brakes revise

[typeh]

we are considered first year and working on our brakes system i almost finished it but want to take advice from older teams to be sure if i'm going in right track and see if anyone have advice to me
1-pedal force it's obvious in rules the 2 KN force but i see it's too much high so built system on normal pedal force with pedal ratio 3 can be applied in all conditions and just used 2 KN to be sure the system will be safe and won't fail. Assumption i did is right as whole calculations done for normal force not 2000?
2-assumed deceleration i had will stop car from 100 Km/hr to zero i.e kinetic energy= 1/2*m*(100)^2 i think it's high too but took it for safety. K.E i took is high or normal? as usually won't stop from 100 km/hr suddenly in competition
3-rotor we designed is from Cast iron and it's brittle material so factor safety need to be a bit high do you think 2 is enough?
4-Temperature rise will affect on brake fluid and pad friction only or are there to take in consideration?
5-Do you have any more advice?


AUM( Alexandria University Motorsports)
http://www.facebook.com/AUMotorsports

[typeh]

we are considered first year and working on our brakes system i almost finished it but want to take advice from older teams to be sure if i'm going in right track and see if anyone have advice to me
1-pedal force it's obvious in rules the 2 KN force but i see it's too much high so built system on normal pedal force with pedal ratio 3 can be applied in all conditions and just used 2 KN to be sure the system will be safe and won't fail. Assumption i did is right as whole calculations done for normal force not 2000?
2-assumed deceleration i had will stop car from 100 Km/hr to zero i.e kinetic energy= 1/2*m*(100)^2 i think it's high too but took it for safety. K.E i took is high or normal? as usually won't stop from 100 km/hr suddenly in competition
3-rotor we designed is from Cast iron and it's brittle material so factor safety need to be a bit high do you think 2 is enough?
4-Temperature rise will affect on brake fluid and pad friction only or are there to take in consideration?
5-Do you have any more advice?


AUM( Alexandria University Motorsports)
http://www.facebook.com/AUMotorsports

[Ben W]

I'll take a stab at this but just so you know, if you use proper grammar and punctuation, as well as better phrasing, it'll help others to understand and they will be more likely to answer your questions.

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content"> pedal force it's obvious in rules the 2 KN force but i see it's too much high so built system on normal pedal force with pedal ratio 3 can be applied in all conditions and just used 2 KN to be sure the system will be safe and won't fail. </div></BLOCKQUOTE>

Not sure what you're saying / asking here. As I understand it, only the pedal needs to withstand 2kN, but you'd better make sure that the pedal box / lines / calipers / etc. don't break during the test. I decided that the required strength was too low for my comfort so I used a higher case load for wost case scenario and a more realistic one for fatigue and stiffness.

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content"> 2-assumed deceleration i had will stop car from 100 Km/hr to zero i.e kinetic energy= 1/2*m*(100)^2 i think it's high too but took it for safety. K.E i took is high or normal? as usually won't stop from 100 km/hr suddenly in competition </div></BLOCKQUOTE>

You have discovered a rub that had bothered me as well... Let's re-phrase the problem in this manner: Assume the car loses no energy except from the braking system (no drag, energy into the tires, etc.) Make a series of acceleration runs with quick stops. There is a finite amount of energy that goes from the engine into the brakes and an average power. If you assume all of that energy is dissipated via convection you'll find that you'll need a very very high convection coefficient, high rotor area, or have your rotors run at very high temperatures.

Taking typical car values and about 90 square inches and 60 inches for the respective front and rear individual areas, the rotors would reach several thousand degrees F.

No the car doesn't go from 100 to 0 often, but the brakes are almost constantly being used. The 100 to 0 calculation is useful for evaluating how quickly your rotors come up to temperature. However, the energy from the engine running HAS to go somewhere, drag, tires, the air, the coolant system, and the rotors. Not all of it will go to the rotors but much of it will. Just how much depends on the track, among other things.

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content"> 3-rotor we designed is from Cast iron and it's brittle material so factor safety need to be a bit high do you think 2 is enough? </div></BLOCKQUOTE>

Being brittle has more to do with what failure theory you use than the safety factor. I use a safety factor of one, but I also do the failure analysis for the absolute worst possible conditions for each rotor AND I designed the rotors to not fail after 120 hours with 50% confidence.

I was told we used a FoS of 3 before my time, but then again we've had one set of rotors fail because we analyzed the wrong material (and I am suspicious we neglected temperature effects).

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content"> 4-Temperature rise will affect on brake fluid and pad friction only or are there to take in consideration? </div></BLOCKQUOTE>

If something can have a temperature EVERY property it has is temperature dependent. Whether it is noticeable is another matter and is up to you as an engineer to decide.

I'd say from experience that the effect of temperature on the properties of brake fluid to not be noticeable from a performance standpoint, until it boils. Just keep your caliper temperatures below the fluid's boiling point.

