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  1. #1
    Senior Member BasicQ's Avatar
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    Default Some interesting reading on Brakes

    BRAKE BIAS AND PERFORMANCE
    by Tom McCready and James Walker, Jr. of scR motorsports.

    Long, long ago in a magazine far, far away, a few renegade brake engineers rallied together to bring forward the following message:

    “You can take this one to the bank. Regardless of your huge rotor diameter, brake pedal ratio, magic brake pad material, or number of pistons in your calipers, your maximum deceleration is limited every time by the tire to road interface. That is the point of this whole article. Your brakes do not stop your car. Your tires do stop the car. So while changes to different parts of the brake system may affect certain characteristics or traits of the system behavior, using stickier tires is ultimately the only sure-fire method of decreasing stopping distances.”
    However, there’s more to the story. Yes the tires stop the car, but improper brake balance can make a complete mess out of even the best components.
    There’s always a “but”, isn’t there?

    In order to demonstrate the concept of proper brake balance, it is usually simpler to analyze a car’s handling characteristics and then apply those principles back to the braking system. (For some unknown reason, people seem to have a much better understanding of handling than they do of braking. Brake guys think that’s not fair, but we’ll try to use it to our advantage here.)
    In theory what everyone is looking for is that all-too-elusive handling balance which makes the car corner as fast as it possibly can. Generally speaking, this is referred to as the ‘neutral’ car and takes the driver directly to victory circle following the race. Rarely do we ever hear of a winning driver explaining that the car was a handling nightmare.
    Of course, no car is ever perfect, so we have ways of expressing how far from optimal the handling balance really is. When a car enters a corner and the front end skids off into oblivion, this is called understeer – the car is turning less than the driver intends. On the other hand, if the rear end breaks free and begins to lead the car through the corner this is called oversteer – now the car is turning more than the driver intends.
    In both cases, when one end of the car breaks traction, or begins to slide, the driver can pretty much bet on the fact that he (or she) has found the maximum cornering speed for that particular corner. Yes, there are a million other factors at play which can govern the handling relationship, but the longer each end of the car can “hold on”, the higher the cornering speeds. Conversely, if one end or the other consistently breaks traction early in the cornering event, corner speeds will suffer dramatically.
    Naturally, as speeds continue to increase something has to eventually give and slide; however, the very best suspensions do a great job of ensuring that both ends of the car break traction at relatively the same time. How far one end breaks traction in advance of the other is ultimately a function of driver preference (this is just one reason why there is no single “perfect” set-up), but if there are complaints of heavy understeer or terminal oversteer you can rest assured that one end of the car is three steps farther ahead than the other.
    Umm…isn’t this an article about brakes?

    So, now that we are all chassis tuning experts, let’s look at how this information can be used to understand our braking system. Grab a pop and a bag of chips and hang on.
    Like the corner carvers, the brake guys are always looking to achieve maximum accelerations, but of course these accelerations are now really decelerations. Stopping distance is everything and every single foot counts. Remember: outbraking your opponent by just two feet every lap for a twenty lap sprint race can result in a three to four car length advantage at the checkered flag. Attention to detail matters.
    As braking force is continuously increased, one end of the car must eventually break traction. If the front wheels lock up and turn into little piles of molten rubber first we say that the car is “front biased”, as the front tires are the limiting factor for deceleration. In the not-so-desirable situation where the rear tires are the first to lock we say that the car is “rear biased”, but the driver would probably have a few more choice adjectives to add. In either case, however, one end of the car has given up before the other, limiting the ultimate deceleration capability of the car.
    Just like the car that pushes its way through corners all day long, a car which is heavily front biased will be slow and frustrating, but relatively easy and benign to drive. On the other hand, like the oversteer monster that people are afraid to even drive around the paddock, a car which is severely rear biased will be a scary, twitchy ride resulting in a bad case of the white-knuckle syndrome. Envision an imaginary co-pilot yanking up on the park brake handle in the middle of every corner, and you begin to get the idea. While a rush to drive at speed, it will be horribly slow on the stopwatch.
    The car with perfectly balanced brake bias will, however, be the last one to hit the brakes going down the back straight. By distributing the braking forces so that all four tires are simultaneously generating their maximum deceleration, stopping distance will be minimized and our hero will quickly find his way to victory lane. Just like neutral handling, balanced brake bias is our ticket to lower lap times.
    All that said, once the braking system has achieved its perfect balance, it is still up to the tires to generate the braking forces. It’s still the tires that are stopping the car, but a poorly designed braking system can lengthen stopping distances significantly, expensive sticky tires or not.
    So why is brake biasing necessary?

    The maximum braking force that a particular tire can generate is theoretically equal to the coefficient of friction of the tire-road interface multiplied by the amount of weight being supported by that corner of the car. For example, a tire supporting 500 pounds of vehicle weight with a peak tire-road coefficient of 0.8 (a typical street tire value) could generate, in theory, 400 pounds of braking force. Throw on a good race tire with a peak coefficient of 1.5, and the maximum rises to 750 pounds of braking force. More braking force means higher deceleration, so we again see the mathematical benefits of a sticky race tire.
    On the other hand, if our race tire was now only supporting 300 pounds, the maximum force would drop from 750 pounds of braking force to 450 pounds of braking force – a reduction of 40%.
    Since the amount of braking force generated by the tire is directionally proportional to the torque generated by the calipers, pads, and rotors, one could also say that reducing the weight on the tire reduces the maximum brake torque sustainable by that corner before lock-up occurs. In the example above, if an assumed 700 ft-lb. of brake torque is required to lock up a wheel supporting 500 pounds, then only 420 ft-lb. (a 40% reduction) would be required to lock up a wheel supporting 300 pounds of vehicle weight.
    At first glance, one could surmise that in order to achieve perfect brake bias you could just:
    1. Weigh the four corners of the car
    2. Design the front and rear brake components to deliver torque in the same ratio as the front-to-rear weight distribution
    3. Win races

    In other words, for a rear-wheel-drive race car with 50/50 front/rear weight distribution it would appear that the front and rear brakes would need to generate the same amount of torque. At the same time, it would look like a production-based front-wheel-drive car with a 60/40 front/rear weight distribution would need front brakes with 50% more output (torque capability) than the rears because of the extra weight being supported by the nose of the car.
    Like most things in life though, calculating brake bias is not as simple as it may appear at first glance. Designing a braking system to these static conditions would neglect the second most important factor in the brake bias equation – the effect of dynamic weight transfer during braking.
    The ever-present weight transfer phenomenon

