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.