In 1839, American chemist Charles Goodyear mixed some rubber together with a sprinkle of sulfur. To his surprise, he solved the challenging puzzle of creating durable rubber. This event was the precursor to a worldwide proliferation of vulcanized rubber tires in every shape, size, and application.
Tire technology today, in contrast, is so advanced that most of it is guarded in proprietary secret and each manufacturer has its own recipe. Effective chemistry is a better descriptive for a high performance sportbike tire than alchemy; however, it's very interesting how little we really know about tires and how they actually work on a motorcycle.
Not unlike mutating iron into gold, modern rubber is astounding in the way it enables a 240 HP motorcycle to transmit power effectively to the ground. Motorcycle tires are able to change their shape from a solid—cold form to a hot sticky mess in a matter of minutes, then cooling back to their original state. The ability to do this over and over again all while providing riders the ecstasy of the internal combustion kind is truly a modern marvel.
Motorsports are a driving factor in the development of auto and motorcycle tires. Every win on the racetrack promotes a brand and furthers technology that eventually “trickles” down to the street. This translates to a major benefit for the John-Q Public and for tires available for everyday use.
Composition and Construction
Not related to those smart folks with pointed ears, Vulcan is the Roman god of fire, hence the term Vulcanization—this clever description of rubber manufacturing has been around since its inception.
The main purpose of vulcanizing is to make a more durable and functional form of rubber that is the base for tire manufacturing. The magic begins when the bark of rubber trees are processed into what is commonly called "Natural Rubber." This rubber (or also referred to a polymer) is then mixed with synthetic polymers, sulfur, and a myriad of other chemicals and oils. Heat and pressure forms the mix into rubber strips to later form the tread and weaved cords of synthetic and steel fibers together. This then creates plies that form the structure of the tire known as the carcass. Testing and ensuring quality of a tire is also an extensive process.
This is, of course, a simplified explanation of how tires are produced, but the takeaway is that the science, effort, and money put into these black donuts have come a long way since the late 19th century.
This is not just exclusive to motorcycle tires—it's also done for auto tires as well. There are a few other ingredients thrown in that enable grip and durability at high speeds. Inside the tire compounds, there are many closely held ingredients due to economic and professional competition.
Pirelli’s Compound Engineer, Fabio Meni, who is involved in designing their World Super Bike tires, gives his explanation of the synthetic polymer-Carbon Black.
“Carbon black is the product of the incomplete combustion of petroleum and it is commonly used inside rubber compound as a strengthener. Thanks to its very high surface area, carbon black can penetrate during the mixing, inside the polymer entanglements, creating strong bonds with the macromolecules (so called bound rubber).”
Carbon Black provides grip at high temperatures, abrasion resistance, and is the component of the tire that makes it black in color. With racing tires, there's a high volume of carbon black in the compound that is required to sustain the extreme heat required to stay in the operating range of grip. That range is where the rubber is extremely hot and sticky (well over 165 degrees F). Without carbon black in a high performance tire, the rubber wouldn’t come close to lasting and would eventually fall apart after a few miles of riding.
Fabio Meni gives insight on Silica, a common polymer in street tires:
“Silica, in the same way of carbon black, gives reinforcement to the compound improving the mechanical properties, but due to its high polarity, it is particularly affine to damp or wet surfaces because the water is also polar. On the contrary, carbon black is a polar, which means that in contact with water, a repulsive force is generated. This behavior drastically reduces grip on wet surface(s).”
Silica is found predominately in street tires due to the requirement for them to heat quickly to provide immediate grip. Silica is good in wet street conditions and reacts differently on a racetrack.
“Silica decreases the working range of the compound, so the tire starts to work at a lower temperature. For the same reason, silica is generally not used in racing compound (apart for rain tire) because lower working range means lower grip at the temperature reached in all racing activities,” said Meni.
With a racing rain tire, the rubber is very soft—almost supple—when freshly off a tire warmer. That high percentage of silica provides a scary amount of grip on water; however, once it's on a dry racing line, the tires will last only a few laps until there isn’t anything left on them.
In contrast to a racing rain tire with the largest amount of silica, as an example, the Bridgestone Battlax S20 hypersport street tire is proven to provide good wet grip due to the silica in the tire compound and has a quick warm up by utilizing the NanoPro -Tech TM polymer.
