Driving characteristics

Contributing factors

How a car handles on the road is determined by a number of different factors. For example, if it is understeered or oversteered, and whether it is front, rear or four-wheel drive.

The condition of the tyres, wheels, brakes, shock absorbers and steering system also have a major impact on the car's driving characteristics and roadholding.

How the car is built is just one of many factors that affect how it handles on the road

External factors, that is, factors that you cannot influence yourself, which also change the car's driving characteristics are: side force, weather and road surface conditions, as well as the condition of the road itself. It is important that you understand what can affect the car's driving characteristics, the ways in which driving characteristics can be affected and what you can do to maintain control of the car at all times.

Side force and roadholding

Side force

Side force is the lateral (i.e., parallel to the road surface) force produced by a vehicle tyre when cornering (taking a curve). Without side forces from the road, the front tyres cannot make the car turn. But if the driving side force on the front tyres is greater than the counteracting side force on the rear tyres, the cornering becomes unstable and potentially very dangerous.

The need for side force increases and decreases quadratically in relation to speed. If you double the speed, the need for side force will multiply four times, and if you triple the speed, the need for side force will multiply nine times. In order to reduce the driving side force during cornering, it is very important that you maintain an appropriate speed in the curve.

Roadholding

In order to keep a vehicle on the road, grip, which is the friction between the tyres and the road, must be sufficient. Grip is reduced by rain, snow and ice, pollution such as oil and unevenness such as gravel. Grip is also affected by the quality and condition of the tyres.

The best grip is on dry asphalt. But if the asphalt is wet and muddy or newly laid, grip can become much worse. In such circumstances the grip on a hard-packed gravel road may actually be better than on an asphalt road.

When deciding which speed is appropriate in a particular situation, you must always take grip into account.

Drive extra carefully in curves and when grip is bad

Kinetic energy

An object's kinetic energy is the energy that all moving objects possess – for example, a car driving forward. The car's kinetic energy is affected both by its weight and its speed. As long as the weight or speed of the car does not change, its kinetic energy does not change either.

If the car's weight changes (for example, by load or passengers) then the kinetic energy changes in proportion to the weight change. A doubling of the weight also doubles the kinetic energy.

If the car's speed changes, the kinetic energy changes quadratically in relation to the speed change. A speed increase from 30 km/h to 90 km/h will multiply the kinetic energy nine times (3 to the power of 2 is 9).

If the kinetic energy is multiplied by nine, braking distance is also multiplied by nine, as braking distance is directly proportional to kinetic energy.

All moving objects contain an energy called kinetic energy

In other words, the forces at high speeds are always great. If you misjudge or underestimate the car's kinetic energy, you run the risk of not being able to stop the car in time – which, in the worst case, can lead to a serious accident.

Understeering and oversteering

Cars are either understeered or oversteered by design, but depending on the circumstances, the same car can quickly switch between under and oversteering.

During normal driving conditions, it is difficult to know if the car is under or oversteered. It is only when the tyres begin to lose grip that you really feel the difference.

Understeering

An understeered car turns less than you want it to and strives to go straight ahead in curves. This is because the front tyres do not have enough grip.

The illustration shows a car understeering in a curve, which makes the car continue almost straight ahead even though the driver is turning the steering wheel

Most modern passenger cars, especially those with front-wheel drive, are somewhat understeered as that is considered the safest. This is because understeering requires less of the driver than oversteering. But if a car for some reason understeers too much, the car will become very difficult to safely manoeuvre.

Understeer can be caused when:

• The front tyres are aquaplaning.
• The driver in a car without ABS (anti-lock braking system) brakes so that the front wheels lock.
• The driver engine brakes or accelerates too hard in a front-wheel drive car.
• The car is too heavily loaded.
• The front tyres are worn or have insufficient air pressure.
• A trailer or caravan is connected.
• The differential is locked.

The car can understeer if the front tyres are worn or have insufficient air pressure

Oversteering

An oversteered car does the opposite and turns more than you want it to. This is because the rear tyres do not have enough grip. Oversteer can result in rear-end skidding which means that the car's rear tries to pass the car's front, so that the front of the car points more and more inwards. This could, in the worst case, lead to a serious side-on collision, which is the most dangerous type of collision (cars are less protected on the sides).

The illustration shows a car rear-end skidding in a curve as a result of oversteering

Oversteer is most common in rear-wheel drive cars and is particularly noticeable when accelerating in curves, but can also be caused when:

• The rear tyres are aquaplaning.
• The driver is braking hard.
• The driver engine brakes or accelerates too hard in a rear-wheel drive car.
• The car is too heavily loaded.
• The rear tyres are worn or have insufficient air pressure.
• A trailer or caravan is connected.
• There are strong side winds.

An uneven road surface can also cause a car to oversteer, even if the driver is only driving straight ahead, and has not turned the steering wheel.

