Using Tires
People who are actually using tires are serious road and rally racers, circle-track racers, and autocrossers. They have learned a lot about tires by trial and error, or their team collected data. Hopefully, these few tips will help and offer some answers that will generate more knowledge among us, normal people and "will be" racers and make them more confident when they're using their tires at the limit. Lower lap times forever!
What is "Tire Give-Up"
A racecar uses a set of tires a length of time, called a tire stint, mainly determined by the number of laps required to use the fuel allowed by the rules. In most race series it takes more time to refuel the car than it does to change tires, so no time is saved by refueling only. In professional racing there is no reason for the tires to have a useful life that is any longer than a fuel stint.
For most tires the first lap or two are the fastest laps those tires will ever produce. During Michelin - Bridgestone good old-fashioned tire Formula 1 war, some Michelin tires appear to increase in performance after five to seven laps and reach peak grip during laps 10-20 before falling off slightly.
The Goodyear tires sold to NASCAR Winston Cup teams, "give up" at least a second in lap time from the second through the tenth lap and continue to deteriorate until the end of the stint. In fact you can see the tires "give up" during the two-lap qualifying runs for the Nextel Cup races. The first lap is almost always the quickest and many drivers just run one lap and go back into the paddock. The amount of give-up from the first to the second lap is 0.05 to 0.1 seconds per mile of lap.
So why do tires give up? The rubber in tires undergoes cyclic stress at very high levels. The work done by these rubbers result in heat generation. The combination of high cyclic stress and high temperature is bound to generate mechanical and chemical changes in the rubber. Some elastomers, and rubber falls into that category, exhibit stress softening and permanent set. Stress softening has been attributed to displacement of polymer network junctions and entanglements and/or the incomplete recovery to original positions of those junctions and entanglements after stress deformation. The presence of fillers [carbon and silica] introduces possible additional softening mechanisms, including breakage of rubber/filler attachments, disruption of filler structure, or chain slippage at filler surfaces. When intermolecular structures are irreversibly disrupted or reform in new positions while the polymer is extended, the result is permanent deformation. The rubber is said to have "taken a set."
A rubber compound is a mix of many materials, including chemically active agents. An excess of certain kinds of active agents can provide an opportunity for a disrupted or broken bond to repair itself by rebonding at the same location or in a new location.
Another mechanism for tire give-up is more simple. The tire might wear to a tread thickness too thin to generate enough heat and the tire temperature falls out of the range for max grip. A thick tread, even a slicks, deforms and the resulting hysteresis generates heat. A thin tread deforms less, generating less heat.
Heat Cycles
Rubber is a complex substance, a mixture of materials and chemicals manufactured with mechanical processes and various heat and pressure cycles. In use, tread rubber sees mechanical working and time at elevated temperatures very similar to the processes it saw as it was manufactured. It makes sense that more of the same processing would further change the rubber.
The material in a new race tire is semi-stable. If the tread rubber had been totally cured it might be too hard to do its job. So stress and heat can continue the curing process. Even small amounts of energy from ultraviolet wavelengths in sunlight, ozone in air, heat, or mechanical working can cause the rubber in a tire to continue its vulcanization process or change in some way.
The first heat cycle is called scrubbing. Every heat cycle changes a tire to some degree, generally in the direction of harder, less flexible, and less adhesive. Race tires can loose effectiveness before the tread wears through if they go through many heat cycles. For some tires three cycles is too many, while others show a performance drop off initially and then maintain a good level of performance until the tread is worn off. Smart race organizers are incorporating long-lasting tires into their rules so that "spec" tires can lower the cost of racing.
Abrasion
The cause, typically, is overly aggressive driving with a tire that is too soft and grippy for the conditions or the driver has overworked the tires before getting them up to a working temperature. Abrasion patterns are not necessarily caused by gross sliding. A requirement for the development of an abrasion pattern is "unidirectional sliding." Sliding in random directions does not produce these patterns. The orientation of the pattern is important because it indicates the direction of relative motion between the tread and the road.
Here's how that pattern gets worked into the rubber. When a new rubber sample is continuously abraded in the same direction, the rubber develops an array of nearly parallel ridges at right angles to the abrasion direction. The shape of the ridges in cross section, seen in picture, is saw toothed, with the teeth pointed against the direction of abrasion. During sliding, deflection waves in the rubber turn into peaks which are bent over, exposing the upstream side to abrasion. The peaks wear thinner into teeth and the tips are eventually torn off.
Sliding on smooth tracks does not always produce abrasion. Abrasion is generally initiated by local stress concentrations at the contact between track asperities and rubber. Abrasion intensity depends on shape rather than size of the asperities. Experiments have shown that road surfaces exhibit wide variations in abrasion characteristics.
