FSAE Braking System Design
The design of the University of Toronto’s 2017 braking system required repeatability, reliability and most importantly, safety. The drivers also voiced their need for the system to be easily adjustable to facilitate their differing sizes and driving preferences. The following major components were designed with these factors in mind.
Pedal Tray System
- To facilitate the rigid mounting of the pedals, master cylinders and balance bar
- Transfer the brake pedal load generated from the driver effectively through a rigid, yet adjustable system
- Tray and adjustable mounting assembly must operate within the foremost section of the chassis base frame
- Must conform to the 95th percentile male, so the foremost tray position must be clearly communicated during the design and manufacturing of the brakes and chassis sections
- Must be capable of taking a 2000N applied load to the brake pedal surface as an emergency load case
The design process began at the driver level, by measuring each of them and fitting them into an ergonomics rig to help both myself as well as the chassis lead to determine the positions in which the drivers would be operating the vehicle. I then had them each apply a set number of successive braking efforts onto a load cell in order to get a feel of their average capable braking force over the course of a race. The braking system would then be designed around this level of force and would be used to facilitate the ability for the driver to lock all four wheels in the event of an emergency.
Taking into consideration many agreed upon design variables such as target vehicle mass and center of gravity location, I was able to get rough estimates of the vehicle’s static individual wheel loads and longitudinal load transfer. I then used these figures along with simulated results of aerodynamic loading to determine the approximated corner weights of the vehicle under braking. Now, using some tire data, I was able to get an estimation of the coefficient of friction of the tires, as well as their rolling radii. At this point, I was able to get approximate figures for the required braking torque that is required to lock up the wheels.
Now working backwards from the wheel, we can use the rotor design parameters, as well as caliper and pad specifications to get the required caliper clamping forces to lock the wheel under the above torque, which then leads to estimated figures for our brake line pressures. These pressures then give us an estimate of the forces that exist at the master cylinder rods, which then translate through the balance bar, and finally we are left with the force at the brake pedal that is required to lock the vehicle’s wheels.
Our task now becomes providing the optimal pedal ratio that will give the driver enough mechanical advantage to safely lock the wheels based on the exerted forces that were measured during the conducted ergonomics studies. This is one of the fundamental considerations that went into determining the brake pedal’s positioning and geometry.
The tray itself was designed to be inexpensive and easy to manufacture. The whole tray was waterjetted from two thicknesses of 6061-T6 aluminum and TIG welded together after each component was slotted into the main base plate, with critical dimensions jigged as required.
The base plate was then mounted to four Igus carriages, which allowed the tray to translate longitudinally along their associated Igus rails, which were mounted to the chassis. This provided the drivers with the level of adjustability that they desired, as two locking pins located on either side of the vehicle could be removed in order to reposition the tray.
The stiffness of the welded pedal tray base assembly and the brake pedal were also simulated under the emergency braking scenario of a driver applying 2000N of force. These simulations were used to minimize weight wherever possible, while maintaining an agreed upon safety factor.
Rotor System Assembly
- To facilitate the application of the driver’s pedal effort, in order to bring the car to a stop, or reduce the given velocity of the vehicle when in motion
- Limited room for radial caliper positioning due to the fixed inner radius of our Kaizer rims and the mounting surface on our wheel hubs
After selecting our calipers based on weight, mounting style and ease of packaging, I worked with the suspension team to determine the inner and outer radii of the rotors such that the entirety of the brake pads would be in contact with the rotor surface. The mounting style of the rotors was also agreed upon to be floating, and would be connected to the wheel hubs via cylindrical bobbins and held in place by retaining rings.
Based on sensor data gathered from the previous year’s vehicle, I had a rough estimate of the steady-state operating temperature of our rotors as they underwent a full race. Therefore, given that the wheels and surrounding suspension geometries were quite similar to the previous year in terms of airflow to the rotors, I was able to select a pad compound that maximized our coefficient of friction given our typical operating temperature range. Based on this information, I ended up selecting the SBS-S-DC3-HH compound.
Now came the step of selecting a rotor design. Three rotor profiles were selected in order to analyze any variations in steady-state operating temperature, pad wear patterns, and effective deceleration.
The first profile was seen as our conservative design in terms of surface area removal. This was expected to see low levels of stress when loaded, however, due to the extension of the slot not covering the length of the pad, there was expected to be high points and possible glazing at the top and bottom of the pad material.
The second profile facilitates the extension of the slot to cover the entirety of the pad surface during a braking application. However, in order to do so while refraining from contacting the inside of the caliper, the area between the top of the slot and the outer diameter of the rotor was minimized.
The final profile used the same shape as the second, but incorporated cross-drilling and inner-disc material-removal. Given our use of 1020 mild-steel, as opposed to a lighter material such as cast iron, this was purely done as a method of un-sprung mass reduction, and another means to analyze wear patterns due to the cross-drilled sections being within the pads’ swept area.
Analyses were performed on each profile to simulate the situation of the wheels locking up under a maximum braking effort. The estimated maximum wheel torque was applied at the rotor-bobbin interface, with the reacting clamping force being applied at the pad-rotor interface. The assumption was also made that the rotors would be operating at their peak operating temperature of 300 degrees Celsius, which would of course have a negative affect on the steel’s stiffness.