Merityre Article Archive

Looking at the above title, you could be forgiven for wondering, ‘What on Earth is a CRF?’ .As it happens, a CRF is one of those Michael Caine things: not a lot of people know what one is. To put you out of your misery, a CRF is a car tyre science value. It stands for the Coefficient of Rolling Friction. Still mystified? Don’t be – all will become clear!

A CRF is a figure that relates to how much force it takes for a car tyre to be driven along the road. The CRF is used to calculate the amount of drag, otherwise known as rolling resistance, that the tyre has in use. If you need a still simpler example of what rolling resistance is all about, imagine that you’re trying to ride a bicycle with flat tyres. Is it easy? No it isn’t, because the tyres have a high CRF.

We can use our new knowledge practically, to calculate how much force a typical car uses to push its tyres along a road. The force is needed to overcome friction and we can calculate it simply: if we know the tyre’s CRF and the weight it is carrying, we can calculate the force used.

In this example, our car weighs 4,000 pounds and the tyres have a CRF of 0.015. If we now multiply one figure by the other, we can find out the force in pounds. So, 4,000 x 0.015 equals 60 pounds. Does this car tyre science figure mean anything yet? Not really, because we’ve one more sum to do.

The next calculation uses an equation that can tell us how much power the car uses to push its tyres along. The equation is simple enough and we can use it with some figures that are, again, typical. So, Power equals Force multiplied by Speed. Adding figures into this equation shows that the power used is dependent on the speed at which the car is travelling.

Our theoretical car tyres, therefore, use up more power as the car’s speed increases. At 55 miles per hour, they use 8.8 horsepower. At 75 miles per hour, they consume 12 horsepower. Moreover, like with our flat-tyred bicycle, the lower the air pressure in the tyres, the higher the CRF.

Apart from using more power and therefore burning more fuel, what does this friction do? As you can neither create nor destroy energy, it changes its form. In fact, it turns into heat energy. Should the road surface be soft, more heat is transmitted to the ground and less builds up in the tyres.

This car tyre theory is interesting but asking the question, ‘what is it for’ brings an equally interesting answer. Tyre manufacturers sometimes publish CRF figures for their various kinds of tyre. The lower the CRF the less the rolling resistance or, to put it simply, drag. The less the drag, the better the economy. This also shows what is costly about running your car tyres at too low a pressure. However, remember that wasted fuel isn’t the only cost of under inflation.

Lastly, here are the CRF figures for some typical wheel and tyre (and just wheel) configurations.

A low rolling resistance car tyre’s CRF is 0.006 to 0.01

A conventional car tyre’s CRF is 0.015

A lorry tyre’s CRF is 0.006 to 0.01

A train wheel has a CRF of 0.001

Meaningless figures? Look at the one for the all-iron train wheel and imagine how much engine power is used to keep it rolling. As the Americans say, go figure!

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Appearances can sometimes be deceptive. Like the first part of this title, for example. In this instance, the article isn’t about money, or the consumer price index. It can, however, be about saving you money. Buying a car tyre inflator can be a costly business. On the other hand, since a good one makes inflating your car tyres easier and quicker, it will tend to make you carry out this simple chore more regularly. This in turn means your tyres will remain at the correct pressure; you’ll save money by not burning extra fuel to push under inflated tyres along, your tyres will last longer and will be less likely to fail or suffer damage from overheating.

What are car tyre inflators about? They’re a simple electric pump for blowing up your tyres. Yet, a quick look through the available inflators reveals a lot. Some inflators are powered by your car’s battery, usually via the cigar lighter socket. Some inflators are rechargeable. Some are claimed to be quiet. Some are billed as being rapid inflators – one example inflating a tyre from totally flat to full pressure in four minutes. Others aren’t so rapid – one on test took 15 minutes to do its work.

The car tyre inflator’s abilities don’t stop there. Many have a number of LED lights, so you can utilise their inflating facility at night, which could be useful in an emergency. Most are digital, in that they have a digital readout for the pressure of the tyre to which they’re connected. This readout can usually be set to give a figure in the desired scale, that is pounds per square inch, Bar or kPa (Kilopascals).

