Art's Articles
Ice Pedal
Ice Pedal --- OMG My Brakes Don’t Work
What It Is, Why It Happens and What to Do About It
by Art Meltzer
Many of us have had the awful experience of Ice Pedal (IP for short) – applying the brakes in a heavy braking zone only to find the brake pedal incompressible and the brakes barely slowing the car. This often results in going very wide around the corner, going off the track, or worse. I’ve heard various explanations of IP which suggests no one really knows what causes it.
IP is not unique to Porsche. Randy Probst describes an incident of IP on youtube while driving a yellow Mazda turbo RX-7 in 1994 at Road Atlanta. Incidentally, I’ve heard drivers of GM and Ford cars describe IP.
Bosch manufactures all Porsche brake components. I contacted Bosch inquiring about IP. All they would tell me is that IP occurs as a consequence of the ABS computer not being able to correctly analyze the wheel speed sensor data so that the computer determines there is significant wheel slip when, in fact no wheel slip is occurring.
Not being an engineer, my adventure into discovering the precipitating factors and reasons for IP began with understanding the fundamentals of production car (OEM) ABS systems. Several companies manufacture ABS systems in the US and abroad but all the systems have a basic commonality.
I was able to find several detailed discussions of how ABS systems work including an ASE tutorial. Combining this understanding with Bosch’s brief statement about IP and information in the owner’s manual for Bosch’s M4 racing ABS system, I was able to formulate an understanding as to how a design limitation in production car ABS systems can, under a very narrow set of circumstances, result in IP.
The ABS system consists of the ABS computer, an accumulator/reservoir (A/R) that under control of the ABS computer can release pressure in the brake lines, a brake fluid modulator consisting of computer controlled solenoid valves that determine the path of the brake fluid as it courses either from the master cylinder to the brake lines or from the brake lines to the A/R, an ABS pump that returns brake fluid from the A/R to the master cylinder, a yaw sensor to determine the angle of the direction of travel of the car relative to the long axis of the car, a steering wheel angle sensor, the master cylinder, brake lines, calipers, brake pads, rotors, wheel speed sensors (WSS), wheels, and tires.
Wheel slip occurs when the force exerted by the brake pads on the rotors exceeds the grip between the tires and the road surface. Wheel slip means the tires do not completely adhere to the road surface (they slip) so that the distance the tire travels (tire circumference times tire RPM) will be less than the distance travelled by the car. An extreme example of wheel slip is tire lockup – no wheel rotation at all.
Maximum tire grip is achieved when wheel slip is in the 5% - 20% range. The ABS is designed to maintain wheel slip within this range as optimum tire grip will result in minimal stopping distance. The ABS utilizes the steering wheel angle and yaw sensor data to sense and correct for car rotation to assure that the car stops in a straight line.
The ABS computer’s algorithm analyzes WSS data to determine when ABS activation is necessary. When activated, valves in the modulator close off the master cylinder to the brake lines and open the brake lines to the A/R. This allows the brake fluid pressure in the brake lines to decompress permitting the slipping wheels rotate freely. Next, the valves return to their initial configuration so that brake pedal pressure is transmitted to the brake lines. If reactivation of the ABS is necessary the cycle repeats.
Reactivation of the ABS will continue (at ten times per second) until wheel slip is within the acceptable range. Brake fluid that accumulates in the A/R is pumped back to the master cylinder by the ABS pump. Under extreme circumstances the return of brake fluid to the master cylinder is perceived by the driver as a vibration of the brake pedal.
Engineers determine the maximum wheel deceleration rate for a car based on the car’s weight, brake design, etc. The predominant parameters that activate ABS are wheel slip and wheel deceleration rates that exceed the maximum calculated values. ABS activation can involve an individual wheel or any combination of wheels. Release of the brake pedal by the driver under any circumstance deactivates the ABS and resets the computer.
Brake Assist (BA) is a standard feature on many production cars. Normally, brake pedal pressure is amplified by the brake booster to create brake fluid pressure. When the speed and force of brake pedal activation by the driver exceeds programmed thresholds, BA augments the brake fluid pressure to further decrease stopping distance. The action of BA may contribute to the conditions that precipitate IP.
