This article originally appeared in the August 2012 issue of Road and Track.
Don't ever think of a tire as a homogeneous glob of molded rubber, because nothing could be further from the truth. In fact, a tire is more properly recognized as a high-performance composite of some 60 different materials. It is quite possibly the single most complex component of your car. Learning about tires, I chatted with specialists at Goodyear, Michelin and Pirelli. I got an automaker's perspective from GM. I consulted my usual collection of SAE International sources, visited a most informative Tire Rack website and perused my Bosch Automotive Handbook. Here's what I gleaned:
An Ever Larger Spider
Four tire contact patches, each about the size of your hand, are responsible for your car's grip of the road, both longitudinally and laterally, both wet and dry. They support the car in comfort with low rolling resistance and without undue noise. And they continue to do this for mile after mile after mile.
In assessing a tire's engineered optimization, specialists arrange all of these criteria into a spider diagram, an example of which resides below. As we delve deeper into this complex composite, we'll see that certain of these criteria are antithetical—tug the spider one way and it loses size in the other.
Grip and wear are evidently such an opposing pair: The softer a tread compound, the better it'll perform its gripping action of intermolecular adhesion. Alas, though, the quicker it'll wear as well.
Yet tiremakers have done an amazing job of lessening these inherent compromises with each generation of new designs. An excellent example in recent years is a tire's contribution to fuel economy by way of reducing its rolling resistance, the energy consumed by a tire supporting its load while in motion. Generally, it's figured that for every 10-percent reduction in rolling resistance, there's a 1–2-percent payback in mpg.
Rolling Resistance And Hysteresis
Rolling resistance comes inevitably with tire deformation. Purely as a thought experiment, you can appreciate that a steel railway wheel exhibits essentially no rolling resistance. By contrast, a tire has hysteresis, inherent lags in its compression and rebound. A high-hysteresis tire— think a gummy tread compound—would exhibit gobs of rolling resistance; one with low hysteresis would have decidedly less. And note, though related, this isn't simply a matter of hardness or softness; rather, it's the characteristic lag in a tire's response to deformation.
In fact, though, the first generation of low-rolling-resistance tires were notoriously hard-riding and certainly not known for their grip. (I've already mentioned intermolecular adhesion; there's a second aspect of grip—hysteretic friction—that's also involved.) Advancements of tread compound, carcass design and tire processing have diminished these particular tugs in the spider diagram.
Deconstructing A Complex Composite
The principal elements of a tire are its tread contacting the road, the underlying belts stabilizing this tread, its sidewalls protecting the tire from curbs and the like, its beads at the wheel interface, its body plies defining the carcass shape and an interliner maintaining inflation pressure. From the onset, each of these has influence on a tire's performance. A thin interliner, for instance, promotes low rolling resistance and light weight (which benefits handling); overly thin, though, and its air retention and long-term durability are compromised.
Carcass design dictates a tire's load capacity and balances its handling, damping and comfort. Body plies of a modern automotive tire run essentially across the most direct path—radially—from bead to bead, thus the name "radial" as opposed to the traditional bias-ply tire. Patented by Michelin in 1946, radial tires came relatively slowly into the U.S. market. Michelin X radials were considered all the rage among rallyists in the late 1950s because their mileage calibrations were more uniform than with prevalent bias-ply counterparts. It wasn't until 1968 that a domestic, Ford, introduced radials to some of its line. Even in Formula 1, as late as the 1982 season Avon and Goodyear (both bias-ply designs) battled Michelin and Pirelli (both radials). The last bias-ply NASCAR race was in 1992 (though stalwarts continue to argue for them today, just as they do for carburetors as well).
These carcass-defining radial cords are typically of polyester. By contrast, the belts residing directly beneath the tread are often of steel cord aligned on the bias as well as circumferentially.
Elastomeric elements of a tire contain both natural as well as synthetic rubber. Indeed, even in these days of syntho-everything, natural rubber offers a toughness thus far unmatched. A race tire's rubber might be 65 percent synthetic; a passenger tire's, perhaps 55/45 synthetic/natural, respectively; an off-highway tire's, as much as 80 percent natural rubber.
