Wed, 01/20/2010 - 03:25 — admin
By Andre Lam
Changing tyres can be a simple or complicated event. The simplest change would be to get the same tyres that the car has as OE (Original Equipment). This would ensure that the qualities that the car manufacturer intended are retained with the new car.
However for budget cars and indeed most of today’s passenger cars, the tyres have been specified based on a minimum set of standards pertaining to ride, handling, longevity and of course minimum cost. So it leaves a lot of room for improvement by changing tyres. At the other extreme, the most expensive luxury or sports cars have strict criteria for tyre replacement as they have been specifically tuned to work with certain tyres homologated for use in these vehicles. The requirements would not necessarily consider cost a priority but must meet the requirements for grip, handling and high-speed durability especially in sports cars like Ferrari or Porsche. On luxury limousines, ride characteristics, low noise and load-durability/capacity are more important.
Tyres are perhaps the single most important item in a car yet their choice of replacement has been taken for granted. They are not just round black doughnuts that help protect the rims. These items are all that stand between you and an accident or seen another way, with a proper set of tyres it is possible to raise enjoyment levels in an otherwise mundane car, all for the price of a tyre change. You can say that your expensive car is no better than the set of tyres it sits on.
It is then surprising that as many trade down as do they trade up. This behavior pattern is alarming because in Singapore we enjoy one of the lowest tyre prices in the world today and yet people choose an even cheaper tyre to use. It is not to say that there are no bargains around but considering that we pay just about the highest taxes for a car in the world it would seem prudent that its worthwhile to invest a little more on a decent set of tyres. Here are some pointers for those intending to swap their OE tyres for something better and some reasons as to why.
The most compelling reason for consumers to upgrade is ironically not for safety or performance but for looks. This is not a bad thing as usually this upgrade comes with a useful lift in performance as well. The smaller the OE wheel and tyre the cheaper it is for the car manufacturer. The tyre industry oddly uses a combination inches and millimeters in the same sentence so do not be confused with those numbers.
The most common upgrade is going upsize by one inch and now going two inches is getting increasingly common. In order to keep the rolling circumference of the new set the same as the original so as not to upset the odometer and other electronic settings for ABS or traction control, it is best to stay close to the OE circumference. This means that since there is an increase in rim diameter, the tyre’s sidewall shrinks in height correspondingly. For those not so technically minded the local tyre shop can help you but it is wise to do a little research before you jump in.
A lower profile tyre is one where the tyre looks squat and broad rather than thin and tall. The reduction of tyre sidewall height has two effects, which are usually diametrically opposed. The shorter the length of rubber sidewall, the stiffer it is and this translates to a stiffer ride but in return it offers sharper steering and a more stable footprint, enhancing performance. Usually these tyres come with a better griping tread compound to further add value. The very latest top-level tyres have an uncanny ability to combine high performance and retain a good ride quality but at a hefty premium.
Since the industry is appearance driven it is not surprising to find a bewildering array of alloy rims available nowadays and those sold here meet some minimum criteria for safety and should be adequate for general use. In order not to upset the vehicle’s suspension geometry too much it is best to stay close, within 5-10mm of the OE offset. Also the width of the rims should be such that the mounted tyre does not foul up against the car’s bodywork.
Note/diagram on RIMS.
Note/diagram on sidewall markings
Note diagram of Speed Ratings
Those seeking performance will undoubtedly upgrade to lower profile tyres. These individuals are more cognizant of their rim and tyre choice. This also means the tyre and wheel manufacturers have to deliver a higher standard product to satisfy these consumers. The Ultra High Performance tyre is also known as ULP or Ultra Low Profile tyre and refers to one, which is generally below 50 Series in profile. Originally increasing the rim size was a technique used in racing which allowed the engineers to fit even larger brakes which required grippier tyres. This concept caught on and although many do not swap their brakes, one can still enjoy the benefits of a wider, lower profile tyre.
The main ingredient in a high performance tyre is undoubtedly the tread compound, a secret blend of various rubbers and filler materials to yield that magical mix of high grip in dry and wet yet possessing a usefully long life though it is usually shorter than the normal street tyre. The compound is invariably softer to be able to key into the road asperities better and indeed some tyres actually feel sticky to touch once warmed up. Typically these high performance tyres sacrifice ride and longevity in exchange for its superior grip and handling. Today’s best examples manage to deliver somewhat improved ride and durability but carry a steep price tag.
