May 23, 2014
There is never a good time for a flat tire. And no one wants to deal with installing a spare, assuming your vehicle actually has one in the trunk (a growing list of cars don’t have a spare). Plus, beyond doing the undesirable, taking the time to change the tire yourself or wait for roadside assistance to come can cause quite a disruption to the schedule you have to keep.
But rather than getting stuck on the side of the road, what if you could keep driving even after you get a puncture that causes complete air loss? What if you could choose where and when you replace that damaged tire?
The idea of a tire that can keep running when flat isn’t new. Bridgestone was one of the first tire companies to pioneer the concept back in the early 80s with Original Equipment tires on the Porsche 959 supercar. Back then the idea of a run-flat tire that could keep rolling even without air pressure was the stuff of supercars, and not for everyday family cars or budgets. Over the two decades that followed, run-flat tires from a variety of manufacturers evolved far enough to make their way onto select premium vehicles, but it still wasn’t quite as simple as using conventional tires. Those run-flat tires required a carefully tuned suspension to accommodate the inherent difference in ride quality versus conventional tires, along with special wheels and a tire pressure monitoring system (TPMS) to alert the driver in the event of a puncture. The concept was good, but the challenges to mainstream integration were big indeed.
Despite these hurdles, temporary extended mobility is at the core of what makes run-flat tires appealing. The ability to keep driving even when there is no air pressure in the tire solves a lot of problems when a flat tire strikes. Over the past 10 years, Bridgestone has been hard at work trying to solve the riddle of combining a run-flat tire’s extended mobility feature in a tire that works on nearly any passenger car, without requiring special wheels or sacrificing everyday ride quality.
Bridgestone’s latest solution is simply called DriveGuard, and draws on their extensive experience tuning run-flat tires for a variety of Original Equipment fitments. DriveGuard uses a unique rubber compound in the sidewall reinforcements plus clever aerodynamics on the sidewall to help cope with the stress and dissipate the heat created when the tire has to roll without any air pressure. Both help minimize the trade-off in ride comfort found in traditional run-flat tire designs. By creating a better riding run-flat, DriveGuard aims to be the simple mainstream solution, providing appropriate road manners along with extended mobility capability that can be used on any passenger car that has a working tire pressure monitoring system. As they say, timing is everything, and making the switch to DriveGuard easier, every passenger car built for sale in the U.S. since September 2008 has included TPMS.
To see how well DriveGuard works, the Tire Rack team conducted a Real World Road Ride and Performance Track Drive using a common mid-sized sedan, comparing it to the Michelin Primacy MXV4, a top-rated conventional tire in the Grand Touring All-Season category.
Our 6.0-mile loop of expressway, state highway and county roads provides a great variety of road conditions that include city and highway speeds, smooth and coarse concrete, as well as new and patched asphalt. This route allows our team to experience noise comfort, ride quality and everyday handling, just as you would during your drive to school or work.
After the road ride, comments from our team were somewhat mixed. One group noticed only a small difference in overall ride and noise characteristics between the Bridgestone DriveGuard run-flat and conventional Michelin Primacy MXV4 tire. For these drivers, the modest increase in firmness and impact noise for the DriveGuard was apparent only because of the direct side-by-side comparison format of our test, and was quickly forgotten as driving distance increased. One driver commented on how the quick transition to not noticing the ride quality difference felt like putting on a pair of brand new shoes that feel different for the first few minutes, but then evolve to feeling like any other shoe on your foot. The other group found the DriveGuard felt noticeably firmer, particularly over smaller bumps and ripples in the road. In many instances these are imperfections that would go mostly unnoticed with a conventional tire, but became somewhat noticeable when riding on DriveGuard. Certainly this is one area where the Primacy MXV4 excels versus other Grand Touring All-Season tires, and for these drivers there was a difference in ride comfort.
When we average the scores from our large and diverse group of test drivers, there was approximately half a point (on a 1-10 rating scale) advantage in ride and noise quality for the conventional Primacy MXV4, indicating there is an observable difference, but one which is on the order of the typical range we see when doing a comparison of several conventional tires in the same category.
One benefit of the extra sidewall reinforcement in the DriveGuard tire came as an advantage in steering feel and handling. When maintaining lane position in traffic, or taking expressway on-ramps and negotiating 90-degree corners, the DriveGuard felt somewhat more direct and responsive than the Primacy MXV4.
