Road Glue

Performance tires offer rolling technology for your BMW.

Photo: Road Glue 1
October 25, 2013

If you’re a performance-minded driver worthy of the blue-and-white roundel, you’ve no doubt been in “the zone.” Maybe it’s a back-road DTM fantasy, where you thread your BMW down to the apex and bounce off imaginary racetrack curbing. A flick of Timo Glock countersteer, a squeeze of Joey Hand-appropriate throttle and your Bavarian machine rockets away toward the next writhing bit of road. With forces delicately counterbalanced, it’s as if control of the vehicle is telekinetic; you’re as much thinking the car into the bend as steering it there.

Defining moments like these are why we own BMWs, and few manufacturers have managed to maintain the purity of the driving experience as has our marque. Much of the magic comes from engineers tirelessly tweaking suspension kinematics and obsessing over shock damping curves, but ultimately it comes down to where the rubber meets the road. It’s the hallowed contact patch, the area of a tire’s tread that touches the road at any given moment. And it’s not a whole lot of surface area: Take all four corners of tread, subtract the void areas for water evacuation and you’re left with a footprint equivalent to a couple sheets of typing paper, plus or minus a few Post-It notes.

It’s imperative, then, to maximize the effectiveness of these precious square inches. And short of a racing slick or a track-day R-compound tire, a high-performance summer tire is the weapon of choice for a street-driven car. If you own a new 328i Sport Package, you have them from the factory. If you’ve got a fourth-hand E30 whose previous owner fitted a set of Walmart house-brand radials, you need this upgrade, as performance tires will yield the single greatest bolt-on improvement in handling, period. So put the hop-ups on next year’s Christmas wish list and read up about performance radials, how they differ from mere mortal passenger tires, and whether you should stray from what BMW fits as original-equipment rubber.

UHP, MP, EP?

Before we can talk differences, let’s more precisely identify what we’re talking about. In ascending order of sportiness, tires are classified as Grand Touring, High Performance, Ultra High Performance, Max Performance or Extreme Performance. The last three—the sportiest—are the most popular for enthusiast-driven BMWs and thus what we’ll be discussing here, but what are the real differences between those categories?

“Extreme Performance (EP) is like Ultra High Performance (UHP) on steroids; by intent, by purpose, by compounding, it is another level,” says John Rastetter, director of tire information services at Tire Rack. “Max Performance (MP) is sort of UHP on performance-enhancing drugs, but not all the way to steroids.”

All are summer tires (i.e. those with tread compounds not suitable for below-freezing temperatures) have aggressive tread patterns and a minimum speed rating of V, which allows for a maximum speed of 149 mph. Of course, an M5 or Z8 will require a more serious speed rating: Z, for speeds over 149 mph; W, good to 168 mph; Y, to 186 mph; and (Y) for speeds in excess of 186 mph.

As a general rule, the more aggressive you go, the larger the tread blocks, the stiffer the sidewall, the softer the compound and the shorter the tread life.

Whatever their performance category, those black pneumatic donuts in your wheel wells may be the single most complex component on your car. For starters, there’s the tread compound, the exact composition of which is a highly guarded secret. In general, it contains natural rubber, a selection of synthetic rubbers, carbon black (a binder), silica (for wet traction), antioxidants, and waxes (that will migrate to the surface after periods of non-use) to fend off ozone.

Working inward, we find belts wound circumferentially atop the basic radial carcass whose bead-to-bead body plies are typically made of polyester or rayon. Those outer belts are of woven steel strands (often brass-plated for better adhesion to the rubber), nylon and sometimes aramid fibers like Kevlar. Working down toward the rim, we find bead filler rubber (whose stiffness plays a large role in sidewall rigidity) and more hoops of steel wire that form the bead.

After extruded rubber sheets are bonded to the ply materials in a process called calendaring, these elements are wound together on a mandrel to form what’s known as a “green” tire; then it’s off to the molds that are intricately tooled with the tread pattern and sidewall markings. While curing in the mold (12-15 minutes at 300° Fahrenheit would be typical for auto tires; a truck tire, for instance, would cure significantly longer at a lower temperature), the vulcanization process occurs where the rubber forms chemical bonds with reagents to produce a stronger, more resilient finished tire.

Photo: Road Glue 2

How does a performance

tire differ?

Besides the rakish low profile and aggressive tread, one of the key differences of a performance tire is the shape of its so-called crown radius. Looking at a cross section of the tire, that radius would be the arc of the tread from sidewall to sidewall. On a generic passenger tire, that radius would be small, resulting in a rounded tread (picture a tundra tire on a Piper Super Cub in the extreme case). But our performance tire has a large-radius crown, almost appearing flat, so it follows that the tread’s shoulders are more squared off.