    Let’s assume we have a 2500 pound car with a 50/50 static weight distribution. If we are only concerned with the vehicle at rest, it’s easy to determine the weight on each wheel. We just need to find some scales and weigh it. The sum of the front corner weights is equal to the front axle weight (1250 pounds), and the sum of the rear corner weights is equal to the rear axle weight (also 1250 pounds). The weight of the vehicle is of course equal to the sum of the two axle weights (our original 2500 pounds), and this weight can be thought of as acting through
    the vehicle’s center of gravity, or CG. Figure 1 sums it up nicely.
    [IMG]https://www.brakes-shop.com/media/wysiwyg/Content/brake-bias-1.jpg[/IMG]
    Note that when at rest, there are no horizontal (left or right) forces acting on the vehicle. All of the forces are acting in a vertical (up and down) direction. But what happens to the vehicle when we start to apply forces at the tire contact patch to try to stop it? Let’s find out.
    During braking, weight is transferred from the rear axle to the front axle. As in cornering where weight is transferred from the inside tires to the outside tires, we can feel this effect on our bodies as we are thrown against the seat belts. Consequently, we now need to add several more arrows to our illustration, but the most important factor is that our CG now has an deceleration acting on it.
    Because the deceleration force acts at the CG of the vehicle, and because the CG of the vehicle is located somewhere above the ground, weight will transfer from the rear axle to the front axle in direct proportion to the rate of deceleration. In so many words, this is the effect of weight transfer under braking in living color.
    This deceleration force is a function of a mechanical engineer’s most revered equation, F=ma, where F represents the forces acting at the contact patches, m represents the mass of the vehicle, and a represents the acceleration (or in our case, deceleration) of the vehicle. But enough of the engineering mumbo-jumbo – just have a look at these additional factors in Figure 2.
    [IMG]https://www.brakes-shop.com/media/wysiwyg/Content/brake-bias-2.jpg[/IMG]
    In Figure 3 (the beginning of what we call a “fishbone diagram” – more on this later), we see how our 2500 pound vehicle with 50/50 weight distribution at rest transfers weight based upon deceleration. Under 1.0g of deceleration (and using some typical values for our vehicle geometry) we have removed 600 pounds from the rear axle and added it to the front axle. That means we have transferred almost 50% of the vehicle’s initial rear axle weight to the front axle!
    [IMG]https://www.brakes-shop.com/media/wysiwyg/Content/brake-bias-3.jpg[/IMG]

    At this point, the brake system we so carefully designed to stop the vehicle with a 50/50 weight distribution is going to apply too much force to the rear brakes, causing them to lock before we’re getting as much work as we could out of the front brakes. Consequently, our hero is going to get that white-knuckled ride we talked about earlier because he creates more tire slip in the rear than the front, and it’s going to take longer for him to stop because the front tires are not applying as much force as they could be.
    So what influences brake bias?

    If we look at the equations we have developed, we see that all of the following factors will affect the weight on an axle for any given moment in time:
    · Weight distribution of the vehicle at rest
    · CG height – the higher it is, the more weight gets transferred during a stop
    · Wheelbase – the shorter it is, the more weight gets transferred during a stop

    We also know from fundamental brake design that the following factors will affect how much brake torque is developed at each corner of the vehicle, and how much of that torque is transferred to the tire contact patch and reacted against the ground:
    · Rotor effective diameter
    · Caliper piston diameter
    · Lining friction coefficients
    · Tire traction coefficient properties

    It is the combination of these two functions – braking force at the tire versus weight on that tire – that determine our braking bias. Changing the CG height, wheelbase, or deceleration level will dictate a different force distribution, or bias, requirement for our brake system. Conversely, changing the effectiveness of the front brake components without changing the rear brake effectiveness can also cause our brake bias to change. The following table summarizes how common modifications will swing bias all over the map.
    Factors that will increase front bias Factors that will increase rear bias
    Increased front rotor diameter Increased rear rotor diameter
    Increased front brake pad coefficient of friction Increased rear brake pad coefficient of friction
    Increased front caliper piston diameter(s) Increased rear caliper piston diameter(s)
    Decreased rear rotor diameter Decreased front rotor diameter
    Decreased rear brake pad coefficient of friction Decreased front brake pad coefficient of friction
    Decreased rear caliper piston diameter(s) Decreased front caliper piston diameter(s)
    Lower center of gravity Higher center of gravity
    More weight on rear axle Less weight on rear axle
    Less weight on front axle More weight on front axle
    Less sticky tires (lower deceleration limit) More sticky tires (higher deceleration limit)
    Perfectly balanced, in theory

    While we can do calculations to determine what the optimum front-to-rear brake bias should be under all conditions, the difficult part is creating a brake system that can actually keep up with all of this. Our hero racer has it a little easier than those of us building cars for the real world. If he knows what his maximum deceleration capability is due to the tires he’s using, he can tune his brake system for that specific deceleration level. The good part is, if he tunes his vehicle for this 1.5g decel condition, because of the way weight transfer works, his car will be more front-biased in lower traction conditions, such as rain.
    Back to the “fishbone diagram” mentioned earlier. Figure 3 shows front and rear axle weight versus deceleration of the vehicle. Now let’s look at it now as a percentage of the total vehicle weight. We can add on top of this chart the front-to-rear balance of the brake system. For example, if we use the exact same brake components at the front and rear axles of the car, they will each perform 50% of the braking, and the chart will look like Figure 4.
    [IMG]https://www.brakes-shop.com/media/wysiwyg/Content/brake-bias-4.jpg[/IMG]
    Evaluating this chart, we see that the vehicle will always be rear-biased. That is, the rear brakes will always be applying more force at the tire contact patch than the weight of the rear axle can sustain. This vehicle will always lock the rear brakes before the front. Not so good.
    Most cars, however, have brakes at the rear that are smaller than the front. There are a lot of reasons for doing this, and one of them is to help provide the correct brake bias. Also, most cars have a proportioning valve which limits the amount of brake pressure seen at the rear calipers. If we look at the same chart with a more realistic braking system (one that takes into account these effects) it might look like the chart in Figure 5.
    FIGURE 5
    [IMG]https://www.brakes-shop.com/media/wysiwyg/Content/brake-bias-5.jpg[/IMG]
    Perfect brake bias is obtained when the front-to-rear balance of the brake system exactly matches the front-to-rear weight balance of the vehicle. Looking at our typical brake system chart, we see how difficult this is to do. However, if we’re trying to optimize a brake system for a particular deceleration level, it becomes much easier. We can tune the system so that the two lines cross (or come close to it) at the deceleration level the vehicle will be operating at most often. This is easy for a non-aero racing vehicle which typically operates at one fixed deceleration level. For a street car, this is almost impossible to achieve, because a car driven on the street doesn’t always operate at one deceleration level (if yours does, you probably don’t get too many repeat passengers!).
    And here’s a free tip – effects of poor brake bias on the street not only include sub-optimal stopping distances, but also include sub-optimal brake pad life. If a car is too heavily front-biased in the deceleration range it typically operates in, it will wear front pads more quickly due to the fact that the rear brakes aren’t doing as much of the stopping work as they could be. However, the rear brake pads will probably last forever…
    Perfectly balanced, in practice