Another benefit of a good car tyre inflator is that it can do some of the work for you. True, you have to connect it to the electricity supply (or make sure it’s fully charged). You also have to connect it to the tyre you want to inflate and you ought to oversee what it’s up to. However, the majority of good car tyre inflators feature an auto shut off facility. When the tyre is at the right pressure, the inflator stops inflating – it’s that simple.

All this begs a rather obvious question. Which one do you buy? Well, this is not a buyer’s guide. However, we all know that the Internet is laden with user reviews as well as advertisements. You can spend a happy hour doing your research, on which car tyre inflator has the best reviews.

There are some aspects of car tyre inflators that you may want to consider before buying. A quick look through some of the web tests of these devices reveals much. One point relates to a very old, well-worn adage: you get what you pay for. This is scarcely surprising and you won’t get a lot for very little money. You will have the choice between quiet inflators, fast ones, rugged ones and so forth. However, one overarching drawback seems to apply to even some of the more expensive car tyre inflators. More than one test website offers the advice to add a top-quality tyre pressure gauge to your shopping list. Quite a number of the tests discovered that the on-board digital pressure gauges were not necessarily accurate. Comparison will out, of course, during your research. On the other hand, you shouldn’t really need to buy an additional pressure gauge. As a responsible motorist, mindful of your own safety, that of others and the way in which incorrect pressure shorten tyre life, you already have a quality pressure gauge. Don’t you.

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Low Cunning

Fashion has followed performance, which is why ultra low-profile tyres have become more common. Every car tyre has an aspect ratio, which is a comparison of its height in relation to its width. A ‘full profile’ tyre has a section height that is 80 to 85 percent of its width; such tyres are rarely seen these days, unless a classic vehicle is riding on them.

Aspect ratios have been falling over the years. 70 percent profiles came along, followed by 65 percent, 60 percent and so on. Nowadays, 45 percent profile tyres can be seen on high-performance cars and the current low figure is a mere 35 percent. Beware though, ultra-low profile tyres may look thrilling and they certainly offer tremendous performance. There is, however, a twofold cost. The lower you go, the more ride comfort suffers. What’s more, wheel damage from touching a kerb, or hitting a pothole or debris in the road, is far more likely.

Formula One Facts

A Formula One car can accelerate from rest to 99.42 miles per hour and decelerate back to zero in four seconds. Even though such a car weighs only a little over half a tonne, the breathtaking forces involved clearly give the tyres a hard life. Here are some captivating facts from F1′s sole tyre supplier, Bridgestone.

The company produces 40,000 of its Potenza F1 tyres each year and takes 1,800 tyres to each grand prix.

During the 2008 grand prix season, 19,435 laps were completed on Potenza F1 tyres.

A Bridgestone tyre fitting team can fit 56 car tyres per hour, that’s 14 sets of tyres. Sixty of the company’s personnel attend each grand prix, taking with them 12 tonnes of specialised equipment.

At just over 186 miles per hour, Bridgestone wet weather tyres displace 61 litres of water per second.

Potenza F1 tyres are built to last – for around 93 to 124 miles under racing conditions.

Back to the Real World

Now for some less esoteric facts and figures, from the everyday world of motoring.

About 75 percent of British motorists are driving on under-inflated car tyres.

What this is doing to their carbon footprints isn’t known but it is costing them over 2 billion pounds every year.

Approximately 12 per cent of the cars in the UK have at least one defective tyre.

Another 12 percent have a tyre with less than two millimetres of tread, which is legal but decidedly unsafe.

Ninety percent of Britain’s motorists don’t know the pressure to which their tyres should be inflated.

Continental Tyres carried out a study, which discovered that 40 per cent of British motorists claim never to have checked the pressure in their car tyres.

More than 40 per cent of drivers admitted that they didn’t know how to check tyre tread depth. Seventy per cent had no idea of the legal minimum tread depth, which is 1.6 millimetres over 75 per cent of the tread’s width, in a continuous band.

About 20 percent of drivers confessed to driving on tyres that would probably not meet legal tread limits.

These sobering figures, apart from highlighting the need for a certain amount of education for UK drivers, suggest something else. It’s comforting that the average UK driver isn’t in charge of Formula One car tyres!

