- Jul 6, 2009
Reviewed: Bridgestone 3G RFT Tires take the shock out of run-flats
Bridgestone 3G RFT Tires – Click above for high-res image gallery
The BMW enters the massive skidpad at more than 100 mph and begins a sweeping right turn along a soldiered row of orange cones. On cue, a pneumatic device attached to the hub of the spinning left rear wheel instantaneously rips its air valve clean out. Abruptly unsettled by the simulated tire "blow-out," and the collapse of the tire, the family wagon plunges into a long and flat uncontrolled spin knocking the hapless cones skyward in its wake. The BMW finally comes to a rest off course, facing 180-degrees from its original direction of travel.
Melodramatic in execution, the tire demonstration was repeated again moments later. This time the BMW was fitted with a set of Bridgestone's new third-generation run-flat tires. The high-speed blow-out again unsettled the car, but riding on sustained tire sidewalls it remained controllable as it tracked within the cordoned lane of orange cones and gently came to a stop.
The demonstration was powerful. Without a doubt, run-flat tires do work. Unfortunately, we've never met a set that didn't ride rough, feel like anchors, or cost us an arm and a leg at the register. We typically avoid them at all costs. Determined to make a change, Bridgestone invited us to Italy to try out its latest iteration of run-flats, promising to alter the way we think about the technology. What is this new tire? Why is it different from its predecessors? And, most importantly, how does it ride? Find those answers and more after the jump...
Photos Copyright ©2009 Bridgestone Tire
Air-filled vehicle tires, or "pneumatic tires" as they are more properly called, were invented in 1846 by Robert Thomson. The European inventor wrapped an elastic belt of rubberized canvas with an outer casing of leather for protection around a wheel and filled it with air. Patented in France, his so-called "Aerial Wheels" were fitted to horse-drawn carriages to improve the ride. Unfortunately, Thompson's wheels were ahead of their time. The technology to build a durable and affordable pneumatic tire had yet to evolve.
It took but another 50 years for the invention of Bartlett Clincher's rims, Schrader's valves, beaded edges, and corded tires to make the original pneumatic tire invention a commercial success. In 1904, Firestone started producing corded reinforced pneumatic tires. Two years later, they were chosen by Henry Ford to be OEM tires on the Model T.
The basic design of a tire-a flexible sealed vessel to hold pressurized air-has changed very little in the past century with the exception of belt construction and tire compounds. Furthermore, the tire's Achilles Heel remains the same; any compromise of the outer casing allows the pressure to escape resulting in a vehicle-crippling flat tire (according to Bridgestone, a staggering 60,000 motorists in the United States are afflicted by a punctured tire each day).
The concept of a tire that can still be "driven on" after suffering a "flat tire" (a complete loss of pressure) has been tossed around for decades. Tire manufacturers have tried self-sealing tires (a layer of flexible sealant inside the tire clogs punctures), dual-lined tires (additional layers of material slow or delay air loss), auxiliary-supported tires (a rigid ring mounted on the wheel holds the tire up at zero pressure), and self-supporting tires (stiffer sidewalls sustain the weight of the vehicle after a pressure loss). To skirt the issue entirely, some manufacturers have even tested non-pneumatic tires that don't require any pressurized air at all.
The most common type of run-flat today is the self-supporting tire. Compared to a conventional tire, the construction of a self-supporting run-flat requires additional material thickness in the sidewalls to prevent the tire's collapse once it has lost pressure. It is this additional reinforcement that adds weight, reduces ride comfort, and increases rolling resistance (these characteristics are the kiss-of-death to performance).
While Bridgestone originally developed run-flat tires for physically disabled drivers, the company's first well-known application of "self-supporting" run-flat tire technology (SSR) debuted in the late-80's on Porsche's 959 supercar. The OEM-spec tires on the 200-plus mph Kevlar-bodied exotic were "Potenza RE71 N0" run-flats. Considered the company's first-generation run-flat (known as the "1G RFT"), the technology utilized Bridgestone's "Denloc" system. Technically ahead of its time, the Denloc system was based on a special wheel design that held the tire's bead in place after a loss of pressure. While the technology worked, the failure to use a standard mass-produced wheel prevented it from expanding to a broad consumer market.