The issue about just how long a tyre can or should last has been recently becoming a question that needs addressing. In the good old days where retreads were common, tyre manufactures were claiming that the carcass could be used again and again as long as there was no serious damage and that the retreading was done correctly. Though retreading is still practiced for aircraft, bus and heavy-machinery tyres, the passenger tyre manufacturers are now singing a different tune suggesting prompt replacement.
Passenger car tyres typically last 40-60,000 km depending on your driving style but another factor seems to be ignored, time. In our small island, it is unusual to rack up very high mileages in a short time but in Europe or USA these mileages are common for a 2-3 year old tyre. So if you find a lot of tread left in your 3 year old tyre it is prudent to look for a replacement in the interest of safety as time has a deleterious effect on rubber. Once fitted on a car tyre rubbers face a very harsh environment of continuous heavy loads, high heat, oil and ozone which all conspire to damage rubber compounds over time.
In Germany lawmakers are looking into the possibility of ensuring that tyres older than 6 years are not allowed to be used on their roads that invariably include high-speed autobahn driving. In Japan recently there has been a push for the compulsory inspection of the tyres during their annual vehicle inspections to request owners change their tyres if they are more than 5 years old. So if your tyres are more than 3 years old they are already well past their best and you should consider a new set in the interest of safety. Can you measure the value of a human life in terms of the cost of a set of new tyres especially if the life may be your own?
Unless you stay right next to a tyre making plant, the likelihood of getting a week old set is impossible. Most imported tyres arrive in about four to six months and due to rotating stock at the local warehouses, end up being about 6-9 months old by the time you get them at retail level. So do not be unduly upset, as this time lag has been considered during the manufacture of the tyres. Many of the big retail outfits have proper storage facilities that keep tyres away from heat, ozone and oil to prolong its shelf life.
With today’s sky high petrol prices, interest on improving the car’s fuel consumption has been growing. Today’s latest crop of tyres has improved fuel consumption by 3-5% over each generation. The industry’s latest swap over to the latest Silica compound tyres have yielded a quantum leap in energy efficiency and manufacturers are constantly doing more to stretch that litre of petrol though it should be said that tyres can only do so much.
Tyre construction also helps here as do the rubber compounds. Currently only the top European brands are quietly touting the energy efficiency of their tyres because fuel saving tyres in its earliest days were terrible, sacrificing vast levels of grip and handling for that elusive 1-2 more km/litre. Today it’s a different story with the latest Silica compounds. This is not to say you can have top performance and still be fuel efficient but the grip levels of energy efficient tyres have returned to normal and afford an adequate degree of safety for normal use. Unfortunately it is still more marketable to tout high performance than penny-pinching fuel efficiency but if petrol prices breach the $2 mark you can be sure this will be the next big thing.
Since you may have just bought a set of tyres, not necessarily the latest energy efficient type, here are some tips how to improve efficiency.
The culprit is called rolling resistance and is determined largely by the tyre compound and construction. What you can do to help is increase inflation pressures. Raising pressures from a comfortable 28 psi to a firm 35 psi can improve fuel efficiency by 5%. Sure comfort may be sacrificed but you do usually gain in steering response and handling. However some new designs work without such high pressures but still benefit a little from raised inflation pressures. A word of caution though, there is a maximum inflation pressure embossed on each tyre’s sidewall and it is best one observes this limitation which is usually between 36 –41 psi. Overinflation will not improve efficiency much more and it becomes damaging to the tyre carcass and can leave permanent damage, becoming a safety hazard.
Other tips include planning your trips such that you accomplish as much of your chores in a single trip, avoid excessive braking and accelerating as much as avoid crawling at slow speeds which may ironically work against efficiency as during start-stop city driving just overcoming inertia (getting the car up to cruising speed) is responsible for 35% of the car’s fuel consumption. Driveline friction accounts for a whopping 45%, aerodynamic drag 5% and the tyres account for 15%. Keeping to a decent and constant speed not only helps you in your quest for better fuel efficiency but it helps others following you achieve the same. Switching to a lower grade of petrol if the car manual says its ok saves you money but may not extend your mileage and in some very high performance cars it will actually increase consumption a little. These cars have engines with turbocharging or compression ratios of 11:1 or more.
A tyre can loose pressure at a rate of one psi per month so monitor tyres every month. Studies have shown that 25% of tyres are under inflated so go check on your tyres today. Also make sure the wheel alignment is up to spec as excessive toe-in and camber not only wears out tyres, they waste energy doing so. Rotate tyres according to the handbook to equalize wear on all four tyres. This will allow you to replace the entire set of four as opposed to just two as this ensures you will have the best safety and performance margin always.