Merityre Article Archive

The star cars that make some movies memorable often start life as completely standard vehicles. They rarely stay that way and they always attract a following. So let us follow in the tyre tracks of some Hollywood luminaries.

Our first diva was in fact six cars, of which four still exist. In many respects it was ahead of its time; by the end of the first movie in which it appeared, it was running on rubbish. This early green machine, Doc Emmet Brown’s time-travelling car was a DeLorean DMC12 and it first burst on to our movie screens in 1985. A trilogy of movies and many years later, Christopher Lloyd (Doc Brown) is 71, and Michael J. Fox (Marty McFly) is 49, has Parkinson’s disease and an honorary doctorate from the Karolinska Instituet. Of the four DeLoreans remaining, two are studio tour stars at Universal, one lives on the ceiling of Planet Hollywood in Honolulu – and the last one is being restored. Some of the cars that John Z. Delorean made still set tyres to tarmac, and some among their number have attained a degree of immortality.

‘So cool it hurt’ is an epithet that applied to the late Steve McQueen, and his handling of the 1968 Ford Mustang GT390 Fastback he used in ‘Bullitt’ did his reputation no harm at all.

During the seven-minute, tyre-burning chase sequence with the baddies’ Dodge Charger R/T440, McQueen did most of the driving and there was no camera trickery.  Both the Mustangs used were given tuning tweaks so they could keep up with the more powerful Magnum Chargers, and one of each was given beefed-up suspension and a full roll cage.

All that drifting car tyre smoke was real, as was much of the San Francisco traffic. One thing, however, was a later addition. The full-blast V8 engine sounds came from an example of the GT40, Ford’s four-times Le Mans winning racing car.

Now for a quirky one. What has massive tyres, ten carburettors with gold plated intake pipes, and a huge, Ford Cobra motor? Need more clues? How about Jahns high-domed pistons and an Iskendarian camshaft? Still too techy? Ok then.

Customizer George Barris built the idiosyncratic vehicle in question. It was based on a lengthened 1923 Model T ford chassis and had a custom body. Still struggling? Alright, here are some names. The car was featured in a 1960s TV comedy starring Fred Gwynne, Yvonne De Carlo; Al Lewis played Grandpa. Those who are thinking of Clive Dunn at this stage can go to dunce’s corner right now. However, the names Herman and Lily might ring some bells. Yes, it’s the Munster’s Koach.

This unique car toured the ‘States extensively in the 1970s, and pre-tour, George ‘Batmobile’ Barris gave it a new paint job. The car now rests in peace, in the ‘Cars of the Stars’ museum, in Keswick, Cumbria.

Finally, a vehicle with just 114 horsepower from its 4.9-litre, six-cylinder engine is hardly likely to set tyres or tarmac alight, especially with its three-speed automatic transmission. However, the very same vehicle had its charm and made us laugh, even if it was slow. The film ‘Dumb and Dumber’ featured this 1984 Ford Econoline van, complete with floppy, furry ears, a lolling tongue and a full fur coat. Jim Carrey and Jeff Daniels’ ‘Mutt Cuts’ van didn’t stay long but the movie wouldn’t have been the same without it!

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There’s nothing in the rule book that says you must keep the wheels your car had on when it arrived. Many people change their wheels, for a variety of reasons. Some want a different style of wheels, some want wider wheels and most want alloy wheels. A change of wheels usually means a change of car tyres. That is no problem in itself but there is a trap into which a lot of people have fallen with a resounding clatter.

The thing is, any car wheel will accept one or more different sizes of car tyre. There is, however, more to car wheels than just rim diameter and width. Ignore the other measurements or get them wrong and disaster looms. In short, you can find there’s a clash between your car tyres and its bodywork. This can happen with steering or suspension movement and often, it happens just the once, immedately before the tyre and bodywork wreck one another.

The first measurment is Pitch Circle Diameter, or PCD for short. This is nothing to do with car tyre to bodywork clearance but if you get this wrong, the wheels won’t fit. The PCD is the diameter of a circle drawn through the centre of your wheel mountings, be they studs or bolts. The wheels obviously need the right number of stud or bolt holes but you also need to ensure that the wheel nuts or bolts have the right thread and profile for your car’s hubs, and that they are correct for use with the wheels you’re using.

