There seems to be alot of information here on why you shouldn't look to go for bigger rear rotors and better rear calipers saying it will throw things off.

Could something as simple as better rear pads be just as a bad? (i.e. The Axxis Ultimates like those in the Stoptech front brake kit) Could rear stainless lines be just as bad? At which point have you gone too far?

 

My thoughts are that the rear brakes need an upgrade. No matter what they are non vented and very small. Since I am a road racer and hold a competition drivers license I feel I know what I am talking about. I have raced my WRX and have witnessed what happends to rear brakes on a stock setup.

My beliefs are that Gary is not having rear brake issues because he is running the stop tech kit up front. This takes a tremendous load off the rear brakes on a race track. He has also changed his rear pads out to a pad that can cope with the higher temps the track produces. I would also be willing to bet Gary is using some expensive high temp rated fluid such as Motul 600. All of this increases the rear brakes resistance to fade but the main thing is that his front brakes are taking the load off the rears.

That said, I will explain the scenario once more assuming a stock car. Again, this is my opinion only because after road racing my car for three events this is what I witnessed... Front brakes fade due to the stock pads not being able to cope with the heat. The rotors are small which does not help heat disapation. There was never a lack of brake torque with the stock brakes, rather fade from heat. When the front brakes diminish the rear brakes start to pick up the slack. Physicaly this is because I had to push the pedal farther and harder because the front brake performance was going away.

After a few 40 minute sessions what I found was Glazed front rotors, Perfect front pads other then some surface glaze. Rear rotors where cracked and pads where reduced to half the thickness along with missing chunks one fell apart in half when it was removed from the caliper.

The point of all of this is because I wanted to explain that I believe people are spending money upgrading the rear brakes for the wrong reasons. Yes, I said the rear brakes good stand to be upgraded above in this post but For different reasons. If you upgrade the fronts like Gary did this problem goes away on the rear because a great deal of work is now not being asked of them.

I do believe the rear system could stand to be upgraded for a few reasons if we are talking about a race car that will see track duty. One being weight savings, Two would be heat capacity and three would be brake control. The little rear rotors and small pad contact do not provide the best control and feel. In fact they are on the verge of lock up many times on my car during hard braking. The last reason would be pad choice.. Many aftermarket kits open up a whole new world of pad choice to test and to play with.

In the end I did not go with the Stop tech kit on my car for two reasons. They offer no rear upgrade path and when called they said they do not plan on it due to demand. Two, I feel the piston sizes on the stop tech front ST40 calipers are not the correct size for the WRX. This is one of those things that is just personal preference. My opinion is that the brake pedal requires way to much effort with the stop techs. While one person may like this and spout back things they have read on the internet about how this is an advantage, I dont believe it is. Initial bite of the stop techs is in my opinion slow and effort is just past what I feel confortable with. I spoke with Stoptech about this through a friend and they offered to custom machine different sized pistons in the calipers for my needs. Of course at an extra cost. Ever since they have offered to do that I have been thinking about going with the stop techs in the back of my mind. They are a nice kit and a classic Brembo Copy with some improvements. Rotors seem to be nice as well and many are happy with them. Then again. I would still have to run stock rear brakes or another vendors rear kit and then possibly have to install a prop valve which I will not do on a car that sees street duty.

my .02

Clark

 

Hi all. New guy here on the board, so please be easy...

My name is James and I am a brake system consultant for STOPTECH. I don't work for them full-time, but I get called in for this kind of thing on a semi-regular basis. Before I answer a bunch of the posts on this board, let me throw out a couple of credentials so you don't think I am a complete ****** (you might anyway, but that's your decision...)

I have been in the brake engineering and brake controls world for a while now. Past employers include Delphi, Kelsey Hayes/TRW (ABS calibration), Saturn/GM (ABS/TCS/base brakes), Bosch (brake control systems), and Ford (brake control systems and brake actuation). I also am a regular guest lecturer (topic: Braking Systems 101) at several Engineering schools and have presented my "travelling brake road show" for SAE. As previously mentioned, I also consult for STOPTECH and have had materials published in several automotive magazines.

I also hold my SCCA Club Racing national license and have been road racing since 1997. My team (I am the driver) has been to the Runoffs (ran in the top 10), has been to the ARRC (4th place in 2001), and has broken track records at Road America and Watkins Glen. In addition, with my degree in vehicle dynamics I was a test driver instructor at both TRW and at Bosch and frequently instruct at club track days in my 2002 WRB WRX Wagon.

