Source: Cycle World
Ask Kevin Cameron (Cycle World/)Years ago when a crew from Dunlop surveyed tire pressures of attendees’ bikes at a big tour rally, they were worried when they found 40 percent of those tires to be underinflated. Why? Rubber is elastic, but imperfectly so. Compress a perfect spring with X amount of energy, then release it. As it expands, it gives back 100 percent of the energy we just put in.Stretch, shear, bend, or otherwise deform a bit of rubber with X energy, then release it. As it resumes its original shape, it may give back only 90 percent of X. The other 10 percent is dissipated within the rubber as internal friction, which the engineers call “hysteresis.” It becomes heat.We’ve all stopped for fuel after a long run on a high-speed highway and happened to touch one of our tire treads. It’s quite warm. Hysteresis is the cause, for a round tire must flex to enter the flat footprint, then flex again as it resumes its round shape, hundreds of times per minute. As the journey begins, this heats the tire, but the rush of wind over the tire also carries away heat. After some time a steady state occurs. The tire has reached a temperature at which the heat put into it by rubber flexure equals the amount of heat lost to the passing air.The faster we ride or drive, the greater the flex energy we put into our tires. Air rushes past the tires faster, causing more cooling, but heat transfer is mainly proportional to the temperature difference between the hot object and the air that is cooling it. This difference is called “Delta-T.” To get rid of more heat, the tire’s temperature must rise to increase the Delta-T or the ambient air temp must decrease.Delta-T is a big concern in MotoGP racing and affects how racers ride. (MotoGP/)This is why the Dunlop techs were frowning at finding so many low tires. The lower the pressure, the more rubber flexure occurs, and the hotter the tire runs—possibly hot enough to threaten tire integrity. Keep a tire pressure gauge handy, and use it.If the day becomes hotter, the warmer cooling medium (rushing air) reduces the Delta-T, forcing the tire’s temperature to rise. It rises until the heat being generated in the flexing tire again equals the heat carried away by passing air.Racing tires behave this same way, but because speeds are higher and extra “flex heating” is added by hard acceleration, braking, and cornering, Delta-T must rise higher to reach a steady state. Race tire operating temperatures are well over that of boiling water (212 degrees Fahrenheit), up in the vicinity of 260 F. Back in the “Goodyear decades” (1964–1984) I saw a TZ750 come off the banking and into the Daytona pit lane. A Goodyear tech stepped forward to push his thermocouple needle into the tread, and I saw the needle swing up to 315 F.Race teams experiment with tire pressure to find the pressure that gives quickest lap time. The higher the tire pressure, the smaller its footprint becomes, with less severe tread flex entering and leaving the footprint. This reduces the tire’s operating temperature but can also reduce grip because higher inflation pressure makes the footprint smaller.At lower tire pressure, the footprint becomes larger. Because rubber friction is not linear, a larger footprint gives greater grip, but only up to the point at which the footprint becomes unstable (buckling), or because tread temperature rises high enough to threaten tire integrity. Racers naturally want to run their tires at the pressures they have found to give quickest lap times.The tire manufacturer uses sophisticated test equipment to explore the relationship between tire temperature and tire damage from heat—the onset of blistering or chunking. In blistering, components in the tread compound reach a temperature at which they volatilize, forming bubbles in the material. They may burst near the surface, creating little blisters. In chunking, heat weakens the bond between tread rubber and the tire’s strong fiber carcass. Chunks of tread material are flung off at high speed.Minimum tire pressures in MotoGP are monitored during the race, and racers and teams are penalized for not adhering to the rules. (MotoGP/)Operators of roller rear-wheel dynos are familiar with chunking because, with the drive wheel strapped down against the dyno’s roller, an abnormally large amount of rubber flex takes place there. It’s easy to let a tire get hot enough to chunk, giving the operator on the bike some nasty bruises.What if the best performance from a given tire is found by the riders to be possible only at an inflation pressure lower than suggested by the maker? That used to occur at Daytona every year in a certain period. The riders on tires of a particular maker would air up to the pressures they had found to give best