Now for the easy ones: rim width and diameter. You could go for the standard measurements here. However, many people want bigger wheels and low profile tyres. Fair enough – just ask the wheel supplier, who you can always blame if something doesn’t fit. It’s also important to make sure that the wheels can accommodate your brakes. Big wheels and low profile tyres were invented, in part at least, to allow for bigger brakes to be crammed in. These days, clearances are close and it’s problematic if your wheels and disc calipers rub each other. This makes only one difference to the car tyres: they won’t work too well if the wheels can’t rotate.

So, we come to the tricky one, the knotty problem of wheel offset. This is most easily understood with a little bit of theory. Imagine a car wheel sawn in half across its diameter. If you were to draw a line through the centre of the wheel rim, and this line was to coincide with the wheel’s hub mounting face, the wheel offset would be zero. If, as is common, the hub face lies outboard of the centre line, the wheel has positive offset. Conversely, if the hub face lies inboard of the centreline, the offset is negative. The degree of offset, apart from altering wheel clearance, has a profound effect on the behaviour of your car tyres.

Visually, positive offset places the car tyre further under the car’s wheelarch. Negative offset makes the tyre carcass sit further outboard. In either case, provided you don’t choose a seriously large figure in relation to what is standard, the car tyres won’t cause difficuties. That said, there is a danger in choosing too radical an offset, as this figure (usually expressed in millimetres) affects steering. You could find that your steering has becomes mysteriously heavy. Worse still, you could find it’s become over-light, to the extent that straight line driving becomes a thing of the past.

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‘We’re on your side.’ This sounds like the tag line in an advertisement but here it refers to the codes on a car tyre’s sidewall. Most of us know the obvious ones like those referring to wheel diameter and tyre section. There are, however, many other pieces of vital information to be found, moulded into the sidewall. Let’s consider some of the more important ones.

Did you know that some car tyres have a marked rating that indicates how well they handle heat build up? If you see a mysterious “A” on a tyre, it has the highest rating. A “C” denotes the lowest.

The traction rating shows how well a tyre can stop you on a wet road. This rating goes from “AA” to “CC”. No prizes for guessing which is the best.

Now for some numbers. Tread wear ratings go from 60 to 600 in increments of 20 and the figure shows how long the tyre is expected to last. In theory, a tyre marked “200″ should last twice as long as one marked “100″. This is, however, a relative rating. The actual tyre life will depend on many factors, including your driving style, the road surfaces encountered, correct wheel alignment and the right tyre pressure. Think ‘benchmark.’

The speed rating a tyre carries is extremely important. Using over-specified tyres is a (very expensive) upside here; tyres for high-speed use can be used at lower speeds. On the other hand, if the speed rating is too low and you have an accident, your insurance company may take a dim view of the tyres your car was wearing at the time. Speed ratings run from “L” (75 mph) to “Z” (150 + mph). Currently, H-rated tyres (130 mph) hold the largest percentage of new tyre sales.

So much for speed, what about load? Yes, car tyres tell you about the load they are capable of carrying, They have a marked load index (LI), which is valid for sub-130 mph speeds. Load indices range from 50 (190 Kg) to 169 (5,800 Kg) but the calculations are a touch complex. Let’s say you have a car that weighs 2 tonnes, or 2,000Kg. Divide this by four to get the weight on each wheel; this equals 500Kg. So far so good, but, do you know that all four wheels carry 500Kg or is one end of the car heavier than the other? In practice, it’s usual to add a percentage to cover such factors.

In our example, then, the car needs tyres with a load index of 84. We can add 20 percent, giving a weight bearing capacity of 600Kg. Therefore tyres with a load index of 90 will do very nicely. In reality, a car tyre’s load rating will exceed its actual needs by a long way. However, when speeds reach 130 mph or more, the load index calculations change dramatically. For reasons far to technical to explain here, the theoretical load rating decreases as road speed rises. In the event you run a very, very fast and heavy car where the speed cameras don’t work, there are experts in such matters. Find one!

