Tread
Tyre tread depths and driving safety.
Loss of vehicle control can occur where tyres with insufficient tread depth are used on wet roads. For this reason it essential all vehicles have sufficient tyre tread depth so that loss of vehicle control is avoided.
What is the function of the tyre's tread grooves?
The tyre's tread grooves disperse water away from the tyre's contact patch when
roads are wet. Where tyres have insufficient tread depth, their ability to disperse
water away from the tyre's contact patch when roads are wet is compromised,
which can cause the driver to lose control of their vehicle.
- When a tyre has insufficient tread depth to disperse water away from the tyre's contact patch, the vehicle's tyres can become
separated from the road surface. The technical term for this is "aquaplaning". As you would expect, where the tyre is separated
from the road surface, the tyre cannot transmit steering, braking and drive forces to the road surface. - It is also a legal requirement for tyres to have a minimum tread depth, which is a depth of 1.5mm.
- Standing water on wet roads reduces the size of a tyre's contact patch, as shown in the image below. In extreme examples, the tyre can "aquaplane" as described above.
The tread depth of the tyre will determine how much the tyre's contact patch is reduced in wet conditions and therefore the amount of grip the tyres can provide. The graph shown below shows an example of how tyre tread depths affect braking distance on wet roads. The tread depth of most passenger car tyres when new is close to 8.0mm. For roadworthy purposes in Australia, tyres with a tread depth 1.5 to 1.6mm are considered worn out. The graph below indicates the practical reason for this regulation.
How do I check the tread depth of my tyres?
All tyres have a tread wear indicators moulded into the tread grooves. The position of the tread wear indicators are indicated on the tyre shoulder, as shown below. Where your tyres are worn down to the tread wear indicators on any part of the tyre, the tyre should be replaced. Ensure you check all of the tyre's tread area, including the inside of the tyre's tread. While you are checking the tread depth of your tyres, It would be conveniant also check the inflation pressures of your tyres at the same time.
Tyre performance in extreme conditions
Heavy rain
Regardless of the tread depth of your tyres, aquaplaning can occur during heavy storms due to the depth and volume of water on
the road. In these conditions, vehicle users need to reduce their vehicle's speed and allow for extended braking distances to
ensure that collisions and loss of vehicle control is prevented.
First rain after a dry period
Regardless of the tread depth of your tyres, grip levels can be very low when during the first rain, after a long dry period. In these
conditions vehicle users need to exercise caution and allow for for extended braking distances to ensure that collisions and loss of
vehicle control is prevented.
Reference material: "Toyo Tire Talk" Nos. 95-002, 95-007 and 97-011. Produced by the Toyo Technical Service Department, Itami, Japan.
