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| Fit for the winter--How winter tires brave the weather | |
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Ever since they were invented in the late 1800s, automobile tires have undergone a tremendous process of development. However, in spite of all the research efforts in this field, there’s still no product that gets top marks in every discipline. For example, as soon as the outdoor temperature drops below 7°C, a switch to winter tires is strongly recommended. That’s because of the chemistry of rubber, as well as the trend toward increasingly powerful vehicles that must transfer their force to the road as efficiently as possible.
On the following pages you’ll find some tips and tricks for changing to winter tires. You’ll also find out how tires have developed from the initial wheel coverings of hard rubber to today’s high-performance products, and how today’s rubber industry is helping to make the tires of the future even better.Prime season for accidents
The road surface is wet, it’s raining cats and dogs, it’s dark, and the oncoming traffic is blinding you. Suddenly a red light flashes in front of you. You slam on the brakes — but the car goes on sliding forwards. In such a situation, you’re lucky if only the car body is damaged. Winter is the prime season for road accidents. Well over half of all traffic accidents in germany happen between October and Easter. According to the initiative to promote winter tires launched by the German Traffic Safety Council, the risk of accidents is six times higher in the winter months than during the rest of the year. There are many reasons for this: poor visibility, muddy streets, and low temperatures that cause summer tires to lose their normal grip on the road surface. The fact is that even if your summer tires’ tread is still good, they won’t do you much good when the weather turns cold. That’s because a tire’s tread alone won’t guarantee a good grip on the road, whether it’s summer or winter. Even extremely knobby treads can’t prevent non-winter tires from slipping on wet roadways — even if drivers scrupulously comply with the ADAC auto club’s recommendation that for winter driving tires should have a tread of at least 4 millimeters instead of the legally required 1.6 millimeters. The true reason behind many a dangerous skid between October and Easter is the composition of the rubber in a car’s tires. Of course tire treads with a good grip on the pavement are part of a vehicle’s basic equipment. A good tread interlocks with snow to provide additional grip, and fine sipes repel water and form gripping edges that optimally lock into frozen mud or snow. But even the best tread doesn’t compensate for a fundamental characteristic of the rubber compounds used for summer tires: they harden at low temperatures.
However, hardness in particular is a crucial factor in optimal road grip. The softer the rubber, the better it can adapt to the unevenness of the road. Soft types of rubber virtually “flow” into the asphalt, interlock with the road surface, and pull the car forward. Something similar happens when the driver steps on the brake: only if the rubber has an optimal grip on the road surface can the driver stop the car in time. As a rule, road grip is not a problem at high summer temperatures, because the types of rubber used for summer tires are soft and can ideally grip the road surface. But when outdoor temperatures turn cool, driving becomes risky, even if there’s no snow on the ground.Temperatures don’t have to plunge to Siberian levels in order to qualify as dangerous. On the contrary, in recent decades tires have become so sophisticated that the sensible division of labor between summer and winter models is at the temperature of 7°C. Below this cutoff point, life becomes difficult for summer tires. And according to the winter tire initiative, these low temperatures don’t occur only in winter. In Germany, for example, they are possible during no less than 185 days a year.
In 2007, approximately 57% of all drivers in germany were smart enough to switch to winter tires in good time. That’s almost 15% more than the figure for 2002. One reason for this increase may be that the legal regulations recently became stricter in germany. Ever since May 1, 2006, drivers using the wrong tires have been legally penalized: those caught driving with summer tires during the winter must pay a fine of €20. If the driver has obstructed traffic because the vehicle’s summer tires cannot proceed on a slick road surface, the fine is raised to €40 and the driver receives penalty points at the central traffic violation registry in Flensburg, germany. And if there’s a collision, the car insurance provider may even refuse to honor the driver’s liability insurance.
The winter pros
How to find the best tires
Changing tires well ahead of time is the smart thing to do, because only winter tires are ideally adjusted to the weather conditions we’re likely to face in the winter months. They are made of rubber compounds that are soft enough to provide an optimal grip even at low temperatures. For example, they contain a larger proportion of natural rubber, which is especially elastic, and special rubber compounds such as those produced by LANXESS.
