SoundGuyDave
This Space For Rent
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There's been a lot of discussion about braking packages, what works, what doesn't, as well as a lot of opinion thrown around about what you NEED to successfully track an S197 chassis Mustang.
BRAKE THEORY:
Just so that we’re all up to speed, the following is a brief-ish description of exactly how brakes work, and a couple of comments on how this impacts us from an open-tracking perspective.
First, the brakes don’t actually stop the car, the tires do. Whether you’re braking, accelerating, or turning, it all comes down to the four tire contact patches on the ground to do all of the work. The brake system simply attempts to slow the wheel rotation, but as anybody that has driven on ice knows, it’s not the brakes that stop the car, it’s the tires. Brakes are a hydraulic system, with the real work being done by the brake fluid, which is non-compressible. When you step on the brake pedal, a rod running through the firewall connects the pedal to the master cylinder, which pressurizes two lines heading to the front of the car. These two lines hit the ABS hydraulic control unit (HCU) and then branch out into four lines, with one line running to each wheel. ABS operations theory is a bit beyond the scope of this short article, so we’ll skip that, but suffice it to say the ABS system can selectively pressurize or depressurize each wheel to keep them from locking up. Once the master cylinder pressurizes the lines heading to the wheels, the pressure is transmitted to the inside of the brake caliper. This pressure forces the caliper piston(s) outwards, pushing the inboard brake pad against the brake rotor. On the OE Mustang GT design, the caliper itself is mounted on a pair of slide-pins, allowing the caliper to move inboard and outboard. Since all the piston pressure is on the inboard side, the caliper slides on the pins, distributing the pressure between the inboard and outboard pads. Assuming the rotor is spinning (car in motion) the brakes apply friction, slowing the rotor down by converting kinetic energy (rotation) to thermal energy (heat). This heat is what interests us the most. The rotor itself acts as a heat-sink, and the lower the mass, the smaller the thermal capacity. Once the thermal capacity of the rotor is approached, this heat is transferred into the caliper through the pistons, and from there, into the fluid. If there is moisture in the fluid, this moisture will flash into steam in tiny bubbles. Steam, unlike brake fluid, IS compressible, and this is how you get the “spongy pedal” feeling after beating on the brakes for a while. In a truly worst-case scenario, you can overheat the pads themselves, and they start to lose their frictional characteristics. Some brake pads fall off rapidly when the maximum operating temperature (MOT) is reached, others start to fade more gradually, but they ALL will fade, given enough heat. “Street” brake pads must work efficiently at low temps, but will not have a particularly high MOT. “Race” brake pads have very high MOTs, but sacrifice bite at lower temps. The brakes that I run really don’t stop the car all that well until the brake pad temps come up to around 250*F, but they’re good for up to 1650*F. Street pads, by comparison, start to completely fall apart in the 400*F range!
Now that we’re all singing from the same sheet of music, we can start to look at parts comparisons with a better understanding of the implications of making changes. First, there is a myth that we need to examine and discard: Better brakes will make your car stop in a shorter distance. This is patently false. Assuming that the STOCK brake setup (pads, rotors, calipers, fluid) can put the car into ABS (or lockup if you have no ABS), then you have achieved maximum braking potential. Period. Your tires are now the limiting factor. If you compare that setup to a completely tricked-out race setup, which is ALSO able to put the car into ABS, they also have achieved maximum braking potential. Your tires are STILL the limiting factor! Before anybody whines, yes, there are other subtleties at work here, like where in the braking curve the ABS kicks in, but for the purposes of general discussion, the ONLY thing that the upgraded brake packages offer is the ability to put the car into ABS more often before failure. Granted, that is hugely important, but the whole myth of being able to go into a corner 100-200’ deeper strictly because of a brake change is purely fiction.