Under ideal conditions, on dry asphalt, ABS activation occurs well before wheel lockup. ABS functioning on dry asphalt frequently involves a small number of ABS cycles that occur at ten times per second (or faster). The frequency and duration of ABS activation is often too fast and too small to be perceived by the driver.
The situation is different when tire grip is compromised such as on snow, ice, rain, leaves, etc. In poor grip conditions the wheels may lockup. Many ABS cycles may be needed in order to resolve wheel slip and excessive wheel deceleration. In this case the driver is more likely to feel ABS pulsations in the brake pedal.
The OEM ABS was designed for conditions incurred during daily driving. Speeds and rates of deceleration experienced on the track far exceed those in daily driving.
It’s notable that computer calculations of vehicle speed from wheel speed sensor data depends on tire diameter and the ratio of the diameters of the front and rear tires. Standard OEM values for a particular car are programmed into the car’s ABS computer. The use aftermarket tires that differ in diameter and front/rear tire diameter ratio from factory recommended tires will render the ABS calculations inaccurate. According to the “Tech Q&A” section of the June 2023 edition of Porsche Panorama, a change of tire circumference or diameter of 4% or more could induce errors in the traction control or ABS systems.
For example, suppose an aftermarket tire that is one inch larger than the OEM tire was put on a Cayman S. At 6000 rpm in 4th gear, the car’s speed relative to the asphalt would be 4 mph greater than the computer’s calculation of the car’s speed because computer’s calculation is based on the OEM tire and the OEM tire is smaller than the aftermarket tire. Combining this with staggered tire sizes seen on many cars, threshold braking, R-compound tires, high-performance brake components and other performance modifications may result in wheel slip and wheel deceleration rates that are beyond the computational facility of the ABS computer.
According to Bosch, there are combinations of wheel speed and wheel deceleration data that can fool the ABS computer into determining that there is significant wheel slip when in reality there is none. This is the root cause of ice pedal. Let’s denote this by “IP-conditions” and see how IP-conditions lead to ice pedal.
Under IP-conditions the ABS algorithm erroneously determines that excessive wheel slip and/or excessive wheel deceleration is present. This triggers ABS activation consisting of blocking off the master cylinder and releasing brake fluid pressure in the brake lines to allow the slipping wheels to rotate freely. Because of IP-conditions, the ABS computer is incapable of recognizing that the wheels are spinning freely meaning it won’t proceed to the next step of directing the valves in the accumulator to return to their initial configuration that restores the relationship of the brake pedal with brake function.
The ABS cycle is frozen. The brake pedal is incompressible, the brakes won’t respond to brake pedal pressure, and the car won’t slow down. Not an ideal condition in a heavy braking zone.
The only way to restore normal brake function is to reset the ABS system. In order to accomplish this the driver must release the brake pedal returning the ABS to its default configuration. Since the window for IP-conditions is narrow, reapplication of brake pedal will hopefully restore normal brake function.
The take away message from this is to understand and be aware of IP so that you can immediately recognize why your brakes aren’t working. Early recognition of IP combined with immediately release the brake pedal will reset the ABS and minimize the time you that your brakes don’t work. Hopefully, all that will happen is a scary corner and you’ll keep the shiny side up.
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- Drivers Education
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Left Foot Braking
By Art Meltzer
Most of us learned to drive on a car with an automatic transmission. We were taught to use our right foot to operate both the brake and gas pedals. Let’s call this one-foot driving (OFD). There is no role for your left foot in OFD. For those who learned to drive on a manual transmission your left foot was limited to operating the clutch pedal.
Probably the most important reason for the limited role of your left foot is safety. Using only your right foot for both the gas and brake minimizes the likelihood that you will step on the gas instead of the brake and vice-versa. Also, OFD rules out the possibility that you will simultaneously apply the gas and brake. Simultaneous application of the gas and brake will unnecessarily wear the brakes and may damage the torque converter in the transmission.
Let’s use the term left-foot braking (LFB) to refer to using your left foot to brake. This article will highlight the advantages of LFB over OFD and provide the reader with a mechanism to learn LFB.
