Brake info/FAQ

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Brake info/FAQ

Post by jamal » Fri Apr 06, 2007 10:56 am

If you're looking for a brake upgrade, other Subaru models are a great start. Most other Subaru brakes will bolt right up to our cars without a problem. But first you should know how brakes work and what the benefits and drawbacks are when changing things around. So I'll start out with brake information and reasons to upgrade before I get into the actual upgrade options.

General Info/ helpful links:

Brake Math Spreadsheet by Josh (Legacy777). This spreadsheet is very handy and will calculate the bias change when using different front and rear brake combinations. It also has information about master cylinders, prop valves, and pedal travel.

nasioc faq/threads of note on brakes. I got a lot of information here and we could probably just sticky a link to that thread. Still it'd be nice to have our own.

white papers on the Stoptech website. Read these if you want to learn about brakes- they are very informative.

Compiled Brake Information All the different brakes that came on all the different Subarus

Brake Interchangeability thread Details how to put Subaru brakes onto other Subarus.

Okay now that all the other things you should be aware of are out of the way lets talk a little about how the brakes actually work
Last edited by jamal on Sun Jun 22, 2008 11:18 am, edited 28 times in total.

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Post by jamal » Fri Apr 06, 2007 10:57 am

When you push the brake pedal, it acts as a lever which pushes a piston into the brake booster. The brake booster uses engine vacuum to create additional force into the master cylinder. A piston in the master cylinder pushes fluid into the lines, through the proportioning valve, ABS (if equipped), and to the brake at each wheel.

The force and lever arm on the brake pedal, booster, and master cylinder diameter determine the line pressure in the brakes. The line pressure is the same as the pressure inside the caliper, so that multiplied by the piston area determines the clamping force. The coefficient of friction of the pad determines the friction force relative to the clamping force the pad exerts on the rotor. So then you can take this force of the pad on the rotor and multiply it by the effective rotor radius, to determine the braking torque generated. This all leads to one very important thing: The TIRE stopping the car.

So, obviously, if you want to stop in a shorter distance, the only thing that will make a significant difference is stickier tires.

Okay. everyone please pause to let that sink in.

Pads don't decrease stopping distance
Slotted or drilled rotors don't decrease stopping distance
$18/pint brake fluid doesn't decrease stopping distance
Giant brake rotors and 12 piston calipers don't decrease stopping distance.

Only better tires or more effective use of the tires reduces stopping distance.

For example, having a giant set of Brembos on the front WILL NOT shorten your stopping distance. They'll make it take longer to stop because your front tires will be at the point of lockup before the rears are doing much of anything.

Okay then now that I've told you how brakes work and why a lot of the commonly believed information is bullshit, I suppose I could go over some reasons why you should, or would want to upgrade the brakes, since that's kind of what this thread was originally about.
Last edited by jamal on Sun Jun 22, 2008 11:19 am, edited 42 times in total.

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Post by jamal » Fri Apr 06, 2007 10:58 am

Bias:

The brake bias is the proportion of front to rear brake. The front brakes do more of the work to stop the car, so they need to exert more force. This is because a) there is more weight on the front wheels so they have more grip, and more importantly b) because as the car decelerates even more weight shifts to the front wheels. So the front does about 70-80% of the work to stop the car, although it depends on grip.

I say depending on grip because the more grip a car has, the more weight gets transferred to the front. So, a track car on slicks actually needs more front bias than a street car on all-seasons, because it can decelerate harder and transfers more weight to the front.

This also helps explain what a proportioning valve does and why cars have them. The [Subaru] prop valve cuts line pressure to the rear brakes once it reaches a certain point. There are charts of this on Josh's spreadsheet. So, under low force braking, there is less weight transfer so the rears get more pressure and do more work. Under hard braking, pressure is cut to the rears because the fronts need to do most of the work.

People talk about cars being front or rear biased. This describes how they act while braking. A front biased car will tend to lock up the fronts first, where a rear biased car will lock the rears. Rear bias isn't good, because it can be kind of like pulling the e-brake every time you want to stop in a hurry (or are braking into a corner).

Bias is very important because under hard braking, you want all four tires to do their share of the work to stop the car in the shortest possible distance. All cars have front bias, because having the rear tires lock up first is not a good thing. Perfectly neutral bias isn't really possible, because grip and weight distribution of the car are constantly changing. Race cars have adjustable bias so that the car can adjust for different track conditions and even the amount of fuel in the car in order to get every last bit of braking performance out of all of the tires..

I've found the stock bias is definitely oriented toward the front. It has to be. However, I didn't ever consider it to be too far forward, and have locked up individual tires on all four corners depending on conditions. When swapping different brakes onto your car, one of the most important things you can do is not move the brake bias any farther forward.