Last season, I'd been tracking a lot of data points from people on track, both members here, as well as others who don't hit the forums. The things I've been keeping an eye on are: brake cooling, compound selection, rotor selection, caliper choice, and driving styles. While it's hard to quantify the last factor, I chose to break down driving style into three basic categories: "track day," "aggressive," and "competitive." I'm classing "Track day" drivers as those that are simply out to have fun, and enjoy their cars, without necessarily pushing things to the edge. "Aggressive" drivers are those that ARE pushing their cars, specifically braking a lot harder, but not focusing on the last few tenths per lap. "Competitive" drivers are those that are really pushing the braking envelope. For the hard data, I'll use my car, as I believe that it's fairly typical of the hard-core but street legal car that hits the track events with some regularity.
From what I can see, there's a pretty simple upgrade path as drivers move from one group to the next, with some interesting results in some areas.
First, we need to realize exactly what it is we're working with! While there are a lot of differing opinions about exactly what the S197 chassis really weighs, I've personally seen eight of them weighed on "official" scales, and excepting stripped track-only cars, they are ALL over 3600lbs in race trim, with driver and safety gear in place. With just a splash of fuel left and a full interior, I scaled at 3605! That really is a LOT of weight to try to accelerate, slow, and turn, and puts a pretty heavy load on the braking systems of the cars.
With the Fox and SN95 chassis, the brake upgrade most commonly seen is a change to the "Cobra" calipers and rotors, which is almost exactly what we start with from the factory; 13" rotor diameter up front, PBR-type dual piston calipers, and a floating mount. Make no mistake, this base package does NOT suck, and can be very successfully tracked. While 12-piston calipers, and two-piece 15" rotors made from unobtanium are available, unless you're just a banzai driver, they are NOT necessary! More on this later.
For drivers that track their cars once or twice a year, or are contemplating their first track event, there are only a few basic upgrades that really need to be done. First is pad compound. The stock pads are okay on the street, but will turn to dust rapidly under the demands of a road course, and I have seen more than one student parked before the end of the weekend due to burned up brake pads. This is important: there is NO "street" compound that can handle the heat of open-tracking! Given the weight of the car, and the power available to get up to speed, you NEED to go to a race compound of some sort. For the entry-level or occasional use car, something like a Hawk HT-10 up front with HP-Plus in the rear is usually sufficient. For Carbotech fans, that would be XP-8 up front, and Bobcat in the rear. The next thing is brake fluid. Again for the occasional or novice driver, simply flushing the system with clean, fresh, factory-type fluid is sufficient, but make sure that the fluid in the car is less than a year old before you hit the track. Honestly, that's it. Mild pads and fresh fluid are really all you need, brake-wise, to hit the track for your first time, or if you only go occasionally. Once you're done with your event, slap the OE pads back in, and you're golden.
For more aggressive drivers, the formula starts to change. Brake cooling becomes critical, aftermarket fluids are indicated, and the pad compound needs to change upwards. Hawk DTC-60 up front, and HT-10 in the rear (Carbotech XP-10 and XP-8), good "racing" brake fluid (Motul RBF600, ATE SuperBlue or SuperGold, Brembo; Castrol SRF) that you flush every six months, and of course, brake ducting. Quantum Motorsports, Steeda, and Agent47 all sell ducting kits, and all of them work fairly well. If you're leaning towards the more hard-core end of the spectrum, though, you may want to roll your own. It'll save you some money, and give you more efficiency in cooling. Butler-Bilt inlets, QMS high-temp duct hose, and fabricated backing plates will let you get more airflow through the rotor, but too much of a good thing can be bad! At the really high-speed tracks, I have to tape off part of my inlets, since I can over-cool the brakes and actually drop the temps below the point where the initial bite is still good. Also, if you are going to track your car fairly regularly, invest in some GOOD braided brake hoses. Yes, they will stiffen up the pedal feel a tiny bit, but the primary benefit is more durability when subjected to the heat, vibration, and occasional contact with "track boogers" being kicked up.
For the truly hard-core, the fabricated ducting setup is the only way to go, along with serious brake pads. Think DTC-70/DTC-60 (XP-16/XP-12), and consider making the move to Castrol SRF fluid. Hugely expensive, but reports indicate that it's all but impossible to boil the stuff, and it won't grab moisture out of the air like the Motul fluid will. I personally just can't afford $70/quart fluids, so I haven't tried it myself...
