GarySheehan

iTrader: (1)

Romanom,

I'm doing some research to make sure I bring the right information to this discussion. I am fairly positive that the Stoptechs do not increase front bias as the piston diameters have been sized to counteract the additional torque arm from the larger rotor. Meaning the piston sizes are larger to require the same pedal pressure as stock to generate the same torque on the wheel. I'm inquiring to find out how closely they have gotten it.

Why would you ever want to increase front brake bias over stock? The manufacturer already biases enough brake to the front to make it incredibly safe (i.e. you will never prematurely lock the rears). There is already enough torque in a stock system to lock the front brakes. More front brake torque will not slow the car down any faster.

About the proportioning valve, it does change brake torque. The prop valve decreases the line pressure on the output side of the valve. This decreases the torque the rear brakes apply to the wheels. It's not a flow limiter, it's a line pressure limiter.

Regarding wearing out the rear brakes, that occured only when we ran out of front brakes at the beginning of the race. Of course the rear brakes wore out. They were the only thing slowing the car down from 130mph. My right leg wore out as well. I've never had to press the brake pedal so hard for so long...


AaronC,

The Stoptech kit should not have any more torque than stock. The stock setup already has more than enough torque to lock the front brakes, even with racing tires. The braking performance would not benefit from more front brake torque.

Gary
Sheehan Motor Racing
www.teamSMR.com

GarySheehan

iTrader: (1)

Romanom,

It is my understanding that a brake proportioning valve is not like the valve on a faucet. It is actually made up of a floating piston in-line with the fluid in the brake line. Once a certain line pressure is reached, the fluid displaces the piston (which is counteracted by a spring). The rear brake line pressure will continue to increase, but at a predetermined rate that is less than the front brake lines.

So once this point is reached, the rear brake line pressure increases at a slower rate, shifting brake bias to the front. This piece definitely limits the torque generated by the rear brakes and prevents rear lock-up.

Gary
Sheehan Motor Racing
www.teamSMR.com

AaronC

Here is one of the best articles I have read discussing brakes.
http://dsr.racer.net/brake_bias.htm

AaronC

There seems to be some misconceptions about the functionality of the proportioning valve. Here is a some information about them. I got it from http://www.thebrakeman.com/valvetechii

The tech line at "The Brake Man" gives us a great opportunity to find out many of the misconceptions that exist in the real world. It seems ironic that in both the oval track racing industry, and the street performance industry, a new disease among the owners and drivers has sprung up. This disease is called "proportioningvalveitis", (owners overwhelming desire to install a proportioning valve for no particular reason, other than he thought he ought to have one because he thought everyone else did). The only characteristic of this disease is that a valve is purchased and installed without bothering to find out if it is needed. In street performance cars, where consistent braking is often the key to a fast performance car, they are NEVER a good idea, because there are better, more consistent ways to modify brake pressure on a car. For example, on most vehicles I checked, about 4 out of 5 now in use are causing more problems and system imbalance than they fixed.


The proportioning valve is the most often misused valve in the brake system. The most important thing to understand about the proportioning valve, is that is was never intended to be used in every brake system, in fact, NO VALVE WAS MEANT TO BE IN EVERY BRAKE SYSTEM. Just like a hammer in your tool chest, you only should use it when you need it. In fact, the chances are far greater that you don't need one, than you do. As with all valves of the brake system, the proportioning valve performs one function for one specific situation, and that's all. You must first see if your system has a problem that a proportioning valve will correct. That problem is imbalance, and even then, the proportioning valve is only one way to fix it. The best way to understand what a proportioning valve does is to look at a graph of pressure output of a brake system over the entire pressure range. Please note the graph in figure one shows a disc front, drum rear combination. This is really the only combination that can best benefit from a proportioning valve, and even then, only if certain conditions exist.


