ABSTRACT

The design report focuses on explaining engineering and design process of brake system in the Baja vehicle that is developed till now. The report also throws some light on the alternatives considered. The design of the brake is in accordance with the specifications laid down by the rule book. This design report is accumulative effort towards explaining the design process to the readers.

INTRODUCTION

The design process of the brake is simple and is based on various engineering and sae rules depending upon the availability, cost and other such factors. So the design process focuses on:

Safety, Service ability, Cost, Standardization, Strength, Driving feel and ergonomics.

The design criterion followed here is design for the worst and optimize the design while avoiding over designing, which would help in reducing the cost.

T h e d e s i g n t a r g e t s o f o u r v e h i c l e f o r B a j a 2 0 1 3 a r e a s f o l l o w s :

1. Maximum speed – 45 km/hrs.

2. Weight – 300 kg

3. Ground clearance – 20 cm or 8 inch

4. Track width – 160 cm or 64 inch approx.

5. Wheel base – 190 cm or 75 inch approx.

6. Braking distance – 1400 cm

7. Turning radius – 240 cm or 96 inch

Further, as designing is based on prevention of failure so let me define the condition of failure of brake system of our vehicle is failure is their inability to lock all the four tires simultaneously.

The baja sae rules for the brakes system are following :

Foot Brake

1. The vehicle must have at least two (2) independent hydraulic braking systems that act on all wheels and is operated by a single foot. The pedal must directly actuate the master cylinder (no cables are allowed). Each hydraulic system shall have its own fluid reserve either through separate reservoirs or by the use of a dammed, OEM-style reservoir.

2. The brake system must be capable of locking ALL FOUR wheels in a static condition and dynamically on pavement and an unpaved surface. Both the independent brake systems may be actuated by a tandem master cylinder.

3. All rigid brake pipes must be mounted securely along the roll cage / other members as required. Loosely hanging / pipes located by using ties / tapes etc shall not be permitted. The bleed point location must lie on top of the caliper cylinders, without exception. Vehicles not meeting this requirement shall not be allowed to start any dynamic event. The decision of the Head of the Technical Committee of Baja SAEINDIA in this regard will be final and binding to all. This is in the interest of safety

4. The brake(s) on the driven axle must operate through the final drive. Inboard braking through universal joints is permitted. Braking on a jackshaft or through an intermediate reduction stage is prohibited.Plastic brake lined are not allowed

Brake Light

The vehicle must be equipped with a red brake light whose lens must be marked with an SAE “S” or “U” rating (i.e.: SAE IPRSTM) or if it is not rated as per SAE J759, it must be equal to or exceed these standards (Eg: OEM brake light assemblies)/. AIS and ISI rated brake lights are also permitted. The determination of whether or not a brake light meets the required standards rests with the Technical Inspectors.

MAIN SECTION

This section includes design consideration, data and calculation to finalize the design.

OBJECTIVE - The goal of the braking system was to provide good pedal feel/travel, effective braking distance, while reducing brake drag.

Braking System

Table 1 : Brake design considerations

Consideration Priority ReasonSimplicity High Overall goal of vehiclePerformance High Capable of decelerating 300 kg vehicleLight Weight High Prevent air bubbls within the brake linesReliability Essential Light weight part to be minimize total weightErgonomics Essential Optimal pedal assembly fitment to suit every driver

The objective of the braking system is to provide a reliable and prompt deceleration for the vehicle. Moreover, the driver must have complete control of the vehicle while the brakes are applied. More importantly, the brakes must be capable of locking up all four wheels while on the pavement, which is one of the requirements stated by SAE rules. Consideration of the brakes can be found in the above table 1. Through research and applications from previous year, disc brake has been selected as the optimal choice for the braking system for the braking system. Disc brakes are far better than drum brakes because of their powerful stopping ability. Disc brake pads handle substantially better in wet weather conditions. The vehicle is equipped with hydraulic braking system.