The bottom line on pads, however, is that if you’re running the stock GT brake package, you’ll need to stagger your pad compounds, running one range less aggressive in the rear, to keep braking balance from biasing rearward. If you run the same compounds front and rear, you will have a tendency to lock up the rears, triggering early ABS activation, and at that point you’re throwing away braking potential.
Please note that ALL of the above packages and recommendations are still based around OE calipers! Now, let's talk about rotors... The more aggressive the brake compound you run, the more metallic content is in the pad, and the faster the rotor wear you get. While there are ALWAYS exceptions, that's a good rule of thumb. Given that, my personal recommendation is to stick with OE, plain-face rotors. Cross-drilled pieces only provide opportunity for localized hot-spotting and cracking. Slots were originally used to provide an off-gas path when brake pads used organic base compounds (think asbestos), with the modern compounds, the whole need to allow a gas path has essentially disappeared.
Up to this point, we’ve been looking exclusively at the OE braking package: 13” vented, plain-face rotors with 2-piston sliding calipers up front. Now, I think we should look at the alternatives.
GT vs. GT500 front brake packages
Since both packages use the base GT rear caliper and rotor, we can focus strictly on the difference in the front brakes. Up to this point, I have been very positive about the stock GT brake package, and I continue to assert that there is really nothing inherently wrong with that package, with one exception: Durability. There are a few drawbacks with the GT front package, and they all revolve around heat. The aluminum caliper castings will, over time, soften in the bridge area between the inboard and outboard pads, and spread. This leads to a taper-wear condition, where the tops of the pads, particularly the outboard side, will wear very quickly, but the bottom won’t. This leaves the pad with a wedge shape when looked at in profile, and dramatically shortens the lifespan of the pad. In addition, since there is relatively limited thermal capacity in the rotor, they tend to surface-check quickly, which can lead to cracking. In 2009, over 16 track days, I went through two sets of calipers, three sets of rotors, and three sets of brake pads. Parts cost, however, really isn’t that high, due to ready access to take-off parts for the rotors and calipers.
The GT500 (Brembo) calipers, however, are a completely different animal. Instead of a floating-caliper design, where the caliper rides on a pair of slid-pins and shifts back and forth with pedal actuation, the Brembo calipers are fixed-mount, bolting directly onto the knuckle, and are equipped with four pistons, two to the inboard, and two to the outboard halves of the calipers. In addition to the much stronger bridge section, which is not nearly as prone to spread as the PBR style caliper on the base GT package, they are engineered for 14” diameter rotors, rather than the 13” stockers. In terms of overall weight, you will gain approximately 5lbs per side, all unsprung, but you will also gain a much larger thermal capacity in the rotor. The three main downsides to the GT500 package are: 1) MUCH higher parts cost. The pads are much more expensive, as are the rotors (no take-off source). 2) More complicated bleeding, as there are two bleeder screws per caliper. 3) Potential for pad knock-back. This occurs when there is play in the wheel bearing or major runout in the rotor, which causes the pads to be pushed away from the rotor surface. The result is a long pedal stroke on initial application, but that can be counteracted by a stab or two with the left foot just before entering the braking zone. The major upsides to the Brembo package are: 1) drastically reduced caliper fatigue. The “bending bridge” problem with the PBR-style calipers is eliminated. 2) more even pad wear, as there is no frictional binding in the slide pins. 3) vastly lower heat transfer into the brake fluid, resulting in reduced need to bleed mid-day. For an experiment, I went as far as I could before the brake fluid boiled badly enough to get a spongy pedal, and it was a total of four track days with no bleeding! 4) demonstrably longer component wear. In 2010, after 13 track days, I was finally forced to change the front pads. The rotors were just starting to show surface checking, and of course, the calipers themselves were fine. According to my spreadsheets, this all equates to reduced net operating cost, despite the higher initial outlay. For track junkies, this will pay off, for the occasional user, perhaps not as drastically.
The brake pad bias will also change with the larger rotor diameter, since the braking torque is increased up front. Instead of stepping down one compound range in the rear, with the 14”rotor testing shows that identical compounds are the best match. I’ve had excellent success with the DTC-60 (or XP-12) compound all the way around.