High performance driver’s education (HPDE) is a sport. Sports generally require using both hands, both feet, and both eyes. In baseball we don’t bat with one eye closed. In football we don’t hop down the field for a pass on one leg. Doesn’t it seem odd that in HPDE we use both eyes to see, both hands on the steering wheel but drive with one foot? Despite these comparisons, baseball and football are unlike HPDE – they’re sports that require only one ball.
The key issue with OFD is time that it takes time, at least 4-5 tenths of a second, to move from one pedal to the other. During the time that you’re switching pedals your car is coasting.
At high speed (~120 mph) the car will travel about 17.6 ft in one tenth of a second meaning you will coast for about 70.4 – 88 ft in the time it takes to switch pedals. At 50 mph the car travel about 7.3 ft in a tenth of a second meaning it will coast for about 29 – 36.5 ft while switching pedals. When coasting you can’t accelerate, slow the car, or influence weight transfer – in other words you can’t control the car.
Sometimes, with OFD, when switching pedals your foot does not “find” the pedal that you’re switching to. This is most problematic when going from gas to brake as improper foot position on the brake pedal can impact upon your ability to slow the car.
LFB means you apply the brake pedal with your left foot whenever possible. In a car with a manual transmission LFB is possible only in braking situations in which you don’t change gears. With an automatic transmission or a PDK, you can LFB everywhere.
One advantage of LFB is that there is no time delay or coasting when switching inputs from the throttle to the brake or vice versa. Consider entering a heavy braking zone at high speed (say 120 mph). OFB will commit you to coming off the gas and coasting for 70.4 – 88 ft before slowing the car. Alternatively, LFB by eliminating coasting, will allow you to remain on the throttle until you reach the point that you want to apply the brakes.
When negotiating sections of linked turns your speed is generally 50-60 mph range. Control of weight transfer in linked turns is a fundamental element allowing you to control the car, maintain momentum, and accelerate upon exiting the section. Oftentimes one must use the gas, brake, or both in traversing a section of linked turns in order to maintain the proper line, preserve speed, and position the car to in order to accelerate when exiting the section.
At 50-60 mph OFD commits you to coast for 29 – 44 ft when switching pedals. While coasting the car slows (engine braking and friction), you can’t transfer weight to the front wheels, plant the rear end or keep the car balanced with brake/throttle inputs. Your ability to control the car and maintain speed thru linked turns is compromised. Alternatively, with LFB your brake/gas transitions are simultaneous giving you the maximum opportunity to attain minimum section times.
Many tracks have high speed turns that require a brush of the brakes while turning in to transfer weight to the front wheels. By eliminating coasting LFB allows you stay on the throttle longer prior to the turn and reapply the throttle sooner after the turn so that you negotiate the turn with maximum efficiency.
Learning to LFB is easier than you think but it requires commitment and practice. I recommend training your left foot (and your right brain) to do something new while driving on the street. At first you will have to think about using your left foot to brake. Your braking will be clumsy and fitful. Smoothness and efficiency will eventually come with practice. Ultimately, you will use your left foot without thinking – meaning you have reached the point where LFB is an unconscious act. At this point, you’re ready to try it on the track.
With some HPDE experience using LFB I’m confident you’ll achieve a significant improvement in your gas/brake pedal transitions and overall improvement in your driving.
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- Drivers Education
- Hits: 107
Heel Toe Shifting
“WHEN THE STUDENT IS REALLY READY THE TEACHER WILL APPEAR” - -Lao Tzu
HOW TO TEACH YOURSELF HEEL TOE SHIFTING
By Art Meltzer
For those of us that drive cars with three pedals, heel-to-toe shifting (HTS) is an essential component of HPDE. HTS is a complex maneuver that requires precise footwork coordinated with an awareness of engine rpms that is performed during heavy braking while preparing for a corner. It’s no surprise that many drivers find HTS difficult to learn and to perform.
This article will discuss the consequences of downshifting without HTS as it relates to the impact on the car and the driver’s ability to negotiate a corner and present a stepwise and safe technique to teach yourself HTS.