Temperature DOES have an effect on friction between the brake pads and the rotors. Just how much depends on the compound, pressure, and sliding speed, among other things. Remember how we found the rotor temperatures after a series of stops? Rotor temperatures are more and more important as your pads are more and more temperature dependent. If you can keep your front and rear rotors the same temperatures, then you can pretty much ignore the temperature dependance of the pads, at least as far as front/rear balance is concerned.

-Ben

[typeh]

sorry for language as i'm not a native English speaker.
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content"> Not sure what you're saying / asking here. As I understand it, only the pedal needs to withstand 2kN, but you'd better make sure that the pedal box / lines / calipers / etc. don't break during the test. I decided that the required strength was too low for my comfort so I used a higher case load for wost case scenario and a more realistic one for fatigue and stiffness. </div></BLOCKQUOTE>
i meant to check safety for all components under 2000 N and don't build system on it .i mean not working on deceleration calculated from it. another point from your comment that you think it is low for comfort. i see it depends on driver and some teams use pedal ratio 1.

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">You have discovered a rub that had bothered me as well... Let's re-phrase the problem in this manner: Assume the car loses no energy except from the braking system (no drag, energy into the tires, etc.) Make a series of acceleration runs with quick stops. There is a finite amount of energy that goes from the engine into the brakes and an average power. If you assume all of that energy is dissipated via convection you'll find that you'll need a very very high convection coefficient, high rotor area, or have your rotors run at very high temperatures. </div></BLOCKQUOTE>
i totally agree with you.

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Being brittle has more to do with what failure theory you use than the safety factor. I use a safety factor of one, but I also do the failure analysis for the absolute worst possible conditions for each rotor AND I designed the rotors to not fail after 120 hours with 50% confidence. </div></BLOCKQUOTE>
my fear is to go in plastic zone so permanent deflection.

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content"> If something can have a temperature EVERY property it has is temperature dependent. Whether it is noticeable is another matter and is up to you as an engineer to decide.

I'd say from experience that the effect of temperature on the properties of brake fluid to not be noticeable from a performance standpoint, until it boils. Just keep your caliper temperatures below the fluid's boiling point.

Temperature DOES have an effect on friction between the brake pads and the rotors. Just how much depends on the compound, pressure, and sliding speed, among other things. Remember how we found the rotor temperatures after a series of stops? Rotor temperatures are more and more important as your pads are more and more temperature dependent. If you can keep your front and rear rotors the same temperatures, then you can pretty much ignore the temperature dependance of the pads, at least as far as front/rear balance is concerned. </div></BLOCKQUOTE>
i meant boiling temperature. i appreciate your comment and opened my mind to new point (balance for heated rotors)

Thank you and i think this post will help others too.

AUM( Alexandria University Motorsports)
http://www.facebook.com/AUMotorsports

[Z]

typeh,

There are two main failure modes for brakes.
~~~o0o~~~

1. The brakes can break (ie. fall apart). The forces that do this come from two directions;

1.1. The driver pushing with 2+kN on the pedal.

1.2. The road pushing on the wheels. This is worst when the wheels have maximum vertical load and are on a high grip road surface, and can thus exert maximum torque on the disc, caliper, etc.

Because brakes are critical safety items, I would suggest you over-design for both these conditions. Bending is the preferable failure mechanism. "Breaking" is undesirable.
~~~o0o~~~

2. The brakes can overheat.

This is common in many racing series. You can tell when this is a problem because the brakes glow bright yellow at mid-corner.

Has anyone ever seen brakes glowing yellow hot in FSAE? Orange? Even dull red?

IMO overheating of brakes is not a big problem in FSAE. If it does become a problem you can use the same solution used on all other racecars;

Make a little scoop that catches the wind and throws it at the disc and caliper.

Z

[Ben W]

I agree that overheating is not nearly as much of a problem in FSAE, but I've seen it happen (soft fade), and I am suspicious that hard fade has happened before on one of our cars.

If you have an old car to go off of, see what temperatures you are reaching and back out a convection coefficient. Otherwise, estimate one using heat transfer theory and estimate what will happen if you reduce the rotor surface area by X%. Keep your rotors happy and your fluid should stay happy as well. If you want to know fluid temperature stick a thermocouple somewhere on the a caliper, preferably away from a direct flow of air. It isn't exactly the temperature of the fluid, but it's close enough for an estimate.

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content"> i meant to check safety for all components under 2000 N and don't build system on it .i mean not working on deceleration calculated from it. another point from your comment that you think it is low for comfort. i see it depends on driver and some teams use pedal ratio 1. </div></BLOCKQUOTE>


Pedal ratio of 1? I doubt it. That would require the master cylinders pointed at the driver's foot horizontally, or that the pedal would extend as far below the box as to the foot from the pivot point. Also, you need to know the mechanical advantage at the rotors AND the hydraulic advantage to know pedal effort, on top of pedal ratio and coefficient of friction.