Here is a great stoptech article about brake bias.
Last edited by jamal on Mon Jun 23, 2008 4:03 am, edited 17 times in total.

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Post by jamal » Fri Apr 06, 2007 10:58 am

There are a few reasons to change/upgrade the brakes, some being better than others. Some of them include:

- Heat capacity
- brake feel
- brake bias
- bling bling
- pad availability

Heat capacity is the ability of the brakes to store and radiate heat. A brake rotor is really just a big heat sink, since the purpose of brakes is to convert the kinetic energy of the car to heat. A bigger rotor means more capacity for heat which means they will take more abuse.

With stock brakes, it doesn't take much to overheat the system.

The more you use the brakes, the hotter they'll get, and when a pad reaches temperatures above it's design capacity, it starts to not work very well. The pad compound exceeds it's maximum operating temperature and basically starts to evaporate. This is called outgassing, and the gases form a layer between the pad and the rotor that prevent them from contacting each other, so all of the sudden it's like "hey I can't stop and there's smoke coming out of my wheels." That's the origin of cross-drilled rotors, because back in the day all pads had a tendency to outgas, and they drilled the rotors on race cars so they would still work. With modern pads, working in their design temperatures. it's not a problem so drilled rotors are not really beneficial. There is still of course some stuff coming off the pads, and it can help clear dirt and water, and possibly delay the onset of fade. Still, not particularly worth buying and with heavy use they tend to crack. More details on drilled rotors

The brake fluid in the system can also overheat. The rotors and pads will heat up the calipers and brake fluid (along with other things in the area). When the fluid gets too hot, it starts to boil. Compressible bubbles in the brake system means "hey I'm pressing down on the pedal and nothings happening."

Brake fade will not occur on the stock system with normal driving, but some hard mountain driving or a track session can overheat the brakes very quickly.

The first step to fix this problem? Better pads and fluid. This lets the brakes get hotter before the pads and fluid overheat, which means you will still be able to stop well, even if your rotors are glowing red. In 02 for example Gary Sheehan raced a WRX with stock brakes and race pads/fluid in the USTCC and did very well.
Gary Sheehan wrote:I raced for a full year on the stock rotors and calipers. We had them set up where they would not fade. Switching to Stoptech's made my life easier because they are more consistent, have a better feel and run at lower temps, thus not ruining everything around the braking system. But they did not drastically decrease my stopping distance. Probably a few feet.
There, straight from a race car driver: You don't actually need bigger brakes. With the right pads and the right fluid you can race our cars and not run into fade. If you have a lot of power and are seriously tracking the car however, you might have glowing rotors, melt piston and slider pin boots, melt ball joint and tie rod boots, destroy wheel bearings, etc.

Now that I've told you why not to upgrade the brakes, I can talk about why you should. Brake feel is very important because it gives you better feedback in the pedal about what the brakes are doing. A stiffer pedal with less travel produces much better feel and is easier to modulate. That, generally, will produce more consistent and shorter braking distance. A well bled brake system in good working order with quality pads is going to feel much much better than an old system on cheap pads. However, there are tons of variables when it comes to brake feel, such as caliper design, the suspension, bushings, and alignment, master cylinder, brake booster, and pedal ratio. The master cylinder is very important when it comes to feel. A larger diameter means the piston has to move less to move the same amount of fluid, but requires more force to produce the same pressure.

But you don't really have to worry about that too much. To start, get some good pads, bleed the system, and make sure all calipers slide freely. Better pads produce more grip on the rotor and compress less. That makes the brakes feel better.

When you get into changing calipers and brake boosters and master cylinders, you have to worry about what it will do to feel, pedal travel, and brake bias.

Upgrades:

So, if you're at the point of looking for a different brake setup, there are plenty of OEM options, but only a few combinations that I would recommend. Just putting on bigger front brakes will give you more heat capacity, but will increase braking distances because you've shifted the bias even farther forward.

turbo cars already have pretty good brakes. They're the same size as the Impreza RS but have vented rear brakes. The upgrade I would suggest for these is front Subaru 4-pots. You have bigger rotors, fixed calipers for better feel, and bias remains the same. Rear 2-pots would be a nice match but are not necessary.

For a non-turbo sedan there are few good combos that will both increase brake capacity and maintain good bias:
-Impreza RS/turbo sized brakes all around.
-WRX front and 00-04 Legacy or 05+ LGT rear
-4-pot front with RS, turbo, 00-04 Legacy, 05 LGT, or 2-pot rears.