    Brake bias can be measured in several ways. One method – the way the auto manufacturers do it – is to actually mount wheels on the vehicle that are equipped with strain gages, so that the actual torque at each wheel can be measured throughout a stopping event. Analysis of the vehicle deceleration data combined with the measured torque values and knowledge of the vehicle parameters mentioned above (wheelbase, CG height, weight on each axle at rest) allow us to calculate brake bias for that particular event. This is the most precise method of measuring brake bias. However, there are simpler and cheaper methods that can be just as effective.
    We know where most auto manufacturers tune brake bias – they like our cars to be front-biased in all conditions achievable by the tires offered on the vehicle. This helps to insure vehicle stability under braking by the mass public. If we measure stopping distance of the vehicle as delivered from the showroom floor, we have a good benchmark for a vehicle with a 5% to 10% front brake bias.
    Now, if we make changes to the car that can effect brake bias and re-measure stopping distance, we can tell immediately if we have taken a step in the wrong direction. For example, it is not uncommon to install more aggressive front brake pads (which will make the car even more front biased) and see stopping distances go up 5% or more. Dedicated race pads can result in even longer stopping distances.
    The most dramatic front-bias impacts are usually brought about by “big brake kits” which are not properly matched to the intended vehicle. Any time that a bigger front rotor is installed, there is a simultaneous need to decrease the effective clamping force of the caliper (installing smaller pistons is the easiest method) to offset the increased torque created by larger rotor effective radius. The objective is to maintain a constant amount of brake corner output (torque) for a given brake line pressure as Figure 6 illustrates. Unfortunately, too many upgrades do not take this factor into account, and those poor cars end up with both bigger rotors and larger pistons which serve to drastically shift the bias even more forward. While rock-solid stable under braking, stopping distances will go up dramatically.
    This is exactly the reason why StopTech performs instrumented testing for every single kit and application they develop. You’re not just buying parts – you’re also buying the assurance that the brake bias has been developed and tested to be optimized for your exact application.
    [IMG]https://www.brakes-shop.com/media/wysiwyg/Content/brake-bias-6.jpg[/IMG]
    The flip side can be seen by making changes to increase the amount of rear bias. Because the auto manufacturers leave a little bit of wiggle room in their designs, it is usually possible to make small changes to increase rear bias and end up with shorter stopping distances than stock. Keep in mind, however, that there is only so much of this wiggle room to play with. After a point, increased rear bias will make the car unstable under hard braking and will consequently drive the stopping distances through the roof.
    The moral of the story

    So, what have we learned? As Figure 7 illustrates, every car has a “sweet spot” for brake bias which will generate the shortest stopping distances possible. Typically, the auto manufacturers design their cars to be 5% to 10% more front-biased than optimum for maximum deceleration, but they provide enhanced brake stability in return. Not a bad trade-off for the public at large, and not necessarily a bad place for a race car in the heat of battle either.
    FIGURE 7.
    [IMG]https://www.brakes-shop.com/media/wysiwyg/Content/brake-bias-7.jpg[/IMG]
    As you go about modifying your car for the street or for the track, be aware that changes in the braking system as well as changes in the car’s ride height, weight distribution, or physical dimensions can swing brake bias all over the place. The only sure-fire way on knowing if your final bias has been optimized is to measure stopping distance both before and after your modification(s).
    In summary, your tires certainly still stop the car, but if your bias is out in left field you might not be able to use everything they have to offer. Your braking system is just that – a system – and keeping an eye on brake bias effects during modification will go a long, long way toward bringing home the checkered flag. Of course, selecting the proper kit from a manufacturer who has already done the hard part for you can make the trip to victory lane that much easier…




  2. #2
    Senior Member BasicQ's Avatar
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    REAR BRAKE UPGRADES. IS BIGGER REALLY BETTER?

    by James Walker, Jr. of scR motorsports


    One of the most common questions received from new owners of our front brake upgrade kits is "Do I now need to upgrade my rear brakes too?" To answer this, we need to look at the role of the rear braking system from a few different perspectives. The answer may surprise you, especially hearing this from a company that sells big brake upgrades!

    REAR BRAKE 101

    One of the many design factors that goes into the development of a base braking system is the mysterious "bias" or "balance." Truth be told, it's a pretty simple concept to grasp: for vehicle stability under braking, it is required that the rear brakes do NOT lock before the front brakes. Simple, right? Most of you probably knew that already.

    OK, so what governs the 'lock up' point of the rear brakes? Drum roll, please:

    1. tire tractive capability (friction)
    2. tire normal force (weight on the tire)

    This can be proven from looking that the fundamental relationship for maximum sustainable tire force: F=µN, where:

    F = the lock up point, or peak force
    µ = tire-road coefficient of friction
    N = normal load sitting on the tire

    So, when the OEM is designing a brake system, they 'size' the system components (calipers, master cylinder, rotor OD, etc.) to generate the proper amount of torque at both ends of the vehicle so that the front brake force ('F' above) exceeds its peak traction first. At this point, the front brakes lock and the car slides in a nice, stable straight line.

    POTENTIAL IMPACTS OF BIG FRONT BRAKES

    Fortunately (from a safety standpoint anyway), when most big-brake suppliers adapt a mondo rotor and caliper package to a vehicle, they end up actually increasing the FRONT bias. How? By increasing the effective caliper piston area and the rotor effective radius, these two factors work together to increase the 'mechanical gain' of the front brakes, building more torque for the same pressure, everything else being equal. So, from a bias perspective we are not pushing the vehicle toward instability, but rather just the opposite - we are underbraking the rear axle! The obvious impact would be an increase in stopping distance - probably the one thing the new owner was actually hoping to reduce. Ironic. So, say you chose to install these big brakes on the front axle but want to maintain the OEM bias. What's the answer? Well, one way would be to invest in big rear brakes too which increase the rear mechanical gain to the point that the system is balanced once again.

