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Do I want 4 or 6 piston brembo?

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Duane Black

Curbs go brrrppp
567
401
Exp. Type
Time Attack
Exp. Level
5-10 Years
Durham, NC
Its partially on the face and partially on the central hub of the rotor, but I was having cracking issues long before brake ducts. I was compromised with where everything went sooo

They rarely crack that badly. That was a new rotor. It was hot and dry during my session, and literally as I hit pit road after checkers it POURS rain for 20 minutes, so I'm sure that was a factor. They usually just get stressed to the point I swap them and occasionally get a hairline crack you can hook a fingernail in .. toss another on the scrap pile
 
1,289
1,113
Exp. Type
HPDE
Exp. Level
5-10 Years
Philly Metro Area

Norm Peterson

Corner Barstool Sitter
939
712
Exp. Type
HPDE
Exp. Level
5-10 Years
a few miles east of Philly
Duane sent me a PM, and by the time I got this far with it I figured it might be worth posting here as well. I'm more than open to comments and corrections because it's been a while since I did any of the kind of analysis involved here.

How much more "heat" and "torque" can bigger and heavier rotors generate and manage? I'm sure torque is a moot point as when right, I can already hit ABS, SO heat is my issue.

I'm currently running a 13.25 inch rotor that weighs roughly 22 lbs. I upgraded from 12.25" rotors already, and no noticable difference in braking, rotor heat, etc.

The 4 pot brembo goes to about a 14" rotor weighing about 26 lbs, but the 15" 6 pot has a rotor weighing about 32lbs.

How much more "heat" can those rotors "manage?" If the pads are bigger (they are) and the caliper heavier (by 7 lbs?) Does that help manage more heat and if so, what % am I gaining?


I'll try, but it's going to be tough to picture or describe heat flow in anything but very general terms. I used to use a company's heat flow computer program to do this sort of thing in detail for power plant piping subject to abrupt changes in the temperature of the fluid inside. I may still have some notes, but if I do they're physically difficult to get at and I don't have the program to try for any percentage improvement kind of answer. I'm probably going to miss a lot here because I'm going off memory of stuff I did anywhere from 10 to nearly 40 years ago.

I'd picked up that you were most likely on the 13.2" rotors (scaled your picture comparing OD to the known bolt circle).

Generally, when you put the same amount of heat (car mass times speed change squared) into a finite heat sink (rotors), a greater mass won't heat up as much - overall. That doesn't say enough about the temperature distribution through the metal - think through the thicknesses of the two 'cheeks' of each rotor. Long story short (because I haven't tried to locate any information I might have kept from my job), the 'skin effect' is still going to be significant because it takes a finite amount of time for heat developed at the face to be conducted through the metal. I'm strictly guessing that rotor face temperatures would be roughly a linear function of actual swept area - X amount of heat applied to Y amount of area rather than Z amount of rotor mass.

Rotor mass does affect the magnitude of the metal temperature swings, with greater mass being associated with smaller swings. Think improved fatigue resistance, which is a whole 'nother complicated topic. On the other side of the coin, local rotor geometries can result in more severe local effects and reduced fatigue resistance (crossdrilled holes for one, places where a thin section suddenly becomes a thicker section being another).

All of that, minus the thick to thin "gross structural discontinuity" part, applies to the pads, except that the thermal properties are different. My guess - through-pad temperature distribution is probably a lot more severe than through-cheek rotor distribution. I've put cracks in XP8 pads while heat-checking 12.4" rotors just shy of fingernail-catch level. Don't remember about the XP10's.


Braking torque is basically a direct function of pad mu and mean radius of the actual swept area. What you want here is both the outer and inner radii to the swept area, so pad "width" matters. Assuming zero caliper flex (that you don't really want but which would tend to move the effective swept radius outward).

There's way more detail involved . . .


Norm
 
I've been also researching brakes for my car and this is what I concluded. The larger rotors and pads are about heat management while keeping the balance of the brake bias.
If you haven't had issues with overheating then the 4 bot / 14" will be fine. Unless you'll run aero or stickier tires which you mentioned you won't.
Other stuff like pedal feel, cooling, weight and bang for the buck will largely be the same.

You said you're having doubts about the 4 because you learned about the 6 but really it isn't better. Like many mods each will be better suited for certain situations.
 
898
544
The S550 6 piston brake pads will last twice as long as the S197 4 piston pads. This is based on my direct observation of both setups on 2012 - 2014 S197 GT / Boss cars. The pads on the S550 setup actually last longer than the 25mm thick pads I have with my 380mm / 15" Brembo Pro brakes I currently have on my Boss.

Bottom line, you will actually spend less money on the S550 pads than the S197 4 piston pads because you will replace them less often.

One recommendation is to make sure you are running the Boss / GT brake booster and master cylinder with either setup. We tried the booster and master offered by Kohr that is supposed to provide a more progressive pedal, but in reality the pedal was way to long / soft. I believe CaptainDistraction had a similar issue with the Kohr setup. With the stock booster and master, the S550 brakes on an S197 are great.

My observations are based on making the S550 swap on three different track cars (Pete Desito's Boss, Steve Espo's 2012 GT - i.e. 21 car, and Ernie Lombardo's car).

Having said all this, Norm is probably right in that the slight weight penalty for the S5550 setup. You would probably have no issues with the 4 piston Brembo calipers with a V6 car.
 

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