Driver Knowledge Tests

Taking care of your tyres

toyota-hilux-sr5-2014-wheelA recent survey found that only 33% of young drivers never check their tyre pressures. Here’s what you need to be doing to keep yourself safe, and keep your tyres in great condition. Where your tyres meet the tarmac is a small area that keeps your vehicle on the road, and how well they work is influenced by the tyre’s width, tread depth and type, pressure, heat, camber, compound, wheel alignment and the road’s surface.

In general, wider tyres mean a larger contact patch between the tyre and the road, and therefore more grip. However, the more tyre in contact with the road, the more rolling resistance there is (i.e. the more fuel it takes to get the vehicle moving and keep it moving), the more difficult it is to turn, the more likely the vehicle is to follow imperfections in the road (“tramlining”), the more expensive each tyre and wheel is, and the greater the unsprung weight (i.e. the weight of each wheel). Therefore tyre and wheel size is chosen to be a compromise to give the best grip possible without huge compromises.

Tyre tread

Slick tyres, like the ones you see on racing cars, have no tread. They are smooth and create their grip by having sticky rubber that gets transferred to the road. They are useless in wet weather, though, as the tyre rides up on top of the water (“aquaplanes”) and this is like driving on ice.

If we have a tyre with grooves (“tread”), and we make it soft, the tread blocks move around excessively and create a lot of heat. Additionally, having very soft tyres means they wear out quickly. However, we need treads to disperse water in the wet.

Therefore a compromise is made: we have fairly hard tyres with grooves that give good wear characteristics and don’t heat up too much. The depth of the tread influences how well the tyre disperses water. If the tread isn’t deep enough, there’s a risk of aquaplaning in deeper puddles. The tyre’s wet weather performance drops off significantly once the tread depth is under 3mm.

Make sure you check your tyres regularly – preferably every month. The legal minimum tread depth is 1.5mm across the whole width. Even though 1.5mm is much less than 3mm, at 1.5mm your grip is seriously compromised in the wet and this lack of grip could cause an accident.

Pressure

Tyre pressure is another compromise. The higher the pressure, the less the rolling resistance and therefore the better the fuel economy, but less of the tyre is in contact with the road, and therefore there is less grip. At low pressures, the tyre has a larger contact patch with the road, but will get hotter and, at very low pressures, is at risk of rolling off the rim if pushed hard. The only time you would let your tyres down is if you are stuck in mud or sand and need a larger contact patch to give you more grip while you get out.

Tyre manufacturers work out an ideal pressure for the tyre in conjunction with the vehicle manufacturer that gives the optimum performance on the road. Check your tyre pressures every 4-8 weeks as your tyres will lose air gradually and will be well under pressure before you visibly notice.

As your tyres heat up the pressure will increase slightly as the hotter air expands. Heat also tends to make the tyre’s rubber slightly more sticky, although negligibly so in a road car. The tyre’s compound is what affects how much grip it has relative to its surface area on the road, and its willingness to give up rubber in exchange for grip. Soft compound tyres will wear out quicker but will have better grip; hard compound tyres will last longer and will have worse grip.

Alignment and camber

The wheel alignment is how true the tyres are to pointing directly forward. Often tyres are given a slight leftwards alignment as this means if you fall asleep at the wheel your vehicle will veer left off the road rather than right into oncoming traffic. If your wheels are too misaligned it will cause uneven wear and excess heat buildup within the tyre. You will notice the vehicle pulling heavily to one side if the wheels are misaligned.

Camber is the deviation in the angle of the wheel from 90 degrees to the road surface. Vehicles with a negative camber have wheels that lean in at the top – you will see this on race cars. This gives slightly more grip in the corner, slightly at the expense of grip in a straight line as during cornering the tyre’s width is flattened out against the road surface by the cornering forces. Negative camber will wear your tyres on the inside more quickly.

Tyre damage

Inspect your tyres regularly for damage. Small stones wedged in the tread can gradually wear a weak spot in the tyre. Remove small stones with a screwdriver or your key.

If the side of your tyre (the sidewall) is damaged, this can lead to unexpected blowouts. The side of the tyre is not that thick. If you notice your tyre is deflating quickly you probably have a slow puncture which will most likely be caused by a nail in the tyre. These can usually be fixed and plugged. The earliest notice you’ll get of this is a kind of throbbing or oscillating sound as you are driving.

The road

The weather and the type of road surface are the biggest factors in the grip you will have. Ice on concrete has very little friction, while coarse chip on a hot day will have a lot of friction. If it has been fine weather for a while a layer of oil and rubber builds up on the road. When it first rains, this forms a slippery film that can be treacherous. Eventually the rain washes this away and the road surface gains more grip, but will always be more slippery than when it is dry.

Be especially careful of black ice, which is ice you cannot see. Any time you are driving and the temperature is less than 3 degrees Celcius there is a risk of ice forming.

Darren is an expert on driving and transport, and is a member of the Institute of Advanced Motorists

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Posted in Advice, Car, Heavy Vehicle, Motorbike
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