Our 1/3-mile per lap test track course includes 90-degree street corners, a five-cone slalom and simulated expressway ramps. Run in both dry and wet conditions, the test track allows our team to experience the traction, responsiveness, handling and drivability normally only encountered during abrupt emergency avoidance maneuvers or competition events.
For the track portion of our test, we used our standard test car platform, the BMW 328I sedan. It’s proven to communicate differences in tire performance very well, as well as stand up to the rigors of multiple laps by our team.
In the dry, both tires delivered appropriate levels of traction and handling for tires in the Grand Touring All-Season category. Following what we found on the road, the DriveGuard felt a little more responsive than the conventional Michelin tire, likely due to the run-flat tire’s sidewall reinforcements. In the wet, both tires were again very similar in overall capability, with a small advantage in ultimate grip and shorter stopping distance for the Primacy MXV4. The DriveGuard offered slightly less wet traction, but felt slightly easier to balance and control at the limit.
Winter weather is often unpredictable; and snow-covered roads change with every passing vehicle as they churn snow into slush or pack it down to polished ice. A constantly changing test surface makes side-by-side comparisons difficult, so we use a dedicated winter testing facility in Northern Sweden with acres of groomed snow that provides the consistency we need to get reliable acceleration and braking comparisons. This facility also has a prepared snow-handling course where we evaluate the stability and control of each tire during abrupt maneuvers. To simulate the icy conditions found at intersections or the black ice experienced out on the highway, we use ice at a local hockey rink and measure acceleration and braking traction.
The Michelin Primacy MXV4 delivered modest overall snow traction that will let you start, stop and maneuver, but anecdotally isn’t at the level of the category’s newest leading options. The Bridgestone DriveGuard provided noticeably less overall traction than the Michelin tire, and took more than a full car length longer to stop in the snow from just 20mph.
Our Real World Road Ride features a relatively flat, 6.0-mile loop of 65 mph expressway, 55 mph state highway and 40 mph county roads along with two stop signs and one traffic light every lap. Our team drove each tire approximately 400 miles over the course of several days. Since we wanted to compare fuel consumption results that typical drivers would experience, our drivers were instructed to maintain the flow of traffic by running at the posted speed limits and sustain the vehicle’s speed using cruise control whenever possible. They did not use hypermiling techniques to influence vehicle fuel economy.
| Tire | Test MPG* | Gallons/Year @ 15,000 Miles |
% vs. Most Efficient |
|---|---|---|---|
| Bridgestone DriveGuard RFT | 32.9 | 455.9 | -5.5% |
| Michelin Primacy MXV4 O.E. | 34.8 | 431.0 | -- |
| *Our evaluation used Linear Logic ScanGauge II automotive computers to record fuel consumption, and Race Technology DL1 data loggers to record true distance travelled. | |||
The conventional Michelin Primacy MXV4 tire in our test is an original equipment tire, and is likely tuned to maximize rolling efficiency. Therefore, it is an example of the results you might see with most newer original equipment tires. Nearly all non-original equipment tires (run-flat or conventional) typically have less emphasis placed on rolling efficiency, and will have lower observed mpg. Coming as not much of a surprise, the Bridgestone DriveGuard showed lower observed fuel economy during our road ride. The extra rubber reinforcement in the sidewalls needed to provide extended mobility adds weight and stiffness, both of which take more energy to roll down the road, even when properly inflated.
Based on our results the 1.9-mile per gallon difference between the Michelin Primacy MXV4 and Bridgestone DriveGuard would result in an annual difference of almost 25 gallons of premium gasoline. At the current cost of $4.00/gallon, it would amount to an annual difference of just under $100 for drivers driving 15,000 miles per year.
It’s important to note our test’s fuel consumption measurements follow consistent procedures designed to minimize variables that could influence the results, however they do not represent an exhaustive long-range fuel consumption study. While our procedures require the test vehicles in each convoy to run under the same prevailing conditions, the week-to-week differences in ambient temperatures, barometric pressures and wind speeds that we experience over a season of testing can influence vehicle fuel consumption and prevent the absolute mpg values of this test from being compared directly against those of others.