Lab experiments with abrasion in an inert atmosphere (nitrogen rich for example), show less abrasion than the same process in air. This leads to speculations that antioxidants in a rubber compound can make it more resistant to abrasion.
Unfortunately, once a graining pattern is worn into the surface of a tire it's difficult to wear the pattern away. The ridges tend to perpetuate as wear continues. Even worse, since the tread is not evenly loaded after it has been grained, the tire loses grip. It's just another way for a driver to mess up. That's why experienced drivers are so valuable.
Graining is generally consequence of abrasion.
Graining
It's called graining because the rubber rolls off into grains. Laying down more rubber will increase grip and help with graining.
Front tire with even wear, little or no graining
Front tire with extreme graining, consequence of abrasion. This tire is finished!
Rear tire with graining on the inside edge, perhaps due to excessive camber. The graining is not present across the entire tread, suggesting slippage during acceleration. More camber would create more graining and the pattern would appear over a wider area of the tread.
Tire with graining on full surface, sometime because grip levels are very low which makes it difficult to generate load and therefore sufficient temperature in the tire due to track surface condition or poorly balanced car
A more accurate description would have been, "It's called graining because the tire surface takes on a grainy texture. The soft, adhesive tread surface digs into the track texture, gets deformed into waves, and as sliding continues the waves turn over wearing rubber off the upstream side of the wave. When the sliding stops the deformed rubber snaps back leaving a peak pointing against the direction of travel.
As more rubber is laid down during the race, grip will be increased and there won't be enough side loading on the tread surface to start the wave formation. Poorly balanced car, maybe one with excess understeer will grain the front tires more readily than a better balanced car. And opposite. Since some drivers need more understeer for comfort than others, driver style can also contribute to graining. Another factor is the relatively light loading of F1 tires. The contact patch is wide and the cars are light. I don't think you'd see that same type of graining at NASCAR events. Of course the rubber formations of Goodyear and Bridgestone are entirely different.
Blistering
Before we described the vulcanization process and mentioned that if the process goes too far, the rubber can "revert" or return to its uncured state. Reversion can also happen when race tires get way too hot. When you see a driver lock up a tire the smoke tells you something bad is happening. The resulting flat spot is caused by frictional heating of the tread rubber causing reversion. The overheated rubber softens and is scrubbed off the tire.
Blisters around all circumference of Formula 1 tire
Another, less severe, form of reversion is blistering. The photo shows a front right tire with a line of blisters around its circumference. Too high cold inflation pressure or too much pressure build-up during use caused the tread to overheat. The heat is generated at the interface between the belt plies and the tread and the rubber melts, causing a local blister.
Some tire companies have developed anti-reversion compounds that resist blistering. That's all I know about it because of course they won't talk about it. And I can see why this would work. If you can run your tire a little bit hotter without blistering that would be more forgiving for the racers using the tire and maybe the tire would give a little more grip at the higher temperature. That's assuming the anti-reversion compound has no negative trade-offs.
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Ripped wide open, the left rear fat-spoted Pirelli P Zero of Vitaly Petrov's Renault R31 shows just what damage a high-speed spin or hard braking can do to a modern-day F1 tire. |
From 2010, only tire change is allowed during pitstop. Refueling is banned, and drivers have to start with all fuel needed for race. This will put additional load to tires because until now, car was loaded with 50 to 60 kilograms of fuel, and now this load is about 150 to 170 kilograms.
And there is another thing to consider. With refueling, pitstop duration was about 6 to 9 seconds, and a plenty room for good strategy and overtaking trough pits. Without refueling pitstop duration is around 2 to 3 second and strategy should be impeccable. Often this short time can't be used for any meaningful strategy.
But anyway, during pitstop in one typical Formula 1 race there were places to be won and lost to other drivers by mistiming the stop, so how did the teams decide?
The teams have a more sophisticated timing system than the one available to the public and media. It divides the lap into ten sectors, rather than the basic three available for rest of us and this gives them a much faster evaluation of the way the tires are behaving. Rather than wait 20 plus seconds to find out whether they are going off, they can tell every (more or less) 8 seconds how the performance is going.
Engineers and computers study the trends and make the strategy calls based on what they see.
This works in two ways; it tells them the precise moment when their tire compound are losing performance, and by reading the sectors of cars already on different compound, they can see what performance gain rivals are experiencing.
After one driver has pitted and his lap times went from 1m 19.7s (old tires) to 1m 17.6s (new tires - different compound). Rival engineers could see immediately the speed advantage he was getting from his new tires and decide to do and when to do their pitstop.
To know more see the articles:
Tires
Tire Compound
Tire warmers
What is the most important part of a racing car?
To have a complete picture of performance driving, take a look at Corners, Setup, Traction circle, Using tires, Left foot braking, braking, advanced braking, WRC braking technique, Slipstreaming, drifting, cornering, shifting, Heel and toe driving technique and steering technique articles
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