What this means at the oh-so-important contact patch is a shape that’s much more conducive to cornering—wider laterally but shorter longitudinally, versus the opposite for Grandma’s tires on the Camry. Mind you, the area of that contact patch won’t be different, but its optimized shape and stouter interface with a firmer sidewall will provide greater cornering bliss.

A tire’s internal construction affects this, too. According to Rastetter, the steel belts on a UHP-and-above tire are oriented at lower angles to the direction of travel: “The way in which the steel belt is put in the tire is different, and that helps stiffen or support the tread area.”

Performance tires take it to the next step by adding reinforcing materials above the steel—nylon, aramid fibers like Kevlar, and polyamide, a nylon/polyester blend. In the early 1990s, tire manufacturers commonly added a nylon belt maybe a half-inch wider than the steel, wrapped around the steel belt package twice, with some overlap at the splice…a good strategy to contain the belt package from growth at high speeds but bad for making a nice uniform, round tire.

These days, wrap-and-splice is the exception rather than the rule.

“Almost universally now,” says Rastetter, “the manufacturer takes a filament (as little as one strand or as much as a half inch) and they spirally wrap it over the tread of the tire to provide that nylon reinforcement. This way, you only end up with a quarter to a half-inch where you have any overlap at all. This eliminates splices, which makes the tires not only perform better at high speed but makes them ride better.”

In addition, Rastetter notes, some tires employ creative variations in wrapping, putting more layers of nylon in the tire’s shoulder where it’s needed, which makes for less mass in the center of the tire where it’s not. The evolution and increased precision of manufacturing extends benefits to the steel belts as well; Rastetter says they’re often joined in a butt splice, with little if any overlap at all.

Designer treads

The tread of a performance tire is all about balance, as you want to maximize the amount of rubber in contact with the road, yet still evacuate enough water for safe hydroplaning resistance. You want, in Rastetter’s words, “enough dry performance to make your ears bleed,” yet still retain reasonable wear characteristics. Add noise suppression and some degree of ride comfort, and it’s a very tall order for the tire engineer.

Photo: Road Glue 3

Some common traits emerge: large, stable outer tread blocks, or even a slick-like outer shoulder with a few vestigial grooves; a solid circumferential rib in the center that’s good for directional stability and low noise, and either a directional or asymmetrical tread pattern. The directional variety may have a slight edge in hydroplaning resistance at higher speeds, but the asymmetric designs have the advantage of conventional or side-to-side rotation to equalize wear, great for BMWs with staggered fitments. And, Rastetter adds, Europe’s recent, more stringent drive-by sound regulations tend to favor the quieter asymmetrics.

Tire manufacturers also employ little tricks to enhance grip here that aren’t immediately apparent. Many performance tires will chamfer the edges of the tread blocks, so as they deflect with cornering load they still maximize surface contact. Some UHP tires have tread blocks that mechanically interlock with side load, sort of a tongue-in-groove approach to stabilize adjoining tread blocks while cornering, yet still providing a sipe or water channel when driving straight ahead.

And cool appearance counts—check out the subtle flames on the sidewall (and even tread!) of the Hankook Ventus R-S3, or the race car-inspired “slick” sidewalls of the BFGoodrich g-Force Sport Comp-2. Some like the Continental ExtremeContact DW even have tread depth indicators, in which both a D (Dry) and a W (Wet) are molded into the tread, each at different depths. When the W wears away and leaves just the D visible, the tire is deemed no longer suitable for wet conditions.

Compound is key

We’d be nowhere if it weren’t for today’s amazing tread compounds, but as mentioned before, each manufacturer guards its recipe as closely as KFC guards The Colonel’s 11 secret herbs and spices. It used to be that the percentage of natural rubber to synthetic rubber increased as a tire moves up the UHP-MP-EP scale.

“I don’t think it’s as sure a bet anymore,” says Rastetter. “Chemists are finding ways to give you a really grippy tire with what in some ways might be considered a hard rubber, but that rubber’s stiffness keeps the tread blocks from flexing and helps the high-speed stability and handling. It’s not like a racing slick where you don’t have to compensate at all for tread pattern.”

Some tiremakers hint at technologies they believe make their concoctions superior. Hankook engineers, for example, put forth that hyper-uniform mixing of the silica in the rubber compound is key, as is getting the silica to properly adhere to the rubber. Bridgestone touts the lengthening of carbon black molecules in its compounds, their enhanced bonds making for a softer yet more tear-resistant rubber.

And your tread might contain more than just one compound; it could be as many as three or four. Bridgestone UNI-T technology places a softer layer of rubber beneath the outer tread, exposing the fresh stuff as the tire wears and that outer compound hardens with use. Michelin uses two compounds across the tread of its Pilot Sport PS2, for example, the outer a carbon black/silica hybrid optimized for dry traction, the inner even richer in silica for wet conditions.

Run-flat tires:

panacea or problem?