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In many respects, modern cars are computers riding on tyres. Much of what happens below decks may seem to be under the driver’s control but in fact, computerised assistance is ever-present and sometimes, the computers take over. Here, we can take a look at some computer-controlled elements that specifically relate to the work of car tyres. We’ll find ourselves knee deep in acronyms but don’t worry, each will be explained.

Lets start with ABS. Which is? The Anti-lock Braking System. ABS first appeared in 1929, when Gabriel Voisin, a French aircraft and car engineer, developed a working system for aeroplanes. In the production car world, Chrysler later teamed up with Bendix to produce ‘Sure Brake’. This, a fully computerised all-wheel ABS system was available on the 1971 Imperial. Not to be outdone, General Motors stopped car tyres skidding with ‘Trackmaster’, a rear-wheel only system that first appeared on their 1971 models.

So how does ABS help car tyres? A typical system has a central electronic control unit (ECU), a speed sensor at each wheel, and hydraulic valves within the braking system. Under hard braking, a car wheel that is rotating significantly slower than its counterparts is on the point of locking. The ECU detects this and commands the appropriate hydraulic valve to reduce the braking force at that wheel. As the wheel speeds up again, the ECU reimposes full braking force – this can happen up to 20 times per second.

What does this mean in practice? When the ABS keeps the car tyres at the point of locking but doesn’t let them lock, the car is being braked to the limit of the tyres’ adhesion. This represents maximum braking efficiency. Moreover, skidding car tyres have actually lost their grip. The driver may be applying maximum braking force and locking the wheels but braking and steering control are lost. Under the influence of ABS, it becomes possible to still steer the car around an obstacle.

Now for another acronym, which describes a system that has evolved from ABS. Electronic Stability Control, or ESC, adds two further sensors to the ABS armoury. The first, a gyroscopic sensor, can detect when the car’s direction of travel doesn’t accord with the position of the second, the steering wheel angle sensor. The ESC’s software can then apply braking force to up to three of the wheels individually. Consequently, the car tyres’ grip is used to assist in making the car travel in the direction the driver has chosen. CBC, or Cornering Brake Control, is a refinement in ESC. This system can control the ABS, telling it that the wheels on the inside of the curve being taken should be braked harder than the those on the outside. It also dictates how much additional braking force is applied.

Paradoxically, an ABS-related system can help car tyres to retain grip under acceleration. A Traction Control System, a.k.a. A TCS, detects a driven wheel that’s spinning and briefly applies braking force to allow the spinning car tyre, or tyres, to regain traction. More sophisticated TCS systems can control the throttle level simultaneously, cutting power to the spinning wheels.

As you can see, these electronic aids to safety are complicated. Different manufacturer’s calling them by alternative names further complicates the issue. For example, some call their TCS an ASR, an Anti Slip Regulator. The above is a very simplistic explanation of such systems and there are other, similar systems in use. The bottom line is that they all help a car’s tyres make maximum use of the grip they offer.

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In the wake of the Budget, we can do our own budgeting and save money on motoring. Attending to everything from your car tyres to the way you drive can save your hard-earned cash and help you avoid a deficit of your own.

1: Check Tyres Pressures

Air is no longer free at many forecourts but checking your tyre pressures frequently can save you much more than the few pence it might cost. For a start, running car tyres at too low a pressure increases their rolling resistance. This makes the engine work harder to push the car along so fuel consumption suffers. Underinflated tyres wear out faster too.

Correct tyre pressures improve a car’s handling, road holding and braking. These checks can cut your fuel bill by 10 percent and increase your car tyres’ useful life.

A major upside of checking tyre pressures in that you can look over your car tyres at the same time. Being caught with unroadworthy tyres can attract a fine of 2,50 pounds.

2: Research Insurance Needs

The recession has increased competition in most industries. Car tyres have become cheaper but insurance premiums have become highly competitive. Shop around for the best deal and make sure you’re paying only for the insurance you need.

3: Don’t be a Drag

Correctly inflated car tyres won’t be too helpful if your car is being held back aerodynamically. Trundling around town with the windows open may be enjoyable but open windows equal aerodynamic drag. The same goes for a roof rack, which can impose a surprisingly high amount of drag, even if unloaded. So, remove the rack when it’s empty. Also, tidy out your car. Carrying anything you don’t need costs fuel.