Also part of the elastomeric mixture are reinforcing materials such as carbon black and silica, the latter beneficial in lessening the tradeoffs among wet grip, dry grip and rolling resistance. Other admixtures are antioxidants/ozonants; others promote adhesion of rubber to steel and polyester cords; others act as curatives and processing aids in the tire's vulcanization, its 12–25-minute curing at more than 300 degrees Fahrenheit.
Tread, Slip Angle, Under- And Oversteer
The tread pattern of a tire affects every aspect of its performance—as well as its appearance. We're all attuned to tires that are asymmetric (non-uniform across their tread centerline) as well as directional (tread patterns favoring a particular rotation). These subtleties optimize everything from handling to noise reduction to water evacuation.
Even in straight-line travel, a contact patch's leading-edge compression and trailing-edge rebound are non-trivial. (That second aspect of grip—hysteretic friction—is part of this.) Add a cornering side load and things get really complex—introducing the concepts of slip angle, understeer and oversteer.
Indeed, it's a pity the word "slip" is used, because this gives the impression that the tire is slipping or sliding—which needn't be the case. Imagine a cornering tire and follow a particular contact patch through rotation (above illustration displays this). Because of the tire's sideloaded deformation, this portion's new contact won't coincide with the direction the tire is steered. The angle between these two is the tire's slip angle at this particular loading. Despite that word "slip," this is a measure of a tire's inherent deformation, not of sliding.
Now imagine a car accelerating in a turn. If its front tires' change in slip angle is greater than that of the rears, then the car is understeering. If its rear tires' change in slip angle exceeds that of the fronts, then it's oversteering.
Accelerating on a constant radius—our skidpad, for instance—an understeering car would require increasingly more steering lock to stay on course; an oversteering car would require less and less. At extremes, an understeering car would plow off nose-first; an oversteering car would loop. One that's neutral would teeter between these two extremes. While neutrality or a modicum of oversteer may be acceptable to those with exceptional car control, the rest of us mere mortals are a lot more confident—and safer—with just a tad of understeer.
Decoding a Sidewall
Tires obviously come in a variety of sizes, so let's decode some of the nomenclature embossed on a sidewall. A 2012 Mazda Miata's standard tire is a 205/50R16 with a V speed rating. The 205 is its nominal section width, sidewall to sidewall, measured in mm. The 50 is its profile or aspect ratio, its sidewall height divided by section width. R identifies this tire as a radial. The tire fits a wheel of 16-in. diameter. Its V speed rating implies security up to 149 mph (240 km/h).
A bit of arithmetic (combined with unmixing the English and SI units) can identify a lot about a given tire size. As shown in the sketch below, our 205/50R-16 has a section width of 8.1 in. Each sidewall, being 50 percent of this, rounds to 4.05 in. Therefore, mounted on a 16-in. wheel the assembly's overall diameter is approximately 24.1 in., thus yielding a circumference of about 75.7 in.
The Miata also has a "Plus 1" option, a 205/45R-17, going to a 45 profile and increasing wheel diameter accordingly. (How does this affect overall circumference? This is left as an easy exercise for the arithmetically unchallenged.) By the way, this higher-performance option has a commensurately higher speed rating, W, with capability up to 168 mph (270 km/h).
Size Matters, But Not How You Think
It's common as well for performance upgrades to fit a wider tire, provided, of course, the added width is compatible with suspension geometry and fender clearances. For example, instead of the Miata's 205/45R-17, why not fit a 235? Wouldn't it give a larger contact patch?
The best I'd say is "not necessarily." It'll certainly be a wider contact patch, but likely commensurately shorter as well—and thus yielding approximately the same contact area. To unravel this oddity, remember that we're dealing with a pneumatic structure. And, as any flat tire displays, without its inflation the tire doesn't support much load.
That is, we can get a fairly good estimate of contact area (though not of contact shape) solely from load and inflation pressure: For example, a tire supporting 1000 lb. and inflated to 35 psi will have a contact area of about 28.6 sq. in. Namely, each sq. in. of inflation pressure supports 35 of the 1000 lb. (I say "fairly good" as this neglects the tire's sidewalls and carcass contribution to supporting the load—but again, remember that flat tire.)