The 4-pots put out less torque than WRX brakes, so IMO any of the rear brake upgrades will work with them. Just putting on the calipers with bigger pistons out of a wagon would be easiest and cheapest, 00-04 Legacy would actually move the bias back a bit, and 2-pots would be the best match.

If you're upgrading the fronts while leaving the rears alone I'd suggest only doing RS/Turbo sized brakes and maybe installing a wagon prop valve. Wagon proportioning valves have a higher split point so that at higher braking forces they receive more line pressure than sedans (and therefore more rear braking torque).

Wagons have bigger rear pistons than non-turbo sedans, so the RS/WRX etc rears are not an upgrade. Turbo gives vented rotors but will keep the rear brake torque the same. This means that putting on the RS/turbo fronts on an n/a wagon is going to move the bias forward a bit, but probably not enough to be a problem. I definitely recommend bigger rear brakes to go with WRX-sized rotors. 00-04 Legacy, 2-pots, and 05-09 LGT would all work great.

Pedal travel:

One thing to keep in mind while upgrading brakes is the pedal travel. When you increase the piston area, you increase the amount of fluid that needs to move to brake the car. Turbo cars have a bigger master cylinder, and when installing WRX or LGT or 4-pots or whatever, there won't be an increase in travel because the piston area stays the same.

With an upgrade from non-turbo brakes to the 2-pot fronts, you're going to see more pedal travel. I don't have that big of a problem with it but I intend to swap master cylinders. The only requirement I know of is that abs and non-abs MCs are different. For non-abs people, a fwd SVX L MC will work. With abs I think any 1 1/16" mc will work (WRX sedan or STi, for example), but if it's post 1999 you'll need the brake booster that goes with it.
Last edited by jamal on Sun Feb 22, 2015 11:59 pm, edited 18 times in total.

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Post by BAC5.2 » Fri Apr 06, 2007 11:41 am

The car Gary Sheehan is talking about is one I've driven too :). The stoptechs he refer's to, are sitting on the floor of the office at the shop.

Good write up! I'll see if I can sticky. If you think of a better title, just let me know.
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Post by skid542 » Fri Apr 06, 2007 7:29 pm

Excellent thread. Great info on the other Subaru brakes and which models they can be found on. Very helpful.

However, I do want to say one thing that I know is controversial. If this is going to cause clutter to your thread then by all means - Mod's delete/edit this out.

Upgrading only brakes, not tires, to a larger size and heat capacity can make a significant difference. I noticed an honest 40 ft difference (70-0) when I upgraded my L-trim brakes to turbo brakes - front and back. My bias was also more even I suspect that helped as well. Albeit I was only running a 205 tire so on the wider tires this effect may be diminished. But still noteworthy.
Lee

93' SS, 5mt swapped, 182k, not stock...
96' N/A OBW 5sp, 212k, Couple mods... RIP
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Post by jamal » Fri Apr 06, 2007 10:25 pm

how did you measure?

I'll be getting to bias with one of the place holders.

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Post by skid542 » Sat Apr 07, 2007 12:42 am

A quick brief of the mechanical changes, tires were the exact same throughout - 205/60/15. Stage 1 was stock L trim with the stock sedan rear caliper and stock pads. Stage 2 was rear Legacy SS slotted, cryo'd, Bradi rotors and Mintex pads with the front being stock with stock pads. Yes this did move the bias to the back, which I found both pee your pants fun and crap your pants scary. But I did lose ~23'. Next was the addition of the SS front rotors with stock pads. I lost another ~18' and the bias felt really balanced. I could hardly tell whether the front or the backs locked first, and swear at times I could hear the backs go first sometimes.

These distances though were measured by making repeated 70-0 stops at the same 250' stretch of road tarred gravel/paved road with a definative begin braking marker, aka buried road sign, and then taking averages. I took an average of over 15 different 70-0's for each brake setup. I measured the distances with a 100' tape from where I stopped - had my passenger get out to mark the exact spot - to the marker. I realize that my reaction/anticipation is included in my distance measurements but by performing the test at the same spot in similar conditions - including ambient temp. - I still get a true differential measurement of the three brake setups. This isn't meant to sound 'snooty/uppity' but I do have an engineering degree and these tests were performed with the mindset of getting a true difference.

Latter I did upgrade the tires to a better all-season. I also removed considerable wieght to where I was down to 2700 - lest not forget I was FWD. I was at that point repeatably getting ~100' 70-0 distances - and yes I stand by that.

To all those who have read this very informative thread this far... Jamal is right in that tires and wieght made the biggest difference but size and balance can help by itself.
Lee

93' SS, 5mt swapped, 182k, not stock...
96' N/A OBW 5sp, 212k, Couple mods... RIP
99' N/A OBW, 4eat, mostly stock.