Electronic stability control (ESC)

Electronic stability control systems such as ESP, ESC or DSTC are systems that detect and assist when the car is about to start skidding. When the system detects loss of steering control, it automatically brakes individual wheels, preventing the skid. Some systems can also throttle the engine if required.

ESC can prevent the car from skidding by braking individual wheels, or throttling the engine

Electronic stability control systems significantly reduce the risk of accidents, but they do not and cannot change the laws of physics. If you drive too fast for the road conditions, you can still lose control – even in a car with ESC. Never solely rely on the car's technology. You must always adapt the speed, distance ahead and your driving according to visibility, road surface condition and the prevailing traffic situation.

On cars with ESC, there is usually a warning light on the dashboard that flashes when the system is actively working. If the warning light comes on without flashing the system is either broken or disconnected.

Tyres

Tyres are a crucial component affecting a car's driving characteristics. To maximise grip and minimise the risk of rear-end skidding and side-on collision, the best tyres should always sit on the rear axle, even in winter. This applies to both rear, front and four-wheel drive cars.

To get the best grip and driving characteristics, as well as the shortest braking distance possible, it is important to change your tyres for the summer and winter seasons: summer tyres in the summer - winter tyres in the winter. Driving with non-studded winter tyres (so called friction tyres) in the summer impairs the car's performance and significantly increases the risk of accidents.

Tread depth

A decisive factor for grip is the tyre tread depth and condition. Tyres older than 10 years should be replaced as tyre friction properties deteriorate over time.

New summer tyres have a tread depth of 7-8 millimetres. When the tread depth has been worn down to 3-4 millimetres grip in wet conditions starts to deteriorate and the risk of aquaplaning increases. For this reason you should replace summer tyres that have worn down to 3 millimetres. You must, however, replace summer tyres that have worn down to 1.6 millimetres.

New winter tyres have a tread depth of 9-10 millimetres. Winter tyres used in winter road conditions must have a tread depth of at least 3 millimetres. To get the best grip, you should replace winter tyres when the tread depth has been worn down to 4-5 millimetres.

Check the tread depth at regular intervals using a tyre depth gauge. In the picture below you can see how it is done.

Summer tyres must have at least 1.6 millimetres of tread depth

Correct tyre pressures

You should check tyre pressures at least once a month. Tyre temperatures and pressures increase during travel, so pressures should be measured when the tyres are cold, that is, when the tyres have the same temperature as the outside temperature.

The correct air pressure is stated in the vehicle's instruction manual. The wrong air pressure can increase fuel consumption, shorten tyre life, lengthen the braking distance and impair the car's driving characteristics.

Check the tyre pressure at least once a month

If the air pressure is too low in one of the front tyres the car may start to pull diagonally. If there is too little air in both front tyres (or if they are worn), the car may start to understeer more than usual. If there is too little air in both rear tyres (or if they are worn) the car can start to oversteer more than usual.

Flat tyres

Getting a flat tyre while traveling is unusual but may happen. You can drive at least a couple of hundred meters on a flat tyre, so it is better to drive to a suitable place to stop than to stop directly in the middle of the road, where you pose a danger to the traffic behind.

Wheels

After a change of wheels it is important to retighten the wheel nuts when you have driven about 50-100 kilometres. Otherwise you risk a wheel nut or an entire wheel coming loose during travel.

This is especially important to remember if your wheel rims are alloy (light metal). Note that such rims should be tightened with a special torque wrench to avoid damaging them.

The wheel nuts need be retightened shortly after a change of wheels

Wheel alignment and balance

Sometimes the wheels need to be adjusted and balanced in a workshop. Wheel alignment means that the wheels' angles are adjusted so that they are perpendicular to the ground and parallel to each other. Wheel balancing means that the wheels are adjusted so that they can rotate without vibrating at certain speeds.

If the car pulls diagonally when driving on a straight road, the air pressure in one of the front tyres is probably too low, or, the wheel alignment is incorrect.

If the wheel alignment is incorrect, the outer edges of the front tyres will also wear faster than the rest of the tyres.

If the steering wheel shakes at a certain speed, the wheels are incorrectly balanced or some part of the steering system is worn.

If the wheels are incorrectly balanced, unevenly worn patches usually occur on the tyres as well.

If the steering wheel shakes at a certain speed or if unevenly worn patches occur on the tyres, the wheels probably need to be balanced in a workshop

Incorrect wheel alignment

• The car pulls diagonally when driving on a straight road.
• The outer edges of the front tyres wear faster.

Incorrect wheel balance

• The steering wheel shakes at a certain speed.
• Unevenly worn patches on the tyres.

