Understanding Brake Calcs

Hello,

Before reading my questions, it might be helpful to know that i am a member of a university fsae team and we build a new car every year. From a brakes perspective, we want to be able to lock all four wheels during brakes testing at competition.

I have completed the first set of brakes calculations- determined target lockup pressures, selected master cylinder sizes accordingly and determined a pedal ratio. Now i have the following questions:

1. Based on experimental data from our previous car on MOTEC, why are we accelerating harder during launch than we are decelerating during hard braking? Shouldn't it ideally be the reverse. For context this is a 2WD FSAE car. The data says we are accelerating at 1.3gs but decelerating at only -1gs during braking?

2. What is the best way to settle on a value for max deceleration to use in the load transfer equation? Is there a way to estimate the max deceleration we could be hitting even before we have a physical car?

3. How to determine the pad coefficient of friction for our lockup calculations? We do not have sufficient temperature data on our rotors to help with that.

4. I determined master cylinder sizes for our car based on the target lockup pressures we are trying to achieve so that (in theory) we achieve only a little more pressure than is actually required to lock the car at the speed. For example, at 35 miles an hour, we need 918 psi to lock our fronts. However, using a driver foot force of 130 lbs and a MC bore size of 0.625" (5/8"), we are able to achieve 930 psi in our brake lines at that speed, which means we will lock in this ideal scenario. My question is if our target lock pressure is 918 psi what pressure should we be calculating to achieve in the brake lines to ensure lock up accounting for the compliance issues that will come into play? Obviously 12 psi overhead seems less.

Thank you!

Do you have the accelerometers correctly calibrated in the MoTeC unit? They should be reading 1.0G on the Z and 0.0G on the X and Y channels when stationary, tipping the unit 90° should result in a ±1G change in the reading.

When it comes to the testing, are you able to do a burnout before the launch? Are you using tyre blankets? Is there more rubber down in the launch box compared to the braking zone? These and other variables can make it so that a vehicle will launch harder than it can brake.

It may be that the way the the load transfer and suspension kinematics work in your vehicle that places more emphasis on accelerative traction, rather than braking performance.

1. The driver can have a huge effect on braking performance. Launching is much simpler, (particularly if you are using the MoTeC launch control). If you had ABS on your braking, that would be similar to having ECU Launch control. Lastly, your front / rear weight bias may give you more 2-wheel traction on acceleration, than in 4-wheel deceleration. Does the braking data show signs of wheel lockup? If not, the driver could press the pedal harder resulting in more decelerating torque.

2. Pick a design target -- nothing wrong with 1.5 Gs if you've seen 1.3 Gs accelerating.

3. Past experience, or a test day with different pad compounds to learn what the driver likes for feel.

4. It doesn't matter. If you design for 130 lbf on the pedal and driver can press the pedal with 200 lbs of force you will easily lock the brakes, drivers can easily choose to use less force. Size master cylinders to get in the ball park, then be prepared to go up/down based on driver feel and car performance.

Good luck -- and remember brakes just slow you down. Using a hard brake compounds and high pedal force keeps less experienced drivers from overslowing entering the corner.

I would add one point to what David said - you didn't mention the rear brakes!

The rears are a critical part of the braking available, as they can provide a large degree of braking on their own, especially with a rear brake bias and/or low CoG and/or limited grip. You're going to be looking at the maximum you can get while avoiding rear lock-up under conditions such as clumsy downshifts (esp') the lower gears - if you've a correcly set blipper that shouldn't be a problem, or even under engine braking in the lower gears and high RPM.

With your vehicle, I would expect a significant rear static weight bias, maybe even increasing at speed as it's often easier to get more rear aero' downforce.

I would start with moving the brake balance rearwards until it starts to slip first, and check the changes against the "G" achieved.

As David said, there are several factors that might give a surprisingly high acc'n figure - I suspect a big part of that is the rear weight bias and load/weight transfer under the acc'n. If the tyre compound/grip and width (! I can't recall if they have to be the same size, or staggered front to rear) is different, that will also skew the results - think of drag cars.

David also mentioned brake compounds - the CoF Vs temperature can have a HUGE impact on braking, and depending on your brake set-up you may get best results with a "soft", or low temperature pad, as speeds are low and there isn't a lot of energy heating the discs/pads.

Remember, empirical ALWAYS trumps theoretical - the latter can get you in the ball game, but the former is for confirming the theoretical and fine tuning.