Introduction To Brake System Criteria
Braking later can create a higher speed differential between cars than anything else during a lap.
The brakes are the most powerful system on the car.
0-60 MPH in 3.0 seconds and 75 horsepower. Fairly impressive statistics for an FSAE engine group, aren’t they? How about 60-0 MPH in 2.2 seconds and well over 100 horsepower in conversion from kinetic energy to heat? Go brake team! Better not tell the engine group. Might hurt their feelings.
Just like with the engine, we are concerned about much more than just the peak torque or power that we can produce. We need it to be easy to control, distribute it to the tires (in this case 4, not just 2) in the most effective manner, not have to completely rebuild the system after a day of testing, and make it effective in a wide variety of conditions for an array of drivers. There is more to it than that. The brake system does much more than just slow down the car!
Here is a common scenario for FSAE teams. The brake system on last year’s car worked fairly well getting them through the entire competition without incident. They do not want to stray far off from what worked before. In fact, they just want to reuse most of the same components. Sounds like a fairly conservative plan… maybe.
What if there is a considerable change in the track next year? What if the weather changes grip on the track, or the ambient temperature changes in either direction? We probably will not be considering producing less aerodynamic downforce and pulling fewer g’s and asking for less power from the engine. Are packaging requirements changing because of other systems? Any new drivers? Hydraulic line pressure traces will show that some demand more from the system than others.
Then there is the driver’s ability to make an accurate assessment of the system performance. A typical comment after driving an FSAE car with a mediocre brake system is, “those are the most awesome brakes that I have ever driven!” Well, the brakes are OK. The impression they provide is nudged a fair amount in the positive direction by the car’s light weight and extremely high-grip tires relative to the student’s daily driver. Of course the driver restraints pull back so hard when you hit that pedal in the middle (or on the left)! Yet, there may be considerable room for improvement. It can take hundreds of laps for a single driver to develop the required talent and some effective use of data acquisition to discover where and how big that potential may be.
When designing the new car, it helps to step back and have a look at the entire brake system and what the new car will demand from it. There will be some common goals, but, different teams are going to have different needs. You need to develop your precise list of needs. Most of those needs/goals can and should be quantifiable. After all, we are engineers. Although not a complete list, let’s look at some subjects that will help us establish some goals and design criteria for our brake systems.
Passing technical inspection - This should be nothing more than a formality. The rules are clear.
Passing the brake test - This should not be a struggle either. A good system has plenty of reserve capacity and should have no trouble producing wheel lock under any conditions. If you find that your fingers are crossed when the car approaches the test area, then you need a system in which you have more confidence. Most of the teams that we see at the competitions that have difficulty passing the brake test are in that situation because their system was assembled, not engineered.
Sensitivity - Racing is about the ability to maintain precise control at the traction limits. If the pedal requires too much force, the driver will not be able to maintain that precise control after a number of laps when exhausted. If the pedal is too sensitive requiring little force to lock the wheels, small pedal force changes produce large brake torque changes making precise control difficult in all situations. A happy medium is needed suiting a range of drivers.
Bias - Developing the correct proportion of braking front vs. rear (brake bias, often called brake balance) is critical to good handling. Too much front brake and the car will understeer while braking. Too much rear brake and the car will oversteer or possibly spin. We might also be able to think of this as maintaining proper yaw control. Keep in mind that much of the braking that you do is also while cornering. Therefore, we are asking the tires to produce deceleration and lateral (cornering) forces at the same time.
Consistency - We want to know exactly how the car is going to react before we depress the pedal. Otherwise, how do we know how hard to push? If the performance is inconsistent, then we have to push on the pedal with our best guess of force, see how the car reacts, and make changes to the pedal force to achieve the response that we really desire. By racing standards, that is a slow process minimizing our control at the traction limits and increasing lap times. Consistency helps and there are several areas in the system design that affect consistency.
A few of the ways brake systems can be inconsistent are:
sensitivity
bias
pedal stroke
feedback
Tuning - Despite the precise calculations that we make, they are often based on estimates for tire grip, brake pad friction level, center of gravity height, aerodynamic forces, final vehicle weight, and weight distribution. Expect to tune the bias to the needs of the final product. The optimum setting can change for different pavements, course layouts, wing angle settings, brake pads, and other factors. Pedal force might need a tweak or two as well to optimize. No new car turns its fastest lap ever on lap one unless it is crashed and destroyed on lap 2. This is why we test and tune.
Adjustability - We noted above that we need to be able to adjust the bias and possibly even the sensitivity. Do we also want the ability to adjust the brake bias while driving? How do we create a system that can be used to its full advantage with a wide range of driver heights, shoe sizes, and strengths?
Temperature considerations - Brakes convert kinetic energy into heat through friction. Therefore, we have work to perform in heat management. What temperature ranges do we want to keep certain brake components within? How does temperature affect balance, sensitivity, feedback, wear, compressibility, stiffness, and general durability? How close to the limit do we want to run in this department? This subject can tie in tightly with weight, consistency, and maintenance.
Component availability - We have limited amounts of time, treasure, and talent and need to use them where they pay off with the greatest returns on investments. Sometimes it makes sense to put in the extra effort where it could possibly pay off with extra performance. Sometimes commonly available parts cost much less and work just as well. Analyze appropriately and decide wisely.
Maintenance intervals and requirements - How often should we have to inspect and perform maintenance to the brake system? How much work does that require? Are tasks that should be quick and simple to perform such as bleeding the brakes easy to perform?
Mass, unsprung weight, and inertia - We want zero. Not going to happen. This one is in conflict with some of the other items that we list. For example, lighter discs generally see higher peak temperatures. Yet, we should have a temperature target that we do not wish to exceed. Calipers and pedal assemblies that are too flexible produce systems that feel vague challenging our ability to control them precisely at the traction limits. We need to determine the best compromises for the various factors.
Packaging requirements - Your brake system must interact with the suspension, steering, frame, and other systems. Compromises will need to be made achieving geometries satisfactory to all. Function, component protection, cooling, and accessibility all need to be addressed. Topping off the brake fluid should not require disassembling the front end of the car.
Failure mode - If the temperature goes too high, what will fail first? Will that be a catastrophic failure or something less ominous? Will you lose both front and rear systems or just one? Will there be any warning that this limit is being approached?
Budget - Most of us are limited by this, even in the professional racing world. How much budget is available? Where will we gain the most performance per dollar spent? A major reason for putting our list together is so that we spend it where it will produce the best results.
A good engineer is not one who has all of the answers. None do. A good engineer is one who has the ability to ask the right questions. Developing the list of needs for your brake system is about asking the right questions. It is the first step in the following processes that we must conquer.
Develop the general list of needs.
Develop specific goals and design targets for each of those needs based on analysis and testing.
Determine your solutions.
Verify through testing.
Optimize and tune as needed based on the test results and further analysis.
Never stop asking questions!
What we have done here is help with that first step. By no means is this “the list” nor is it prioritized. The intention is to make us all aware that there is quite a bit to consider in this system and that there may be room for much improvement relative to previous team efforts regardless of what previous drivers thought. A highly optimized brake system properly managed by the driver allows longer wide-open throttle times, shorter brake zones, and higher average cornering speeds. This is how we reduce lap times.