None of this makes me an expert nor does it mean you should listen to a single thing I have to say, but I hope I am out of the '******' category.

Now, to address a few of the comments on this thread:

1. STOPTECH balances every one of their brake systems to ensure that the front/rear bias is optimized for every platform. Of course, the factory bias is the logical starting point, but as every car is a compromise, STOPTECH tests multiple piston sizes on every car to ensure that the factory bias was optimized as well. In most cases, the mathematical bias generated by the STOPTECH brakes is within 5-10% of the factory setting, but is always finely tuned for the auto enthusiast.

In other words, for a given line pressure the STOPTECH brakes will generate the same amount of torque that your stock bakes would. We call this maintaining the P-T relationship. This is quite different from some manufacturers who build (or buy) a single caliper and bolt it on to several applications without taking this critical sizing implication into consideration.

2. On a front-engined, production-based street/race car (i.e. not a tube-frame creation), there is rarely ever a need for "more rear brake." Our tech article found here...

http://www.teamscr.com/rearbrakeupgrades.html

...goes into detail behind this bold statement. It speaks for itself pretty clearly.

3. If you are contemplating a rear brake upgrade because (a) your front brakes fade meaning (b) your rear brakes need to work harder so (c) your rear brake fall apart, isn't a better solution to upgrade the front brakes? Upgrading the rear brakes will not address the root cause of the problem in this case...

To make a long story short, on a typical front-engined, production-based street/race car (like a WRX), under limit braking the front brake will be providing up to 90% of the total deceleration capability of the vehicle through its tires. When you compare the biased torque output and thermal masses of the front and rear brakes, it becomes pretty apparent that for the amount of 'work' to be done, the rear brakes are sized much more generously than the fronts. Sure, there are rear kits out there and people will buy them for whatever reasons the marketing people will conjure up, but for the educated customer, there is a better solution.

Finally, just look around the paddock at any production-based race series event - Gary's car is a great example. Don't you think that if bigger rear brakes was going to make them faster they would do it in a heartbeat? After all, racing is about looking good on the podium, not in the paddock.

Heck, look at the fabled Car & Driver test WRX - with an adequately sized front kit, the car never faded, period.

We could go on and on about this topic, but what it comes down to is this: if you want big rear brakes, go ahead and do it. We agree that they will most likely look sweet as hell, but don't expect the car to stop any faster. The laws of physics don't care how good your car looks, after all.

This is a great thread. Keep the questions coming, as we all need as much good information as possible to make the best decisions for our own, unique applications. As it may sound, there is no 'right' answer out there - just the one that is the best compromise for YOU.

 

James. Thanks for the long post. It was filled with information. I have a question that maybe you can answer. If I did decide to go with Stop techs and take them up on the offer to use smaller pistons to reduce pedal effort would that in turn knock the balance out of whack? Again, Its just personal prefference but the pedal effort is just to hard for my tastes.

Clark

 

What if you didn't use the stainless lines?

(and put some air/water in them for the extra squishyness)

I was serious on the first question.

have a nice day.

p@
www.achtuning.com

 

Replying to your questions...

Q: Clark - If I did decide to go with Stop techs and take them up on the offer to use smaller pistons to reduce pedal effort would that in turn knock the balance out of whack?

A: Yes. Absolutely. However, if your goal is to REDUCE pedal effort, then you would be asking for LARGER pistons in the calipers. Currently, the WRX kit is sized to approximately 90% of the stock front brake torque reaction for a given brake line pressure, and the next step up in the piston food chain would land you at close to 98%. Because in the WRX's case the factory bias was a bit too conservative for the enthusiast driver, we chose to design the bias a bit farther rearward than stock. By going back to a stock bias not only will you ultimately impact minimum stopping distances, but your pedal TRAVEL (not EFFORT) will also go up because of the increased piston seal area.

If lower efforts alone are your goal, a better path to take might be to select a slightly more aggressive or less compressible (yes, they squish) brake pad. Now, this will also impact the balance, but it's a more 'reversible' solution than buying a pair of new calipers. Note that when changing pads there is always a new noise/wear/dust/bite/effort set of expectations, and the pad that gives you the effort you desire might not meet your other needs and/or wants. But hey, it's easier than changing piston sizes.

 

Still replying to your questions:

Q: Akiata - What if you didn't use the stainless lines?