Larger differences in consumption between tires may indicate a difference that might be experienced on the road, while smaller differences should be considered equivalent. As they say, your mileage may vary.
Contrary to urban legend, run-flat tires are not bullet proof, and therefore cannot drive indefinitely without air pressure. DriveGuard tires are designed to travel up to 50 miles at up to 50 mph without air, but like any other run-flat tire they sacrifice themselves in the process. That sacrifice means you can get to a safe spot or where you are going, but the tire will need to be replaced. So whether you chose to keep driving when the TPMS warning light comes on, or stop immediately and have your spare tire installed, the choice is up to you.
When any tire is driven without proper inflation pressure the vehicle’s load is transferred to the tire sidewalls, causing them to flex more than when the tire is properly inflated. With standard tires the sidewall gets pinched between the wheel and pavement from significantly greater deflection than they were designed to handle, and the sidewall quickly shreds after a short drive. The sidewall of a run-flat tire has special reinforcing inserts to temporarily carry the weight, but the added stress and heat of running without proper air pressure takes its toll on the components in the sidewall, which is why run-flat tires like DriveGuard have a limited range when running without air. Remember, tires don’t heal, so once the damage is done the tire has to be replaced.
We took the opportunity to test the run-flat promise of DriveGuard by driving on one flat tire for multiple laps of our road evaluation course. The tire performed as promised, achieving the 50-mile range promise at an average speed of around 45 mph. Our drivers commented on a slight pull towards the flat tire, and some additional road noise heard mostly at lower speeds. As speed picked up the clues that a tire was flat diminished, making it harder to tell one tire didn’t have any air pressure. Had this tire been on the rear it might have gone completely unnoticed. This underscores the need for a working tire pressure monitoring system in any vehicle using run-flat tires, as without the alert on the dash you might not know you are riding on a flat tire, and could easily exceed the 50 mile range promise and risk the tire reaching its failing point.
Another very important point is the unseen damage that occurs when using the run-flat tire’s ability to drive without air pressure. You can’t simply fix the puncture, air back up and be on your way.
After our drive on the flat tire there was very little sign on the outside that the tire had been driven on
without air.
But once dismounted, a look at the innerliner shows the internal sidewall damage, and the telltale ring of
wrinkling/shredding seen whenever a tire has experienced too much deflection from underinflation (or overloading).
Driving on a conventional tire without air has more immediate and similarly devastating effect on the tire,
leading to a quick demise. Driving on a tire that is chronically underinflated causes the same sidewall damage,
but it just takes longer for the tire to finally fail. In many cases, the evidence of damage is hardly visible on
the outside, but on the inside the tire is being damaged from the stress and excessive deflection in the sidewall.
Remember, air at the proper pressure carries the load. The tire itself is simply the vessel to hold that
all-important air.
The Bridgestone DriveGuard’s extended mobility feature gives drivers of passenger cars a back-up plan when facing the inconvenience of a flat tire. And while it’s not without a modest penalty in ride quality and fuel economy, the Bridgestone DriveGuard opens up a new option for drivers looking for peace of mind. In addition to a small penalty in ride comfort, DriveGuard doesn’t provide the winter performance drivers in the snow belt will be expecting.
Back in the 80s when run-flat tires first arrived in the market, the idea of mainstream passenger cars riding on run-flats seemed out of reach. Perseverance and advancing technology has made that dream a reality today. Bridgestone is hoping their run-flat tires will advance to the point that in the future every car will ride on them. Only time will tell.
Bridgestone DriveGuard (Grand Touring All-Season Run-Flat): Choose to keep moving. The DriveGuard is Bridgestone’s Grand Touring All-Season, self-supporting, replacement run-flat tire designed to satisfy drivers looking for a longer wearing, better riding option to replace Original Equipment run-flat tires, as well as safety-minded drivers whose cars were not originally available with run-flat tires. Read more.
Michelin Primacy MXV4 (Grand Touring All-Season): Primacy MXV4 tires are Michelin’s Grand Touring All-Season tires developed to meet the needs of coupes and sedans, as well as family minivans and crossover utility vehicles. Michelin Primacy MXV4 tires are designed to offer a quiet, comfortable ride, long tread life, responsive handling and all-season traction, including in light snow. Read more.
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