lap times, but at pit-out a tire company tech would inflate them to the officially “recommended” higher pressures. No problem; down at the far end of pit lane, riders had placed mechanics with pressure gauges, to return the tires to “best-lap-time” pressures.But in MotoGP, tire pressures have been made mandatory, backed up by penalties. This is made complicated by the fact that if you are in a drafting pack of bikes, your front tire is being “cooled” not by fresh ambient air, but by air heated by the 300 hp engines of the bikes ahead of you. This is air heated by passage through water and oil coolers, or directly by mixing with much hotter exhaust gas.This hotter air reduces the Delta-T that is cooling the front tire, so it has to become hotter to achieve enough tire-to-air temperature difference to stabilize at a higher constant tire temperature. That causes the tire’s footprint to shrink, and its grip to decline. Riders know this is happening when the front tire begins locking during braking in places where braking was normal on previous laps. They know that continuing this way will keep on raising tire pressure until its footprint shrinks enough that it can no longer support cornering loads. Then it will slide, adding even more frictional heat.The amount of heat coming off a MotoGP bike is a real issue regarding front tire pressure and heat for those right on the heels of the bike in front. (MotoGP/)To avoid this, the rider pulls out of the draft, into fresh air. Or falls back far enough that the hot air from bikes ahead is enough diluted by surrounding air to become much cooler. The rider may also “rest” the tire from time to time. Fascinating for the spectator! Using any of these methods also slows the rider’s lap times.Riders having this experience may resolve to start their next race at lower tire pressure, so that the temperature rise from drafting won’t go as high, but that is limited by the tire maker’s mandatory lower limit of 1.8 bar (26.5 psi). Tire pressure monitors inside every tire report not only to the rider’s screen, but also to officialdom. If they see your tire has operated below the minimum pressure for X percentage of laps, you are sanctioned.There is one good way around this. Lead from the start and lead every lap to the finish. No drafting, no hot slipstreams, no front locking or closing.I’m sure riders would like the tire maker to just make a better tire that is safe to operate at the tire pressure that gives best race performance.That’s racin’ in the 21st century.
Full Text:
Ask Kevin Cameron (Cycle World/)
Years ago when a crew from Dunlop surveyed tire pressures of attendees’ bikes at a big tour rally, they were worried when they found 40 percent of those tires to be underinflated. Why? Rubber is elastic, but imperfectly so. Compress a perfect spring with X amount of energy, then release it. As it expands, it gives back 100 percent of the energy we just put in.
Stretch, shear, bend, or otherwise deform a bit of rubber with X energy, then release it. As it resumes its original shape, it may give back only 90 percent of X. The other 10 percent is dissipated within the rubber as internal friction, which the engineers call “hysteresis.” It becomes heat.
We’ve all stopped for fuel after a long run on a high-speed highway and happened to touch one of our tire treads. It’s quite warm. Hysteresis is the cause, for a round tire must flex to enter the flat footprint, then flex again as it resumes its round shape, hundreds of times per minute. As the journey begins, this heats the tire, but the rush of wind over the tire also carries away heat. After some time a steady state occurs. The tire has reached a temperature at which the heat put into it by rubber flexure equals the amount of heat lost to the passing air.
The faster we ride or drive, the greater the flex energy we put into our tires. Air rushes past the tires faster, causing more cooling, but heat transfer is mainly proportional to the temperature difference between the hot object and the air that is cooling it. This difference is called “Delta-T.” To get rid of more heat, the tire’s temperature must rise to increase the Delta-T or the ambient air temp must decrease.
Delta-T is a big concern in MotoGP racing and affects how racers ride. (MotoGP/)
This is why the Dunlop techs were frowning at finding so many low tires. The lower the pressure, the more rubber flexure occurs, and the hotter the tire runs—possibly hot enough to threaten tire integrity. Keep a tire pressure gauge handy, and use it.
If the day becomes hotter, the warmer cooling medium (rushing air) reduces the Delta-T, forcing the tire’s temperature to rise. It rises until the heat being generated in the flexing tire again equals the heat carried away by passing air.
