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I have read going with just Porsche Boxter brakes on the front increases braking distance.
If I get S4 rears 265x22 would that improve the balance and actually improve the braking distance performance?
Would going to 300x22 be unbalance it more?
Kinda confused on this.
BBK up front, braided lines, good rotors and pads in the rear is your best bet. You can out brake P-Cars with A8 setup and the rears as I've mentioned. PM Onemoremile if you want an honest and track person to discuss braking in detail with you.
Boxster / A8 setup upfront increases stopping distance? I don't think so.. it might not decrease stopping distance by much but definately doesn't throw the bias out enough to increase it.
Much better response and control when threshold braking with the stiffer 4 pot caliper
More heat capacity from larger rotor
Definately worthy mod. To balance the rear rotor though you can do the 255mm (stock 245mm) rotor upgrade with b6 carriers. The 3.0lt B6 comes 312mm F 255mm R stock..
Honestly, for a street car I think a quality set of rotors and pads combined with stainless steel lines and good brake fluid is terrific. Cheaper then a BBK kit as well.Quote:
Originally Posted by Squarrl
Something like this
I already have the stock system pretty dialed in.
as long as the pads and tires can handle it, and all other things being equal, larger rotors will always decrease braking distance.
But by how much???Quote:
Originally Posted by ModifiedA4
by the ratio of the diameters of the rotors.Quote:
Originally Posted by gotaudi
Quote:
Originally Posted by ModifiedA4
really? So if you increase to diameter of the rotor by 33% you decrease the breaking distance by 33% ???
sure, all else being equal.Quote:
Originally Posted by gotaudi
Another thing crossed my mind..what kind of tires do you have on your car? A good set of tires can make a huge difference in braking performance.
toyo proxies 4 atm going with General Exclaim UHP for my next set
Quote:
Originally Posted by G_Match
They will increase stopping distances. They have larger/more pistons and generate more brake torque than stock given the same line pressure. So when the fronts are doing their thing the rears are left with less pressure than normal and do correspondingly less work. This may only be an overall braking power decrease of a few percent but it does increase stopping distance.
are you saying you need a bigger hydraulic pump for your braking system?
no.
Can you lock up your tires? (ie ABS kicks in)
Do you have problems with brake fade?
If you answered yes and no, dont bother doing anything. Tires are the limiting factor.
i dont understand the last statement ... how does it lower the rear brake pressure?
do you mean the actual force being applied to the brakes via the lines or the work being done by the brakes?
I believe he is saying it takes more pressure to move 2 pot over 1 pot. So you're loosing the extra psi it takes to operate the rears from the front 2 pot calipers.
I don't understand what he is saying either. Pressure remains constant through the system. Two pistons on a bigger surface would improve clamping force and heat dissipation which would definitely shorten stopping distance.
We're going to make up numbers to illustrate the point. Assume that it takes 100psi of line pressure to get the job done with stock brakes. Also assume that the fronts do 80% of the work and the rears do the other 20%. The 4 piston calipers will get more work done with that 100 psi. That proportionally lowers the amount of work the rears do. If it now takes 75 psi line pressure to generate the same amount of brake torque (as the stock fronts at 100 psi) then the rears are now limited to 75% of their potential.
In a short stroke hydraulic system the piston area determines how much work gets done. Psi is pressure per square inch and larger pistons have more square inches. The fronts will grab harder with less pressure and that is what makes them feel so good. They feel tight and powerful and most of the time that is just fine but when you really need to stop now the rears are down on power.
It gets even trickier when you bring this all into the real world. For example, it is much harder to lock up tires at 100 mph than it is at 10 mph. Everything is a variable.
Larger diameter rotors increase the length of the lever that generates brake torque. It also increases the distance swept by the pad which increases rotor life, exposes more surface area to open air, and increases mass which makes for a larger heat sink but also increases unsprung weight.
Thicker rotors increase mass and/or improve internal vaning for better cooling. The air rushing past a wheelwell pulls air from the wheel area which can improve brake cooling. So can running air to the center of the hats or the outer edge depending on how your rotors are vented. Some new rotors are even center hung so they pull air from both sides in through the center.