Simply put, the engine converts the potential energy stored in the chemical bonds of gasoline into kinetic energy in the form of a rotating crankshaft and flywheel. The car’s transmission consists of an input shaft (IS), internal gearing, and an output shaft (OS). The IS is driven by the engine when the clutch is engaged and the OS is mechanically connected to the axles, wheels, and tires. The tires connect the car to the road. The internal gearing of the transmission translates the rotational speed of the IS to the OS so that the rotational speed of the OS provides maximal mechanical advantage to propel the car.
The clutch consists of the clutch pedal, clutch springs and the friction disc. The friction disc is mechanically connected to the IS and contains friction material that allows the friction disc to grip the rotating flywheel. When there is no pressure on the clutch pedal (meaning the clutch is engaged) the clutch springs press the friction disc against the flywheel so that the friction disc and the flywheel are locked and rotate in unison. Stepping on the clutch pedal moves the friction disc away from the flywheel. With the clutch pedal fully depressed the friction disc is completely disengaged from the flywheel so that the engine is mechanically separated from the drivetrain.
SHIFTING WITHOUT HTS
Let’s say a car is traveling at 110 miles an hour in fifth gear approaching a turn. You want to downshift to third gear and enter the turn at 50 mph. Also, assume that at 50 mph in third gear with the clutch engaged, the crankshaft, flywheel and IS all rotate at 4500 rpms.
At some point after beginning braking, the driver must downshift. Let’s assume that the driver downshifts when the car is traveling at exactly 50 mph and the engine is at 4500 rpms. Without HTS the sequence of events involved in downshifting consist of depressing the clutch pedal, moving the stick to third gear, and releasing the clutch pedal.
In the time it takes to depress the clutch and move the stick the engine will slow to idle speed, approximately 850 rpms. At the same time, the transmission is mechanically connected with the rear wheels that are travelling at 50 mph.
With the transmission in third gear, and the rear wheels (at 50 mph) driving the OS, the internal gearing of the transmission causes the IS and the friction disc to rotate at 4500 rpms. This differential in rpms between the friction disc and the flywheel (in this example 4500 rpms vs 850 rpms) is referred to as rev mismatch.
With this degree of rev mismatch, no matter how carefully the clutch pedal is released, there will be a forceful and abrupt exchange of energy between the rear wheels/transmission/friction disc (high energy) and the engine/flywheel at idle speed (low energy). The energy is transferred through the drive train and results in a powerful braking effect on the rear wheels as the car’s momentum is utilized to increase the engine rpms. This is referred to as engine braking. The powerful and jarring forces of engine braking are damaging to all of the components of the drivetrain, of many of which are delicate and expensive to repair.
There is significant forward weight transfer with engine braking. This forward weight transfer causes the nose of the car to dive and the rear end to become light. Sometimes the rear wheels will momentarily lock up. All of this occurs under heavy braking just before turn-in at a point in which proper balance of the car is essential and the driver’s concentration needs to be focused on negotiating the corner. Needless to say, unsettling the car and distracting the driver just before corner entry is not optimal.
Because of this, some drivers that do not utilize HTS will delay downshifting to just before turn-in to minimize the detrimental effects of rev mismatch. This may lessen the magnitude of engine braking but does not resolve the issue.
HEEL TOE SHIFTING
HTS eliminates rev mismatch by blipping the gas pedal with your right foot just as you’re releasing the clutch pedal at the same time your right foot is applying pressure to the brake pedal. This is harder to do than patting your head and rubbing your tummy at the same time.
I will refer to using your right foot to simultaneously brake and blip the throttle as the “one foot, two pedal” (1F2P) maneuver.
REV MATCHING
Rev matching corrects rev mismatch. In the example above, the rev mismatch consisted of the friction disc rotating at 4500 rpms with the flywheel rotating at 850 rpms. In order to attain rev matching one blips the throttle to increase the engine rpms and releases the clutch pedal in such a way that the clutch disc and flywheel engage when the engine rpms and the clutch disc rpms are roughly equal. This allows the clutch disc to seamlessly mate with the flywheel without any appreciable transfer of energy.
Contrary to the rule that all pedal inputs should be smooth, blipping the throttle requires that you “stab” the gas pedal. Your goal is to quickly increase engine rpms. As you blip the throttle with the clutch pedal depressed, stabbing the gas pedal does not upset the car.