MASTER CYLINDER

MASTER CYCLINDERS - To conform to the rules set forth, an individual braking circuit is employed for each axle. A dual master cylinder is used. The 5/8 Bore provides adequate hydraulic pressure to actuate the brake calipers with minimal pedal effort. The master cylinders are actuated using mount brake pedal. Both master cylinders are mounted to the pedal bracket. To vary the amount of force applied between the two master cylinders, the balance bar on the pedal assembly can be adjusted, to suitably divide braking force between the front and rear braking circuits. To fine tune this calibration, an adjustable proportioning valve is also used in series with the brake lines. Though all wheels must be able to be fully locked upon heavy braking, it is possible to slow the vehicle with light pedal pressure by applying more of the pressure to the front brakes. The rears will be able to spin freely, and allow for a theoretically faster acceleration after braking

A CNC Dual Cylinder Brake Pedal Assembly was used as the pedal assembly of the vehicle. The unit composed of dual circuit tandem master cylinder, thus the implementation of this feature is easily accomplished. To incorporate a brake bias, a brake bias bar was installed into the pedal assembly. CNC Dual Cylinder Brake is used because if one fails then other will keep working.

Pedals

The pedals were designed that they be light weight but complete each task without error. The brake pedal was designed to withstand a high amount of force created by the driver and transfer that input force to the master cylinders through the brake circuits. The brake pedal must withstand a high amount of force created during panic scenario without fatigue. Deflection in the pedal would lead to an un-safe drop in the drivers’ control of the vehicle. The pedal is a push mechanism pinned at its pivot point to the base plate using a shoulder bolt to decrease wear on the material as well as prevent the loosening of the fixture. It also contains a spherical inside along with locking spacers using the pedal to prevent any unwanted lateral motion. The spherical is directly connected to the bias bar and master cylinder mounts completing the attachment. Both the spherical placement and base plate pin are attached at the distances for an efficient braking ratio used in the braking force calculations in Appendix .The pedal is also designed to fit comfortably in the foot box and against the sole of the drivers’ foot. A foot plate is attached along with grip tape to increase the surface area and decrease any slip of the driver’s foot

during use. Lever Ratio for the brake pedal is 4 times which provide better lever force and less driver effort. We will use aluminums brake pedal because it is light in weight and high tech appearance with the elimination of rubber boot, reduction in machining requirement.

BRAKE CALIPERS

The final, crucial components of the braking system are the calipers. Since the SAE Baja application does not call for not a high speed, heavy duty brakes, we chose the Wilwood High-Performance Disc Brakes - Calipers due to its compact size, light weight, and low cost. It is a single piston caliper, but it is more than adequate for the vehicle. Multiple piston calipers would likely be too powerful, and cause the wheels to lock with even the slightest pedal pressure. The calipers are also compact, to provide enough clearance with the wheels that were selected.

BRAKE ROTORS

Flat surface rotor can be used because it is cheap easy to replace. The rotors are .200" inches thick, with an outside diameter of 7”. The diameter was chosen for front wheel packaging reasons and the thickness was dictated by the caliper specifications. It was also used because it has no slots or holes cut into it. It was the experience of previous teams that dirt and mud serves as an abrasive to brake pads. In muddy environments it was found that the mud would occupy the slots and/or holes, and score the pads excessively.

Brake Linings

Brake linings are the consumable surfaces in brake systems. The output of any brake is directly related to the coefficient of friction (µ) between the lining and the disc.

equation for a disc brake is

Brake line

Arguably, one of the most important brake parts in turning pedal action into stopping power is a vehicle's brake lines. The fluid is stored in the master cylinder. When the brake pedal is applied, it moves fluid from the master cylinder to the brake calipers, forcing them to clamp down on the brake rotors to slow the car. That fluid is carried through the brake lines, making them a rather critical brake part. If your brake lines don't work, your brakes won't work and you (and your car) will be in a heap of trouble. Because brake lines carry brake fluid from the master cylinder to various other brake parts, it's important to keep them in good repair. Brake lines should never look frayed, cracked, brittle or corroded. Sometimes brake lines can become worn from rubbing against metal parts of the car (or even another brake part) so pay particular attention to the areas of the line that come in contact with other components. Any leaks should be dealt with immediately -- before the car is driven. If you can't fix the leak on your own and without driving the car, you should have the car towed to a licensed repair shop. Malfunctioning brake lines are nothing to fool around with.

