Since most of the braking is done by the front wheels, a proportioning valve is used to li
Master Cylinder Pressure
Most hydraulic brake systems operate at a safe maximum of 1,000 to 1,200 psi, with regular braking pressures below this figure. The hydraulic pressure in the system is a function of input pressure from the pedal and the piston area of the master cylinder. The way to calculate this pressure (P) is to divide the force on the pedal (F) by the area of the piston face in square inches (A), or P=F÷A. Knowing that a 1-inch bore master cylinder has an area of 0.7854 square inches (šR2), and with a known force of 600 pounds on the plunger in the master cylinder, we come up with: 600÷0.7854=764 psi in the braking system. Likewise, substituting a larger or smaller diameter master cylinder bore in this scenario, such as 1 1/4-inch or 7/8-inch, will produce 489 psi or 997 psi, respectively. So a larger volume master cylinder (a bigger bore) will produce lower pressure than a master cylinder with a smaller bore.
Proper pressure, which is critical to the correct function of any braking system, can be adjusted as described with different size master cylinder bores and pedal ratios. A smaller master cylinder can be used if higher system pressure is needed. However, switching to a smaller master cylinder will also increase the pedal travel. If the system can't accommodate this travel (i.e., the pedal hits the floor), then this is not a good option. In that case it might be better to increase the force on the master cylinder by raising the pedal ratio to obtain more pressure in the system.
Master Cylinder Volume
When changing the diameter or the stroke of a master cylinder the volume of fluid displaced is changed as well. But since the volume of a closed system remains the same and brake fluid can't be compressed, the stroke of the cylinder is limited. On any brake system in proper operating condition the pads or the linings are very close to the rotors and drums, so not much movement is needed for them to make contact. After clearances are taken up, pressure rises in the system as the pedal is pushed down, but the actual fluid displacement within the system remains the same, regardless of the diameter of the master cylinder. The volume of fluid displaced equals the stroke of the piston times the surface area, so a larger-diameter master cylinder needs a smaller stroke to displace the same amount of fluid as a smaller-diameter one. This is one case where bigger isn't always better. If the stroke and the diameter of the master cylinder and the pedal ratio can't be changed, then adjusting the pressure within the system can sometimes be accomplished by using larger wheel cylinders or bigger calipers.
Any brake system must contain the proper brake fluid and be completely bled of air. Even t
Here's a simple formula to determine the volume of displacement necessary to safely operate a brake system. Proper capacity is determined by the equation D=S x A, where D is the total cylinder displacement, S is the stroke, and A is the piston area in square inches. To see how this works, we'll use a theoretical four-wheel disc system where the front calipers each displace 0.075 cubic inch and the rears each displace 0.050 cubic inch. The total volume of all four calipers is 0.25 cubic inch. ([0.075 x 2]+[0.05 x 2]). After multiplying by a safety factor of 100 percent to account for swelling, leakage, and deflection, we arrive at a total of 0.5 cubic inches of needed cylinder displacement. The master cylinder stroke is 1.2 inches, a common length. Solving for the unknown variable, where D÷S=A, we get 0.5÷1.2=0.417, or a piston area of 0.417 square inch. That translates to a bore size of between 11/16 and 3/4 inch (Piston area=šR2).
The End Result
To design and build the perfect braking system for any particular truck, many additional factors must also be considered. The coefficient of friction of the pads or shoes, the brake torque of such, the vehicle's center of gravity and total weight, and so on. Do real people actually sit down and measure all these factors and figure out exactly how to build a brake system? You bet they do! But the old seat-of-the-pants method usually comes into play once the general design work is finished. Most builders set up a rig and then go out and test it on a deserted road with no nearby telephone poles or cliffs. Changing any or all of the aforementioned variables a little at a time is essential to dial-in proper braking for each individual vehicle, especially custom-modified ones.
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