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Post by jamal » Wed Apr 11, 2007 9:47 pm

how much of an effect on pedal travel did that cause? I'll have that much piston area soon.

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Post by DLC » Wed Apr 11, 2007 10:05 pm

My 00 GT Limited did not have the 294x24mm fronts, and I'm nearly certain that the 00-01/02 GT/GTLTD had the same front brakes as the 98-01 Impreza RS, which according to cars101, were 10.7", versus the Legacy Turbo, which is stated as 10.9".

I replaced my fronts first with WRX, then 05 Legacy GT parts, so the difference in size has been noticeable each step, but mostly so on the latter.

I did reuse my stock GT calipers when upgrading to the WRX brackets/rotors/pads, as they were identical to the WRX parts. Everything was replaced when going to the significantly larger LGT setup, however.
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Post by skid542 » Thu Apr 12, 2007 12:25 am

Pedal travel was definately effected. Upgrading the rears was noticable but not largely so. Upgrading the fronts took a little bit to get used to. Once I got used to it I found it very workable and having that travel did make it easier, or so it seemed, to keep the tires closer to the edge without locking (no ABS). I will say that whenever I'd drive my dad's 95' Altima, the first few stops were pretty abrupt and when he drove my car he complained slightly about the travel. But I always thought his was car was touchy to begin with. I certainly wouldn't worry about upgrading your cylinders but I think SS lines would help.
Lee

93' SS, 5mt swapped, 182k, not stock...
96' N/A OBW 5sp, 212k, Couple mods... RIP
99' N/A OBW, 4eat, mostly stock.

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Post by jamal » Thu Apr 12, 2007 10:03 am

DLC wrote:My 00 GT Limited did not have the 294x24mm fronts, and I'm nearly certain that the 00-01/02 GT/GTLTD had the same front brakes as the 98-01 Impreza RS, which according to cars101, were 10.7", versus the Legacy Turbo, which is stated as 10.9".
Hmmm, well I know that some of the 00-04s got the 294mm rotors.

Brembo lists the same part number for Legacy turbo and the later 2.5RS etc rotors.


skid, cool. I'm going with SS lines and am thinking of fabbing up some sort of a MC brace. If it really bugs me I can also go with a non-abs SVX mc, which is SS/WRX, etc size from what I've heard.

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Post by DLC » Thu Apr 12, 2007 8:48 pm

The 03-04 GT definitely got the 294mm fronts, but the 00-01 didn't. Not so sure about 02.

It wouldn't surprise me if Brembo made one part for two different sizes. If they're really 10.7" and 10.9", something in between could service both if the thickness was identical.
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Post by 86ruguy » Fri Jul 27, 2007 6:59 pm

So, if I understand correctly.......

The easiest brake upgrade for an N/A wagon is to put turbo front rotors and 2pot calipers on.

Right?
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Post by jamal » Fri Jul 27, 2007 10:15 pm

Yes. Any of the 2-pots and 276mm rotors are essentially the same except for pads and how the caliper mounts to the brackets, so you can find any of those sets of brakes.


Also clicking on the rotor dimensions now takes you to the DBA spec sheet.

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Re: The giant brake information thread

Post by jamal » Wed Nov 07, 2007 2:46 am

oops I didn't mean to post here.

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Post by IronMonkeyL255 » Wed Nov 07, 2007 3:02 am

Just a minor correction:

The Legacy Turbo front pads are d-plate number D563

The rears are D471.

I found this spreadsheet on EBC's site which has d-plate numbers for just about anything.

http://www.ebcbrakes.com/Assets/USA%202 ... motive.xls
Disclaimer: If anything I post is inaccurate, please correct me. I do not wish to add to the misinformation floating around on the internet.

That being said, everything I post is accurate to the best of my knowledge.

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Post by jamal » Sat Nov 10, 2007 1:48 am

Thanks.


That reminds me that I need to change some things. The nasioc version is more up to date...

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Post by slowjoe » Mon Mar 03, 2008 9:28 am

about shorter stopping distances. wont a reduction in weight decrease the stopping distance as well as better tires?
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Post by vrg3 » Mon Mar 03, 2008 3:50 pm

Yes. Both of those things will make a significant difference. Upgrading the brakes is usually done to reduce fade.
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Post by jamal » Tue Mar 04, 2008 9:47 am

Well, generally stopping distance relates to the coefficient of friction of the tires. So if a tire can pull a g of braking force, it theoretically shouldn't matter if the car weighs 500lb or 5000lb. However, the coefficient of friction of a tire varies with normal force; generally as the force increases, the Cf decreases. So for example a tire might generate one g with a 750lb load, but only 0.9g with a 1000lb load. Once again I'm putting the responsibility on the tire (where it belongs).