As you can see, in a system without valves on a stock vehicle, the front brake torque is simply represented by a straight line. This straight line represents the increase in torque that is produced by a disc brake. That is to say, pressure in is a linear function when compared to torque output. The curve representing torque from a self-energizing drum brake is represented by the curved line, which indicates two interesting characteristics of a drum brake. First, you can see that the disc brake started creating brake torque as soon as applied. In comparison, however, the drum brake does not begin to produce torque until the system pressure is over 100 pounds. Why? Because the rear drums have strong return springs to hold the shoes away from contact with the drum. Until the line pressure overcomes the spring tension, the shoes will not even move into contact with the drum. Once that does happen, the torque rises dramatically, even faster than the input of line pressure. This is because, on a drum brake, there is a characteristic known as self-energizing, which means the shoe is actually pulled into the drum surface. This causes the pressure to rise much faster than the increase in the disc brake torque and the lines actually cross at about 1000 pounds per square inch.

AaronC

A stock vehicle, in most cases has more of the weight on the front wheels. When you combine this with the fact that there is an approximate weight shift of another 10% of the vehicle weight to the front during braking, you can see the rear brake will lock up first if this condition is not corrected. When the rear brakes lock up on a vehicle, the rear wheels are now traveling faster than the front wheels, which are still in full contact with the road. The result is a car that has the back end pass the front end, better known as "spinning out".


Now before you panic and go buy a proportioning valve for your car, you need to know one additional piece of information. You may not need it. Why? Because the height, (not the width), of the tire on the back of the car must be compared to the front tires first. Take a measurement from the center of the tire, (where the spindle is located), to the ground, both front and rear. This will tell you the "rolling radius" of the wheel. If the rear is greater than the front, then you will need to leave the line pressure higher on the rear because, the larger the tire, the more brake required to stop it from turning. If the tire on the rear is substantially larger than the tire on the front, then major increases in rear brake pressure are required.


When a vehicle is built at the factory, the brake system is balanced by the size of the caliper and rotors chosen for both the front and rear. Unless you are removing rear drum brakes and upgrading to disc brakes, the valves will most likely not require changing. However, if you do remove drum rears in favor of discs, take note of two important items.


You must locate and remove the 10 pound residual valve. It will ruin the rear disc brakes. Make sure you do not improve rear brake performance when making this change, because if you install better rear brakes, the front brakes must be improved too.

If you study the graph in figure 1(below) closely, it will alert you to another important and potentially damaging problem. Under low pressure stops, (those normally encountered under 30 mph, cruising situations), the front brakes are doing all the work in stopping the car because the rear drum brakes have not yet received sufficient pressure to overcome the return springs. This will result in extremely premature front pad wear. In a stock passenger car, imbalance is corrected by a metering valve. The metering valve stops the first 100 pounds of pressure from reaching the front brakes thus allowing the rear brakes to overcome the return springs and apply at approximately the same time as the front. This is the valve that is truly needed to balance the system.

Figure 2 illustrates how torque output from the front and rear brakes compare when a metering valve and proportioning valve are installed on a stock system. Don't forget that large tires on the rear would require more, and that the proportioning valve would be robbing the vehicle of available brake it really needs.


A proportioning valve NEVER gives you more brake, it only takes away available brake effort. You should be sure that your situation benefits from a less brake condition.


Since many of the most important factors in balancing the brake system are never the same from one vehicle to the next, (weight ratio, tire size etc), the only proper way to determine whether you have developed a well balanced system is to first plan what you need. Then, based on your system components, test it when you feel you have it correct. Take the car to an isolated location so you can perform several hard decelerations. Start at low speeds, 20 mph, and perform a fast stop, similar to one you may do if a light turned red and you were very close to the intersection. Then try the same stop at 30 mph, then 40 mph, allowing the brakes to cool for 5-7 minutes in between. Continue this up to 60 mph, or any speed you expect to encounter during the vehicle's actual usage. If at any time you encounter premature rear wheel lockup, or front wheel lockup, then the system is not yet properly balanced. If the rear wheels lock up, then you will need a proportioning valve to reduce rear pressure. Most of you will find this is not the case and no proportioning valve will be needed. The biggest problem I am seeing right now in getting safe brake systems on any vehicle, is the approach of the car owner. It is not enough just to get the car to stop. It must be balanced throughout the range of speeds and pressures. There is also a misconception that taking a complete system off a stock car will give the owner everything they need to set up a safe system on their vehicle. Nothing could be further from the truth. Unless your car has exactly the same weight ratio, tire size, wheel base, center of gravity, etc. etc. the system will have to be modified, maybe excessively, to work on the new machine.