BRAKE THEORY:
Just so that we’re all up to speed, the following is a brief-ish description of exactly how brakes work, and a couple of comments on how this impacts us from an open-tracking perspective.
First, the brakes don’t actually stop the car, the tires do. Whether you’re braking, accelerating, or turning, it all comes down to the four tire contact patches on the ground to do all of the work. The brake system simply attempts to slow the wheel rotation, but as anybody that has driven on ice knows, it’s not the brakes that stop the car, it’s the tires. Brakes are a hydraulic system, with the real work being done by the brake fluid, which is non-compressible. When you step on the brake pedal, a rod running through the firewall connects the pedal to the master cylinder, which pressurizes two lines heading to the front of the car. These two lines hit the ABS hydraulic control unit (HCU) and then branch out into four lines, with one line running to each wheel. ABS operations theory is a bit beyond the scope of this short article, so we’ll skip that, but suffice it to say the ABS system can selectively pressurize or depressurize each wheel to keep them from locking up. Once the master cylinder pressurizes the lines heading to the wheels, the pressure is transmitted to the inside of the brake caliper. This pressure forces the caliper piston(s) outwards, pushing the inboard brake pad against the brake rotor. On the OE Mustang GT design, the caliper itself is mounted on a pair of slide-pins, allowing the caliper to move inboard and outboard. Since all the piston pressure is on the inboard side, the caliper slides on the pins, distributing the pressure between the inboard and outboard pads. Assuming the rotor is spinning (car in motion) the brakes apply friction, slowing the rotor down by converting kinetic energy (rotation) to thermal energy (heat). This heat is what interests us the most. The rotor itself acts as a heat-sink, and the lower the mass, the smaller the thermal capacity. Once the thermal capacity of the rotor is approached, this heat is transferred into the caliper through the pistons, and from there, into the fluid. If there is moisture in the fluid, this moisture will flash into steam in tiny bubbles. Steam, unlike brake fluid, IS compressible, and this is how you get the “spongy pedal” feeling after beating on the brakes for a while. In a truly worst-case scenario, you can overheat the pads themselves, and they start to lose their frictional characteristics. Some brake pads fall off rapidly when the maximum operating temperature (MOT) is reached, others start to fade more gradually, but they ALL will fade, given enough heat. “Street” brake pads must work efficiently at low temps, but will not have a particularly high MOT. “Race” brake pads have very high MOTs, but sacrifice bite at lower temps. The brakes that I run really don’t stop the car all that well until the brake pad temps come up to around 250*F, but they’re good for up to 1650*F. Street pads, by comparison, start to completely fall apart in the 400*F range!
Now that we’re all singing from the same sheet of music, we can start to look at parts comparisons with a better understanding of the implications of making changes. First, there is a myth that we need to examine and discard: Better brakes will make your car stop in a shorter distance. This is patently false. Assuming that the STOCK brake setup (pads, rotors, calipers, fluid) can put the car into ABS (or lockup if you have no ABS), then you have achieved maximum braking potential. Period. Your tires are now the limiting factor. If you compare that setup to a completely tricked-out race setup, which is ALSO able to put the car into ABS, they also have achieved maximum braking potential. Your tires are STILL the limiting factor! Before anybody whines, yes, there are other subtleties at work here, like where in the braking curve the ABS kicks in, but for the purposes of general discussion, the ONLY thing that the upgraded brake packages offer is the ability to put the car into ABS more often before failure. Granted, that is hugely important, but the whole myth of being able to go into a corner 100-200’ deeper strictly because of a brake change is purely fiction.
Last season, I'd been tracking a lot of data points from people on track, both members here, as well as others who don't hit the forums. The things I've been keeping an eye on are: brake cooling, compound selection, rotor selection, caliper choice, and driving styles. While it's hard to quantify the last factor, I chose to break down driving style into three basic categories: "track day," "aggressive," and "competitive." I'm classing "Track day" drivers as those that are simply out to have fun, and enjoy their cars, without necessarily pushing things to the edge. "Aggressive" drivers are those that ARE pushing their cars, specifically braking a lot harder, but not focusing on the last few tenths per lap. "Competitive" drivers are those that are really pushing the braking envelope. For the hard data, I'll use my car, as I believe that it's fairly typical of the hard-core but street legal car that hits the track events with some regularity.