F∝ rR

×2×µ× AwAm

×(Rp×f )

So we will be using steel brake lining for our car. Steel Brake Lines advantages are First, for serious off-road drivers, puncturing a brake line is always a concern. With soft brake lines, a rock or twig could easily make a small puncture in a brake line that could end up being catastrophic. A second advantage is that steel brake lines don't swell the way a flexible line might. With repeated use, a flexible brake line can stretch from the pressure of the fluid running through it. This is called brake line swelling. It may not seem like a big deal, but once the line is stretched, the line pressure lessens, which weakens braking performance. Over time, this decrease in braking performance will only become more if a problem. In a panic situation, you definitely don't want swollen lines. Steel brake lines can't swell and your brakes' performance will remain strong.

Steel brake lines may be strong, but they aren't perfect. They're subject to corrosion and breakage just like other brake parts on your vehicle. Steel lines are also less flexible than other types of brake lines, so their connections to each brake part in the system should be checked more often.

Brake Fluid

Brake fluid is used to transfer force into pressure. It works because liquids are not appreciably compressible - in their natural state the component molecules do not have internal voids and the molecules pack together well, so bulk forces are directly transferred to trying to compress the fluid's chemical bonds. Brake fluids must have certain characteristics and meet certain quality standards for the braking system to work properly.

We will be using DOT-4 types of brake fluid because as DOT 3 are hygroscopic and will absorb water from the atmosphere. This degrades the fluid's performance, and if allowed to accumulate over a period of time, can drastically reduce its boiling point.

Dry boiling point Wet boiling pointDOT 3 205 °C (401 °F) 140 °C (284 °F)DOT 4 230 °C (446 °F) 155 °C (311 °F)

BRAKE

The braking forces calculated in Appendix were determined using a number of parameters such as: input force from the driver, pedal ratio, weight of vehicle with driver, weight distribution, master cylinder bias, as well as tire and brake pad coefficients of friction (COF). The tire to ground COF can range between 0.5 and 1 depending on type of asphalt and tire used. Since the Baja is equipped with off road motocross tires the COF used for calculations was 0.8. Research of materials used for the rotors and brake pads the

COF was determined to be 0.6. Research done by the SAE Baja team tested and recorded a range breaking forces to use in their calculations and derived table 2 below.

Light Braking Force Average Braking Force Panic Braking Force45 N 135 N 270 N

Other statistics have shown that panic forces can reach three times that shown in the table but they have been recorded in real life scenarios. These parameters along with other constraints such as wheel diameter, caliper and master cylinder selection the final calculations were determined.

Brake Plate design

The design of the throttle and break system was conducted with three main goals; that the system be light in weight, compact, and simple adjust. The decision to have a pin jointed push mechanism allowed the system to fit in the small area of the foot box and allow the pedals, master cylinders, and bias bar to be contained on one mount plate modeled below Figure .All pieces are fixed except the threaded bias bar allowing minimal error allowed during adjustments.

Figure 1: Brake Assembly

The base plate was designed to withstand the loads forced upon it by the brake and master cylinders. The systems bias bar is added to allow the driver some adjustability in the braking force to the front and rear calipers .The bias bar contains a spherical connected to a threaded rod. That rod is then spun to relocate the spherical within the one inch bore of the brake pedal changing the contact point of the pedal and moments on the master cylinders. The closer the spherical is to the master cylinder the more

force is applied to that braking circuit be rear or front calipers. Shown below inFigure2 is the bias illustrated in two different positions; 50:50 and 60:40 ratios front/rear or rear/front.

