Anti-lock braking systems (ABSs) should be a familiar feature to anyone owning a modern vehicle, whether a car, truck or motorcycle. Mandated since 2004 in the European Union and 2013 in the U.S., ABS is widely credited with reducing both the number and severity of vehicle crashes. ABS prevents the wheels from “locking up,” or ceasing rotation, during braking. As noted in a previous column, in order to steer a vehicle, the wheels must be turning. By keeping the wheels turning, steering force is maximized, thus allowing the driver to have maximum control of the vehicle direction during deceleration.

A common misconception, however, is that ABS always reduces stopping distance compared to manual braking. Stopping distance is, in fact, reduced on relatively hard and smooth surfaces, such as concrete and asphalt. But on loose surfaces, such as sand, gravel and deep snow, ABS actually increases stopping distance. The reason for this somewhat paradoxical behavior is that loose materials will accumulate in front of a tire that is not rotating, creating a wedge effect that slows the vehicle more effectively. In spite of this disadvantage, ABS still gives the driver steering control and is considered the safer option on soft surfaces.

In the pre-ABS days, drivers were taught to “pump” the brakes to avoid a skid—that is, repeatedly press and release the brake pedal. Unfortunately, most drivers (especially inexperienced ones) would fail to do so during panic stops, and even those who did could only pump the brakes a few times per second, varying from full application to full release, which is not ideal. ABS, on the other hand, monitors the rotation rate of the wheels and can apply and release pressure to the brakes 15 times per second, and vary the pressure to prevent lock-up and maintain maximum steering control.

Another feature of all modern vehicles is the tire pressure monitoring system (TPMS). Also mandated in the U.S. since 2008 and the European Union since 2013, this system is intended to warn the driver when a tire is under-inflated, typically by 20% or more of the recommended minimum pressure. Improper tire pressure affects a number of vehicle performance characteristics, such as ride quality, handling, fuel economy, tire wear and even stopping distance. Most vehicles today use sensors inside the tire to directly measure pressure and relay the information to the vehicle via radio. So-called direct systems have several advantages, namely independent readings for all tires, absolute values of pressure and real-time monitoring of the pressure. The downsides include the expense of the sensors and transmitters, and the fact that the transmitters require power, usually supplied by battery, which eventually become discharged and require replacement.

A different kind of TPMS, the so-called indirect system, was actually more popular at first, and is still used by some vehicle manufacturers today. Instead of sensors inside the tires, this system uses the information on wheel rotation already supplied by the ABS sensors. As tire pressure falls, the tire becomes ever-so-slightly smaller in diameter, which means, to maintain the same ground speed, the tire must rotate ever-so-slightly faster. By measuring the change in rotational speed over time, and comparing the tires to each other, the onboard computer can determine when a wheel is turning faster than the others and send a signal to the driver (usually a warning light on the dashboard).

Clearly one advantage of this indirect system is cost, as no additional hardware is required (assuming the vehicle is equipped with ABS). The disadvantages, however, are numerous. The indirect system does not measure absolute pressure and cannot detect changes that effect all the wheels simultaneously, such as pressure changes due to temperature and elevation. Also, the indirect system is sensitive to anything that causes a wheel to turn at a different rate than the others, such as uneven tire wear. In addition, the system must be recalibrated anytime something that affects rotation rate is changed, such as adding air, buying new tires or rotating tires. This calibration typically takes about an hour of drive time and requires the driver to initiate the process, meaning the tire pressure must be manually checked and verified to be within specification before the calibration process.

Which system do you have? If your vehicle displays the pressure in each tire, it has direct TPMS. If you only get an alert to “check tire pressure” and cannot see the individual pressure values, you have the indirect TPMS.