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content"> my fear is to go in plastic zone so permanent deflection. </div></BLOCKQUOTE>

If you design for service life, you won't see plastic deformation in the short term (if you did it right). If you don't want to go through the fatigue calculations, use a reasonable factor of safety.

-Ben

[typeh]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Z:
typeh,

There are two main failure modes for brakes.
~~~o0o~~~

1. The brakes can break (ie. fall apart). The forces that do this come from two directions;

1.1. The driver pushing with 2+kN on the pedal.

1.2. The road pushing on the wheels. This is worst when the wheels have maximum vertical load and are on a high grip road surface, and can thus exert maximum torque on the disc, caliper, etc.

Because brakes are critical safety items, I would suggest you over-design for both these conditions. Bending is the preferable failure mechanism. "Breaking" is undesirable.
~~~o0o~~~

2. The brakes can overheat.

This is common in many racing series. You can tell when this is a problem because the brakes glow bright yellow at mid-corner.

Has anyone ever seen brakes glowing yellow hot in FSAE? Orange? Even dull red?

IMO overheating of brakes is not a big problem in FSAE. If it does become a problem you can use the same solution used on all other racecars;

Make a little scoop that catches the wind and throws it at the disc and caliper.

Z </div></BLOCKQUOTE>

Thanks Z
calculation done based on 2 KN and this is mechanism i can imagine and what i found in books i read about brakes that braking torque is highest load on rotor and must be designed on it.
i don't agree with 1.2 as loads fed into tires and from it to springs through push/pull rod and rotor almost only affected by torque on it

i totally agree with 2

Alexandria University Motorsports
FSG 2012

[typeh]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Ben W:
I agree that overheating is not nearly as much of a problem in FSAE, but I've seen it happen (soft fade), and I am suspicious that hard fade has happened before on one of our cars.

If you have an old car to go off of, see what temperatures you are reaching and back out a convection coefficient. Otherwise, estimate one using heat transfer theory and estimate what will happen if you reduce the rotor surface area by X%. Keep your rotors happy and your fluid should stay happy as well. If you want to know fluid temperature stick a thermocouple somewhere on the a caliper, preferably away from a direct flow of air. It isn't exactly the temperature of the fluid, but it's close enough for an estimate.

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content"> i meant to check safety for all components under 2000 N and don't build system on it .i mean not working on deceleration calculated from it. another point from your comment that you think it is low for comfort. i see it depends on driver and some teams use pedal ratio 1. </div></BLOCKQUOTE>


Pedal ratio of 1? I doubt it. That would require the master cylinders pointed at the driver's foot horizontally, or that the pedal would extend as far below the box as to the foot from the pivot point. Also, you need to know the mechanical advantage at the rotors AND the hydraulic advantage to know pedal effort, on top of pedal ratio and coefficient of friction.

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content"> my fear is to go in plastic zone so permanent deflection. </div></BLOCKQUOTE>

If you design for service life, you won't see plastic deformation in the short term (if you did it right). If you don't want to go through the fatigue calculations, use a reasonable factor of safety.

-Ben </div></BLOCKQUOTE>
i found out the pedal ratio 1 here in forum during searching but i see it will be very hard on driver foot, hope someone from those teams explain their point of view here,i imagine they do it for the 2000 N in rules

for life what i studied in fatigue that part can fail on fatigue or static so normally check both and design part to withstand so if part will fail on static load it's totally independent on life or we consider load in brakes completely reversed load and design on fatigue only?

Alexandria University Motorsports
FSG 2012

[Warpspeed]

I agree, red hot glowing discs and boiling brake fluid are not very likely in FASE, the kinetic energy and horsepower are just not there to do it.

But what is far more likely is pad fade caused by out gassing of fresh new pad material.

Bed your fresh new pads with stops of increasing severity, and really cook those pads. They will then be fine, until the bedded surface layer wears away with repeated wimpy braking.
Then when you do that really big stop, they could fade again, because the brakes have not been worked hard enough for quite a while.

Real race teams bed sets of brand new pads during testing, remove them, mark exactly where they came out from, put them back in the box, and keep them for the start of the big race.

Aggressive driving on brand new pads is just begging for brake fade.
Probably nothing at all wrong with the brake system design.
It's just something you need to know about, because it's a trap waiting for the unsuspecting.

[JT A.]

We did some testing where we tried to make our rotors glow, just to see what it would take.

It ended up taking about 10 minutes of repeated 70mph to 0 braking tests, followed by a couple minutes of driving around at 20 mph using about 50% brakes and 50% throttle. Also, this was at night time. It was a very dull red that probably wouldn't have been visible in daylight.