The other large and just as important facet to a motorcycle tires construction is the carcass. In basic terms, it's the body of the tire that gives it its shape. Steel and aramid are two common materials used as cords that form the plies or layers comprising a tire carcass. These layers are woven in varied ways to provide a type of rigidity and profile for each type of tire (shoulder and center line flex). In very basic terms, the carcass is bonded with the rubber like an eraser to a pencil. In extreme applications such as in racing, a tire is worn to the carcass over a 45-minute period. In the 2013 Sepang GP, Jorge Lorenzo road his Bridgestone rains to the limit of sanity, scrubbing off almost the entire tire down to the carcass.
Tread is applied in varied ways to street and DOT race tires. This is to assist in mechanical grip, running on rainwater, and it's a way of measuring tread depth. Manufactures have their own ideas about tread groove geometries; they are cut to prevent tire deformation under braking and acceleration in regards to mechanical grip. One could scream down the highway with a set of Dunlop NTEC slicks on board a streetbike and probably do fine until gravel, stoplights, and rain come along the path of travel. The more uneven (or fouled) and wet a roadway is, the better it is to have some tread on the tires (it also tells you that there is some rubber left.)
The gem in all of this is what a modern motorcycle tire provides. Stability on acceleration and braking, varied profiles, shoulder grip for cornering, quick warm up time of the tread rubber compound (thermal conductivity), lightweight construction, stability at high speed, and mechanical resistance of the rubber to the ground are what is important in motorcycle tires. Each manufacture builds a tire for different applications: sport touring, track/street, race DOT and slicks, rains, sport and cruising to name a few. Each application has its own ingredients in the rubber compound, carcass construction, profile, speed rating etc.
On the Track
A balance between grip and endurance is what everyone wants in a motorcycle tire. On the racetrack, grip may outweigh endurance because it’s hard to have the best of both. In a race tire, heat is generated in the tires by the increased pace versus a street tire that requires less abuse to come up to temperature.
The rubber compound of a high performance tire heats to extremes then cools, returning to its original shape. This is not necessarily due to the actual speed of the rotating black buns, but due to accelerating and braking to and from high speeds and cornering. The carcass of the tire is designed to flex, and it produces heat just like repeatedly bending a paper clip. The more rapid the flex, the more heat is created in the tire. The cords in a tires carcass produce and hold heat as well—on top of providing for rigidity—both flexing and resisting flex.
That heat is also held in the wheels (which will be covered here in a bit). With more physical rubber on the tire's carcass, the better the performance of the tire will be. The thickness of the rubber holds in heat and makes for sticky rubber equaling mechanical grip. The heat activates the chemicals in the compound and the rubber starts to change. This heat also provides that sticky rubber to interact with the road or track surface—all done on a patch of rubber the size of a credit card. The less and less the rubber is in thickness, the less the tire holds heat from the carcass and the wheel. In regards to abrasion and wear, cornering, braking and accelerating increases abrasion and this is what primarily wears tires.
With track day and race tires, there are several differences when compared to street or sport touring tires. When we look at race tires, like the Dunlop D211 GP-A, the profile is higher to allow quicker turn in, and has increased shoulder area for grip at higher corner speeds, mid corner stability, and exit traction. The lower the lean angle (upwards of 50 degrees!), the higher lateral force becomes on the tires and suspension. A track or race tire is built to withstand that abuse and perform to a high level of heat and abrasion, but still afford grip.
Most motorcycle tires of every application have multiple compounds on separate surface areas of the tire. In racing tires, for example, the center strip of rubber cools quicker than the edges and should give stability at speed as well as for accelerating and braking. On a street tire, the center is designed to last thousands of miles, and to give good feel and assured grip at lower temperatures and on wet roads.
The front tires differ dramatically from the rear. On the front tire for a GP bike, there is over 1.6 g’s of force pushing down on it. A street tire is literally in no shape or form built to sustain this type of abuse—and it doesn’t need to. On a race track there is no droning down the straight at 55 mph like on a highway, and the shoulders of a race tire need to be hot and sticky all the time, or close to it to guarantee grip at every corner. The major virtues to look for in a track tire are what you want to do with it and which tire suits that need.