From what I can see, there's a pretty simple upgrade path as drivers move from one group to the next, with some interesting results in some areas.
First, we need to realize exactly what it is we're working with! While there are a lot of differing opinions about exactly what the S197 chassis really weighs, I've personally seen eight of them weighed on "official" scales, and excepting stripped track-only cars, they are ALL over 3600lbs in race trim, with driver and safety gear in place. With just a splash of fuel left and a full interior, I scaled at 3605! That really is a LOT of weight to try to accelerate, slow, and turn, and puts a pretty heavy load on the braking systems of the cars.
With the Fox and SN95 chassis, the brake upgrade most commonly seen is a change to the "Cobra" calipers and rotors, which is almost exactly what we start with from the factory; 13" rotor diameter up front, PBR-type dual piston calipers, and a floating mount. Make no mistake, this base package does NOT suck, and can be very successfully tracked. While 12-piston calipers, and two-piece 15" rotors made from unobtanium are available, unless you're just a banzai driver, they are NOT necessary! More on this later.
For drivers that track their cars once or twice a year, or are contemplating their first track event, there are only a few basic upgrades that really need to be done. First is pad compound. The stock pads are okay on the street, but will turn to dust rapidly under the demands of a road course, and I have seen more than one student parked before the end of the weekend due to burned up brake pads. This is important: there is NO "street" compound that can handle the heat of open-tracking! Given the weight of the car, and the power available to get up to speed, you NEED to go to a race compound of some sort. For the entry-level or occasional use car, something like a Hawk HT-10 up front with HP-Plus in the rear is usually sufficient. For Carbotech fans, that would be XP-8 up front, and Bobcat in the rear. The next thing is brake fluid. Again for the occasional or novice driver, simply flushing the system with clean, fresh, factory-type fluid is sufficient, but make sure that the fluid in the car is less than a year old before you hit the track. Honestly, that's it. Mild pads and fresh fluid are really all you need, brake-wise, to hit the track for your first time, or if you only go occasionally. Once you're done with your event, slap the OE pads back in, and you're golden.
For more aggressive drivers, the formula starts to change. Brake cooling becomes critical, aftermarket fluids are indicated, and the pad compound needs to change upwards. Hawk DTC-60 up front, and HT-10 in the rear (Carbotech XP-10 and XP-8), good "racing" brake fluid (Motul RBF600, ATE SuperBlue or SuperGold, Brembo; Castrol SRF) that you flush every six months, and of course, brake ducting. Quantum Motorsports, Steeda, and Agent47 all sell ducting kits, and all of them work fairly well. If you're leaning towards the more hard-core end of the spectrum, though, you may want to roll your own. It'll save you some money, and give you more efficiency in cooling. Butler-Bilt inlets, QMS high-temp duct hose, and fabricated backing plates will let you get more airflow through the rotor, but too much of a good thing can be bad! At the really high-speed tracks, I have to tape off part of my inlets, since I can over-cool the brakes and actually drop the temps below the point where the initial bite is still good. Also, if you are going to track your car fairly regularly, invest in some GOOD braided brake hoses. Yes, they will stiffen up the pedal feel a tiny bit, but the primary benefit is more durability when subjected to the heat, vibration, and occasional contact with "track boogers" being kicked up.
For the truly hard-core, the fabricated ducting setup is the only way to go, along with serious brake pads. Think DTC-70/DTC-60 (XP-16/XP-12), and consider making the move to Castrol SRF fluid. Hugely expensive, but reports indicate that it's all but impossible to boil the stuff, and it won't grab moisture out of the air like the Motul fluid will. I personally just can't afford $70/quart fluids, so I haven't tried it myself...