Things changed in 2004 when Bridgestone introduced its second-generation run-flat technology (2G RFT). Significantly updated, the tires were designed to fit standardized wheels, thus releasing the need for expensive specialty rims. Lighter than their predecessors, and with improved sidewall-reinforced rubber, the 2G RFT became an OEM tire for many BMW, MINI, Toyota and Infiniti vehicles. The product line also expanded to high performance vehicles such as the Ferrari 612 Scaglietti and BMW Z8.
Consumers embraced the additional mobility and safety benefits of the 2G RFT, but they never warmed to the handling compromises as a result of the additional unsprung weight, the harsh ride (the sidewalls are a noticeable 15 percent harder than a conventional tire, says Bridgestone), or the expensive replacement cost (up to 30 percent higher, in some cases). Frustrated, many vehicle owners exchanged their run-flat tires with conventional tires even before it was time for their replacement.
Meanwhile back in the laboratory, Bridgestone engineers were hard at work developing the third-generation run-flat (3G RFT, above) designed to address these concerns head-on. Their research and prowess resulted in three new cutting-edge technologies:
- Unique ply construction: The reinforcing layers of a tire are called the "ply." Bridgestone developed a tire ply that uses the heat generated by a deflated tire to contract and curb deformation. In simple terms, the material in the new sidewalls automatically shrinks to abate damage from abrasion and heat. When the tire cools, the ply automatically returns to its original state.
- New rubber compounds: Laboratory-engineered rubber compounds, Bridgestone calls them "NanoPro-Tech," are also used in the sidewalls to limit heat. Conventional tire compounds warm through friction between the carbon and polymers (two common tire ingredients). By optimally distributing the polymers, friction and heat are minimized. Of course, this also reduces energy loss increasing fuel economy.
- Innovative heat control: Bridgestone developed so-called "cooling fin" technology for the new tire. Molded into the sidewall are small protrusions (think of them as miniature spoilers). While this seems rather rudimentary, they effectively disrupt the airflow at the surface to help radiate heat and cool the tire. Thermo-vision (heat sensing) photography of the "cooling fin" technology in action shows a dramatic improvement when compared to smooth sidewalls.
Bridgestone utilizes these technologies independently, or combined, based on the specific design parameters and size of each 3G RFT tire application. For example, the lower sidewalls of a sports car (e.g., BMW Z4) will unlikely receive cooling fin technology, but a higher-profile tire with more weight to bear (e.g., non-sport BMW X5) would have the cooling fins. Regardless of the formula, Bridgestone says all of its new 3G RFT tires are designed to operate safely with zero pressure for up to 80 kilometers (about 50 miles). But, more on that a bit later...
Leaving the technical jargon in the classroom, Bridgestone encouraged us to try its new third-generation run-flat (3G RFT) from behind the wheel. Lacking sensitive test equipment to capture objective differences between the 2G and 3G tires, we were understandably apprehensive approaching this tire "comparison." After all, how does one subjectively "feel" through fast-food sculpted posteriors the subtle nuances in ride quality that reportedly differentiate the two generations of run-flat tires? The answer was Bridgestone's European Proving Grounds in Italy.
The European Proving Ground (EUPG) is to a tire enthusiast what an unattended pharmacy cabinet would be to a drug addict – pure bliss. Located 50 km (about 30 miles) south of Rome, the purpose-built facility is a sprawling high-security testing ground for the latest in Bridgestone tire technology. Set within the outer perimeter gates of the four-year-old complex is a glass-smooth four-lane oval nearly 2.5-miles in length with 35-degree banking in each corner. Inside the oval, and connected for high-speed runs off the main track, is a massive skidpad (an amazing 22 acres of smooth concrete!), two independent short road courses, a wet handling course, and several side-by-side lengths of paved straight-aways designed to mimic various smooth and rough pavement surfaces found around the world. The whole package is wrapped in neatly manicured grounds sprawling with blooming Italian wildflowers.