Aluminum hats can offset the weight gain of the larger rotors but they are really expensive.
so you would have to proportionally have it set to more bias to rear to offset larger fronts via pressure not by increasing the rears size?
this is what i know to be true about brakes, simplified:
hydraulic pressure in a system is constant, no matter how many pistons there are. more piston area = more force, be it via one large piston or many small ones. it doesnt matter.
brakes are energy conversion 'machines'. they take the kinetic engergy of a car and convert it to heat engergy. how much heat? 1/2mv^2's worth.
to do the energy conversion brakes do work. F*d = work. d is the circumfrence of the rotor*# of rotor revolutions. F is the friction force of the brake pads. F = Fp*u. u is the coeff of friction of the pads on steel, and Fp is the piston force.
# of rotor revolutions = # tire revolutions. This is why stopping distance reduction is related to rotor size. The more linear distance the pads act over per revolution (larger rotor), the more work is done converting energy per revolution.
Likewise shorter stopping distances can be achived by changing the F in the F*d equation. F itself is made of Fp and u. So grippier pads (more u) or more piston force Fp(via more piston area) are the other two ways of making brakes convert energy better.
The 'lever arm' people like to talk about is a red herring. All that does is change the braking 'feel' or onset of force to the rotor. As long as the brakes are not locking or the tires skidding, the lever arm has no impact on braking.
to bring it all back around full circle, 1/2mv^2=F*d/s (s is the time it takes to stop)
Finally to answer the original poster's question again...rotor circumfrence is Pi * D . D is the rotor diameter. when comparing the ratio of two rotor sizes, the Pi's cancel, leaving only the diameters left to matter.
i hope this clears up the confusion on how brakes function. provided the brakes aren't fading, tires skidding, or the brakes locked...the above applies.
So should I still do the A8 brake upgrade in the front?Quote:
Originally Posted by onemoremile
I've run the A8 fronts with stock rears for years. They work great. Last year I installed the Tyrol front caliper bushings which was a nice upgrade.
http://www.stoptech.com/products/BBK/balance.shtml
http://www.stoptech.com/tech_info/te...e_papers.shtml
Rear Brake Upgrades, Is Bigger Really Better?
I could not agree more!Always loved math!Quote:
Originally Posted by ModifiedA4
I would only like to add the importance of rear bbk in conjunction to front BABK when it comes to brake fade.On autobahn or autostrade when cruising 200+km/h and braking hard 2 to 3 times one realises how bigger/biggest is better/best.I personally consider the A8 upgrade and the Boxster upgrade insufficient for our cars.IMHO
I know that people are concerned about bias affecting larger brakes.
Try unplugging your abs module and do a panic stop. The rear tires WILL lock.
(I don't really recommend doing this.)
My point is that on these cars the abs module handles the brake bias.
I would think the computer is smart enough to keep all 4 tires in the threshold.
Maybe I am wrong but does anyone have any actual before and after stop times?
I'd like to see facts before I believe upgrading the fronts only will not help with stopping distances.
-richard
it does tend to explain the world around us :)Quote:
Originally Posted by rogerius
And its a standard sysem for everyone, no matter the language.Quote:
Originally Posted by ModifiedA4
Switching to 12.3" brakes with Hawk HPS pads really made a strong impact on my brakes. I can hit the brakes at 80 mph and tires will chirp. After 127k miles, the braking system does feel sloppier than new ofcourse but will still put out.
Quote:
Originally Posted by audinutt
Yes and no. The ABS system is reactive and doesn't do anything until a wheel has locked up. If you upgrade the fronts so the rears are doing even less work then there is less chance of them locking up. The ABS would only help manage relative traction levels if the rears were overpowered and prone to locking up.
I gave you the facts. See that link to Stoptech's white paper section? All the empirical data you need to get off the fence.
SCC magazine tests Stoptech calipers on an RSX and it doesn't perform the way they though it should. They change the piston size slightly and suddenly it works. Hydraulics are what make the brakes work.
http://www.sportcompactcarweb.com/te...ech/index.html
http://www.sportcompactcarweb.com/ed...kes/index.html
http://www.stoptech.com/tech_info/wp...formance.shtml
http://www.stoptech.com/tech_info/Th...%20Systems.pdf
i just finished reading the scc articles, they're pretty weak. the fact that they were testing ABS stopping and changing piston sizes makes it an apples to oranges comparison, imho.Quote:
Originally Posted by onemoremile
in addition they've got some glaring problems with the methodology they used, heres one i noted:
the variance between their test setups was 6% (their number) yet:
huh?Quote:
If we assumed that the A-Spec pads and the Axxis Ultimate pads have the same friction coefficient
so unless the different pads have friction coeffs much better than within 6% of each other, their conclusions cant be valid. [o_o]
for what we're talking about here, the A8 front upgrade, i dont see how any of the SCC stuff applies.
ps: i didnt get a chance to read the stoptech stuff, thanks for the links though!