That doesn’t create any disadvantages for drivers. If you’ve got the right tires on your car, fast driving is not a problem even in the winter months. Nor is it necessary to sacrifice driving comfort — and the time is long past when changing tires was a complicated procedure requiring an expert mechanic. A set of good winter tires, including expert advice, costs approximately €300. Major manufactures even offer programs on the Internet that help customers choose the perfect partner. However, if the price is well below this figure, it’s advisable to look very closely at the products involved: many of the inexpensive manufacturers’ products tested by the german automobile club ADAC did not receive good marks.Of course, €300 is a lot of money — but it’s an investment in your own safety, and it pays off very quickly. Unused summer tires can be stored either at your tire dealer’s, in most cases at little cost, or in your garage. If the wheel rims are left on, the tires can be hung up or stacked — but don’t forget to change their position once a month so that the weight is distributed as evenly as possible over all of the tires. Tires without a wheel rim should be stored upright and also turned every few weeks. It’s important to store tires in a dark and cool place, as they can be damaged by heat sources, ozone, ultraviolet light, and puddles of oil. Temperatures between 15 and 20°C are ideal.
The bonus for you is that this compulsory vacation extends the lifetime of your summer tires. If it is properly stored, a tire can serve you well for eight to ten years, provided that the tread doesn’t get worn down.
However, you should be careful if you intend to equip only individual axles, or even individual wheels, with winter tires. In some countries this is legally permitted under certain conditions, but in practice it’s not recommended. The ADAC is very clear on this point: “This procedure obviously does not make sense, because a car’s safety in the wintertime is defined by the weakest tire of the vehicle.”
Even though the tire change itself is not a huge problem, determining the optimal time to carry it out can be a complicated process. The experts advise changing tires as early as the beginning of October, when the first cold days are in sight but tire dealers are not yet being mobbed by customers. In addition, there’s still a wide range of products to choose from. And there’s yet another advantage that few drivers are aware of: tires that are already run in have a better grip on the road. The german consumer protection organization Stiftung Warentest points out that new tires should as a rule be run in before they are subjected to fast driving. That’s why it’s better to change to winter tires before they are actually needed.
The only thing drivers absolutely should not do is to leave winter tires on the whole year through. That’s because they become too soft in the summer and get worn down very quickly, just as though you were rubbing an eraser across sandpaper — and that’s a waste of your money. However, drivers who ignore this advice and are lucky enough to live in a temperate climatic zone — where it’s unlikely that a solid layer of snow will lie on the ground for weeks at a time — will find that all-year tires are a good compromise. Such tires are in effect “winter tires that are appropriate for summer use,” according to the ADAC. They too offer a “legal” option for avoiding the fall rush at the tire dealers.But compromises bring with them certain disadvantages. For example, all-year tires with the M+S designation are “slow,” so the owners of fast cars may have to paste a sticker somewhere in their field of vision to remind themselves not to drive at top speed. Worse yet, the ADAC points out that tires that are specially made for driving on icy roads in the wintertime may be inadequate in other respects: in tests carried out by the auto club, they did not perform well on snow. So when will we be able to buy tires that fulfill every driving requirement? Possibly never. But the products on the market are improving, thanks to the help of natural rubber and rubber chemistry, which work right down to the molecular level to make tires that grip the road better, are increasingly skid-proof, and save energy. The following two sections will tell you more about these topics. A child of the 50's The modern reliable tires that get us safely through winter weren’t always around. In fact, until quite recently winter tires didn’t exist. Although first conceived around 1914, they weren’t introduced until the middle of the last century. In other words, they had to wait until the era of the sweeping tail fins and curving windshields. Naturally, those precocious winter tires were nothing like their counterparts of today: they squelched through the snow on an especially knobbly tread — and on roads free of snow they made a lot of noise. They also made the eyes of gas station attendants light up in expectation of a good sale. High speeds were simply impossible. However, a thick tread was still better than snow chains — and far better than “spikes.” The latter, which had to be screwed into the tread surface, scarred many of Germany’s roads before the government banned them in 1975. Even in the 1950s people were aware that a thick tread wasn’t the only thing that distinguishes a winter tire from its summer counterparts. The true recipe for success lay elsewhere: in special rubber mixtures that were devised especially for use at low temperatures. For example, with the help of a higher proportion of particularly elastic natural rubber – of the sort that tolerates water especially well and exhibits good grip on wet roads.Black art Working in the magic triangle Despite all the research, the ideal tire for all requirements doesn’t exist yet. A type of rubber that would provide good grip on packed snow may be entirely wrong for summer driving in St. Tropez. And even Formula 1 drivers still need different tires for dry versus wet pavement – never mind all the high