Wheels and Tyres Frequently Asked Questions
What are the metal clips on my wheel rims?
As part of your car maintenance schedule it is recommended that you run your air-conditioning regularly, even if only for relatively short periods of time. This ensures the lubricating oil in the system is correctly distributed within the system and helps prevent premature compressor wear or even failure.
What is Toe In?
Toe in is the difference in distance between the front of the wheels and the rear of the wheels. Most vehicles have some inward pointing of the wheels when stationary, the wheels become approximately parallel when the vehicle is in motion.
What is Castor?
The relationship between the steering axis and true vertical is the castor. Zero castor is when the steering axis is vertical. When the steering axis is tilted backwards (by far the most common design) it has positive castor. Unequal castor from side to side causes steering pull to the side which has the least amount of positive castor.
What is Camber?
Camber is the measurement in degrees of the angle of the tyre from vertical when seen from the front or rear. A tyre tilting out at the top has positive camber, inward tilt is negative camber. Positive camber is normal in a stationary road car. The tyre becomes approximately vertical once the vehicle is in motion.
How does wheel alignment affect my tyres?
Poorly balanced wheels can cause your car’s steering to vibrate badly as the tyre hops along the road and limits adhesion. Tyres need to be balanced on their wheels so that they run smoothly and wear evenly. Out-of-balance wheels accelerate wear on the car’s suspension joints and shock absorbers, and also shorten a tyre’s life.
A correct wheel alignment can save you plenty on the cost of tyres. Badly-aligned suspensions wear tyres out rapidly and are also unsafe, as they affect your car’s handling. Camber and castor angles which are“out” cause uneven wear on your tyres and your car is unable to track “true”. We recommend a wheel alignment at the time of fitting new tyres and thereafter at regular intervals. Wheels which may be only a fraction out of alignment can scrub or drag along the road instead of rolling along it.
What is Under-inflation?
Too little air in a tyre produces abnormal deflection of the tyre and results in excess wear in the shoulder area as well as excessive heatbuild-up. The extra strain increases the risk of fabric damage and separation failures.
What is Over-inflation?
The tyre is over-inflated and not sitting squarely on the road. The normal flexing of the tyre will be reduced and result in undue wear in the centre of the tread. Over inflation makes the fabric pile more susceptible to damage.
What is Understeer?
This can occur when you drive a car with normal tyre pressure, quickly around a corner. The car doesn’t turn as fast as you turn the wheel and tends to “plough-on”. It affects front-wheel drive cars more than rear wheel drive or four-wheel drive and increases front tyre wear. In both understeer and oversteer, the tyres are losing their ability to grip the road surface. This usually happens because the speed and weight of the car is exerting a force on the tyre and deforming it so that part of the tread is no longer in contact with the road. Slowing down will correct the situation but do not brake, slow down gradually.
Correct tyre pressure helps to ensure maximum wear from your tyres. It also optimises ride comfort and fuel economy. Warning: Driving with uneven pressures on different sides of your car could mean loss of control when braking or accelerating. The balance of your car is also upset and tyre wear will be uneven.
What is Oversteer?
When oversteer occurs, the back tends to swing around, sending the car into a spin. It is more likely in rear-wheel drive cars and increases in likelihood if the car has a rear engine, as this raises the weight carried by the rear tyres.
Should I change the tyre pressure when towing?
Carrying heavy loads or towing boats or trailers puts pressure on your vehicle’s tyres. You can check you are not overloading them by referring to the load-carrying capacity marked on the tyres. For example, an '86' on a passenger tyre means it can take a maximum load of 530kg, or 545 kg if it’s marked with an '87'. This applies at maximum tyre pressures only, or about 240 kpa (35 psi) and emphasises the need to add pressure to your tyres if your vehicle is fully loaded with passengers and luggage. A general rule is to add 14 kpa (2 psi) if fully laden. It should be noted that moving loads, such as a boat or caravan, can multiply the static load by 3 or 4 times and can very easily exceed tyre ability to carry the load. Large loads also put a strain on steering gear, axles, wheels and springs, so keep those pressures up and don’t forget the spare
Speed can kill tyres. Load and speed generate heat and together they can destroy a good, new tyre. At 80 kph on a trip, your tyres will last almost twice as long as they would if you were doing 100 kph and at that speed, you'd only save 30 minutes on a 200-kilometre journey. Before embarking on a long trip, ensure you have five tyres which match and which are at normal pressure. Then increase the pressures 14 kpa (2 psi) to cater for the conditions of speed, distance and the extra load. Tyres have a 'memory' and if on a previous trip your radial tyres were under-inflated, they would be fatigued to an extent that could cause a future failure.
What pressure should my tyres be?
For everyday motoring, tyre pressures should never be lower than the manufacturer’s recommended pressure. This information is on a tyre secification placard usually mounted on the drivers door edge or pillar, or in the glovebox (a requirement by law since 1973). Stability, tyre life and fuel economy is adversely affected if a tyre is under-inflated. The life of the tyre is also affected.
When is the best time to check my tyre pressure?
Tyre pressures should only be checked when the tyres are cold. For your convenience, it is better if you purchase a good quality tyre pressure gauge so you can check pressures before driving off on the tyres and thereby warming them up. If it is only possible for you to check your tyres when they are hot, add at least 13.8kpa (2 psi). Tyres heat up when in use and while the air expands, the space in the tyre remains constant, resulting in higher pressure.This pressure build-up is one of the reasons a tyre with a structural weakness can burst at speed, causing a blow-out.
Always make sure your caps are applied tightly to the tyre valve. You can test the valve for sealing by putting a dab of saliva on your finger, applying it to the valve and watching if a bubble rises. If it does, your valve needs tightening. You can use a small spanner or the top of a specially-designed dust cap. If the leak persists, the valve may need replacing.
Why do I have to check my tyre pressure regularly?
All tyres must be inflated with air. It provides the cushioning effect between your car and the road. It is important to keep the amount of air in the tyres close to the vehicle manufacturer’s recommended pressure - that is, not too high and not too low. Unfortunately, all tyres slowly deflate and therefore it is up to you to check the air pressure at regular monthly intervals.
How do my tyres affect fuel consumption?
Tyre choice plays a major role in fuel economy and simply changing from crossplys to steel-belted radials can help improve fuel economy. Radials have less rolling resistance than cross-plys and because a car moves more easily on them, the engine doesn’t work as hard and needs less fuel.
When should I change my tyres?
It is illegal to mix radial tyres with cross-plys on the same axle, due to the greater adhesion of radials which could cause your car to spin when braking hard. A spin could also result if you mix brands on the same axle, due to varying grip from brand to brand. Even the same brand can grip differently.
All tyres over six years old should be checked professionally at a Jarvis Service Centre. Even relatively new tyres need regular checks for sidewall damage, such as cuts or bulges, and treads need checking for nails or tears. Although the minimum amount of tread required for a car to be roadworthy is 1.5 mm, it is wise to replace tyres at 2 mm, as wet-weather grip is diminished when less of the tread is remains.To check tread depth, put a match head into the tread grooves, and if any part of the head shows above the grooves it’s time to replace the tyre. Remember, always check the spare when carrying out tyre inspections.
What do the numbers on the tyre mean?
The numbers on the wall of the tyre show the size and aspect ratio of the tyre. The aspect ratio is the ratio between the tyres width and its side wall height. That is, a 205/60/15 size tyre has a 60% aspect ratio which means its side wall height is 60% of its width which is 205mm. Note that this is a ratio, not an absolute measurement so a 205/60 has a different side wall height measurement to a 195/60 even though the ratio is the same. The 15 refers to the wheel size in inches.
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