    SO, WHAT'S THE HARM IN DOING THAT?

    Well, let's look at why we upgraded the front brakes in the first place. Contrary to popular belief, the real reason sports- and racing cars use big brakes is to deal with heat. Period. There has been a bunch of stuff published which will disclaim this, but when you look at the braking system from a design standpoint, making them 'bigger' doesn't fundamentally do anything for stopping distance. It's all about the heat. So, you upgraded the front brakes because of thermal concerns but as a hidden surprise got a shift in brake bias. As a band-aid to this condition, you now spend thousands more on a rear brake upgrade because the front system was not sized correctly in the first place. Sure, it looks great, but there is another option...

    WHICH IS?

    When upgrading your front brakes, it is possible to size the caliper pistons and rotor effective radius to maintain the original brake system's pressure-torque relationship. Yea, it takes more engineering know-how and you can't sell the same part to everyone anymore, but you are not altering the base brake balance from what the OEM intended. This design philosophy stands behind every brake upgrade kit STOPTECH manufactures. Now, if you sized the front brakes correctly, why would you need to change the rear brakes? Good question. If there are no thermal concerns with the rear brakes (and on a front-engine street car there rarely are) then by installing a rear big-brake kit all you are doing is (a) spending money and (b) adding unsprung weight. This is not usually viewed as favorable, unless you like driving a heavy, expensive car.

    OH YEA - ONE MORE THING…

    Finally, under an OEM bias condition, the rear brakes only contribute about 15-20% of all the braking force the vehicle generates, and when you install sticky tires you actually DECREASE the amount of work they need to do. Why? Because at the higher deceleration levels afforded by race tires, there is more weight transfer taking place, reducing the normal force on the rear tires and increasing it on the front (remember F=µN from above?). If anything, we now want to decrease the rear effectiveness. Ironic once again.
    Of course, if you decide to upsize your rear brake system components you can also impact the front-rear torque relationship, and consequently you can "bias" the "balance" more toward the rear. Go too far, and the rear brakes could lock before the fronts. Again, not the end result you were expecting, right?
    It has been said that "The folks at STOPTECH should consider developing a rear kit to match their front setup. They'll be very happy with the performance improvement if done properly." Well, since our FRONT systems are designed properly, we save you the need to spend your money on the back axle.

    Let's reword that quote to reflect the STOPTECH philosophy: "Our competitors should consider developing a FRONT kit to match their stock bias condition. They'll be very happy with the performance improvement if done properly, AND will save their customers the cost of a rear brake upgrade in the process."
    Last edited by BasicQ; 30-04-2021 at 10:08 PM. Reason: Remove some corporate references

  3. #3
    been here .......too long Smitty2's Avatar
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    yup.. pretty much get most of that
    and
    thats part of the reason I spent 2 seasons getting brakes and handling reasonably
    (it is a Commodore!) sorted when I went back to the circuit racing game.

    interesting.. it does not touch on that one thing that scares circuit racers shirtless -
    brake fade... a horrid feeling that when you push the middle pedal harder, retardation
    or deceleration does not increase.

    This is a topic on its own... as I found out when I started asking questions (of brake shops and suppliers)


    ps.. it is not only stickier tyres that stop you sooner, but any situation where the contact patch
    (tyre to surface) area is increased. Wider tyres also work well, as do tyres with lower pressures
    (up to a point that is)
    Last edited by Smitty2; 29-04-2021 at 04:25 PM.
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  4. #4
    Senior Member BasicQ's Avatar
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    Default Some interesting reading on Brakes

    Quote Originally Posted by Smitty2 View Post
    yup.. pretty much get most of that
    and
    thats part of the reason I spent 2 seasons getting brakes and handling reasonably
    (it is a Commodore!) sorted when I went back to the circuit racing game.

    interesting.. it does not touch on that one thing that scares circuit racers shirtless -
    brake fade... a horrid feeling that when you push the middle pedal harder, retardation
    or deceleration does not increase.

    This is a topic on its own... as I found out when I started asking questions (of brake shops and suppliers)


    ps.. it is not only stickier tyres that stop you sooner, but any situation where the contact patch
    (tyre to surface) area is increased. Wider tyres also work well, as do tyres with lower pressures
    (up to a point that is)
    Brake fade isn't much fun, even feels like you go faster as you expect the feeling of slowing down.

    And yes to the p.s. Whatever increases friction coefficient.
    Last edited by BasicQ; 29-04-2021 at 05:17 PM.

  5. #5
    Do you ever leave?
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    hi
    Here is other factors to consider . In booster assisted brakes the scenario of low vacuum / to smaller booster will drop F/R performance alot also reducing for potential lockup . Re imbalance exists but psi is not high enough for it to show up as rear wheel lock up.

    Yes the advantage for a street car with rear discs is actually limited , issues canbe .
    undersize disc warp
    oversize disc can make brakes run cold

    servicing rear calipers
    Internal H/brake ------- NOT Commodore style called Banksia brake. [internal drum style]
    low movement rear brake operation compared to front this causes seizing of caliper parts
    internal hand/brake units have a small likely hood of seizing if left on for long periods .
    Serviced caliper slides at every pad change correctly the above does not happen or at least every 40,000-50,000kms 2-3 years
    brake fluid flush every 2-3 years

    Cast Iron PBR F/R calipers are far different in service requirements and need alot of work considering age . The calipers fail in some unique ways .

    Binding rear caliper slides often cause poor braking . This can be by design as the parts are not machined properly . Evident in EA Falcon rears and many similar designs .

  6. #6
    Senior Member BasicQ's Avatar
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    A little bit more.

    BRAKE SYSTEMS AND UPGRADE SELECTION

    By Stephen Ruiz, Engineering Manager and Carroll Smith, Consulting Engineer at StopTech LLC



    While almost every current passenger car is capable of a single stop from maximum speed at or near the limit of tire adhesion, the braking systems of most passenger vehicles and light trucks and some sports cars are not adequate for hard or sport driving or for towing. Most stock brake systems lack sufficient thermal capacity - the system's ability to absorb and transfer heat by conduction, convection and radiation into the air or surrounding structure during severe driving. In addition many stock calipers and their mountings are structurally not stiff enough at higher line pressures and the resultant higher clamping loads. That is why even though there is enough front brake torque to lock the front wheels at highway legal speeds, caliper flex at the increased system pressure required to stop the car from high speed may prevent wheel lock up. Needless to say, most OEM brake pads are also not designed for severe use, since cold stopping performance and quiet operation typically are considered more important to new car buyers.

