Comparison Tests
Finding the right tire for your ride is like trying to pick the right mattress for a good night's sleep. You know that buying the wrong one can have dire consequences, but you can get dizzy just wading through the jungle of choices. And unlike mattresses, you can't take new tires for a test run. That's where we come in.
First off, we decided against evaluating the popular all-season tires in favor of an examination of 11 high-performance ones, what are labeled in the industry as "maximum performance" tires. If you're like us, you want to know which tires will best contribute to the performance of your vehicle, accepting the inevitability of having to switch to snow tires when the white flakes appear. Even if your home turf is snow-free, you still have to deal with driving in the rain. Hence, all the tires here were put through three wet tests and three dry tests.
Testing tires is a difficult job (and probably the reason we haven't conducted such a test since 1989). But tires are a most critical part of your car, your only connection to the road. There are many difficult-to-control variables that affect tire performance and make accurate testing a challenge. For one, the temperature of the track surface, which can have a significant effect on how a tire performs, can vary widely over the course of a day.
To minimize these variables, we enlisted the help of the Tire Rack, a tire and automotive-accessory business that was created in 1979 by a couple of autocrossers—Pete Veldman and his son-in-law, Mike Joines—who found it tough to locate high-performance tires for their hobby. The business took off, and in 2001, the Tire Rack built a 530,000-square-foot warehouse and a small test track in South Bend, Indiana. The company keeps a fleet of three BMW 325i's that it uses to perform its own tire tests to educate its sales force and regularly posts the data on the Web at www.tirerack.com (888-541-1777).
The Tire Rack generously offered us the use of its BMWs for our test. The 325i is a wonderfully balanced car, and its handling is so consistently predictable that we felt sure we'd get a clear idea of what the tires were doing as we put them through the tests. The required tire size—225/45R-17—is made by the 11 brands we wanted to evaluate. We used BMW's recommended tire-pressure settings of 29-psi front and 35-psi rear for all our testing.
Although the track we used is small—its longest straight portion is only 456 feet—it's lined with sprinklers that can soak the asphalt and simulate driving in rain. The 0.3-mile autocross course has five timing beacons that provided a total time for each lap and segment times along the way. One of those segments was a long, constant-radius turn that allowed us to calculate maximum lateral grip during each lap.
It took three days to perform all the tests. On day one, we did wet and dry brake tests. We accelerated to 50 mph and then braked to a standstill. (We were unable to drive to our traditional 70-mph mark because there simply wasn't enough track.) There was a benefit to that lower speed: It ensured that we were measuring the braking performance of the tires and not just brake fade.
Over the next two days, when we performed the wet and dry autocross tests, we were joined by Spencer Geswein of Full-Lock Industries. Geswein and Brian Smith formed Full-Lock in 2001 and offer a variety of driving-related services that include instruction, testing, and racing (www.full-lock.com). Both men spent at least 10 years testing tires for Michelin, so we figured that kind of experience would uncover some subtle tire traits that we might miss. And in case you're thinking Geswein might play favorites, he asked that we not tell him which tires he was driving on, so his testing of the 11 brands was done blind.
Geswein drove three laps through the autocross course, and then we drove three laps—in dry and wet conditions. We averaged the six dry and six wet laps to come up with a time for each tire's performance on wet and dry surfaces. In the end, we had performance results for six tests: braking, autocross, and lateral grip—each in wet and dry conditions.
We gave the top-performing tire in each test a score of 100 and scored the rest on their relative performance. For example, the tire that had the highest dry lateral grip of 0.95 g scored 100 points, and the tire placing last with 0.88 g received 92.6 points (0.88 is 92.6 percent of 0.95). We then added the scores from all the dry tests to arrive at a dry-performance rating and did the same for the wet-test results.
Things were more complicated when it came to determining the finishing order of the 11 tires. In addition to factoring in the wet and dry scores, we gave points based on a tire's price (we used the typical selling price in our calculations) and tread-wear grade, which is a rough estimate of how long a tire will have usable tread. For the price and tread-wear ratings we used the same proportional method.
But the categories were not all weighted equally. Our test focused on measuring performance, so we decided that results in the dry—lateral grip, for example—would carry the most weight. Wet performance is important, but less so for the purposes of this test. The price and the tread-wear scores had nothing to do with performance, but who doesn't consider what something will cost before buying?
After a lot of debate, our scheme worked like this: The proportional weighting for tread-wear and price scores was cut in half, the wet scores were not changed, and we doubled the dry-performance scores. And that's how a tire's overall ranking was calculated for this test.
Should you disagree with our rating method, we've presented all the data in graphical form so you can decide what's important to you.
Before you jump to the results, a few more things: The BMWs are rear-wheel drive, and our subjective comments might not be completely applicable to front-drive cars, which typically have handling characteristics different from the BMWs'. We didn't test the tires for ride or noise characteristics, two traits tiremakers may consider as important as performance. We figure those two traits would be secondary for drivers interested in speed.
When the three days ended, we had a mountain of data—and sore backs from changing tires 33 times. We hope you find the information useful.
What Makes a Tire Work?
Tires are much more sophisticated than they look. Although the basic construction of every tire here is the same—there's a carcass of polyester or rayon fabric with steel cords baked inside the rubber—there are countless detail differences that affect how a tire performs. The most obvious is the tread design. Tires with large grooves generally expel water from roadways more effectively. The larger and deeper the grooves, the better the tire is able to resist hydroplaning.
But those large grooves don't help on dry surfaces, where the more rubber a tire puts on the road, the better it grips. Think of a slick racing tire that has no grooves. Then take, for example, the BFGoodrich tire in this test. It has large tread blocks, not many grooves; as a result, it's great in the dry but not so hot in the wet.
We're oversimplifying here, because the chemical makeup of the rubber—known as compound—also has a profound effect on grip in both wet and dry conditions. Usually, a compound that's effective when the road is wet can overheat and get greasy when the road is dry. That's why we experienced a few tires that excelled in the wet but not in the dry—and vice versa.
A tire, like most parts of a car, is a compromise. Make it great in the wet, and it more than likely sacrifices dry performance, and that's why we didn't come across one tire that was able to ace all the tests.
Still, there are countless other variables for tire engineers to consider. For example, the construction of that inner carcass can be made stiff for precise feel or compliant for a good, comfy ride. And the construction can have a marked effect on performance.
Don't forget noise, durability, and appearance considerations, too. It's enough to make us glad we're not tire engineers.