Lastly, you should never use a tyre more than 6 years old. The date of manufacture is part of the DOT code marked on one sidewall. For example, a tyre marked “2709″ was made in the 27th week of 2009.

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Car tyres are car tyres, right? Well, they’re all round, black things that go between your wheels and the road. However, choosing the right tyres for your car and, equally important, for your needs is imperative. Here are some pointers to help you make the right choice.

The Right Type of Tyre

There are five distinct tyre types you can choose from and it isn’t always a case of one tyre type for one car; the choice is also about usability.

Standard car tyres, otherwise known as all-season tyres, are the least specialised. They are suitable for year-round use, in wet or dry conditions. In some respects, run flat tyres lie in this arena. Run flat tyres can do exactly what their title suggests; you may continue using them, albeit at a given maximum speed over a specified mileage, even after a puncture.

The third variety of car tyres differs in the conditions to which they’re suited. Winter tyres can handle colder conditions better than standard tyres and they generally have a tread pattern with larger blocks in a more open pattern, for better grip. Our fourth tyre type is also based on the conditions to be encountered, specifically in terms of terrain, rather than climate. All-terrain tyres can cope with mud, sand and gravel and are often found on 4×4 vehicles used off the road. However, the additional grip in adverse conditions has a cost. All-terrain tyres are noisy when used on ordinary tarmac roads.

Lastly in this quintet, performance tyres are for performance cars. As well as offering superior grip in wet or dry conditions, performance car tyres usually have a higher speed rating.

The Right Tyre Size

When it comes to car tyres, it is anything but a matter of one size fits all. The manufacturer’s recommended fitment may be found in your car’s handbook and if in doubt, you could always have replacement tyres like the existing ones. However, small changes in tyre section and width can be accommodated, often beneficially. The staff of a good car tyre retailer will be happy to recommend the best tyres for you.

The Right Tyres for your Driving Needs

While remaining within the manufacturer’s specified parameters, you can ring the changes by using car tyres that suit your driving style and/or particular needs. You may need economy tyres for better mileage from your gallon. Certain tyre types are particularly quiet in use and therefore better for motorway use. Similarly, some tyres are well suited to stop/start motoring. As mentioned above, you’d be well advised to consult the tyre retailer.

The Right Price

The price of car tyres varies considerably but here, commonsense must play an important role. You can’t for example, have economy tyres fitted to a high-performance sports car and expect good results, not to mention longevity. Conversely, there’s little point in having high specification tyres fitted to a car that covers a small yearly mileage. However, if you use your car daily, economy tyres could represent false economy; budget tyres may not last as long as better quality items.

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Car tyres are tough, which is fortunate, as they have to be tough to survive. That said, there are ways that your tyres can be given an easier life. Using them can save you money in the long run.

Pressure salesmanship

Running car tyres at the wrong pressure is a surefire way of shortening their life. Underinflated tyres flex more, which causes them to run hot, which makes them pensionable long before their time. Underinflation wears out tyre shoulders, and the lack of air pressure makes them less able to defend your wheels from assaults by speed bumps and potholes.

As car tyre pressure reduces, rolling resistance increases. Meaning? Soft tyres take more fuel to push along. Simples, as that meerkat says.

In its way, overinflation is just as bad. Tyres running on tiptoe sacrifice grip and ride comfort. Overblown tyres also wear out the centre of their treads. Paradoxically, over inflation doesn’t enhance the tyres impact resisting abilities.

A weekly tyre pressure check is hardly onerous. Buy a good pressure gauge and find a forecourt that isn’t so cheeky as to charge for air. You’ll find information about the correct tyre pressures in your car’s handbook and probably on a sticker on one of the car’s door pillars.

