Stopping Distance and the Role of Tyres

• Tyres-Safety-Tips/Guides

It is the general belief that it is your brakes that will enable you to stop suddenly to avoid a crash. Not so, says mechanical engineer Jason Fenske, the smarts behind Engineering Explained.

Brakes are the system responsible for turning the kinetic energy of your moving car into heat (science talk for ‘slowing it down’), but a car’s tyres are ultimately responsible for how quickly this happens.

Most modern passenger cars are equipped with anti-lock braking systems (ABS) which prevents your front wheels from locking up under hard braking and allows you to retain control over the steering in an emergency. 

When ABS is activated, it tells the onboard systems that your wheel has stopped rotating and that you have exceeded the maximum stopping force of the tyre. Your brake may be capable of applying more pressure, but the tyre has lost grip and more pressure serves no purpose. In other words, your stopping distance is limited by the tyre. In a car where the ABS is working properly, improvement in stopping distance will be extremely minimal with larger, more powerful brake kits.

What is Stopping Distance?

The stopping distance, or braking distance, is the distance a car travels before it stops completely after you have applied the brakes fully. It is typically given as a 100km – 0km figure.

1. Factors that affect stopping distance

1. Tyres

Worn tyres are unable to produce enough friction which reduces the traction coefficient and causes them to slide over the road surface without stopping regardless of the quality of the braking system.

• Over and under inflation are key factors in wearing out tyres and reducing tyre to road contact.
• Under inflation causes excessive flexing of the tyre sidewall which leads to overheating and ultimately, causing break-up and tread separation.
• Over inflation makes the tyre more susceptible to impact, penetrations and abrasion.

The tyre compound or makeup can also change braking distance. High performance tyres typically offer better adhesion under heavy braking and won't break loose or skid as easily as harder tyres. 

2. Braking and suspension systems

According to road safety advocates Arrive Alive, worn brake drums, rotors, pads, shoes or leaky brake lines all have an adverse effect on braking.

Since weight transfer has so much to do with braking, worn shock absorbers and springs will allow the weight to move to the front of the car which will add distance to stopping distance when you hit the brakes hard.

3. Road conditions

Dirt and gravel roads don't offer good traction and will therefore increase braking distance. Freshly paved asphalt offers the best adhesion. 

4. Weather conditions

Wet, and snowy or icy roads decrease traction and commensurately, increase braking distance. 

5. Driver ability

Driver reaction time is a critical component in calculating and determining stopping speed. The average reaction time estimate varies between 1.5 - 2 seconds and is as high as 2.5 seconds in older drivers.

Driving experience and being familiar with your car are equally key to safe and quick braking.

How is stopping distance calculated?

Braking distance is a mathematical calculation of:

• Original speed of the vehicle
• The type of brake system in use
• The reaction time of the driver/rider
• The coefficient of friction between the tyres and the road surface (also known as the traction coefficient). 

The experts agree that traction coefficient is the principal function of braking distance. You require enough friction between the road surface and the tyres to bring the car to a standstill after braking. The higher the traction coefficient, the shorter the braking distance.

2. Quickly calculate stopping distance

Incorporating an easy way to calculate your estimated stopping distance from cars or objects around you to avoid an accident can save lives and money.

Researchers working under good and dry road conditions as well as with good tyres and brakes, have calculated braking distance at10 km/h to be 0.4 meters. In developing a simplified formula, calculations are therefore based on a braking distance of 10 km/h which is then increased quadratically with the increase in speed.

Simply put: Remove the zero from the driving speed, multiply this figure by itself and then multiply by 0.4.

Example of calculation with a speed of 10 km/h:

10 km/h ⇒ 1

1 * 1 = 1

1 * 0.4 = 0.4 metres braking distance

Example of calculation with a speed of 50 km/h:

50 km/h ⇒ 5

5 * 5 = 25

25 * 0.4 = 10 metres braking distance

Basic rules about tyre safety

The critical importance of tyres in braking efficiency underscore the need to keep them at their optimum level of functionality. This can be done by observing the following basic tyre safety rules:

1. Only use tyre sizes recommended by the manufacturer.
2. Ensure that the tyre brand, size and tread pattern is the same on each axle.
3. Keep tyres, including the spare, at the recommended inflation by checking them at least once a week and at low ambient temperature first thing in the morning.
4. Always look for the SABS stamp of approval to safeguard you from illicitly sold used tyres that may be beyond retreading or counterfeit copies of respected brands.
5. Regularly inspect your tyres for:
   1. punctures, penetrations, cuts and bulges
   2. cuts that could allow damp to penetrate the casing plies and harm the textile and steel casing
   3. misaligned wheels that could lead to excessive wear.
   4. inner- and outer-edge wear which could be a sign of too low pressure.
   5. irregular bald spotting that could be caused by worn shock absorbers, worn suspension bushes or loose wheel bearings
6. Wheel studs /nuts must be torqued to the correct setting when new tyres are mounted and watch out for loose or missing wheel nuts.
7. Check tread depth on all tyres and replace well before they reach the regulatory minimum depth of 1.00 mm.

Supa Quick has over 250 tyre fitment centres in Southern Africa and a national team of more than 4,000 auto fitment experts trained to assist you. Find a store close to you and get your car checked today.

Also read:

The Importance of Brakes in Stopping Distance

Factors that Affect Stopping Distances on the Road