    Several factors should be considered in the selection of high performance aftermarket braking systems. Some have to do with performance and safety, some with ease of installation and some with cost. The object is to select the system that will reliably fulfill your long-term needs with the least trouble and the least cost.

    There are a few basic facts that must always be kept in mind when discussing brake systems:

    1) The brakes don't stop the vehicle - the tires do. The brakes slow the rotation of the wheels and tires. This means that braking distance measured on a single stop from a highway legal speed or higher is almost totally dependent upon the stopping ability of the tires in use - which, in the case of aftermarket advertising, may or may not be the ones originally fitted to the car by the OE manufacturer.

    2) The brakes function by converting the kinetic energy of the car into thermal energy during deceleration - producing heat, lots of heat - which must then be transferred into the surroundings and into the air stream.

    The amount of heat produced in context with a brake system needs to be considered with reference to time meaning rate of work done or power. Looking at only one side of a front brake assembly, the rate of work done by stopping a 3500-pound car traveling at 100 Mph in eight seconds is 30,600 calories/sec or 437,100 BTU/hr or is equivalent to 128 kW or 172 Hp. The disc dissipates approximately 80% of this energy. The ratio of heat transfer among the three mechanisms is dependent on the operating temperature of the system. The primary difference being the increasing contribution of radiation as the temperature of the disc rises. The contribution of the conductive mechanism is also dependent on the mass of the disc and the attachment designs, with disc used for racecars being typically lower in mass and fixed by mechanism that are restrictive to conduction. At 1000oF the ratios on a racing 2-piece annular disc design are 10% conductive, 45% convective, 45% radiation. Similarly on a high performance street one-piece design, the ratios are 25% conductive, 25% convective, 50% radiation.

    3) Repeated hard stops require both effective heat transfer and adequate thermal storage capacity within the disc. The more disc surface area per unit mass and the greater and more efficient the mass flow of air over and through the disc, the faster the heat will be dissipated and the more efficient the entire system will be. At the same time, the brake discs must have enough thermal storage capacity to prevent distortion and/or cracking from thermal stress until the heat can be dissipated. This is not particularly important in a single stop but it is crucial in the case of repeated stops from high speed - whether racing, touring or towing.

    4) Control and balance are at least as important as ultimate stopping power. The objective of the braking system is to utilize the tractive capacity of all of the tires to the maximum practical extent without locking a tire. In order to achieve this, the braking force between the front and rear tires must be nearly optimally proportioned even with ABS equipped vehicles. At the same time, the required pedal pressure, pedal travel and pedal firmness must allow efficient modulation by the driver.

    5) Braking performance is about more than just brakes. In order for even the best braking systems to function effectively, tires, suspension and driving techniques must be optimized.

    For maximum brake potential, vehicles benefit from proper corner weight balance, a lower CG, a longer wheelbase, more rear weight bias and increased aerodynamic down force at the rear.

    To go further it is necessary to understand some of the physics involved, and that requires some definitions.

    1) Mechanical pedal ratio: Because no one can push directly on the brake master cylinder(s) hard enough to stop the car, the brake pedal is designed to multiply the driver's effort. The mechanical pedal ratio is the distance from the pedal pivot point to the effective center of the footpad divided by the distance from the pivot point to the master cylinder push rod. Typical ratios range from 4:1 to 9:1. The larger the ratio, the greater the force multiplication (and the longer the pedal travel).

    2) Brake line pressure: Brake line pressure is the hydraulic force that actuates the braking system when the pedal is pushed. Measured in English units as pounds per square inch (psi), it is the force applied to the brake pedal in pounds multiplied by the pedal ratio divided by the area of the master cylinder in square inches. For the same amount of force, the smaller the master cylinder, the greater the brake line pressure. Typical brake line pressures during a stop range from less than 800psi under "normal" conditions, to as much as 2000psi in a maximum effort.

    3) Clamping force: The clamping force of a caliper is the force exerted on the disc by the caliper pistons. Measured in pounds clamping force, it is the product of brake line pressure, in psi, multiplied by the total piston area of the caliper in square inches. This is true whether the caliper is of fixed or floating design. Increasing the pad area will not increase the clamping force.

    4) Braking torque: When we are talking about results in the braking department we are actually talking about braking torque - not line pressure, not clamping force and certainly not fluid displacement or fluid displacement ratio. Braking torque in pounds-feet on a single wheel is the effective disc radius in inches times clamping force times the coefficient of friction of the pad against the disc all divided by 12. The maximum braking torque on a single front wheel normally exceeds the entire torque output of a typical engine.

    A few things are now obvious:

    1) Line pressure can only be increased by either increasing the mechanical pedal ratio or by decreasing the master cylinder diameter. In either case the pedal travel will be increased.

    2) Clamping force can only be increased either by increasing the line pressure or by increasing the diameter of the caliper piston(s). Increasing the size of the pads will not increase clamping force. Any increase in caliper piston area alone will be accompanied by an increase in pedal travel. The effectiveness of a caliper is also affected by the stiffness of the caliper body and its mountings. It is therefore possible to reduce piston size while increasing caliper stiffness and realize a net increase in clamping force applied. This would typically improve pedal feel.

    3) Only increasing the effective radius of the disc, the caliper piston area, the line pressure, or the coefficient of friction can increase brake torque. Increasing the pad area will decrease pad wear and improve the fade characteristics of the pads but it will not increase the brake torque.

    FRONT TO REAR BRAKE BIAS

    Stability and control under heavy braking is at least as important as ultimate stopping capability. All cars, from pickups to Formula One, are designed with the majority of the braking torque on the front wheels. There are two reasons for this - first, if we ignore the effects of aerodynamic down force, the total of the forces on each of the vehicle's four tires must remain the same under all conditions. When the vehicle decelerates, mass or load is transferred from the rear tires to the fronts. The amount of load transfer is determined by the height of the vehicle's center of gravity, the length of the wheelbase and the rate of deceleration. Anti-dive geometry does not materially effect the amount of load transferred - only the geometric results of the transfer. Second, when a tire locks under braking, braking capacity is greatly reduced but lateral capacity virtually disappears. Therefore, when the front tires lock before the rears, steering control is lost and the car continues straight ahead - but this "under steer" is a stable condition and steering control can be regained by reducing the pedal pressure. If, however, the rear tires lock first, the result is instantaneous "over steer" - the car wants to spin. This is an unstable condition from which it is more difficult to recover, especially when entering a corner.