A question of balance

When checking your car tyre pressures, you may notice little metal blocks attached to your wheel rims. These are balance weights; they’re there for a reason. Anyone who’s driven a car with a lost balance weight will tell you what the results feel like. A seriously unbalanced wheel will put the integrity of the fillings in your teeth at risk. Equally, anything more than minimal braking will make you wonder why the ABS has deployed, making the brake pedal vibrate like the firing pedal of an ack ack gun.

In practice, even a mildly unbalanced combination of car tyre and wheel will be costly. The increased level of vibration puts stress on several components. The tyres itself will wear faster. The need for the suspension joints to cope with additional strain will cause them to wear. Perhaps more noticeably, you’ll feel the strain too; tyre imbalance increases driver fatigue.

Toeing the line

Wear and tear can put a car’s wheels out of alignment. Misalignment makes cars suffer, and not only in an obvious way. Car tyres like to run straight and if they can’t, they object by wearing out faster. Too much toe-in (cross-eyed wheels) makes the tyres’ outer shoulders wear, and toe-out (diverging wheels) makes the inner edges wear. Apart from being costly, this affects steering sharpness and braking.

Having your wheels aligned and balanced when necessary is a lifesaver for your tyres. Moreover, it is far cheaper than new tyres and possibly a suspension and steering overhaul.

Lastly, drive sympathetically, or you’ll burn out your tyres in no time. Make a point of inspecting your tyres regularly and remember that ten yards running flat will kill a tyre. Your tyres look after you; if you look after them, they’ll continue returning the favour.

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Merityre.co.uk are one of the  leading UK independent suppliers of car tyres. Why not visit their website for an online tyre quote or contact your nearest fitting centre.

In these dark times, we must all tighten our belts, as well as having them tightened for us by the powers-that-be in Whitehall. It therefore makes sense to do what we can to save on our motoring costs, from saving fuel to making our car tyres last as long as possible. Here are a few hints to deter the wolf from ringing your doorbell too frequently.

Turning fuel into forward motion is a conversion of energy, from potential to kinetic. Maintaining as much of the former while benefitting from the latter involves applying efficient practices. Maximizing mpg starts with starting the car. Cold car tyres are softer than hot ones and cold engines are distinctly fuel-inefficient. So, if it’s a short trip, don’t use the car. Equally, you can save by being gentle on the gas when the car is cold.

Once the engine (and the tyres) are hot, keep to the highest possible gear. This doesn’t mean accelerating hard until you can use top gear. Instead, keep the rev counter needle low on the scale and change up sooner to keep it there. At the top end of the speed scale, observe the speed limit. We all know that speed camera or police radar gun fines aren’t cheap and they increase insurance premiums. It’s also the case that pressing on costs; the difference between 60 and 70 mph cruising can be up to 5 mpg in some cars.

There are several ways in which life can be a drag or rather, drag can be a costly luxury. Underinflated car tyres give greater rolling resistance and increase fuel consumption. Got a roof rack? If you have and it’s empty, dismount it. The air drag of even an empty roof rack is quite startling and you can do without it, and the weight of the rack itself. Similarly, air conditioning is lovely, especially on a muggy day, but it costs. Specifically, it can cost you to the tune of an 11 percent reduction in mpg. This is about balance. Open car windows create aerodynamic drag too. In general, open the windows to cool down around town but at over 40 mph, use the air con.

Underinflated car tyres aren’t the only things that weigh down fuel consumption figures. Some people carry the essentials but if your car is full of junk, every kilogram is costing you fuel. Figure out how likely you are to need golf clubs, dog blankets, Thermos flasks and half a dozen old newspapers on the way to the supermarket. This is a question that answers itself!

Some savings are more obvious than others. The number of miles of tarmac that pass under your car tyres is probably a given. That said, are you using the most fuel-efficient route? Try to avoid traffic if you can and keep up to date with en route road works. Motorways and dual carriageways, without all that thirsty stopping and starting, save fuel, as does the shortest route. Remember that a motorway detour, though a little longer, could be less costly overall.

Lastly, hidden costs. This is all about shopping around. Many things, such as new car tyres, may be what’s called a distress purchase. However, the firms selling the tyres, car insurance, spare parts and even fuel are in distress too. It’s worth looking out for the best deal.

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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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