A: SS lines really only have a few impacts on the braking system:

1. The SS braid protects the nylon tube from damage (the reason they were invented in the first place).

2. The decreased compliance of the SS lines can (depending on the application) decrease the mechanical response time of the brake system. This is 'felt' by the driver as a 'quicker' or 'firmer' response when the pedal is applied because we are expanding less components prior to the building of pressure.

On some cars, this decrease in response time can actually have a measurable impact to minimum stopping distances, for the time to reach optimum brake system pressure is reduced. In English, you can get to max decel faster. However, a car with three feet of stock rubber hose will probably notice a much bigger impact than a car with six inches of stock rubber hose. The impact to every car is different. Usually, it's always a positive impact, but the extent of improvement can vary quite a bit.

3. Tied to #2 above, the decreased compliance of the SS lines can (depending on the application) decrease the amount of pedal travel required to reach a given system pressure. This is 'felt' by the driver as a 'higher' pedal because we are expanding less components prior to the building of pressure...therefore it takes less fluid volume to achieve a given pressure.

Finally, from our website article on braking systems:

"As pressure begins to build in the braking system, the various components in the system will flex until all clearances have been taken up. During this time seals, clearance parts, and other flexible components will stretch and deform, effectively increasing the hydraulic volume in the brake system.

To picture this, imagine blowing up a balloon inside a pop can. The balloon will expand freely until it comes in contact with the sides of the can. This is the stage of compliance…and the bigger the can, the greater the compliance. Once the balloon has taken up the volume of the can completely (similar to the brake system components completing their flexing), it gets much, much harder to blow more air into the balloon. This is the stage of pressurization.

In the braking system, compliance is highly undesirable - because it requires extra hydraulic fluid to fill this increased volume before pressure can build. In the pop can analogy, one would want the smallest pop can possible in order to minimize the amount of time required to get past the compliance stage and directly into the pressurization stage."

Link to the full article here:

http://www.teamscr.com/grmbrakes.html

So, to answer the question (at last, right?), by NOT using SS lines, you lose the benefits of 1, 2, and 3 above. However, in the case of the STOPTECH front BBK, the lines also provide the correct LENGTH of hose, so mechanically you need them to make sure the hose is not ripped from the body bracket or from the caliper itself. You NEED the front lines, which is why they are part of the kit.

In the rear, it's up to you. Benefits, yes. Greatest thing since sliced bread, no. On the WRX it is a noticeable impact, but is far outweighed by the impact of the front kit. Do both, however, and you never have to think about it again.

 

Still replying to your questions:

Finally, replying to the comment "Then maybe look into a master cylinder with more pistons or smaller pistons"...we explicitly do NOT recommend using master cylinders of a non-stock arrangement with our BBK's without consultation. Because we strive to maintain a stock balance (within our optimization window), we also by default attempt to maintain the stock pressure-volume (P-V) ratio. Switching to a master cylinder with a smaller piston diameter can certainly 'firm up' the pedal feel, but if the stroke is not sized accordingly, you can actually bottom out the pistons prior to achieving the maximum brake line pressure required for a worst-case condition of thin pads/worn rotors/aging brake lines/old fluid/etc. The master cylinder is a variable that we do not advocate changing without a ton of research and experience.

Hope this answers all of your questions. Sorry for the length, but we feel that an educated owner is our best ally

 

I need to get this added by default. Oops...

james walker, jr.
teamscR@flash.net
http://www.teamscR.com

 

Thanks James and Gary! It is interesting to note that Stoptech did feel that there was a some gain to be had by some rearward transfer of braking capacity.

Jame I posted these questions earlier in the thread, could you give them a look?:

[I have had so many Subaru brakes on hand at one time(5 complete front and rear sets about 2 months ago) that I have been amazed at how Subaru interchanges components on vehicles with widely different weights, weight distributions, and tires. It makes me wonder a couple of things: 1. How sensitive is the braking system to interchanging? I mean does Stoptech make a different front brake setup for the GDA vs the GGA WRXs? RS vs WRX? 2. If this a highly sensitive system where do the manufacturers fine tune out the differences? The proportioning valve? For instance, the GC series of Imprezas came with front brakes ranging from 1pot 260mm to 4pot 294mm and rear brakes ranging from drums to 290mm 2pots.

