4 piston calipers

[shawnBaek89]

Hello, I'm Lee from Kookmin university's racing team in south Korea.

We are gonna participate in 2015 FSAE in Michigan. I have a question for 4 piston calipers.

Our team has used 2 piston calipers for front but, this year, we will use 4 piston caliper from APracing for front upright. The problem is the heat that causes the boiling of brake fluid(Motul DOT5.1). So, we mounted rotor surface temperature sensors. After driving 5 laps in endurance track, rotor temperature rises to 400 degrees.

Actually, there was no problem with 2piston calipers

We didn't have any wings for front and rear.

How do you guys overcome the heat from 4piston calipers? Several teams mount air duct for calipers.

I look forward to get you guys reply. Thank you.

[Z]

Lee,

I suspect your problem is NOT directly because of the change from 2 to 4-piston calipers.

I take it that your rotor temperature of "400" is in degrees Celsius? That is NOT very high. If 400 Fahrenheit, then barely warm!

Can you post some pictures of your whole car, and close-ups of the brakes? That is the best way for others to see if there are any obvious errors.

Over to more experienced Brake-Guys...

Z

(PS. Maybe water in brake-fluid???)

[MCoach]

I would call 400C barely warm as well...

Do you have any data to indicate that the brake fluid is boiling? Which two piston calipers were you using previously and did you change your rotor specifications at all? What is the mass of each of your front brake rotors (this will help me have an idea of how to evaluate your issue).

I have a feeling that you may not actually be boiling the fluid and if so there are some underlying issues here. We regularly record temperatures over 900C for our vehicle.

[shawnBaek89]

Thank you for all of you guys comments.

Rotor temperature rises to 400 celcius degrees in just 5 laps.

After finishing endurance event for test drive, we could see some white smoke from brake pad......I thought it was on fire:(....
The problem is...when a driver hits the brake pedal while rapid like hard driving, he feels the inextension of pedal travel. But, there is no inextension pedal travel when the vehicle stops.

According to the thesis from SAE, inextension of pedal travel while driving means the boiling of brake fluid.

The limit temperature of APracing CP4227 caliper is 260 celcius degrees. I'm really worry about O-rings in calipers......

I posted upright picture of my team. Is there anybody had thermal problem with brake system? 588 589

I look forward to get you guys reply..

[Jonny Rochester]

What is your process to bleed air from the brake lines?

A small amount of air in the brake lines will make the brakes a bit spongy, but this feeling would be exaggerated when hot.

Smoke from the brake pads means they are getting too hot, and this will cause pad-fade also. You can probably upgrade to racing pads with a higher metal content and higher operating temperature.

What is the thickness, or mass of your brake discs? I'm just interested for my own reasons.

[Ahmad Rezq]

I would call 400C barely warm as well...

Do you have any data to indicate that the brake fluid is boiling? Which two piston calipers were you using previously and did you change your rotor specifications at all? What is the mass of each of your front brake rotors (this will help me have an idea of how to evaluate your issue).

I have a feeling that you may not actually be boiling the fluid and if so there are some underlying issues here. We regularly record temperatures over 900C for our vehicle.

I searched for MOTUL DOT 5.1 and found these specs

Dry boiling point 272 °C / 522 °F
Wet boiling point 185 °C / 365 °F
https://www.motul.com/system/product_descriptions/technical_data_sheets/2712/original/DOT_5.1_Brake_Fluid_(GB).pdf?1335380510

[Claude Rouelle]

Lee,

Boiling brake fluid is a consequence of either bad braking fluid but most probably of overheating.

If there is heat there is friction. But do you have friction only when you brake?

Ask you driver to run 10 laps. Stop him and lift the car up. Without the driver pressing the brake pedal are the wheels moving freely?

Are you sure your brake discs is “running true”? In other words is your brake discs plane perpendicular to your hub. It won’t: that is because of compliance (hub, hub bearing, upright, caliper, caliper mount on your upright etc…). That is why you need floating discs. You seem to have floating discs on the picture but how much play do you have?

Similarly the center line of your caliper should be right where the brake disc plane is supposed to be. Is it? A wrong mounting or machining of the caliper attachment on the upright could be an issue.

It also could be you have hysteresis in your caliper piston seals.

Also make sure you do not have too strong anti-knock pack piston http://www.apracing.com/Info.aspx?InfoID=110&ProductID=7

Measure the thickness and carefully make notes of each brand new braking pad at 5 points: each corner and in the center. Run 50 kilometers and make the same measurements; it will give you an idea of how parallel or not your brake pad friction plane is to the brake disc plane.

Finally typical mistake: make sure that when the driver release the brake pedal the seal ahead of the piston of the master cylinder does not close the communication to the brake reservoir. It is very important that this hole is fully open when the pedal is at its rest position. Otherwise, any expansion in the fluid will be prevented the fluid from going to the reservoir. And that creates brake binding.

See you in Michigan soon with hopefully your brake problems solved.

PS: Did you make a simple brake kinetic to thermal energy + heat transfer calculation to see if 400 degrees is "normal"?