Air-conditioning also saps power; switching it off unless you actually need it saves fuel. That said, aircon is a more efficient cooling medium than open windows at above town speeds. It’s a question of balance.

4: No Short Trips

Cars run inefficiently when cold. Even the most efficient can return single fuel consumption figures for a surprisingly long time when cold. So, try to use your car only when you know it will warm up fully. The same aspects apply when warming your car, particularly on cold mornings. Drive off as soon as you start the engine – a car engine running at tickover from cold uses a lot of fuel, warms more slowly and can suffer more wear. Here, the car tyres make no difference!

5: Plan Your Journey

Longer routes, stop-start driving, unnecessary mileage and diversions all have a negative effect on fuel consumption. Use the Internet, the radio, TV and your satnav to plan your journey. Avoid rush hour traffic jams, steer clear of roadworks or accidents, and try to use motorways wherever possible. Bear in mind that the shortest route may not be the most efficient; more miles under your tyres can work out cheaper in some cases.

6: Observe Speed Limits

Speed limits have always been in place for public safety, even if many have also become a political issue. Remember that travelling at 60 mph costs less than travelling at 80 mph and wears tyres less. Equally, remember that fines and points on your licence also increase insurance premiums. Moreover, you generally can’t argue with a speed camera or police radar gun.

7: Drive Carefully

With a few exceptions modern cars are powerful enough to be lively but burning your car tyres’ rubber on a regular basis, though fun, can be expensive. Fifth gear can give 20 percent better fuel economy than third can, provided you don’t select it too early. Tyre-shrieking starts and lurid cornering are best left to your imagination – and they wear your car tyres out faster.

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Car tyres obviously play a huge part in letting cars take corners. Few people consider how they do this in a scientific way but knowing about the forces at work is both interesting and educational. Why? Because understanding what’s involved and the terminology that applies can help your driving.

In road tests – particularly in track tests – cars are referred to as having understeer or its opposite characteristic, oversteer. These terms are directly related to car tyres, but what do they mean? The short, smart answer is simple: understeer means you’ll see what you’re going to hit, oversteer means you won’t. The less drastic explanation refers to the behaviour of car tyres when great demands are being on them. An understeering car’s driver will feel the need to apply more steering input to make the car go round the corner. An oversteering car’s driver will have to apply less steering input to successfully negotiate the corner. In drastic oversteer, he’d have to add counter steering, or opposite lock. Most cars understeer, for safety’s sake.

What is the science behind understeer and oversteer? Once again, it’s about car tyres; specifically, it’s about their slip angles. What are slip angles? Something that is easily understood when they’re explained to you.

Every car tyre has a contact patch, the area of its tread that’s in contact with the road surface. Imagine the tyres of a car that’s taking a corner. Each tyre will be following a path in the direction the driver is steering. Each contact patch will be following a path but it won’t be the same path the tyre’s carcass is following. This is because the tyre’s carcass deforms in response to the forces acting on it – the lateral force imposed by the weight of the car, and any forces of braking or accelerating. So, the tread in the contact patch follows a tighter curve than that the wheel/tyre combination is following. The tyre tread’s ability to deform locally also has a bearing on the slip angle. The slip angle is the difference between the two paths.

What, then, do slip angles do? In fact, they add grip, up to a point. Slip angles increase with increasing forces, speed included. As they increase, grip improves, until the car tyres begin to lose grip.

When car tyres begin to lose grip, one of three conditions results. When the slip angles of the front and rear tyres are the same, neutral steering occurs. If a car has 50/50 weight distribution and if its front and rear tyres offer identical levels of grip, it will have neutral handling. At the limits of grip, a four-wheel drift will occur; competition cars aim for these characteristics. A car in a four-wheel drift is cornering as fast as it can, nearly at the point of sliding off the track.

Should the car tyres at the front have a larger slip angle than the rear tyres have, understeer will result. As well as being more controllable, understeer can be used to scrub away excess speed on a racetrack. After the ultimate limit of grip, an understeering car will leave the track while going forwards; it can’t corner tightly enough.

