2009 - 2010 SAE Mini Baja

Northern Arizona University

Brakes

The first step in designing the brake system was to identify the design objectives. These objectives include reducing the weight of the pedal assembly, and making a redundant system to ensure performance.  A thorough analysis of the previous NAU Baja SAE brake system was conducted to determine possible areas of improvement. The investigation revealed that the master cylinder was oversized for the application and the pedal was excessively short, requiring over 100lb of force to lock up the tires on asphalt.  Using existing Polaris disc brake hubs, calipers, and rotors, a 750lb vehicle weight, and 25” tall tires as a starting point, the correct master cylinder size and pedal ratio were calculated1. Table 3 shows the results of the calculations used to determine the size of the master cylinders2.

 

Table 3. Brake calculations for master cylinders

Pedal Effort Desired (lbs)

60

Pedal Ratio

7

 

Calculate Manual MC Pushrod Force

 

Manual Pushrod Force (lbs)

420

 

Calculate MC size required (manual)

 

Area of MC piston required (in^2)

0.3462

MC Bore required (in.)

0.6641

 

Use actual MC size and calculate pressure

 

Actual MC piston size (in.)

0.6250

Actual MC piston area (in^2)

0.3068

Actual pressure produced (psi)

1,369

Hydraulic Ratio - Front

8

Hydraulic Ratio - Rear

8

 

The brake system uses two Wilwood Engineering master cylinders, one supplying pressure to the front brake calipers, and the other supplying pressure to the rear calipers. This creates two independent braking systems, which gives the car the ability to stop should one of the two master cylinders fail.

 

The Wilwood Engineering brake pedal assembly has a 7:1 pedal ratio and is made of cast aluminum, making it light weight. The 7:1 ratio pedal requires only 60lb of pressure to lock up all four tires on an asphalt surface from the vehicle’s top speed of 35 mph.

 

The brake pedal also uses a balance bar to adjust the brake bias towards the front or rear of the car. This adjustment is useful for setting up the brake system so that the front and rear brakes lock up at the same time, improving braking performance.