Strapped in near identical late-model BMW 5 Series vehicles, Bridgestone offered us the opportunity to drive three different variants of its high-performance RE050A tire back-to-back: Conventional, 2G RFT, and 3G RFT.
We drove the conventional RE050A first. A high-performance tire fitted to such cars as the Nissan 370Z and Lexus IS-F, the standard rubber was comfortable on the smooth sections. The course/rough pavements didn't provide much of a challenge either as the compliant sidewalls absorbed the abuse without drama. We've put thousands of miles on these high-rated tires in the real world with few complaints.
Next, we tried a set of RE050A "2G" run-flats, the OEM application found on thousands of late-model BMW and MINI models. Owners appreciate the run-flat technology, but have raised issues with ride quality initiated by the stiff sidewalls and additional mass rotating on each wheel. While they were comfortable on the smooth sections, the compromises of the 2G run-flats were immediately evident when we entered the first sections of rough pavement. What had been damped by shock-absorbing sidewalls on the standard tires was now transferred into the cabin in the form of sharp, and rather uncomfortable, impacts. This test reinforced what many owners have expressed and what we have personally experienced.
The final run through the test course was done with the all-new RE050A "3G" run-flats. Eureka! Surprising even the skeptics among us, the third-generation tires were nearly imperceptible in ride quality from the standard tires (non run-flat) we had driven on merely minutes earlier. The ride was very comfortable. According to Bridgestone, test instruments reveal that the slight difference in ride quality was a near-imperceptible 5 percent change in harshness.
Bridgestone also allowed us to drive a 3G RFT technology-equipped vehicle with a flat tire through a simulated slalom course. The car handled sloppy, and the tire howled when pressed, but it was entirely controllable. Afterwards, the tire wasn't even very hot to the touch.
Driving their message even further, we were shuttled by bus out into the Italian countryside for some real-world observations. Another fleet of late-model BMW 5 Series vehicles were fitted with 2G RFT and 3G RFT tires for back-to-back comparisons. Bridgestone held us close (with pace cars front and rear) as we toured 2-lane roads across farms and into the surrounding hills. The roads were in fairly lousy condition, but they again demonstrated the newfound compliance one can expect with the third-generation run-flat.
While Bridgestone states that all of its new 3G RFT tires are designed to operate safely with zero pressure for up to 80 kilometers (about 50 miles), the reality is that a run-flat tire is designed to get you safely off the road and out of harm's way to where the tire may be safely serviced or replaced. In truth, a virtual clock starts ticking the moment a RFT loses pressure (the frictional stress and heat literally "cook" the service life out of the rubber compounds). Consequently, high vehicle loads, high ambient temperatures, and high speeds accelerate the clock, while light loads and cold temperatures slow it (when we experience a loss of pressure with a run-flat, we immediately pull over safely in the hope that the tire may be properly repaired before it is destroyed by driving on the vehicle). Of course, run-flat tires should never be fitted to a vehicle without a tire pressure monitoring system (TPMS).
The track sessions and test drives made it clear that Bridgestone has eliminated the biggest objection to run-flat tires – abusive ride quality. We never had an opportunity to push the performance envelope of the tire, but Bridgestone says the tread compound of the RE050A 3G RFT is identical to that on the conventional tire, so the grip levels should be very high. Lastly, the cost of RFT technology has come down a bit. While pricing has not been announced as of yet, Bridgestone expects the 3G RFT to carry a 20-percent price premium over the equivalent conventional tire, a small price to pay from a safety aspect.
Bridgestone's third-generation run-flat technology also seems to have clearly addressed the issue of comfort that plagued its predecessor. Moreover, the new "NanoPro-Tech" polymers and "Cooling Fin" technologies will undoubtedly allow the company to expand the fitments and marketability of the new 3G RFT product line, likely helping to bring the cost down even further.
While the first two iterations of zero-pressure tires from Bridgestone delivered the safety and mobility they promised, their negatives frequently outweighed the benefits... particularly in performance applications. Bridgestone's third-generation run-flat tires seem to represent a solution with far fewer compromises. It is a path we are finally willing to take.
Photos Copyright ©2009 Michael Harley / Weblogs, Inc.