The run-flat concept is a brilliant one, at least in theory. Take a nail squarely through the tread, and you’ve typically got 50 miles of range, at a speed of 50 mph or less, to drive to a tire shop or dealer for replacement. No wrestling with a jack, lug wrench or spare (and no need to have them in the car at all); no dangerous exposure to Interstate traffic hurtling by at 80 mph. But there are downsides: higher replacement cost, degraded ride and, probably most important to aficionados of precision handling, a considerable extra chunk of unsprung weight. Depending on tire size, a run-flat can weigh 5-10 lbs. more than a comparable conventional tire of similar construction.

Photo: Road Glue 4

The reason is the run-flat’s internal self-supporting structure inboard of the sidewall body plies, what Rastetter calls “the wedge.”

“When the tire loses pressure, rather than having the sidewall cords flatten out to the point where they create sharp angles and destroy themselves,” explains Rastetter, “the wedge maintains a rounded profile so that the body cords can temporarily hold up the weight of the car.”

That’s where the additional weight and ride harshness come in, creating an extra challenge for the suspension to keep the tires planted to the road surface over bumps. The additional dynamic loads may require stronger (likely heavier) suspension control arms, shocks, bushings, etc., negating some of the savings of losing the jack and spare.

Yet the situation is improving with each generation. Early run-flat efforts earned a reputation for an especially thumpy ride because, Rastetter believes, they were overbuilt. He cites a Bridgestone effort in the early 1990s with run-flats on two Callaway Corvettes running in the One Lap of America event. One was a serious competitive entry, and the other an experimental entry to prove the efficacy of the run-flat concept. The latter car ran the entire 2,800-mile event on de-pressurized run-flats without destroying the tires, though Rastetter acknowledges that a couple of rainy nights and nighttime transit stages helped to keep the tires cool.

“The development engineers were over-designing the tires because no one wanted to earn the reputation of having a run-flat tire that wouldn’t survive its basic duties,” offered Rastetter. “Today we’re seeing manufacturers more than meet the promise, but backing down on some of the design elements that took away ride quality.”

BMW has embraced run-flat technology, fitting them first to the light-armored 7 Series Protection cars of the late 1990s and following with their installation on series production cars starting with the 2003 5 Series. According to Dave Buchko, BMW NA spokesman for Advanced Powertrain Communications, all BMW models beginning with the 2004 Z4 (with the exception of the M models) have been engineered from inception to use run-flat tires exclusively. Buchko acknowledges the weight penalty issue, but echoed Rastetter’s comments that the run-flats are getting lighter and smoother-riding all the time: “It’s a challenge, to be sure, but we think the safety benefits so far outweigh any of the downsides that it’s worth the effort.”

The ultimate street/track tire?

Much like a dualsport motorcycle, which won’t be particularly brilliant on either the trail or the street, no one tire is the ideal “biathlete,” super-civilized on the road and capable of turning sizzling laps on the track. Yet the latest batch of Extreme Performance tires like the BFGoodrich Rival, Bridgestone Potenza RE-11A, Hankook Ventus R-S3, Yokohama Advan AD08 and Dunlop Direzza RII seems to break the mold with both vessel-bursting grip and wholly acceptable ride comfort. They’re an excellent option for the owner whose primary mission is commuting but occasionally likes to indulge in hair-on-fire autocross laps without hauling around an extra set of track-dedicated tires.

Of course, even these tires are not for everyone. Typically, their hyper-aggressive tread patterns aren’t optimized for hydroplaning resistance, and their sometimes shallower out-of-the-mold tread depth complicates matters. A half-worn set of these track-worthy tires are going to be sketchy when you hit standing water at freeway speeds. But their gummy tread compounds make many of these quite good in damp conditions, provided the ambient temperatures aren’t too low. Get down to the near-freezing range (the actual temperature varies depending on tread compound) and the rubber hits what’s called the glass transition temperature, where the tread blocks essentially lose all their elasticity and can’t conform to the road surface’s tiny irregularities. The loss of grip here can range from dicey to downright dangerous.

Last, these Extreme Performance tires have a UTQC treadwear rating that ranges from 140 to 200 (for comparison, a Ultra HP tire might be rated at 280; an 80,000-mile passenger tire at 700), so with aggressive driving you might see only 10,000-15,000 miles on a set, rather less if you drive on the track. That’s an accepted cost if your focus is centrifuge-worthy cornering, which these tires deliver. With optimized suspension and camber settings, each is capable of generating in excess of 1.0g steady-state lateral acceleration.

It’s easy to take tires for granted, whether you’re simmering along a favorite canyon road or lapping Laguna Seca at full boil. We have that implicit trust that a steering input will result in a well-planted change of direction, away from the Armco and into the esses. But just think about the simple act of motoring along in a straight line…Rastetter points out that in 5,000 miles of driving, a tire will spin through four million revolutions. And with a BMW, every revolution is another chance at that DTM fantasy, best enjoyed with a set of performance tires worthy of your car.

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