AaronC

In stock production cars, the proportioning valve and metering valve are usually combined into what is referred to as a combination valve. Different manufacturers combine different valves into the combination valve. In some cases they're combined with a failure warning light switch. In other cases, they are combined with the residual valve. The two most normal stock locations for finding these valves are: 1) on the firewall towards the center of the car from the master cylinder, 2) On the left frame rail usually about even with the front tire.
In both cases the valves can be located by tracing the brake lines from the master cylinder. These valves should not be used unless you are positive that all system components in that combination valve fit your needs, exactly.
Figure 3 and Figure 4 have been included to show you the relative torque outputs of an all disc system and an all drum system. As you can see, since in both cases, the lines already follow each other closely, a proportioning valve would do more harm than good.
There is an old saying, "when you are up to your butt in alligators, it is difficult to remind yourself that your initial objective was to drain the swamp". If you apply that logic to your brake system, remember that your objective is to balance the brake torque to each wheel, based on the weight on that wheel during deceleration. In other words, you want each wheel to do its proportionate share of stopping the vehicle without locking up prematurely. If you don't have the ability to consider these factors, then get someone knowledgeable to help.
I think we all share the concern of wanting our car to stop safely, especially our own and those immediately behind us. If you have built a project car, and are proud as can be of how it looks and runs, add one requirement to your goals. Make sure it will stop under any, and all conditions. Sometimes it only means a little more work and attention to detail to make that happen. The proper combination of valves will make this happen. Do not install parts you are not sure you need. When an owner calls and asks my advice on which components to use, I must look at that car and its individual merits before answering. In order to achieve the best results, you must do the same. A doctor must treat their individual patient's particular needs. If you attempt to grab a generic remedy, you will probably not be happy with the results. Your car is most likely one of a kind. The brake system must fit its needs.

FIG. 1. THIS GRAPH ILLUSTRATES WHAT KIND OF TORQUE OUTPUT WOULD BE CREATED BY A TANDEM MASTER CYLINDER PRODUCING THE SAME PRESSURE TO A FRONT DISC, REAR DRUM TYPICAL BRAKE SYSTEM WITHOUT ANY TYPES OF VALVES PRESENT. AS YOU CAN SEE, THE DISC BRAKE FRONT WOULD IMMEDIATELY BEGIN TO GENERATE TORQUE, WHILE THE REAR DRUMS WOULD BE CONSISTENTLY LOWER UNTIL THE PRESSURE REACHES ABOUT 1000 POUNDS LINE PRESSURE. AT THAT POINT, IT WOULD ACTUALLY BE PRODUCING MORE TORQUE THAN THE FRONT BRAKES. SINCE 1000 POUNDS LINE PRESSURE IS ONLY SEEN IN SEVERE PANIC STOPS, THIS WOULD RESULT IN REAR WHEEL BRAKE LOCKUP SINCE THE WEIGHT HAS SHIFTED TO THE FRONT WHEELS. THIS IS CLEARLY AN UNSAFE SYSTEM!