All corners were not created equal. Appropriate engine rpms for rev matching varies depending on the situation. You need higher engine revs when downshifting for a fast corner as compared to downshifting for a slow corner.
Determination of engine rpms while rev matching is done by listening to the motor. With practice you will learn to listen to the motor and to be able to determine the optimal time to release the clutch after blipping the throttle.
If rev matching results in significant engine braking, then the engine was spinning too slowly when the clutch was released. There are two possibilities. One is that your blip did not sufficiently increase engine speed. The other is that your blip was sufficient but you missed the “sweet spot”. You didn’t time the clutch release with peak engine rpms and clutch engagement occurred too late, after the engine rpms significantly decreased.
If the car lurches forward when you release the clutch then you “over blipped” the throttle. The engine rpms were too high when the clutch was released so that the engine drove the car forward. The solution is to recognize, by listening, that you “over blipped” and wait an instant for the engine to slow before releasing the clutch.
Learning to rev match can be accomplished while driving on the street. The process involves the clutch pedal, stick, and throttle. The brake is not involved. I don’t recommend learning rev matching when downshifting into second gear because second gear is too low. Practice rev matching when downshifting into third or fourth gear.
The exercise consists of driving in fourth gear and letting the car slow to an appropriate speed for third gear. Depress the clutch, move the stick, and blip the throttle. Listen to the engine and release the clutch quickly when engine rpms peak. At first you may want to keep your eye on the tachometer to learn to coordinate engine sound with rpms. Releasing the clutch at 4500 - 5000 rpms is usually optimal. If clutch engagement was not smooth figure out why and make appropriate adjustments to your technique. Repeat this until your downshifting is consistently seamless.
TWO PEDAL ONE FOOT MANUVER
What works for me is to shift gears when the brake pedal is maximally depressed just prior to trailing off the brake pedal. Maintaining maximal brake pressure while blipping the throttle with your right foot is hard to do but can be learned. Trailing off the brake while blipping the throttle is much harder and may not be possible to do smoothly.
You want to wear your driving shoes when practicing the 2P1F because you want to simulate driving on the track. The 2P1F exercise is performed with your car standing still. Initially, practice with the engine off. When you’re comfortable with your technique practice the exercise with the engine running.
To perform the exercise start with your right foot on the gas pedal, move your right foot as quickly as possible to the brake pedal making sure that the ball of your foot is centered on the brake pedal, put the brake pedal on the floor, and blip the gas pedal with your right foot.
For optimal engagement of the brake pedal you want the ball of your foot to be dead center on the brake pedal. You want the ball of your foot to exert all the pressure on the brake pedal. You don’t want to use toes or the sole of your foot.
The reason for this is that when the ball of your foot is on the brake you will be able to use the large muscles of your leg and buttocks to create force on the pedal. This maximizes your ability to slow the car.
With the brake pedal on the floor try to find the gas pedal with your right foot. Experiment with different techniques until you find the one that works the best for you. Some drivers like to roll their foot from the brake to the gas pedal. Some drivers use their right heel. Whatever technique you use you must maintain constant pressure on the brake pedal while blipping the throttle. It’s imperative that you engage the brake and gas pedals exactly the same way every time with your right foot. Practice until you can perform the 2P1F exercise consistently, smoothly, and quickly.
Now start the car, leave the car in neutral with the emergency brake engaged. Practice the technique listening to the engine and watching the tachometer. You want to consistently be able to rev the engine to approximately 4500 rpms – 5000 rpms while keeping constant pressure on the brake pedal.
Gas pedals are skinny. This makes 2P1F more difficult. There are a number of bolt-on aftermarket gas pedals that change the size and shape of the gas pedal in order to facilitate 2P1F. The disadvantage is that if you don’t center the ball of your right foot precisely on the middle of the brake pedal you may inadvertently engage the gas pedal while braking.
At this point you are ready to HTS at speed in the braking zone at the track. Not only will your downshifts be silky smooth, but your cornering technique will improve greatly as you will enter the corner with a balanced car, at an appropriate corner entry speed, in the right gear, and without distractions.
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- Drivers Education
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