Maybe the tire I mentioned that will generate one g at 750 lbs only generates 0.9g with a 500lb load. That is actually how it works. A specific tire will have an optimum amount of grip based on load and pressure. But unless you're under-tired, less weight might not actually reduce stopping distances. My car, for example, is probably over-tired for most situations with 225-50/16s. They're heavy and wide and make my car slower. Mostly I have them because it makes me feel cool. For all I know the optimum grip level occurs at higher loads and temperatures than my car is capable of generating. It's possible that if I dropped to a 205/50, or added weight, I would see shorter stopping distances (no not really, it would have to be a really extreme case for that to be true).

But, the #1 principle of handling is that less weight transfer equals more grip. Less weight means less weight transfer, so it's never a bad idea to lighten the car. For my next rallyx my roof rack, driving lights, rear seat, trunk carpet, and a/c are all coming out so that I can be faster.

In conclusion,

Less weight could theoretically decrease stopping distance, but with good tires the amount of weight you could drop probably won't make a difference. Weight loss, however, is a good idea. It will improve handling and acceleration and probably braking, so do it if you can.

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Post by skid542 » Tue Mar 04, 2008 3:13 pm

F = m*a

F in this case is the force your tires can generate. If you decrease your mass you will increase your deceleration. Increasing your deceleration will decrease your distance.

So now the question is, does F reduce more than m, by percentage.

I'm putting my bet on mass and that your distance will decrease. Search for braking under my user name and you will find my results with my old FWD Legacy that curbed at 2700lb.
Lee

93' SS, 5mt swapped, 182k, not stock...
96' N/A OBW 5sp, 212k, Couple mods... RIP
99' N/A OBW, 4eat, mostly stock.

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Post by Legacy777 » Tue Mar 04, 2008 8:50 pm

The only catch to reducing mass (weight) is that the friction force is dependent upon the normal force (same as weight). The friction force is what gives your tires the ability to stop, turn, accellerate.

Ideally, you'd want to reduce weight and increase friction force. The way to do that would be to go with stickier tires.

This link has a neat little animation/description.
http://regentsprep.org/Regents/physics/ ... efault.htm
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Post by jamal » Wed Mar 05, 2008 6:12 am

skid542 wrote:F = m*a

F in this case is the force your tires can generate. If you decrease your mass you will increase your deceleration. Increasing your deceleration will decrease your distance.

So now the question is, does F reduce more than m, by percentage.
Actually F = µ * N

F being the force the tires can generate, µ being the coefficient of friction of the tire, and N being the normal force on the tire. So if the tire can generate one g of braking force, it doesn't matter how much weight there is. A heavier car will have more kinetic energy, which translates to more heat in the brakes, but should stop in the same distance.

Of course, the coefficient of friction of a tire on a given surface varies with load, pressure, and temperature, so in the real world less weight could possibly result in a shorter stopping distance. But the amount of weight you could drop realistically compared to all the other variables is why I left it out of the original post.

Also, to stop from 70 in 100' requires nearly a 1.7g deceleration rate. I think R-compounds have a Cf somewhere in the neighborhood of 1.4.
Last edited by jamal on Wed Mar 05, 2008 9:26 am, edited 3 times in total.

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Post by skid542 » Wed Mar 05, 2008 6:43 am

The friction of the tires is the force that creates heat to dissappate the energy. It is the dissapation of the energy that stops the car. This heat disappation occurs at two variable locations within the system, all others are factors that are essentially a constant i.e. bearings, drive train, etc.

Your tires will disapate the heat between the rubber and the road.

The other source of dissapation occurs between the rotor and the pad. If the friction of the tires were the only factor effecting braking distance then upgrading to a large rotor with a higher thermal mass/dissapation rate would have no effect on stopping distance. Larger rotors have been proven many times to have a measurable effect on braking distances.

You are correct that kinetic energy is a function of mass. Hence if you reduce the mass you reduce the engery you have to dissapate. Hence a shorter stopping distance.

Finally, and please understand I mean this with all due respect, I offered my measured observations for the benefit of the community. I will quit making references to my study as over the course of time here it has been made clear that no matter how many times I have justified and explained my process, it simply is not accepted.

Mods, I do not wish to dirty a great resource thread, please feel free to trim my responses if you see fit.
Lee

93' SS, 5mt swapped, 182k, not stock...
96' N/A OBW 5sp, 212k, Couple mods... RIP
99' N/A OBW, 4eat, mostly stock.

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