Racing tires behave this same way, but because speeds are higher and extra “flex heating” is added by hard acceleration, braking, and cornering, Delta-T must rise higher to reach a steady state. Race tire operating temperatures are well over that of boiling water (212 degrees Fahrenheit), up in the vicinity of 260 F. Back in the “Goodyear decades” (1964–1984) I saw a TZ750 come off the banking and into the Daytona pit lane. A Goodyear tech stepped forward to push his thermocouple needle into the tread, and I saw the needle swing up to 315 F.
Race teams experiment with tire pressure to find the pressure that gives quickest lap time. The higher the tire pressure, the smaller its footprint becomes, with less severe tread flex entering and leaving the footprint. This reduces the tire’s operating temperature but can also reduce grip because higher inflation pressure makes the footprint smaller.
At lower tire pressure, the footprint becomes larger. Because rubber friction is not linear, a larger footprint gives greater grip, but only up to the point at which the footprint becomes unstable (buckling), or because tread temperature rises high enough to threaten tire integrity. Racers naturally want to run their tires at the pressures they have found to give quickest lap times.
The tire manufacturer uses sophisticated test equipment to explore the relationship between tire temperature and tire damage from heat—the onset of blistering or chunking. In blistering, components in the tread compound reach a temperature at which they volatilize, forming bubbles in the material. They may burst near the surface, creating little blisters. In chunking, heat weakens the bond between tread rubber and the tire’s strong fiber carcass. Chunks of tread material are flung off at high speed.
Minimum tire pressures in MotoGP are monitored during the race, and racers and teams are penalized for not adhering to the rules. (MotoGP/)
Operators of roller rear-wheel dynos are familiar with chunking because, with the drive wheel strapped down against the dyno’s roller, an abnormally large amount of rubber flex takes place there. It’s easy to let a tire get hot enough to chunk, giving the operator on the bike some nasty bruises.
What if the best performance from a given tire is found by the riders to be possible only at an inflation pressure lower than suggested by the maker? That used to occur at Daytona every year in a certain period. The riders on tires of a particular maker would air up to the pressures they had found to give best lap times, but at pit-out a tire company tech would inflate them to the officially “recommended” higher pressures. No problem; down at the far end of pit lane, riders had placed mechanics with pressure gauges, to return the tires to “best-lap-time” pressures.
But in MotoGP, tire pressures have been made mandatory, backed up by penalties. This is made complicated by the fact that if you are in a drafting pack of bikes, your front tire is being “cooled” not by fresh ambient air, but by air heated by the 300 hp engines of the bikes ahead of you. This is air heated by passage through water and oil coolers, or directly by mixing with much hotter exhaust gas.
This hotter air reduces the Delta-T that is cooling the front tire, so it has to become hotter to achieve enough tire-to-air temperature difference to stabilize at a higher constant tire temperature. That causes the tire’s footprint to shrink, and its grip to decline. Riders know this is happening when the front tire begins locking during braking in places where braking was normal on previous laps. They know that continuing this way will keep on raising tire pressure until its footprint shrinks enough that it can no longer support cornering loads. Then it will slide, adding even more frictional heat.
The amount of heat coming off a MotoGP bike is a real issue regarding front tire pressure and heat for those right on the heels of the bike in front. (MotoGP/)
To avoid this, the rider pulls out of the draft, into fresh air. Or falls back far enough that the hot air from bikes ahead is enough diluted by surrounding air to become much cooler. The rider may also “rest” the tire from time to time. Fascinating for the spectator! Using any of these methods also slows the rider’s lap times.
Riders having this experience may resolve to start their next race at lower tire pressure, so that the temperature rise from drafting won’t go as high, but that is limited by the tire maker’s mandatory lower limit of 1.8 bar (26.5 psi). Tire pressure monitors inside every tire report not only to the rider’s screen, but also to officialdom. If they see your tire has operated below the minimum pressure for X percentage of laps, you are sanctioned.
There is one good way around this. Lead from the start and lead every lap to the finish. No drafting, no hot slipstreams, no front locking or closing.
I’m sure riders would like the tire maker to just make a better tire that is safe to operate at the tire pressure that gives best race performance.
That’s racin’ in the 21st century.
