1)What kind of deceleration g's do formula sae car typically experience without an aero package?
Seeing the results of the deceleration event in Formula SAE Austria (you may see the results at the bottom of the post)I'm considering to design our car's braking system to brake at 1.5 g's. However, one of our new members who was part of another university team, pointed out that Formula student cars typically see a deceleration of 1.0g's in competition. I need to know this value to account for weight transfer properly. If I can't know what kind of deceleration our car will be capable of, our brake system will be designed to brake at certain range of braking g's. Using the bias bar would be one of the ways of designing for that range of deceleration.
If a driver was decelerating at 1g on our team, I would simply ask them to brake harder. 1.5 g's is a good reference.
2)Can FSAE car tires have a coefficient of friction of more than 1.4 ? Do I need tire consortium data or will a rule of thumb suffice?
Rule of thumb is fine unless you want to really complicate things. Which you really don't need to. 1.5g reference it is.
3) Has anyone used Dynalight or Dynapro Calipers from Willwood in their brake setup ? If yes, did they provide too much of braking torque ?
Calculations were done using Dynalight and Dynapro calipers in all four tires. It was determined that this kind of setup would be adequate to decelerate at 1.5 g since it would not put a lot of strain on the driver. However this setup would be excessive if the tires can only handle a deceleration of 1.0g's before slipping. On top of this these calipers also cause the car to have a lot more unsprung mass.
We used to use the Dynalight on our cars back in like 2006. That's a beastly caliper you have there. Considering it's size, I'm guessing you want to use 13 wheels. Plenty of room for big brakes. However, I would choose something much smaller. The brakes we ran last year had a 1" diameter piston, 4 front each, and two rear each. Smooth as a baby's bottom on braking. The rear calipers we ran weighed half a pound each loaded with brand new pads.
4) How do PS1 Calipers perform in formula SAE applications ?
Matlab calculations revealed that using PS1 calipers to decelerate at 1.5 g's could put a big strain on the driver. In fact using a pedal leverage of 5, our driver would need to input about 800 N (!!) of force to lockup the wheels. We tried using smaller inlet bore diameters in order to decrease our Lockup pedal force. But we went over the recommended maximum master cylinder pressure rating while doing that.
Matlab also revealed that PS1's would be adequate enough to brake at 1.0g's.
Max line pressure is not really a hard limit....more like a suggestion. Just take that and multiply by 1.5x. There, now you have a new maximum line pressure reference.
I think AP used to rate their motorcycle calipers to 4000 psi line pressure (2000 psi?) and have since reduced it to 1000 psi without changing the design, just looking for a larger safety factor to sell by. The PS1's are adequate to use for even an aero car. Toledo used them on all four wheels last year.
5) What is maximum pedal force that a driver can input in the pedal box ?
Through research, it was determined that a typical driver can input about 150lbf in a panic situation. However the next question is, what would be an ideal pedal force to have the tires lockup at ?
Our guess would be to have the brake system lockup the wheels when the driver inputs about 120lbf of force (80% of the maximum pedal force). Would that be reasonable ? Are there any autocross or endurance racers who could shed some light on this ? What would be your ideal pedal lockup force ?
We have a "motivational design poster" hanging in our office of mistakes and things to look out for when designing. On that poster we have written, "a frantic driver can put 250lbs into a gear lever or pedal" with 250lbs struck out and 450lbs written in it's place. I can leg press upwards of 300lbs and I'm not a very big guy. Think of what a driver weighing 220lbs and built to play football can do...because you might end up with one of those guys, we did.
I've personally designed between 75lbf - 120lbf pedal input force with some success. Maybe cite an ergonomic paper or something similar. We just did so out of keeping the drivers comfortable. I don't want to attempt to crush an elephant when using the brakes, nor trying to sneak past my parents room coming home late from a party; I want something comfortable that feels natural.
6) What kind of rotor thickness is appropriate for an FSAE car ?
Again I'm thinking of using 0.19 in or a 0.25 in thick rotor. However, I have no reasoning behind those numbers. What would be a good step to validate a rotor thickness decision ?
That rotor thickness sounds beefy as well, more suited for something like a Formula Ford. Since 2011, I've varied our iron-based rotors from 3mm (.118") to 5mm( .196") in thickness. I don't replace them until they've worn to 2.5 mm (.098") which seems to blow some peoples mind.
*poof*
Want an easy way to simulate a rotor use? I've suggested this in other threads, but don't want to be bothered to dig it up. Think back to physics 1. It's all about F=ma.
F=ma
F=ma
F=ma
Good. Now, that that is clear. The force you are looking for is the force of the car decelerating (negative acceleration if you're a phys or math major, they hate that word). How do you solve that? Easy, there is one unknown. F = (mass of car {including driver!}) x (deceleration of car). You've already assumed a deceleration, 1.5Gs. Now I'm going to assume a mass, 600lbs. 1.5 x 600 lbs = 900lbs of force. That's what you're trying to stop. But, it's not just that, it's kinetic energy. Back to phys 1 again.
KE = 1/2mv^2.
Sound familiar? It should. We want to get rid of all of that energy. But we really can't, we can only
convert it to some other form of energy. We want to use the brakes to change that energy into heat using friction, just like rubbing your hands together to generate heat on a cold day in the heartland of Detroit. Let's assume there is 0% cooling, 0% losses, lumped sum mass, and 100% efficiency in our system. This reduces our brake model to two equations that equal our input and output. We already have the input, let's check out the output:
The output will be heat. How much does your rotor heat up? Glad you asked! The temperature of an object is governed by several aspects of the material itself.
Q = Cp(m)(delta T)
Q = change in enthalpy
Cp = heat capacitance of the material. This will be different for steel, cast iron, titanium, plastic, coal, a fuzzy bunny, etc.
m = mass of the material.
delta T = change in temperature.
You can use any units you like, just make sure they are consistent. So, these two equations will constitute our thermal brake model.
I'm going to make a small tweak here to our equations.
1/2mv^2 --> 1/2m(v1 - v2)^2
Now we can find the energy generated from a differential in speed, so 60mph to 30mph or whatever you're interested in.
The input to the rotor will be equal to the energy that needs to be dissipated by the car:
KE = Q
1/2mv^2 = Cp(m)(delta T)
([1/2]*m*(v1-v2)^2)/(Cp * m) = delta T
Using this, you can find what the temperature rise is over a single stop, run it several times for several straights and add cooling equations to find what it will do during a lap, and you'll see a lot of what's going on. Typically you want to keep your rotor under melting and in the range of whatever your brake pads work at.
Hope this helped. Feel free to PM me.