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Opinion

Bragging Rights

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A major test of brake systems took place in 1902 on an unpaved road in New York City. Driver Ransom E. Olds had arranged to test a new brake system against the tire brake of a four horse team and the internal drum brake of a Victoria horseless carriage. His Oldsmobile sported a single flexible stainless steel band wrapped around a drum on the rear axle. When the brake pedal was applied the band contacted to grip the drum. From a thunderous speed of 14 mph the Olds stopped in 21.5 ft, the Victoria in 37 ft and the horses (which may not have been going 14 mph, but had no engine braking to aid them in 77.5 ft) (SAE Test Standards were sparse).

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Since the early days of the automobile, selling performance has been one of the stalwart marketing strategies to move the metal. Whether on the secret drag strips of your locale or blaring from the front page of many of the popular magazines that inform us about the next new vehicle, performance showdowns are always popular. I’ll leave it to others to extol you on the details of 0-60, 1/8th or 1/4th mile. However, after all those runs everybody is counting on the brake guys to bring them home safe, quick and short!

Virtually every new vehicle evaluation includes a stopping distance test. Usually, an initial speed and distance is reported and the bragging rights begin. The tests are relatively simple tests using fairly straight forward equipment.

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The common stopping distance test requires you to accelerate the vehicle to a given speed and slam on the brake and then measure how far it took to bring the vehicle to a stop. Most evaluations use a target speed of 60 mph, 70 mph or 80 mph. Of course the European magazines will run at 100 kph (62.5 mph). It is common to run a series of 5-8 stops and report either the best or average distance.

Simple enough.

The distance is measured with a “5th wheel.” Traditionally, this was a fancy bicycle wheel that dragged behind the car with a transducer that counts revolutions of the wheel. Multiply the revolutions by circumference of the tire and you have your distance. The starting point can be normalized by a variety of methods. Most common is to start the count by monitoring the brake light switch.

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Some systems will trigger at a given speed. For example, start braking at 62 mph and the system triggers at 60 mph.

On a side note, the measurement of distance in automotive testing has changed dramatically over the years. At the speeds Mr. Olds was traveling, I would suspect that they attempted to start braking at a target in the road and measure from there. This evolved into more exciting systems that actually shot a paint dot on the road (triggered by a brake light switch). The bicycle 5th wheel has passed the torch to optical, inertial and even GPS devices to measure distance.

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The Physics
Now that we understand this relatively simple test, what is the significance? All will accept the shorter the stopping distance, the greater aide to safety and performance the brake system offers. Therefore, it is important to understand what influences the brakes to stop the car and what are the practical and theoretical limits.

To understand the theoretical limits, you must first realize that there is a direct relationship between speed, distance and deceleration.

If you want a shorter distance, either go slower at the start or generate more deceleration.

Weight is also a factor in how much deceleration a vehicle can produce. To understand how we can increase the deceleration, we look to Mr. Newton for advice. He tells us that the amount of deceleration we can have is equal to the mass (weight) of our vehicle and the amount of force we can apply to it (F=ma). The force we can apply is generated by the brake system. This force is created at the brakes and transmitted to the road through the tire and road interface.

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This torque force attempts to slow down a spinning wheel. But, as the wheel is slowed relative to the speed of the vehicle a characteristic called “slip” is developed.

Slip is characterized in a percentage and ranges from zero to 100 percent. By definition, zero percent represents a free rolling wheel and 100 percent represents a skidding or “locked” wheel. While the tire is slipping it is generating a stopping force. The magnitude of this force is a function of two primary things. First, the frictional relationship between the tire and road plays a large role. Second, more slip is not a good thing!

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The peak friction that occurs between a tire and road occurs at about 15-25% slip. A locked wheel generates a significant amount of slip but not the maximum the tire is capable of. A typical curve of this condition is shown in figure 1.

If you want the best stopping distance a particular tire and road can generate, you must maximize the percentage of the stop that operates at the peak slip level. The brake system contributes to this in three ways:

  • Insuring that the torque necessary to reach the 15- to 20-percent slip zone can be generated by the brake system.

  • How fast the brakes will generate this torque and the corresponding level of slip.

  • How accurately the brakes can maintain this optimum slip level over the entire stop.

The Mechanics
The mechanics of the brake system can be tuned in order to achieve the correct performance. The parts that can be tuned include caliper size, friction levels and rotor diameter.

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The second factor in braking distances comes down to response time and is dominated by the mechanical factors like the applied speed of the booster and the amount of fluid movement required to generate the desired torque action at the wheel.

Restrictions including contamination or pinched lines and hoses can degrade the stopping distance because it can slow the flow of the brake fluid between the master cylinder and caliper.

The last controlling factor in stopping distances is the job of ABS. On the simplest level, the ABS system calculates the slip and controls the pressure to keep it at the ideal value.

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Actual Results May Vary…
If the basic physics, mechanical and ABS elements are properly tuned, the stopping distance is very close to its theoretical limit. Virtually all properly functioning brake systems optimize these factors to within a few percent.

Look at any summary in a car magazine and you will see that stopping distance from one vehicle to another can vary drastically. So how do we account for this? Two other factors come into play.

First, the magnitude of the peak braking efficiency. Essentially it comes down to the tire. A “stickier” tire has a higher peak and therefore has the potential to generate more stopping force and thus shorter distance. Secondly, All four wheels must be operating at their individual peak simultaneously to achieve the true theoretical limit. Here is where the weight balance of the vehicle and the corresponding relationship between front and rear brake sizing become important.

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It is common to find brake pad advertising dramatic improvements in stopping distance. Claims of improvement are often seen boasting 20-, 30- even 45-percent improvement. With the above understanding, these claims should be viewed with a healthy dose of professional skepticism.

If the governing factors were designed properly and functioning correctly, there is very little room for improvement. Since the laws of physics employed here date back to Mr. Newton in the 1600s and are well known within every brake department at all car companies and brake companies, it is doubtful there are many stones left upturned. If a product can really generate the claimed improvements in a true comparison, it should be clearly understood what compromise the vehicle engineer chose or was forced to make to create the sub-optimal result in the test.

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The measured stopping distance is a very critical performance value to establish a vehicle’s performance. Typical numbers from 60 mph are in the range of 120-170 ft. Chief engineers and brake engineers love to brag about these values. However, this is only a partial contribution to overall safety and accident prevention. In the real world acceptable stopping distance is one foot shorter than what ever the thing was you didn’t want to hit. Treating stopping distance in this manner requires us to consider several other factors.

These become the human factor contribution. If you assume the available distance to stop is established at the instant the obstacle/threat exists, the driver must mentally process the event and judge that it is indeed a threat and requires action. Studies have shown this takes between 0.5 and 1.5 seconds. At 60 mph, this represents 44-132 ft of road. Next, the brain must tell the leg to move and the leg must actually move to the pedal. This can be another 0.5 – 0.1 seconds (another 5 to 10 ft). Then the mechanical losses in the system must be overcome and now, finally, the brake light switch is tripped (another 4-8 ft).

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As a result of all of this, in most cases you will require as much as double the published stopping distance to actually avoid a real threat on the road. This front half of the true stopping distance really represents the true area of opportunity for future improvements in driver safety and accident prevention. Forward looking radar, night vision, driver alertness systems, driver training and even autonomous braking all have the potential ability to significantly improve the distance required to identify, react, and execute the stop. I wonder if Mr. Olds had any notion of how far his duel would have evolved?

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