AKRON—It's been 15 years since Ford Motor Co. and Bridgestone Americas jointly recalled approximately 14.4 million Firestone-branded tires linked to a series of fatal sport-utility vehicle accidents, but the impact on the industry can still be felt today in the form of tire pressure monitoring system (TPMS) technology.
The recall prompted the National Highway Traffic Safety Administration (NHTSA) to develop a Federal Motor Vehicle Safety Standard requiring the installation of TPMS on new vehicles. In the years that followed, auto makers have explored a variety of solutions.
Steve Landis, head of product management, TPMS and electronics innovation - NAFTA, for Continental Automotive Systems Inc., told Tire Business the combined TPMS replacement and OE markets today comprise between 20 and 30 brands—including Continental's own VDO brand—and it continues to grow.
However, not all TPMS units are created equal.
In an interview, Mr. Landis discussed differences between the types of TPMS available and the advantages and disadvantages of each.
Many of the earliest tire pressure monitoring systems were indirect, utilizing wheel speed sensors already in place as part of the vehicle's anti-lock braking (ABS) or electronic stability control (ESC) systems. According to Mr. Landis, indirect systems are attractive to car makers because they require virtually no maintenance and are relatively cheap and easy to produce.
In addition, service kits for indirect systems generally aren't offered as it's not something a dealer likely would have to replace.
This level of simplicity comes with its fair share of limitations—one of the biggest of which is that a vehicle equipped with indirect TPMS needs to be in motion for it to function, Mr. Landis said.
Unlike direct systems that measure air pressure in each individual tire, indirect TPMS never actually knows what a tire's pressure is, according to Mr. Landis. Rather, the system works via tire diameter evaluation, using the wheel speed sensors to compare each tire.
“The vehicle is comparing tire to tire and it says, 'Well, wait a second, why is this one tire rotating at a much slower rate of speed versus another tire?'” he said. This comparison process is how low tire pressure is flagged.
That comparison can be impacted by a number of external factors.
“Say you had your system for OE wheels and tires at 32 psi,” Mr. Landis said. “With plus sizing, now perhaps you would do 35 psi. When you do that type of thing, whenever you change pressure in your tires—even if you're not changing your wheels and tires, but just because you're changing the pressure—you should do a reset.
“...You have to recalibrate the vehicle so that the actual system knows what it's supposed to be when everything is good so that it can compare when something is bad,” he said.
The process of recalibration itself can present problems, though.
“If somebody recalibrates the tires when they're at low tire pressure, indirect will never have any idea that there's a critical situation because the system was recalibrated wrong,” Mr. Landis said.
Other potential issues that can impact the dependability of indirect systems include differences in tread height on a vehicle, tire slippage on icy roads and even all four tires losing pressure naturally at the same rate. Indirect systems also won't notify drivers which tire is low on pressure.
“There are definitely some faults with it, but it's an interesting system,” Mr. Landis said. “It's effectively not using any additional hardware. It's just using software with algorithms to compare.”
Technological changes in the last few years have increased indirect TPMS' accuracy and reliability, he said. While earlier systems relied exclusively on tire diameter evaluation, new versions are capable of detecting vibration frequency and amplitude in the tire, leading to improved results.
Still, most vehicle models sold in the U.S. today have gravitated to direct systems, said Mr. Landis, who estimated direct TPMS accounts for 95 percent of new vehicle sales.
In other areas, like Europe, the story is different. There, direct systems account for about 70 percent of the market, while indirect ones cover the remaining 30 percent, he said.
“In Europe the legislation they have is much later than in the U.S. and the legislation is based on fuel efficiency, whereas in the States it's based more on safety,” he said.
Recently the U.S. market has seen a “slight shift” from some auto makers toward indirect systems again because of improvements in accuracy and cost savings, according to Mr. Landis. Some Honda and Mazda models and “a pretty good number” of Volkswagens continue to produce vehicles with indirect TPMS.
“I don't think indirect is ever going to bump direct systems, but on lower-end vehicles, just because of that cost savings and because of the improvements in accuracy, it's not dead,” he said. “And I don't think it's going to die. It'll be around for a while.”
Direct systems are more complicated than indirect ones because of the addition of dedicated tire pressure sensors and the need for occasional service.
“There's an investment in it, but what you get with that investment is you don't have to have motion.”
Today there are two primary types of direct valve sensors being produced: snap-in and clamp-in models. Both are designed to attach to the tire's valve.
According to Mr. Landis, clamp-in sensors typically are made of anodized aluminum and are torqued into place with a hex nut, making them able to handle higher speeds than snap-in models. However, he cautioned that dealers need to be careful with service kit parts when installing replacements.
“When you torque it down, you wear off the anodization,” he said. “You really need to keep up on the service parts, but if you're looking for maximum performance that's really the answer.”
Snap-in models are made of rubber and feature a traditional-looking stem, and the replacement process is similar. They usually are more forgiving than clamp-in models.
“Just like with traditional stems before TPMS, you should replace the service kit parts for the stem for TPMS-equipped vehicles, but you don't have to be quite as careful with the snap-in (models) because it's brass and rubber,” he said.
Speed of installation is another benefit, both for vehicle manufacturers and dealers who service the tires.
Both types of sensors are capable of accurately measuring pressure in each tire and typically will warn drivers of a drop in pressure before an indirect system could, Mr. Landis said. They also can detect gradual air loss over time, one of the shortcomings of indirect systems.
While indirect TPMS has continued to improve, so have direct systems.
Two of the newest advanced technologies being built into modern sensors are auto-locate and auto-learn capabilities, allowing the sensor to “know” which corner of the vehicle it is mounted on and letting the vehicle learn each sensor ID automatically.
In the future, Mr. Landis speculated we will see sensors mounted directly to the inside of the tire. VDO already is producing glue-in sensors that attach directly to the inner liner via a glue-in patch for use in Europe. He added that tires eventually may come pre-equipped with pockets in the inner liner for these sensors.
According to Mr. Landis, mounting TPMS sensors directly to the tire offers many potential advantages in terms of data collection.
“If you could locate the sensor there, what you could actually do via this enhanced monitoring of pressure is you can also get the actual tread temperature, you can detect load and tread depth and with this information you can actually then—if you have it processed with the architecture of the vehicle—you can enhance suspension, the powertain and the braking characteristics,” he said. “You can further optimize all that.
“That's something you would never be able to do with indirect systems, but you also can't do that today because the sensor needs to be relocated to a different place,” he said.
So how far away is this particular technology?
“If one of the auto makers buys into the technology, you're really only talking something like three to five years,” Mr. Landis said.
To reach this reporter: wschertz @crain.com; 330-865-6148; Twitter: @Will_Schertz