What happens if the rear tyres’ slip angles are the greater? Under these circumstances, the grip of the rear tyres will be less than that of the front. The car’s tail will slide towards the outside of the corner. In extremis, the car will spin.

In practice, many aspects affect understeer and oversteer. These include which wheels drive the car forward (front, rear or all, four), the weight of the car and how it’s distributed, whether the car is accelerating, cruising or braking, and the speed and loads involved. However, this explanation gives an insight into your car tyres’ dynamic performance.

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Who makes a movie? Usually, it’s a joint effort. In some films, however, it isn’t only the actors, director, crew and backroom staff that make the result memorable. It’s a car or cars and here are some we all know and love…

Roald Dahl and Ken Hughes scripted the 1968 film ‘Chitty Chitty Bang Bang’. Dick Van Dyck and Sally Anne Howes appeared in it but its specially-built star made it come alive. There were in fact six Chittys and not all ran on car tyres. One was the car in its dilapidated state, another appeared in all its rebuilt glory. Of the four others, the ‘hovercraft car’ is probably best remembered. This variation was lightened with aluminium copies of many of the brass parts. When the car had to be filmed suspended from a crane or helicopter during ‘flying’ sequences, its engine was removed to lighten it further.

Could you create a legend in just over seven weeks? In August 1965, US custom car builder George Barris was given a job to do. He had to produce a special car for TV and film work, as fast as possible. Luckily for him, a division of the Ford Motor Company had built a concept car ten years earlier. The Lincoln Futura made an ideal starting point. Barris fitted the 390 cubic inch (6.4 litre) V8-powered car with 15 inch cast alloy wheels, carrying Firestone Wide Oval tyres. Into the recipe went Cumming & Sander impact safety belts, a Moon aluminium gas tank, 2 Deist parachute packs with 10ft. diameter parachutes, and the ‘exhaust turbine’ was made from a five gallon paint can. If you need any further clues, the giveaway lies in the colour scheme. Threequarter inch fluorescent cerise stripes relieved the ‘Velvet Bat-Fuzz Black’ surface finish. Yes, we’ve been looking at the original 1966 Batmobile.

Now for a collection of vehicles, all but one of which set tyres to tarmac in reality. In 1969′s film ‘The Italian Job’, it wasn’t a real Lamborghini Muira that went into a river. It was a bodyshell on a crash-damaged chassis. However, when the film crew returned to pick up the pieces next day, they’d been stolen overnight!

Speaking of car tyres, the production crew got through quite a number with the Italian Job Minis. These were Mk.1 Austin Mini Cooper S cars; in the hands of stunt driving team L’Equipe Remy Julienne, they became legendary.

The film’s two E-Type Jaguars were Series One models, bought for a measly (for the time) £900 apiece. The red convertible registered 848 CRY, however, was restored and survives to this day. Michael Caine’s character’s 1961 Aston Martin DB4 convertible, scooped off its tyres by a digger, really did go down the mountainside – twice. In fact, the first take was unsatisfactory because the car was supposed to burst into flames (it didn’t because the man with the ‘explode button’ had to run for his life as the Aston plummeted towards him.) The ‘Aston Martin’ that we did see destroyed was a Lancia Flaminia 3c Cabriolet, dressed up by an excellent and obliging bodywork firm in Turin.

To close this little trip down memory lane, we can look at what is arguably the world’s most famous coach – ever. The six-wheeler that ended up with its rear tyres clawing at empty air at the close of The Italian Job was a Harrington Legionnaire. Perhaps the notion of reinforcing the cab so that the boarding Mini Coopers didn’t squash the driver was heralded with this: ‘Listen lads, I’ve got a great idea – er – er’.

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Here in the UK, 52% of our days are overcast. Rain is a given and it is frequently very heavy. Consequently, we should be used to it and there is no excuse for ignoring it. It is also a fact that even car tyres in prime condition inflated to the recommended pressures can struggle to cope, especially in particularly heavy rain and floods.

