1 and DOT3 and DOT4 brake fluids are compatible.

The Department of Transportation rates brake fluid in several classifications; always check with a brake manufacturer for its recommendation. DOT3 provides the characteristics for most cars; for spirited performance and on-track days, DOT4 is recommended.

DOT5 is a silicone-based fluid which, while not designed for high-performance driving, is a favorite among hot rodders because it will not remove paint if it gets spilled or splashed compared to DOT3 and 4, which are highly corrosive to paints and coatings.

Brake fluid must hold its operating parameters through quite a few different requirements. It must have an extremely low freezing point as well as an extreme boiling point. During those varied operating parameters, it must have a constant viscosity and not maintain its compressibility. Add to the list its ability to lubricate the moving components and prevent corrosion, it makes for a pretty tall order. Believe it or not, the fluid also needs to be able to absorb any moisture that collects in the system.

It’s always a best practice to fill a fresh brake system with new brake fluid. A brake fluid canister that has been open on a shelf will absorb moisture in the air. By pouring it into an existing brake system, you would be introducing more moisture. It is important to note that mixing DOT3 and DOT4 fluids is acceptable. However, never mix a DOT5 synthetic fluid with a glycol-based fluid. Component deterioration as well as the transfer of pressure will not function as well when combined.

TBM Brakes offers a high-end brake fluid for the most severe applications. This DOT5.1 fluid is suitable for long races and whenever critical conditions are present. Rated at a 612°F dry boiling point, it has slow moisture absorption and is compatible with all DOT3 and 4 brake fluids.

Master Cylinder

The master cylinder is one of the most important components of your brake system and plays a direct role in the resulting pedal effort, modulation, and the overall braking effectiveness of the system. When selecting a master cylinder, it is highly advised to use the recommendation of the brake system manufacturer due to the number of variables in caliper, booster, or drum designs and fitments.

The master cylinder of the braking system is the heart of the system. It has a reservoir to hold the brake fluid and it converts the mechanical effort from the brake pedal into hydraulic pressure to activate the brake calipers or drums. To simplify its operation, inside the cylinder is a piston that is pushed through a bore by a pushrod connected to the brake pedal assembly. As the piston is pushed into the cylinder, it pressurizes the system with brake fluid.

Within the cylinder, there are small ports that direct the fluid to the proper brake circuit. This creates pressure that acts upon a slave cylinder (wheel cylinder or caliper piston), which in turn pushes the brake pad against the rotating drum or brake rotor.

Many older vehicles were equipped with a single-channel master cylinder, which should be one of the first items updated on any vehicle that will be driven. The reason is simple: safety. A single reservoir is responsible for maintaining both the front and rear brake circuits, and if one circuit is compromised, it will affect the operation of the other. For example, if a rear brake line fails, not only would the rear brakes be inoperative, the front brakes would also diminish! A single reservoir can easily be upgraded to a dual master cylinder.

Upgrading to a dual reservoir in place of a single-chamber unit is an important upgrade. This example does not have any power assist and simply required a new line and proportioning valve to slow the application of the rear brakes. The bracket shown in the photo allows the master cylinder to be mounted under the floorboard, a common practice in street rods.

There is nothing good about a single-line master cylinder. If there is a leak in one of the four wheel cylinders or damage to a line occurs, the vehicle’s overall braking capabilities will be lessened. Stepping up to a dual-port master cylinder that separates the front and rear brake circuits is a much safer system and should be high on the list of future upgrades.

A dual master cylinder, often referred to as tandem, splits the front and the rear system, which keeps the two systems separate to maintain braking function if one system fails. These have two outlets, one for the rear with the other for the front.

For street applications, go with a tandem design that has separate outlets for the front brakes and the rear. When selecting a master cylinder, determine whether you plan to run power assist or stick with manual brakes. There are a few typical bore sizes, such as 7/8-, 15/16-, or 1-inch diameters and the varying bore size will have an effect on the pedal feel.

In a tandem master cylinder, brake fluid is secured in two independent reservoirs. If one brake circuit fails, the other will continue functioning properly. Notice the ports that direct fluid into the cylinder chamber.

A popular place to mount the master cylinder on street rods is to the chassis right under the driver-side floor pan. This bracket assembly is being fit on an early 1950s Chevy.

A larger bore will create more displacement or volume but requires more pedal force, while a smaller bore will produce more pressure. There is no one guideline when selecting a master cylinder because there are so many variables on each vehicle, such as the calipers, weight, suspension, and even seat position. Use components from a single source since manufacturers design and engineer their products to work together.

Selecting a Master Cylinder

Selecting the proper master cylinder for your application is key to the overall performance of the system and feel of the brake pedal. Review your goals with the brake manufacturer before making a choice.

Selecting a master cylinder is not only important for the operation of the brake system but it also affects the pedal feel. This is where knowing the pedal ratio and the force used comes into play. You may like a firm pedal feel, but what if you’re building a street rod and your better half drives it, will they welcome the stiff pedal as well?

Keep in mind that a larger cylinder bore creates more fluid volume, while a smaller cylinder produces more pressure. If you’re going to have a very short pedal ratio, a smaller cylinder may create a brake system with little feel or modulation. In fact, it will become more of an on/off feel with little in between. Conversely, a softer, longer pedal may be slow reacting and take too much movement to effectively slow the vehicle.

This is where it is important to discuss your plans and goals with the manufacturers, especially when it comes to the master cylinder. ■

Power Assist

In the brake system world, there are either manual brakes or power-assisted brakes. Manual brakes have a harder, definitely firm, pedal feel because you’re dealing strictly with the mechanical contact and hydraulic force within the master cylinder and the geometry of the pedal and rod assembly.

Many original high-power muscle cars were ordered with manual brakes for simplicity and less weight (or had cams that were too big to make enough engine vacuum to support a booster). Older street rods also may choose the manual route simply due to the packaging under the hood. However, most rods these days (and even many muscle cars) are going with some form of power assist on their brake systems. Remember, disc brake systems require more pressure than drums (approximately 900 to 1,200 psi) to function as designed, so in the majority of cases, assist is a nice feature.