    Most mid-engine pure racing cars are designed with 55-60% of the total static load and 45-50% of the total braking torque on the rear tires. These cars feature literally tons of rear aerodynamic down force and the footprints of the rear tires are always significantly larger than those of the front. Most passenger cars are front engined; none of them have any appreciable download and almost all of them have the same size front and rear tires. In extreme cases (front wheel drive) they may have 70 % of the total static load on the front tires. They are therefore designed with a preponderance of front brake torque. Most current production cars feature anti-lock brake systems (all cars should). Sophisticated ABS systems ensure that, under heavy braking conditions - even braking with tires on different surfaces - each tire is braking at something very closely approaching its maximum capacity while the ABS system prevents lock up.

    THE REAR BRAKE LINE PRESSURE-LIMITING VALVE

    Since the load transferred from the rear tires to the fronts under braking decreases the braking capacity of the rear tires, a rear brake line pressure-limiting valve (often referred to a proportioning valve) is utilized to prevent rear wheel lock up on most passenger cars that do not feature ABS. Its function is to limit the amount of pressure transmitted to the rear brakes under very heavy braking. Assuming a tandem master cylinder with equal bores, front and rear line pressures are the same until some pre-determined threshold is reached. After this point, rear line pressure, while it still increases linearly with pedal effort, increases at a lower rate than the front. In a graph it appears as a distinct "knee" point where a further rise in pressure after the valve is noticeably diminished. The purpose is to avoid rear wheel lockup and the attendant unstable over steer at maximum deceleration rates when the weight transfer is greatly reducing the dynamic load on the rear wheels. It is not a good idea to remove the limiting valve from a road going automobile. Remember, under steer is stable, over steer is not. Without an effective anti-lock braking system, in any panic braking situation we must be absolutely certain that the unloaded rear tires cannot lock first. Therefore materially increasing the rear braking torque is not a good idea for highway use. If you feel that you must do so, consider removing the OEM rear brake line pressure-limiting valve completely and replace it with one of the adjustable units manufactured by Tilton Engineering or Automotive Products (now part of Brembo). Do not place a second pressure-limiting valve in line with the OEM unit.

    BRAKE PEDAL FIRMNESS AND MODULATION

    The human brain/body system modulates most effectively by force, not by displacement. The side control sticks on current fighter aircraft hardly move. The feel of the brake pedal should approach the firmness and consistency of a brick. There are several factors at work here:

    1) Brake hoses: Optimum pedal firmness cannot be achieved with the stock fabric reinforced rubber flexible hoses which swell under pressure - decreasing pedal firmness while increasing both pedal travel and brake system reaction time. The first step in upgrading the braking system of any vehicle is to replace the OEM flexible hoses with stainless steel braid protected flexible hoses of extruded Teflon. Make certain that they are designed for the specific application, are a direct replacement for stock and are certified by the manufacturer to meet USDOT specifications. A claim that aftermarket hose are certified by the DOT is a caution flag. The DOT does not certify anything. Manufacturers certify that their products meet DOT specifications and legitimate suppliers can produce reports from DOT approved testing laboratories. When upgrading your brake hoses, replace both the front and rear hoses. Due to their swelling under pressure the stock hoses take a measurable amount of time to transmit pressure to the calipers. Replacing the front hoses only will result in a built in lag time to the rear brakes and may also adversely effect the microprocessor control algorithms of the ABS system.

    2) Master cylinders and Caliper piston diameters: While it is true that the most effective master cylinder arrangement is the twin cylinder with adjustable bias bar that is universal in racing, replacing the OEM master cylinder on a road going car is simply not practical. When selecting an aftermarket system, make sure that the caliper bores are designed for the specific application.

    3) Disc run out and thickness variation: Run out in excess of six thousandths of an inch (0.006") can be felt by the driver as can more than 0.001" of thickness variation and any amount of material transfer from overheated pads. Run out is caused by poor design of either vanes or the junction between the friction surfaces and the mounting bell, by poor machining, by thermal stress or by any combination of the three.

    4) Caliper and caliper mounting stiffness: Clamping force tries to open the opposing sides of the calipers - resulting in a longer than optimum pedal travel and uneven pad wear. The only solution is optimal mechanical design and material selection - there is no effective development fix for "soft" calipers. Also, the stiffest caliper will be ineffective if its mounting lacks rigidity.

    5) Out of balance discs (or tires): The driver cannot modulate the brake on a bouncing wheel. Compared to tires, disc diameters are relatively small, but all discs should be balanced. As the installation of balancing clips will interfere with airflow the preferred method is to remove material from the heavy side. Significant core shift in the casting (visible, as thickness variation on individual friction surfaces will result in incurable dynamic imbalance.

    6) Pad "bite" and release characteristics: For efficient modulation the pads must "bite" immediately on brake application and must release immediately when the pedal is released. This is purely a matter of pad selection. It is seldom a good idea to use different compound pads front and rear and never a good idea to use a pad with more bite or a higher coefficient of friction at the rear.

    BRAKE FADE

    Repeated heavy use of the brakes may lead to "brake fade". There are two distinct varieties of brake fade:

    1) Pad fade: When the temperature at the interface between the pad and the disc exceeds the thermal capacity of the pad, the pad loses friction capability due partly to out gassing of the binding agents in the pad compound. Pad fade is also due to one of the mechanism of energy conversion that takes place in the pad. In most cases it involves the instantaneous solidification of the pad and disc materials together - followed immediately by the breaking of bonds that releases energy in the form of heat. This cycle has a relatively wide operating temperature range. If the operating temperature exceeds this range, the mechanism begins to fail. The brake pedal remains firm and solid but the car won't stop. The first indication is a distinctive and unpleasant smell that should serve as a warning to back off.

    2) Fluid boiling: When the fluid boils in the calipers, gas bubbles are formed. Since gasses are compressible, the brake pedal becomes soft and "mushy" and pedal travel increases. You can probably still stop the car by pumping the pedal but efficient modulation is gone. This is a gradual process with lots of warning.

    In either case temporary relief can be achieved by heeding the warning signs and letting things cool down by not using the brakes so hard. In fact, a desirable feature of a good pad material formula is fast fade recovery. Overheated fluid should be replaced at the first opportunity. Pads that have faded severely should be checked to make sure that they have not glazed and the discs should be checked for material transfer. The easy permanent cures, in order of cost, are to upgrade the brake fluid, to upgrade the pads, or to increase airflow to the system (including the calipers). In marginal cases one of these or some combination is often all that is required.