Since the A8 kit keeps the factory calipers it is exempt from all the conversation on hydraulics and piston sizing. That is the beauty of the A8 kit.
The SCC article is anecdotal and has some explanation of how and why things work. To discount the entire article based on one assumption in what they admitted was not a scientific test is poor judgment. There is a lot of other good info in there. You'll learn a lot from the Stoptech articles.
ehh, thats a cop-out, i'll call a spade a spade. it was a poor article. i dont think they understood what was happening. im willing to bet that changing the piston size screwed up the ABS pulsing calibration.Quote:
Originally Posted by onemoremile
think about it, larger pistons will take more fluid displaced to apply a force. during manual stopping your foot/brain automatically compensates for this by pressing the pedal further. in the short time periods that ABS pulses, more fluid displaced = more time per pulse required for the same force. the time pulse in the ABS computer is fixed from the factory. so, i think its entirely possible that the peak pressure applied during the larger piston stoptech runs was less than the peak pressure applied on the normal piston...hence the discrepancy in performance.
the whole brake bias can-O-worms they opened up doesnt makes alot of sense. anyway...
(disclaimer) i realize in a perfect hydraulic system that the force application is instantaneous, but a braking system has mechanical tolerances, which are compensated by displacement of the piston(/disclaimer)
Nothing you said makes any sense. The force generated is based on pressure not the amount of fluid moved. The amount of additional fluid needed to make larger pistons move is very small. Stop making things up and read the white papers already. [:/]
well how about i do this... and using equipment from the physics department take measurements, with both setups. then we can have some experimental to go with the theorys.
Then we can yell at my experimental setup :O
perhaps its not making sense to you, because you cant understand it? hmmm.[o_o]Quote:
Originally Posted by onemoremile
did you read my disclaimer at the bottom of my post? im perfectly aware of how ideal hydraulics work.
Apparently not. Bigger pistons equals more power. That is why the fronts overpower the rears. There is nothing more to be said on the subject. It is so basic that I can't believe we've wasted this many words already.Quote:
Originally Posted by ModifiedA4
No need to do any experiments or extrapolate strange ideas into something convinving. Read the damn reports filed by brake engineers. The best information available is right in front of you and you can't be bothered to read it because you'd rather make up asinine scenarios.
I understand what you're trying to say and it is incorrect. If it was a flow based system you could be on to something but it isn't - it is pressure based and therefore the amount of fluid required to move larger pistons is irrelevant.
The white papers aren't theory or conjecture or up for debate. They are pure post experiment physics.
really dude. [rolleyes]Quote:
Originally Posted by onemoremile
wikipedia:
its a fundimental property of hydraulic systems. you are essentially trading distance for force. changing the output piston size means that piston wont displace as much. (im not going to explain why tolerances require a certain displacement of brake piston on the output, thats basic mechanical engineering.)Quote:
An interesting aspect of hydraulic systems is the ability to apply force multiplication. Imagine if cylinder one (C1) is one inch in diameter, and cylinder two (C2) is ten inches in diameter. If the force exerted on C1 is 10 lbf, the force exerted by C2 is 1000 lbf because C2 is a hundred times larger in area (S = πr²) as C1. The downside to this is that you have to move C1 a hundred inches to move C2 one inch.
like i said above, i dont think you're getting the fundimentals of what is going on. maybe its my fault for assuming everyone has an engineering background, and im sorry for that. [wrench]
So Wikipedia overrules Stoptech? Nice. Maybe we can all skip class and ask the random guy on the corner for a lesson instead? The word is fundamental.
FWIW, I've got more experience with this than you might believe. Not the keyboard or book type but the bloody knuckles and greasy fingernails type. Crewing on race teams, helping the crew chief sort chassis and brake systems, four corner Wilwoods on the Cobra, etc.. After all that I still realize that my personal knowledge is pathetic compared to that of Steve Ruiz and the Stoptech guys. I'm still working towards understanding everything they've written.