Then why do wider tires improve cornering? Because the shape of a contact patch is as important as simply its area. A wider tire's wider contact patch is better at combating side loads and thus provides enhanced cornering.
Another thought on this: All bets are off if a tire's carcass or sidewalls are specifically engineered to carry more load (see ahead to "runflats").
Profile Tidbits—Including a Tiremaker's Success Story
Generally, a lower profile brings sharper turn-in, more precise handling and, in fact, more predictable performance. Initially, though, the concept was burdened by a huge tradeoff in comfort. Back in the 1980s, when the standard tire had a sidewall-to-width ratio of around 78, it was easy to recognize a "low-profile" 60-series tire by watching the car's windshield wipers dance around in response to anything but mirror-smooth roads. Now, perfectly comfortable sedans run 45 series.
The trend, particularly at auto shows and with the aftermarket, is toward increasingly lower profiles and taller wheels. A 255/20R-24 is an extreme example, and with extremes come trade-offs. Rubber being lighter than most metals, there's a point at which the larger wheel's metallic contribution becomes overwhelmingly unfavorable. Also, minimal pneumatic volumes make the tire and wheel more vulnerable to potholes and the like. Below 30 series, for example, or beyond 20-in. wheels, they're style, not function. (Not to knock style—a whole generation grew up thinking cars with fins were really neat.)
Another bit of profile trivia: If low profile tires are so much better, then why don't Formula 1 cars use them? Because the FIA says they can't. In fact, F1 regulations define the tire/wheel package very tightly, with the result being fairly tall tires: roughly 245/65R-13s front, 325/50R-13s rear. Curiously, they run at a relatively low inflation pressure of 1.4 bar (20.3 psi). Also, F1 regulations limit the inflation gas to air or—as is more likely used—nitrogen.
[image id='5d29bce9-17ab-4386-a8c2-c7c99b314b3f' mediaId='01a90c24-d9cc-49fb-b47d-a935c4159d0a' loc='C' caption='Tire Storage' share='true' expand='true'][/image]
Nitrogen For The Rest of Us?
Our atmosphere is composed of 78 percent nitrogen/21 percent oxygen plus traces of water vapor and other gases. Nitrogen molecules are larger than oxygen's, and thus a tire filled with this gas would be less susceptible to leakage. And eliminating the moisture helps maintain uniform pressure over a wide temperature range.
Dealers have been known to offer nitrogen fills (some, even gratis) for road-going tires. Less leakage is a plus. However, specialists say the moisture argument isn't compelling at typical road-tire temperatures. Also, they note that a meaningful moisture-free/nitrogen fill requires first pulling a partial vacuum on the tire to evacuate the air that's already in it.
More important is regularly monitoring your car's tire pressures. These days, newer cars come with Tire Pressure Monitoring Systems (TPMS). However, not all TPMS are created equal. Indirect systems depend on ABS hardware to assess rolling circumferences and identify an underinflated tire. Note, though, indirect measurement will not catch gradual underinflation of all four. Direct systems monitor inflation pressure within each tire, but alas they're not immune to false alerts. Your own good-quality pressure gauge is an excellent investment.
Also, of course, watch for a tire's signs of end-of-life. Wear bars across a tread pattern signal when it's down to around 1.6 mm/0.063 in. This is just about 2/32 in.—the time-honored depth of ensuring that part of Lincoln's head is covered when you invert his 1-cent image into the tread groove.
What Have You Done With My Spare?
Automakers hate spare tires. They take up room, add weight and cost money. Tiremakers don't mind offering a fifth full-size product, but even they sense the spare is an endangered species. It's as much a matter of marketing as security and safety. SUVs and light trucks tend to retain their fifth standard tire. Most passenger cars these days get by with temporary/emergency mini spares or even inflator kits.
And then there are runflats. The carcass and sidewalls can be engineered for deflated limp-home capability. Thus far, these Extended Mobility Tires, as they're sometimes called, tug fairly hard on other aspects of a tire's spider chart, particularly in handling and comfort. With each generation, though, they're getting better.
Here's to larger spiders.