    TAPERED PAD WEAR

    Similar to brake fade, there is more than one distinct type of tapered pad wear - radial taper and longitudinal taper.

    1) If a caliper lacks stiffness and tends to "open" under clamping force, at elevated temperatures, the outboard surface (edge with the longest radius) of the pad with respect to the disc (axle), center will wear faster than the inboard (edge with the shortest radius), and the pad will be tapered in its cross section when viewed from the end. This is termed "radial taper".

    2) The trailing area (portion) of the pad, to some extent "floats" on the entrapped gasses and particulate matter generated from the leading portion of the pad. The leading portion of the pad will always be hotter than the trailing portion and so will correspondingly, wear faster - resulting in a pad that is tapered when viewed from the edge. This phenomenon is termed "longitudinal taper".

    The differential in heat generated across the pad surface, leading to trailing, is characteristic regardless of caliper and pad design. This is why all racing calipers and most high performance street calipers have differential piston bores. Most high performance pads also feature a tapered leading edge.

    3) In the case of new very thick pads like the type used for endurance racecars, longitudinal taper will sometimes occur because the pad literally tips inward at an angle against the disc during "off brake" conditions. When this happens, there is a small amount of force pushing the pad leading edge in the direction of the disc as a result of the contact and the friction generated. At the same time, the trailing side of the pad is wedged back into the corner of the pad cavity in the caliper and against the abutment plate, which further promotes contact at the leading edge. This situation is exaggerated with new thick pads since the increased offset of the pad friction surface from the backing plate, results in a relatively larger constant force vector in the direction of the disc.

    4) Taper can also be seen where the disc is solidly fixed to the hat or where the hat and disc are one piece. In either case, the taper created will appears as more wear on the outer diameter of the outside pad, and the inner diameter of the inside pad. This is due to operating the brakes at high temperature and the resulting thermal expansion forces on the annular outer ring structure of the disc called the friction plates. The center of the disc or hat limits the expansion of the outer structure on only one side where it is joined, typically at the outside friction plate. As a result, the disc cones so that it is concave as viewed from the outside (See also "Floating Discs"). Subsequently due to the coning, the pad contacts unevenly when the brakes are applied or remains in contact with the disc in the regions mentioned and even higher temperatures and wear are the result.

    AIR COOLING

    Most of the enormous amounts of heat generated during deceleration must be dissipated into the free air stream.

    Most high performance (and/or heavy) cars today use some variation of the "ventilated" brake disc in which air entering the center or "eye" of the rotor is forced through the interior of the rotor by the pumping action of the rotating assembly. The most efficient practical way yet devised to accomplish this is through the use of the "curved vane" ventilated brake rotor originally designed for the LeMans winning Ford GT 40s in 1966. In this design the interior vanes are curved to form an efficient pump impeller. They also stabilize the rotor from distortion and serve as very effective barriers to stop the propagation of cracks due to thermal stress. In laboratory testing STOPTECH's innovative design developments in the 48 vane rotors have increased air flow through the rotor by an astounding 61% over some OEM rotors and from 10-15% over racing rotors of the same size. This results in a cost effective but very stable direct replacement rotor that runs typically 15% cooler than stock and 7% cooler than racing designs.

    TITANIUM CALIPER PISTONS

    Caliper pistons manufactured from Titanium do a really good job of insulating the fluid in the caliper from conductive heat transfer from the pads. Unfortunately it is not a simple substitution. The design and manufacture of brake caliper pistons is a complex engineering exercise. If the piston material is to be changed, the designer must take into consideration the difference in thermal coefficient of expansion between the OEM material and the new material. The right grade and condition of Titanium must be selected. The surface finish and treatment must be compatible with the seals. If the seal groove is in the piston, the groove geometry must match the OEM design. As a point of interest, virtually all-serious racing cars use Titanium caliper pistons with an anti-galling surface treatment, which changes the color from a natural almost dull silver to gold. The fact of the matter is that a simple Titanium button placed inside the OEM piston does about 70% of the job at a fraction of the cost with no risk of damaging anything by disassembling the caliper.

    DRILLED VS SLOTTED ROTORS

    For many years most racing rotors were drilled. There were two reasons - the holes gave the "fireband" boundary layer of gasses and particulate matter someplace to go and the edges of the holes gave the pad a better "bite".

    Unfortunately the drilled holes also reduced the thermal capacity of the discs and served as very effective "stress raisers" significantly decreasing disc life. Improvements in friction materials have pretty much made the drilled rotor a thing of the past in racing. Most racing rotors currently feature a series of tangential slots or channels that serve the same purpose without the attendant disadvantages.

    PAD AREA

    We have seen that brake torque is directly proportional to Piston Area, System Pressure, Friction Coefficient and Effective Radii and is not affected by pad area. Pad area and geometry are however important for several reasons:

    1) Pad service life. Since pad material is consumed, an increase in pad area results in an increase in the time interval between pad replacements. OE designs often make slight sacrifices in pad life by including tapered ends for reduction of noise, vibration and pad taper. In some OE designs the pads on the two sides of the caliper are even shaped differently, with the inside pad being shorter in arc-length in the direction of rotation and wider radially than the outside pad for system design and integration reasons.

    2) Heat dispersion and dissipation over a larger surface area and greater mass. Although in the case of a larger pad, the pad masks a larger portion of the rotor face, absorbing more radiant energy and shielding the area from cooling that may cancel any actual benefit.

    3) Geometry: Since rubbing speed between the disc and the pad is greater at the periphery of the disc, the pad geometry will sometimes be designed to reduce the area toward the center of the disc. This is done in an effort to produce even temperature and pressure distribution across the face of the pad.

    INCREASING DISC DIAMETER

    The problem with increasing the effective radius of the discs is that, since the designers used the largest rotor that would fit inside the wheel. Typically, increasing the rotor diameter means increasing the wheel size. The expense involved is only one objection. A major issue is the impact on of the OE suspension geometry.

    The camber curves and roll resistance characteristics of any proper suspension system are designed for tires with a specific sidewall height and stiffness. Increasing the wheel diameter means decreasing the sidewall height and the compliance of the tire. Carried to an extreme, this will hurt cornering capability and might actually result in a loss of braking traction due to "edging" the front tires under heavy braking. And although technology is making possible ultra low and stylish tire side wall heights, it does not necessarily result in ultimate performance, just take a look at the sidewall height of Formula One and Indy cars.