In heavy rain, you need two things, grip and visibility. Good wiper blades and a full screenwash bottle are essential. Doubling the distance to the vehicle in front is equally necessary. When the rainfall approaches biblical proportions, you could encounter standing water. Car tyres are designed to offer a grip by clearing water from the area that contacts the road surface. In extreme conditions, they can no longer sweep water away quickly enough. This can lead to a phenomenon called aquaplaning, in which the tyres skim along the surface of the water. Should this happen, you may find out the hard way.

How? Try to brake and very little will happen because the tyres have no grip; the same goes for steering control. A sudden, suspiciously light feel to the steering is an excellent clue to aquaplaning and the ‘cure’ is to gently lift off the accelerator; you will feel the tyres regaining their grip. Nobody who’s experienced it would deny that aquaplaning is frightening. Should it happen, it will quickly educate you into matching your road speed to the prevailing conditions.

Sometimes, rainfall becomes so heavy that the roads’ and their drainage systems can’t cope with it. Here, flooding is not only a risk, it is practically a certainty. The secret of coping with significant flooding is to employ forethought. ‘Look before you leap’ is a long established proverb and in many respects, it applies to flood driving. You shouldn’t attempt to drive into floodwater without knowing how deep it is. Tall 4x4s apart, modern cars have an engine air intake positioned quite low down. The snag is that water, unlike air, is incompressible. Should water enter the engine, the pistons will be stopped dead in their tracks. This will completely wreck the engine.

So, what do you do when you see floods ahead? A simple rule of thumb is that being able to see the road surface means you can see how deep the floodwater is. Assuming you can see the depth of the water, flood driving goes like this.

Drive straddling the white line. Being on the crown of the road puts you where the water is shallowest.

Drive in a low gear, at a slow pace. Creating an impressive bow wave is a very good way to drown your engine but keeping the engine revving will prevent water’s entering the exhaust and causing a stall.

Should the engine come to a dead halt, it’s likely the worst has happened. Depress the clutch immediately and do not attempt to restart the engine. Damage may already have occurred; turning the engine will make it worse.

Should another vehicle be approaching from the opposite direction, make every effort to let it negotiate the floodwater first. This is for two reasons. One, if you both have to move across to let one another pass, you’ll both be in deeper water. Two, if the oncoming vehicle ingests water and dies mid-flood, you’ll be prompted to not try negotiating the flood yourself.

Finally, remember that brake pads can ‘aquaplane’ on wet discs, leaving you without brakes. On leaving the flooded area, apply the brake pedal for a short while, to dry the discs.

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‘Caveat emptor’ is Latin. It means ‘Let the buyer beware’ and it makes the perfect maxim for anyone intending to buy a secondhand car. The rules for a successful purchase are simple enough and making a matchless deal is easy if you use these pointers.

1: Check the tyres. Car tyres save your life on a daily basis, simply by doing their job. Tyres with 3 millimetres or less of tread need replacing. Tyres can also be excellent tell tales. If they’re worn unevenly, the car’s suspension, steering or brakes could be at fault. Cracked tyres are generally pensionable and lumps or bulges in tyre carcasses suggest internal damage. Remember to check the spare tyre and bear in mind that tyre failure can be a killer.

2: Check the VIN. The car’s Vehicle Identification Number should match that in the logbook. If it doesn’t or is absent, the car could have been stolen, ‘rung’ or be two half cars welded together.

3: Check the history. This advice counts in two ways. Dealers must offer an HPI check by law. If buying privately, have one carried out yourself. An HPI check is to validate the car’s history and would reveal previous damage, theft, insurance claims, writings off and more.

Service history is equally important. If a main dealer or an approved concern has carried out servicing, the car’s automatically a better prospect.

4: Check the deal. Often, dealers can offer a warranty. The car may have some remaining manufacturer’s warranty and there may be ‘sweeteners’, such as included accessories. Be very careful if buying from a private seller. Once the car is yours, you have very little comeback if it proves to be a bad buy.

5: Check the car. Unless professionally executed, accident repairs are easy to spot. Look for poor panel gaps, paint colour and texture mismatches, non-factory additions (e.g. lights) and damaged exterior trim. Take particular note of the car’s extremities: the nose and all four corners. Suspiciously thick, new-looking underseal can hide a lot, as can the boot carpet.