I think my 9 patents and work with the best in the world qualify as an engineering background. Maybe I'd need to get a degree, maybe not. Regardless, just read the damn Stoptech info. Everything you are looking for is there written by engineers that specialize in brake theory and application. They've tested everything and found that piston size changes the balance front to rear.
I know you are an engineer because you cling to false concepts and close your mind to real knowledge. That stubbornness is an unfortunate trait in the industry and one of the reasons I left it. It seems that all I did was fix problems created by degreed engineers. For example, a few of those guys fought a hole tolerance on an engine block and it never came in quite right. They spent a fortune trying to get it perfect before we happened to overhear. When I asked what went in the hole they had no clue. After prodding them to research it they came back and said it was a rubber fitting and the specified hole tolerance was about 10 times tighter than it needed to be and they never had to do anything. They still lost about a million bucks though. The guy that put the tolerance there had a masters in mechanical engineering and just wanted everything to be perfect. I love engineers that are too full of themselves to realize how much is left to learn. Michigan is full of them too. So are the Ford guys that use three different sets of datum holes on a magnesium radiator shroud as it goes through three different plants.
Seriously, I've got nothing against you but I've put the best information possible right in front of you twice and you come up with conjecture and wiki-assumptive info. I've read the same info from Stoptech as from Brembo and Alcon. Some of it is also in the Bosch Automotive Handbook which is a handy little reference. If you had read and understood the info in the white papers those other posts would never have been created. Please just read the white papers and build on a foundation of solid knowledge. Is that too much to ask?
Quote:
Originally Posted by gotaudi
Absolutely not. Increasing rotor size does nothing to increase the tire's ability to slow the car. That ability is limited by traction and nothing more. Picture a powerlifter with an injured back or shoulder. No matter how great the force he can only apply a portion.
Increasing rotor size does do a few nice things including adding more mass to act as a heat sink, more interior and exterior cooling area, more leverage (brake torque), larger and/or longer area swept by the pads, etc. This is offset by the larger diameter which means you have a larger flywheel to accelerate and decelerate. This drawback is rarely a big deal and is often offset by weight reduction through the use of an aluminum hat.
This is a subject that quickly gets buried in data and that is why it is hard to understand. We've got to deal with static and dynamic weight distribution. If a car has bad brake dive then it puts more weight on the fronts which let them do more work and takes weight away from the rears so they can do less. If you limit brake dive this weight transfer is limited and the rears can do more work. How much of a difference is some labor intensive math best left to computer modeling.
For example, this is my car threshold braking at Gingerman Raceway. It is shown on stock sport suspension with A8 brakes, Carbotech Bobcat pads, and 235/45-17 Pilot Sports. The brake dive is quite bad and the rears didn't lock up or trigger the ABS. That was mostly because I let off at about 30-40 mph for the corner. With the coilovers (balanced KW V1) and everything else the same my brake distance dropped quite a bit. I didn't measure it but there are three brake markers and I was about even with the first rather than hitting the brakes while still approaching it. Keeping the car flatter lowered the stopping distance.
Now it gets a little trickier. I switched to more aggressive track pads (Carbotech Panther) and Toyo RA1 track tires but left everything else alone. I even ran the same rotors. The stopping distances were insanely short. An instructor rods with me to help me tighten up some lines and every time we passed the first and second brake markers he was stomping the floor and bracing for impact like a panicked driver's ed instructor. We never once dropped a wheel off the track. Again I didn't measure but I'd guess the distances were 50-100 feet shorter and we were coming in hotter since the tires let me carry more speed out of the previous corner. Where the math gets tricky is that the track tires have more grip which leads to more weight transfer so the rears were doing less work than with the street tires. If I switched to track pads in the rear they might have locked up or not been able to get up to operating temp and been even worse. The only way to really find out is to experiment or do the long math which still only gets you in the ballpark.
http://i23.photobucket.com/albums/b3...ngforturn1.jpg
One thing I've always pondered is the way a lot of VWs sit lower in the rear. In theory that jacks weight to the rear and under threshold braking the rears would carry a little more weight and be able to do a little more work. I've got no idea how this would work in real life and it may compromise cornering which could lead to even slower lap times.
Anyone is welcome to ride along for a demonstration. I can be at the track most summer weeknights for test and tune and we might be able to borrow a Porsche for comparison. All I ask is that you have a helmet, no polyester, and an empty stomach. [;)]