tech. This development isn’t finished by a long shot. On the contrary, there’s still much to do. “You’ve got to realize first of all that tire manufacturers have to deal with very conflicting goals,” says Dr. Thomas Groß, a tire rubber specialist at LANXESS. Most notorious among tire development specialists is the “magic triangle,” a stubborn engineering challenge that causes a good many headaches. It relates to the fact that the three important tire properties of abrasion, rolling resistance (which equates with fuel consumption), and wet grip are closely interdependent. Of course the softer the rubber, the better the grip a tire can have on wet pavement. But soft types of rubber (just like an eraser) abrade faster. The bottom line is that, until recently, anyone striving to tug on one corner of that triangle had to let go of the other two – in other words, accept some curtailments there. To understand the value of such tests, one has to know what really happens in rubber when it’s subjected to stress. This is actually rather difficult to determine, because the numerous constituents of rubber behave very differently from, say, the atoms in a chunk of metal. One of the places where this can be demonstrated is in Leverkusen, Germany, in the special rubber analysis laboratory at LANXESS, the technology leader in synthetic rubbers. Here you’ll find machines that pull, twist, heat, and tear apart pieces of rubber – all in the service of making better tires. But despite all those curves and diagrams displayed on the computer screens of the lab types, the development of new kinds of rubber is one of the last segments of the exact science of chemistry where gut feeling is still important. Even the rubber experts themselves sometimes refer to their work as a “black art.” Some aspects of rubber chemistry, or course, are plain facts, and chemists can make the best of those. In greatly simplified terms, all kinds of rubber consist of enormously long molecules assembled in a process called polymerization from building blocks that are always identical. But these molecules can be linked in many different ways. For example, rubber isn’t always synthesized in the shape of long, straight molecules resembling an infinitely long string of wool yarn under a powerful microscope: its molecules may have many bends and kinks. “The rule of thumb is that the more linear a rubber molecule is, the more elastic it is. And the more elastic it is, the less energy it stores and uses when stressed,” explains Dr. Groß. If you slam a hammer on a rubber cork and on a stone, the stone will shatter. The rubber, on the other hand, will cause the hammer to rebound and will return to its original shape. That’s because the rubber reflects the energy of the hammer blow, while the stone does not: it uses the energy to shatter into fragments. Applied to tires, this means that the more elastic the tire’s rubber, the more energy it releases again after a deformation. The energy that the rubber doesn’t release again but uses up for “other purposes,” like the shattering stone, is referred to as “rolling resistance” by tire developers, and is just a senseless waste of fuel. Conversely, the more linear a rubber molecule is, the more elastic it is, and the lower the rolling resistance. The “most linear” rubbers the industry is capable of making today include neodynium-polybutadiene rubbers (Nd-PBRs), which are developed and sold by LANXESS, among others. They contain 300 times fewer “kinks” than conventional tire rubbers. That’s how clever chemistry can help reduce fuel consumption!And what about the silica fillers? Have there been any advanced developments here? Yes, indeed: “It’s enormously important for the performance of a tire that these filler particles adhere well to the rubber molecules,” says Dr. Groß. “It used to be necessary to knead the materials in the rubber compounder very thoroughly for a long time to cause the molecular strings to somehow adhere to the rough surface of these particles. Now there are better ways.” Today’s chemists have learned a lot more about that and have modified the rubber molecules to endow them with small “sticky spots” that let them cling to the filler particles like geckos on a rock wall. LANXESS chemists have even succeeded in distributing these spots all over the molecule. “We’re hoping that they’ll adhere to the filler particles even better than when those anchor spots are located only at the ends of the molecules, which has been the state of the art until now. Initial tests indicate that we’re actually on the right track.” Not only do the new rubbers adhere better to the hydrophilic fillers: they also coat the hard filler particles with a thick layer of rubber. As a result, chemists think that the particles can further stabilize the rubber, but without inhibiting its mobility as much as in previous types. So the tire tread can better grip the road surface in those fractions of a second when the rubber has to “flow” onto the road. What’s more, chemists also postulate that, due to their greater adhesion, the rubber molecules are less likely to shift in position relative to the filler particles. As a result, less energy is wasted due to internal friction. This also reduces the rolling resistance. “Because these rubbers are more firmly knit together, abrasion values are also very good. That’s important not only in terms of a long tire life, but also in view of the public discussions about sources of fine particulates,” Dr. Groß concludes. These new special rubbers are already functioning in a laboratory setting. If all goes well, they will soon enter the market – and do their part to further enlarge the “magic triangle,” creating entirely new development opportunities for tire makers. One new type of rubber – many new possibilities. “It goes to show how complex rubber chemistry is,” Dr. Groß points out. “Even minor changes can affect many important properties at the same time.” 11/11/2007 | |
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