Look under the bonnet, for a clean engine bay with no rust or coolant stains. Dip the oil, which shouldn’t be like black treacle, and look underneath the car for spots of leaked fluid.

6: Check the mileage. The car’s MOT certificates should show a believable progression of mileage figures and the speedometer’s mileage reading should tally. Modern electronic mileometers can be easily tampered with so checking that the speedo screws aren’t butchered doesn’t always reveal a clocked car. That said, plastics and modern trim can wear. If the steering wheel rim and the pedal rubbers are shinier than the bodywork, chances are the car has an intergalactic mileage in reality. A worn, bursting drivers seat, is a dead giveaway, as are badly worn driver’s door mechanisms. Don’t forget that these parts could have been replaced, so if they look too new, be wary.

7: Check the performance. Have a realistically long test drive, using the vehicle as you expect to on a daily basis. Try the brakes, steering, transmission and electrical items. Should something be not as expected, be careful to not fall for the old ‘They all do that’ routine!

8: Remember these three golden rules. Unless the car you’re examining is so rare that there simply isn’t another, you can always find a rival example. Rule two: it’s a buyer’s market at the moment and you can always walk away. Rule three is simpler still: buy with your head, never your heart!

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Car tyres do a magnificent job. Few would argue with this but there are those who seem to believe that their car’s tyres can perform miracles. Most people who’ve seen the consequences of this belief would argue with it. Now that winter is upon us with a vengeance, knowing what car tyres can and can’t do, particularly on snow and ice, becomes particularly helpful. Here are some basic facts to begin with…

* Car tyres, like many shoe soles, are made of materials that provide grip in most conditions. On snow, car tyres provide much-reduced grip and on ice, can offer practically no grip.

* Treating the road surface, with a mixture of salt and grit, enhances the available grip.

* This enhanced grip is nowhere near that provided by even a wet road surface.

* Should the treated surface re-freeze, the level of grip will decrease radically.

* Loss of grip equals loss of control.

Ok. For those who would argue that their car has anti-lock brakes and traction control, there are further facts. Electronic aids notwithstanding, a car (or a van, truck or whatever) can only obey the laws of physics. ABS, DSC and other driving aids help but like car tyres, they can’t perform miracles.

So, how do we deal with snow and ice? The short answer is very carefully but there are ways of driving in such conditions. Let’s take a look at the most useful…

* When setting off, you can lose traction. A car with furiously spinning wheels won’t be going anywhere except possibly downwards. Be subtle with the controls and in really bad conditions, try setting off in second gear.

* Should your car tyres insist on spinning, you may be able to progress by using wheel spin but only as a last resort. Turn the steering repeatedly from lock to lock and you might, just might, find a little traction.

* In adverse conditions, driving becomes very much a matter of the balance between grip and sliding. This is an immensely delicate balance that’s easily upset. Your fingertips and the seat of your pants give the best feedback about what’s happening at ground level.

* Despite all the theoretical talk, your car can only skid in three ways. In understeer, the front of the car loses grip and runs wide. In oversteer, the rear loses grip and tries to pass you. In a four-wheel slide or skid, all the wheels lose grip. If you’re skidding, you’re going too fast for the conditions.

* If the front wheels skid (understeer), reduce the amount of steering you have applied. In a rear wheel skid (oversteer), steer in the direction the tail of the car has taken. In all cases, four-wheel skid included, lift off the accelerator but avoid hard braking.

*If your ABS cuts in under braking, you may have a limited amount of steering control. Again don’t expect miracles.

* Where traction and grip are radically limited, stopping distances increase vastly. Never, on any account, follow a vehicle too closely and be aware that what’s behind you could have only a minimal chance of stopping. Should a vehicle be following too close, pull in and let it pass.

* Remember that even a treated surface can refreeze. Pay attention to your car’s ice warning and act on it. Black ice can remain in shadowed areas long after sunrise.

Above all, be realistic. Should conditions be dreadful, accept the fact and don’t travel. With the abovementioned driving aids, not to mention seatbelts and airbags, you’re likely to survive an accident. However, surviving the consequent insurance issues and the inconvenience of repairs can be a major hassle.

If in any doubt at all, you need answer one question, the one that posters carried during World War Two: Is your journey really necessary?

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