I would postulate, while I know there is some variation is piston diameter in an apples to apples comparison of Subaru brakes (ie 2pot fronts vs 2pot fronts) there is not enough of that to account for the wide variety of models upon which the same brakes are placed. This leads me to believe that one of the following things is true:

1. Subaru custom designs proportioning valves for each seperate front to rear to vehicle brake setup.
2. Small changes in weight, weight distribution, equipment doesnt upset the apple cart enough to be a problem.]

Focus on the weight distribution parts please. Wagon vs sedan of the same model etc. Stoptech does recommend their kit on both the GDA and GGA correct? Meaning the wagon is gonna have more weight on the rear tires....

k.

Again great discussion.

 

Kobayashi -

Like any manufactured device, brakes are engineered to give the best value to the end user. For this reason, when a car is being designed the manufacturer will many times select brake components from other existing platforms which save cost and reduce engineering workload. If every manufacturer had to redesign an entire brake system for every possible body/chassis/powertrain combination, we wouldn't be seeing new models every four years.

That said, when you pick and choose from the bins of parts, you don't end up with exactly the most optimum set-up for each individual combination (sedans and wagons, for example). So, in order to fine-tune the hard parts of the system (calipers/rotors/etc), the manufacturer will generally use friction materials (brake pads) and the proportioning valve to 'dial it in.'

Even with this fine-tuning, all of the factors in question can certainly have an impact to the optimum brake distribution (bias). Heck, just loading three of your friends and their luggage for a weekend trip throws the loading off the deep end of the scale!

Consequently, when the factory bias is established, it is usually (almost always) set for a worst-case condition of front weight. This means driver, no passengers, and just about no fuel. If the car is designed to be front-biased in this condition (which it is per government regulations), then it will only get "safer" as rear weight is added. In other words, it will become more and more front biased as rear weight is added. The manufacturer really must make sure that the car never goes rear biased. It's not quite that cut and dry, but for the sake of this forum, it will do.

Now, when you go and start making changes to your braking system you can certainly change the bias. Pads, lines, BBK's...they all have an impact. However, your factory bias has some robustness built in, so not every change will put you in a world of hurt. However, something as radical as a caliper and rotor swap can certainly have a negative impact if the change was not well thought out.

This is why the STOPTECH kits usually run a little more rear biased than stock, but not by much. We take up the factory fudge factor and give the enthusiast every bit of braking power we can. This is also why we do not like to deviate from our proven set-ups...we know from our testing that stopping distances will go up every time if we change the recipe.

Because of the built-in factory robustness, we also are able to sell a product that works in several applications. For example, our WRX kit is suitable for both sedans and wagons. The differences between the two are small enough that our testing confirms this kit is appropriate for both platforms. However, because the non-WRX models have different base brake systems, we do not sell this same kit for those platforms, even though the parts would physically bolt up.

So, how much is too much? Don't know...every car is different. This is why we test every kit we sell - we KNOW that if you buy STOPTECH hardware the parts are a well-matched system. While others may follow this same practice in whole or in part, with our stuff there is no second-guessing. It's right.

Hope this answers your questions. Enjoy!

james walker, jr.
teamscR@flash.net
http://www.teamscR.com

 

now thats what I call laying some smoke down.

Well done James!

That is an excellent - no BS - cut to the chase answer. I had thought much of that to be true but you definitely cleared a lot up. So the Stoptech kit is really an exclusive WRX part(GDA or GGA,) that does make good sense. That also excludes my Legacy.

Your analysis makes perfect sense, I can understand the logic of testing a nearly empty car with a single occupant(driver.) Such that if the rear brakes wont prematurely lock in that circumstance, weighting the rear moves even farther from prematurely locking them.

That does help explain a bit of the behavior of my wagon too. I usually do drive alone, but consistently refill the tank, have some extra tools in the storage area as well as a ~50lb trailer hitch hanging out the back. Even with the 290mm rear rotors I still got consistent front lock first. And the "feel" is quite a bit different, before under heavy braking the car always felt a little like it was gonna pirouette around the nose. Now its kinda flatter, like the whole car is into this braking thing together. I am pleased with the result, but now I have a better idea how and why I got there.

So lets just say for argument's sake that someone with a WRX sedan(stock front brakes) and a full tank of gas and maybe a DynaBatt up front and a full size spare or some subs in the trunk might be ok with the 290mm rotors out back. Cmon you could speculate, we wont hold you to it .

k.

Oh and I had posted earlier I would measure the piston diameter in my GT-B rear brakes. It appears to measure out to 1.5", exactly the same as the piston size in the stock WRX(and several earlier) rear calipers.