Emergency wheels

Most cars are equipped with a spare wheel or an emergency wheel. An emergency wheel is a wheel that is lighter and narrower than regular wheels (to save weight and space). Emergency wheels are designed for short distances, usually 80 kilometres or less, and relatively low speeds, usually 80 km/h at most. The maximum speed an emergency wheel may be driven at should be indicated on it.

Emergency wheels should have higher air pressure than regular tyres. The recommended pressure should be indicated on the wheel and in the instruction manual.

Emergency wheels are usually designed for a maximum speed of 80 km/h

Brakes

Failure of a car's brake system can result in dangerous driving characteristics. When braking, the front wheels should brake harder than the rear wheels so that, in the event of a wheel lock, the front wheels lock before the rear wheels. If the brake force distribution is incorrect, so that the rear wheels lock first, there is a great risk of a rear wheel skid.

You should occasionally check that the brake force distribution is correct by testing the brakes on an unpaved gravel road at low speed. Push the brake pedal harder and harder until wheel locking occurs. If the brake force distribution is correct the front wheels should lock first.

Shock absorbers

A car's shock absorbers contribute to better road holding and comfort by minimising rocking movements and impacts on the car's chassis (undercarriage).

Worn shock absorbers result in impaired grip – especially in curves and when braking – and braking performance. If the rear shock absorbers are in poorer condition than the front, the risk of rear wheel skid increases.

You can check the condition of the shock absorbers by pushing down on the front of the car. The car should spring back softly without rocking more than once. You can also test the car's shock absorbers by driving slowly while pushing the brakes a few times. If the car rocks sharply after each braking, it is time to replace the shock absorbers.

The condition of the shock absorbers is crucial for the car's grip and braking performance

Drive systems

A car can either be front, rear or four-wheel drive. Modern cars are usually front-wheel drive because it is considered to provide better driving characteristics than rear-wheel drive on slippery roads. But if you accelerate too hard in a front-wheel drive car on a slippery road, the risk of front-wheel spin is still great. If this happens you lose all steering ability. To regain your grip it is usually enough to release the accelerator.

Rear-wheel drive cars are becoming less common because of their inferior driving characteristics on slippery roads. In the worst case scenario, a drive-wheel spin results in a dangerous and difficult-to-control rear-end skid. Advantages of rear-wheel drive cars are that they are usually easier and cheaper to maintain and that they are better suited for heavy trailers.

Four-wheel drive cars are, on the other hand, becoming more common. They have better grip on ice, snow and other difficult terrain. Contrary to many people's beliefs, however, four-wheel drive does not shorten the braking distance. If all other factors are the same, a four-wheel drive car thus has the same braking distance as a two-wheel drive car.

A four-wheel drive car consumes about 5-10% more fuel than a front or rear-wheel drive car of equivalent size and model.

If you drive a lot on ice and snow or other difficult terrain, a car with four-wheel drive might be the best choice

The steering system

The steering system's task is to transfer the movement of the steering wheel to the wheels so that the car changes direction.

To check if the steering system is worn or loose, you can stand beside the car and turn the steering wheel with the engine turned off. If the system works as it should, the wheels react immediately.

If the steering wheel shakes at a certain speed, the wheels are incorrectly balanced or some part of the steering system is worn.

Power steering

Power steering is a help system that makes it easier to turn the steering wheel. The system is operated and lubricated using power steering fluid (sometimes called hydraulic fluid). The power steering only works when the engine is running, which means steering becomes more sluggish while being towed.

Power steering makes it easier to turn the steering wheel

To check that the power steering is working properly you can lightly turn the steering wheel while starting the engine. You should experience a slight jerk. Turning the steering wheel should then become much easier the moment the engine starts. If you do not experience any difference after starting the engine, something is wrong with the power steering.

If you experience "steering wheel jerks" while driving, and if steering alternates between easy and sluggish, dirt or air has probably entered the power steering system.

On some cars, a warning light on the dashboard comes on if something is wrong with the power steering system (for example, if the fluid is leaking.)

If you suspect that the power steering fluid needs to be replenished (for example, if the power steering feels sluggish or makes whining noises) you should let a workshop do it, as it can be difficult to fill it yourself. Incorrect choice of fluid can also cause damage to the system.

Hand position on the steering wheel

Placing your hands on the steering wheel correctly is important both in order to manoeuvre the car in the best possible way and to minimise the risk of being injured if the airbag in the steering wheel is deployed during a collision.

If you imagine the steering wheel as a clock, you should keep your hands firmly on the steering wheel in the quarter-to-three position. Keep each hand on either side of the steering wheel, and do not cross one hand over the other when turning.

Keep your hands on the steering wheel in the quarter to three position, just like the driver in the picture

In the past, it was said that the correct way to hold the steering wheel was in the ten-to-two position, but that was before cars were equipped with an airbag in the steering wheel. If you hold your hands in that position during a collision, the airbag can knock your hands into your face, causing serious injury.