FIG. 2 THIS GRAPH ILLUSTRATES THE SAME CONDITIONS AS FIGURE ONE, ONLY NOW WE HAVE ADDED A METERING VALVE AND PROPORTIONING VALVE. AS YOU CAN SEE, THE METERING VALVE DELAYED THE FRONT BRAKES UNTIL THE REARS HAD A CHANCE TO JOIN IN THE JOB AT ABOUT THE SAME TIME. ALSO, SINCE A PROPORTIONING VALVE HAS REDUCED THE AMOUNT OF PRESSURE THAT THE REAR BRAKES RECEIVE, THE TORQUE OUTPUT HAS RISEN MUCH MORE GRADUALLY, REMAINING LOWER THAN THE FRONT THROUGHOUT THE CRUCIAL RANGE.

FIG. 3 THIS GRAPH COMPARES AN IDEALLY BALANCED BRAKE SYSTEM THAT HAS 4 WHEEL DISC BRAKES. A METERING VALVE IS NOT NEEDED TO STOP PRESSURE TO THE FRONTS, SO IT IS NOT PRESENT. THE RESULT IS A BRAKE SYSTEM THAN BEGINS TOGETHER AND CLIMBS APPROPRIATELY. THE REAR DISC BRAKES HAVE SMALLER PISTONS, WHICH RESULTS IN A LINEAR REDUCTION IN BRAKE TORQUE. THIS IS EXCELLENT, BECAUSE AS THE STOP BECOMES MORE SEVERE, PRODUCING HIGHER LINE PRESSURES ALSO MEANS THAT THE WEIGHT ON THE REAR IS SHIFTING TO THE FRONT. AS THE REAR GETS LIGHTER, LESS BRAKING FORCE IS REQUIRED TO STOP THE WHEEL FROM TURNING. SINCE THE SMALLER PISTONS ON THE REAR CALIPERS PRODUCE LESS TORQUE, NO PROPORTIONING VALVE IS NEEDED FOR BALANCE.

FIG. 4. FIGURE 4 ILLUSTRATES A FOUR WHEEL DRUM BRAKE VEHICLE PROPERLY BALANCED. SINCE BOTH THE FRONT AND REAR WHEELS HAVE RETURN SPRINGS, NO METERING VALVE IS REQUIRED TO DELAY EITHER END. ALSO, SINCE BOTH HAVE A SELF ENERGIZING DRUM BRAKE CHARACTERISTIC INCREASE IN TORQUE, ONLY A SMALLER WHEEL CYLINDER IS NEEDED ON THE REAR TO PROPERLY BALANCE THE SYSTEM. HENCE, NO PROPORTIONING VALVE IS NEED HERE EITHER.

GarySheehan

iTrader: (1)

Our cars come with proportioning valves from the factory.

Gary
Sheehan Motor Racing
www.teamSMR.com

Wingless Wonder

iTrader: (7)

I still don't understand why it doesn't automatically append when I specify it in my preferences. Sorry to go off-topic, but I want to try this out...
__________________
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"Go wingless, young man." - Horace Greeley

AaronC

Not adjustable ones though.

GarySheehan

iTrader: (1)

AaronC,

Right. There's no need for an adjustable proportioning valve for a streetcar. The prop valve is to balance the front and rear braking bias. Once it's figured out for the front and rear caliper design and rotor diameter, it's set. No need to adjust it.

Unless, of course, you change the brake system. Well, MOST brake systems. The Stoptech is sized to work with the stock WRX prop valve and rear brake calipers and rotors.

The only reason I have an adjustable proportioning valve in my racecar is to compensate for dramatically different conditions. If it's raining out, I can't brake hard enough to transfer a lot of weight to the front of the car like in the dry. So, putting a little more bias to the rear will improve the overall braking performance for that situation.

I can also add in a litte more rear braking if I'm having difficulty getting the car to rotate into a corner. BUT, this is only a bandaid and is a bit dicey as it forces the rear of the car to lose a bit of traction when what you really want is the front of the car to turn in better.

Interesting post, Aaron. I had no idea that there was a big interest in installing aftermarket proportioning valves in streetcars. As the article stated, it's just not necessary and is actually a bit dangerous.

Gary
Sheehan Motor Racing
www.teamSMR.com