On the Street
The rubber rolling along the ground as a “race tire” does not create enough heat at slow speeds to make mechanical grip required for commuting to and from work along Interstate 5. This sounds strange, but imagine running WSBK Pirelli slicks on your S1000RR. Even that rowdy BMW won’t bring those tires to temperature—unless you arrive to work with half of the CHP in your mirrors. Street tires will warm quicker than race ones, but with less capacity for grip at higher temperatures and speeds.
The compounds in street tires not only warm quickly, but also are advertised to be last longer—with the capacity for less punishment. There are tires like the Dunlop Q3 that are good for street and track given the right care and use, but even these types of tires will take a rider only so far on the track. There are many choices today for street tires and almost not a bad one to choose from. This is a subjective statement, but off the showroom OEM tires provide more than enough grip and performance for legal road use.
The common tire choices in sportbike circles are full race tires that require warmers, track day or canyon tires (that could use warmers but don’t require them), sport touring tires (good in the rain) and everything else. That everything else is OEM-issued donuts that will work for commuting and riding like a normal, everyday, well-adjusted adult. Depending on how you push tires and what you ask of them should be predicated on what they are designed to do. Some will argue tires like religion, but when choosing a tire it comes down to application, your ability to ride them correctly, and your self-confidence in doing so.
Chemical composition of tires can change and degrade as the agents in the rubber evaporate from the tire. This is not the main cause for loss of grip, however. It still comes down to how much rubber is hitting the road and at what temperature.
One heat cycle includes a race tire reaching its operating temperature (for most, it's over 165 degree F), and then falling back to its static and cold temperature level. A street tire will perform hundreds or more heat cycles, but with less degradation compared to a high performance race or track tire. A track day or race tire will last with less heat cycles, and ultimately will begin to degrade to unusable past two or three days of continuous use. This is not all due to the heat cycle—less rubber on the carcass equals less performance. But again, with the extreme heat to cold transitions, the tires will lose performance over time.
Tires are also part organic. Sunlight and other factors will degrade the rubber if a tire is exposed for long periods of time. Just like a mushroom, keeping them in a dark and cool space is the best way to store them—just don’t feed them sh*t. Everyone loves the smell of rubber right?
Tire warmers, when used on tires that require them, are a very valuable. They prevent fewer heat cycles, and in turn, increases the longevity of the tire.
The other reason to use tire warmers is at the track. Keeping tires hot between practice sessions or before a race makes sense in many ways. The number one reason is that warmers enable the tires to reach optimum operating temperatures quickly and safely. The predictability of good, proven tires straight off of the warmers gives confidence to a rider and keeps him or her from having a cold tire incident. Tire warmers are not cheap, but are a very good investment even with using them on street/track tires.
Keeping rubber hot is important, and there are some factors to consider when monitoring the thermostat. Besides checking the temperature gauge, feeling a warm touch by hand is a good indicator if the tire warmers are getting the tires up over 160 degrees F. Tire warmers are now available with side skirts to keep the wind off the wheels while the bike sits on the stands, ensuring that loss of heat doesn't occurs.
Air Pressure and Suspension
For a street bike, your optimum and safest bet for performance and handling is to run the recommended tire pressures. When taking your bike to a track day there is some room for adjustment, but the OEM suggested psi is fine (if you're running OEM tires).
Track tires, DOT and slick race tires are a different story. Each manufacturer is different, but when you purchase the tires track side or off the Internet, there are suggested air pressures given for the best performance and safety of the tire. Track temperature, abrasion, personal preference and other factors are considered when adjusting the air pressure in a race tire. The tire vendors at the track are the guys to listen to when you want to know the ground truth.
Tires are very pressure sensitive and a motorcycle's suspension and chassis also react to the air pressure in the tires. With that said, overinflating or underinflating tires can cause undesired things to happen. Overinflation will cause increased wearing in the center of your tires and underinflating can cause tearing (or blowout) in a sidewall and/or an overall loss of handling. On the race track, air pressure (too much or too little) can create abnormal tire wear. The shoulder of a rear tire can tear very quickly if the carcass is put to the test—creating heat beyond the operating temperature of the compounds in the rubber. Suspension settings, a heavy throttle hand, and other variables such as track temp and tire temperature come into play as well.
In the canyons, track days, or racing most sportbike tires will perform well enough to have fun. Tires are not all created equal. Depending on how far they are pushed combined with the riders skill level, there are thresholds to which they lose grip and performance.
In the end, tires are just one of the many amazing facets of motorcycling—understanding them makes every rider safe and faster.