Figure 2: Brake Plate

The spherical is adjusted to the desired position based on driver feedback then locked in place by lock nuts and spacers to prevent any lateral movement. The design of the plate also allows the system to be manufactured easily to since it would only need to be mounted three times in a vice and be completed in less than two hours.

Thermal Analysis of the Rear Disc

The rear brake disc was analyzed to obtain a better understanding of the heat transfer of a brake disc in a Baja vehicle; the heat created in the disc under braking conditions, the heat transfer rate of the disc to the air, and the conduction of the brake disc to the hub were all analyzed. The velocity of the car was used to compute the angular velocity of the brake disc before braking. The air velocity was assumed to

be full of the rotor velocity due to the rotation of the disc giving a positive and negative tangential air velocity.

Figure 3 - Diagram of Heating Analysis

The following assumptions were made for the braking analysis

1. Steady State

2. Lumped Mass

3. Heated volume was equal to three times the contacted section of the disc. Due to the area in the center of the disc being empty.

4. Convection is uniform across surface.

5. Air velocity is assumed to be half the speed of the car.

6. Air flow is turbulent (see Reynolds number calculations)

Force(lbf) 1200

Car speed 45 km/hr

AirTemp(F) 70

Break Disc Density (lb/in^3) 0.284

Brake Heating mass(lb) 4.359

Cp Disc (BTU/lb-F) 0.12

HeatVolume(in^3)0.12

Area (in^2 ) 0.25

Table 17 –Critical Parameters for Equations

E n e r g y B a l a n c e

Where Ein the braking heat is generated by the caliper, Qconv is the about of heat transferred through convection to the air, and Qcond is the about of heat transferred through conduction to the hub. This

can be stated due to the steady state assumption. It is noted that a transient analysis would serve a better approximation of this problem with a relationship of the disc temperature over time.

Braking Analysis

is the force on the brake disc, is the coefficient of kinetic friction, x is the position equation

, is the starting angular velocity, is the final angular velocity, t is time. Equation 3 is used to calculate the total heat produced in the disc from normal braking. The force calculations can be seen above in the braking selection section. Once total heat was discovered, a convection analysis of the disc to the surrounding air was done.

C o n v e c t i o n A n a l y s i s

is the temperature of the surface , is the temperature of the fluid (air) at infinity, A is the area (assumed to be twice the area of a face of the disc), q is the heat transfer rate from the disc to the air.

V is the average speed of the car, L is the approximated length of the air flow over a cylinder, kf

is the conductive coefficient, Nu is the Nusselt number, Pr is the Prandtl number, Re is the Reynolds number, and h is the coefficient of convection. Equation 6 and 7 were used to determine the convection coefficient ; 0.023 and 0.8 are for steady state turbulent flow, which is proved with solving equation 5 for the Reynolds number (Re >> Recr) , and 0.4 is for heating of a fluid as opposed to 0.3 for cooling a fluid. The Nusselt number was found to be 121.3, the Prandtl number was 0.713, the coefficient of convection was equal to 14327, and the Reynolds number was found to be 53202.15. Once h was found the heat transfer rate and total heat transferred could be found using equation 4. The total heat transferred from the disc to the surrounds also includes a conduction analysis from the disc to the hub. Equation 8 was used to determine the total heat transferred down one spoke of the four spoke hub

C o n d u c t i o n A n a l y s i s

X is the distance from the outer disc to the hub, A is the cross sectional area, is the difference in temperature from hub to the disc. The cross sectional area was of one spoke of the hub (about 0.25 inches2)

R e s u l t s

The results are as follows; solving for Tdisc a temperature of increase 114 °F was found on the surface of the disc with a start speed of 45 kph, a stopping speed of zero and a braking time of 15 seconds. This is a close approximation that can be used to allow further analysis of the use of other types of materials for brake disc in a Baja vehicle such as aluminum.