The bottom line on pads, however, is that if you’re running the stock GT brake package, you’ll need to stagger your pad compounds, running one range less aggressive in the rear, to keep braking balance from biasing rearward. If you run the same compounds front and rear, you will have a tendency to lock up the rears, triggering early ABS activation, and at that point you’re throwing away braking potential.
Please note that ALL of the above packages and recommendations are still based around OE calipers! Now, let's talk about rotors... The more aggressive the brake compound you run, the more metallic content is in the pad, and the faster the rotor wear you get. While there are ALWAYS exceptions, that's a good rule of thumb. Given that, my personal recommendation is to stick with OE, plain-face rotors. Cross-drilled pieces only provide opportunity for localized hot-spotting and cracking. Slots were originally used to provide an off-gas path when brake pads used organic base compounds (think asbestos), with the modern compounds, the whole need to allow a gas path has essentially disappeared.
Up to this point, we’ve been looking exclusively at the OE braking package: 13” vented, plain-face rotors with 2-piston sliding calipers up front. Now, I think we should look at the alternatives.
GT vs. GT500 front brake packages
Since both packages use the base GT rear caliper and rotor, we can focus strictly on the difference in the front brakes. Up to this point, I have been very positive about the stock GT brake package, and I continue to assert that there is really nothing inherently wrong with that package, with one exception: Durability. There are a few drawbacks with the GT front package, and they all revolve around heat. The aluminum caliper castings will, over time, soften in the bridge area between the inboard and outboard pads, and spread. This leads to a taper-wear condition, where the tops of the pads, particularly the outboard side, will wear very quickly, but the bottom won’t. This leaves the pad with a wedge shape when looked at in profile, and dramatically shortens the lifespan of the pad. In addition, since there is relatively limited thermal capacity in the rotor, they tend to surface-check quickly, which can lead to cracking. In 2009, over 16 track days, I went through two sets of calipers, three sets of rotors, and three sets of brake pads. Parts cost, however, really isn’t that high, due to ready access to take-off parts for the rotors and calipers.
The GT500 (Brembo) calipers, however, are a completely different animal. Instead of a floating-caliper design, where the caliper rides on a pair of slid-pins and shifts back and forth with pedal actuation, the Brembo calipers are fixed-mount, bolting directly onto the knuckle, and are equipped with four pistons, two to the inboard, and two to the outboard halves of the calipers. In addition to the much stronger bridge section, which is not nearly as prone to spread as the PBR style caliper on the base GT package, they are engineered for 14” diameter rotors, rather than the 13” stockers. In terms of overall weight, you will gain approximately 5lbs per side, all unsprung, but you will also gain a much larger thermal capacity in the rotor. The three main downsides to the GT500 package are: 1) MUCH higher parts cost. The pads are much more expensive, as are the rotors (no take-off source). 2) More complicated bleeding, as there are two bleeder screws per caliper. 3) Potential for pad knock-back. This occurs when there is play in the wheel bearing or major runout in the rotor, which causes the pads to be pushed away from the rotor surface. The result is a long pedal stroke on initial application, but that can be counteracted by a stab or two with the left foot just before entering the braking zone. The major upsides to the Brembo package are: 1) drastically reduced caliper fatigue. The “bending bridge” problem with the PBR-style calipers is eliminated. 2) more even pad wear, as there is no frictional binding in the slide pins. 3) vastly lower heat transfer into the brake fluid, resulting in reduced need to bleed mid-day. For an experiment, I went as far as I could before the brake fluid boiled badly enough to get a spongy pedal, and it was a total of four track days with no bleeding! 4) demonstrably longer component wear. In 2010, after 13 track days, I was finally forced to change the front pads. The rotors were just starting to show surface checking, and of course, the calipers themselves were fine. According to my spreadsheets, this all equates to reduced net operating cost, despite the higher initial outlay. For track junkies, this will pay off, for the occasional user, perhaps not as drastically.
The brake pad bias will also change with the larger rotor diameter, since the braking torque is increased up front. Instead of stepping down one compound range in the rear, with the 14”rotor testing shows that identical compounds are the best match. I’ve had excellent success with the DTC-60 (or XP-12) compound all the way around.
This should be a sticky