    FLOATING DISCS

    All metals "grow" when heated. The diameter of cast iron brake discs can increase as much as 2mm (0.080 inch) at elevated braking temperatures. When the disc is radially restrained from growing (as in all one-piece discs) the friction plates are forced into a cone shape as temperature increases, adversely effecting both temperature and pressure distribution within the pads and the feel of the pedal. Racing and high performance street discs are mounted on separate hats or bells, usually of Aluminum. The fastening system is designed to allow radial growth and minimal axial float resulting in a mechanically stable system. Hats or bells should be made from 7075 or 2024 heat-treated aluminum billets that are pre-stressed and relieved, not from 6061 or from plate stock.

    SUMMARY

    If the braking system is only marginal, upgrading the pads and brake fluid and/or getting more air to the system will probably cure the problem at minimal cost. Replacing the stock rubber flexible hoses with stainless braid armored Teflon hoses will improve the ability to effectively modulate the braking force at moderate cost. When a decision is made to upgrade the braking system, make sure that the replacement components and system have been properly engineered and designed for your specific application ask technical questions and expect valid technical answers.
    1) Discs should have curved vans and both greater thermal storage capacity and better airflow characteristics than OEM - otherwise you will not have achieved anything worthwhile. Depend on actual test results, not advertising claims. Discs should be mill balanced to less than 0.75 ounce-inch (54 g-cm), run out should be less than 0.002" (0.051 mm) and thickness variation should be less than 0.0007" (0.018 mm). On race applications these tolerance are typically reduced to .25 ounce-inch, 0.0005" and 0.0001" respectively.
    2) Calipers should be stiff at elevated temperature. Again, look at laboratory test results, not claims. Calipers must be mounted true to the plane of rotation of the rotor.
    3) Multi-piston calipers should have differential bores to reduce taper wear. Piston area should be consistent with master cylinder size.
    4) Ideally no modifications to the knuckles or uprights should be required for installation.
    5) Front to rear brake torque bias should be consistent with the dynamics of the specific vehicle.

    DRIVING CONSIDERATIONS

    1) In order to brake effectively, the tires must comply with and grip on the road. Your braking system is no better than your tires and suspension. The best money that you can spend is on really good tires and really good shocks.

    2) Proper corner weight is crucial for effective straight line braking. Optimum corner weight for braking is when the cross corner pairs are equal. That is to say the total of the left front and right rear equals the total of the right front and left rear.

    3) If you smell brake lining or if the pedal starts to go soft, ease off.

    4) Use at least a 550 degree non-silicone brake fluid and make sure that your brakes are bled properly and, when used hard, often. Brake fluid is hygroscopic in nature - given any chance at all it absorbs water. A fraction of one percent of entrapped water lowers the boiling point of any brake fluid dramatically - and causes corrosion within the system. Replace all of the brake fluid in the system at least once a year - more often if you constantly use the brakes hard.
    Last edited by BasicQ; 30-04-2021 at 11:21 PM.

  7. #7
    been here .......too long Smitty2's Avatar
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    ... no disagreement again from me

    2 points though!
    firstly, the last point 4 is not followed by the car makers (ESPECIALLY those who plumb their clutch fluid into the brake master... looking at you GM) 5 years or 120,000km is the 'recommended' service interval
    and I get the 'you are a PITA when I ask the dealer to do it every 2 years'.. why? we have to bleed your clutch hydraulic system too!
    and
    drilled v slotted

    I have spoken to various racers (guys with GT3 Porsches to techs or drivers with old style mid 90s Supercars) and Porsche Racing (the factory) recommend and supply their race cars with DRILLED rotors
    but most others seem to have gone to slotted (in various configs.. curved, straight, j hook) including me
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    Hi
    street rotors that I`ve seen tend to fracture around vent holes . Slotted IMO is far better .
    Every 2 yrs 40,000 km on brake fluid flush has been the factory recommendation .Toyota ,Ford etc Also u can buy b/fluid testers cheaply these days.
    Silicon fluid still needs changing . Unlike normal b/fluid that absorbs/disperses the water thru out the fluid apparently silicon fluid eventually deposits the moisture in one place ,sounds weird but as a precaution flush every 2 yrs .
    Silicon fluid can be aerated by ABS pulsations as well .

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    Holy crap, I haven't read it all yet but I will. I've just slapped a Willwood 4-pot kit on the front of a Torana and the 9" rear has a VS disc setup. Clearly I don't know what I'm doing re proportioning to to the front and rear.

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    been here .......too long Smitty2's Avatar
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    Quote Originally Posted by RedTaxi View Post
    Holy crap, I haven't read it all yet but I will. I've just slapped a Willwood 4-pot kit on the front of a Torana and the 9" rear has a VS disc setup. Clearly I don't know what I'm doing re proportioning to to the front and rear.

    .. even if you used the road legal Wilwood kit (Wilwood make 2... road legal and track only) that sounds like a very unbalanced system

    what have you done re the master and booster? standard? or ?
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    Quote Originally Posted by Smitty2 View Post
    .. even if you used the road legal Wilwood kit (Wilwood make 2... road legal and track only) that sounds like a very unbalanced system

    what have you done re the master and booster? standard? or ?
    Road legal kit with the dust boots. Only mod so far is to delete the ducks bill check valve in the master that holds a bit of pressure for the original rear drums. Booster standard for now. I installed willwoods up front in my VH when I was a boy and it was an impressive upgrade with the std (basically same as VS) rear discs. Torana though has a proportioning valve after the master?

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    hi
    Need to check pressures at master outlet to see if system is making high pressure or close to it . By applying 15 to 18 inches of vacuum to booster psi will increase . Does your torana in an A9x version run a bigger / double diaphragm booster???
    Once thats done prop valve / balance can be looked at .

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    LH,LX Torana proportioning valve 1974-76 is PBR # P6720.

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    Quote Originally Posted by swampy View Post
    hi
    Need to check pressures at master outlet to see if system is making high pressure or close to it . By applying 15 to 18 inches of vacuum to booster psi will increase . Does your torana in an A9x version run a bigger / double diaphragm booster???
    Once thats done prop valve / balance can be looked at .
    6cyl booster at present. Cars not running yet. Bigger booster just means less pedal effort. Doesn't alter braking performance. I don't like overly assisted brakes. If the wheels are going to lock up I wanna be off the seat.

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    Quote Originally Posted by GtoGeoff View Post
    LH,LX Torana proportioning valve 1974-76 is PBR # P6720.
    Yeh I've got the std proportioning valve but unsure if it will be ok for rear discs over drums.

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