While most people measure tyre pressure, few measure tread depth. A tread depth gauge is an essential tool

Is it worth running tyres down to their minimum legal tread depth before replacing them? Sure you’ll be legal but is it really saving you money – and is the tyre really safe? No, writes FleetWatch technical correspondent Dave Scott, who backs his claims with research facts.

South Africa’s economy relies on mobility. According to e-NaTIS, over 9 million licensed vehicles roll on our roads every day and that means many millions of tyres are trying to stop movement in time to avoid an expensive collision. 

That’s the key to understanding stopping distance – it’s not what one hears so often that ‘the brakes failed’ but loss of adhesion between tyre footprints and road surface that made the stopping distance longer than a driver expected. Add a dash of lubricating water onto an oily road surface and many factors come into play such as tread depth and ability to disperse water from the tyre grooves. 

The official South African road-death toll in 2007 exceeded 15 000. One in four vehicles insured will have an accident claim. How much more could those figures have been reduced for a few mm more tread depth? South Africans have a good record of taking things to the limit. Most fleet operators are under the false impression that since 1mm tread depth satisfies legal requirements for tyres on the road, so it’s therefore ok to run down to 1mm left on the tread pattern so as to squeeze the last bit of rubber out of running costs. The 1mm only applies if a tyre is not equipped with a tread depth indicator, but all high-quality tyres have tread depth indicators. 

Regulation 212 (j) (ii) specifies that tread pattern may not be level with the tread depth indicator. Now this will elude roadside enforcement but does not escape the attention of insurance assessors who are looking for a loophole to refuse a claim. 

But is it worth running tyres to the legal limit? Definitely not! Research data released by leading European tyre manufacturer Continental makes a conclusive statement: “Results of braking tests for more than 1 500 drivers confirm: As tread depth decreases, braking distances increase disproportionally. At three millimeters and less, summer tyres on wet roads become hazardous.’ 

Tread depth indicators are around 1,6mm above the bottom of a tyre groove but 50% less than the 3mm that Continental recommends. We are only eight weeks away from early summer rains. 

A false sense of safety 
ABS – anti-lock braking systems – prevents wheel lock up while ESC - electronic stability control systems – regulate the rotation of a vehicle’s individual tyres when braking and steering, braking the wheels to maintain driving stability. You cannot have ESC without ABS and the two systems combine to really improve braking and stability in an emergency. 

It’s a bit like people who don’t wear a seatbelt because a vehicle is equipped with air-bags – ABS and ESC cannot make up for a lack of tread where the rubber meets the road. This is because: 

• Brakes only slow down the speed of rotation of the wheels. 

• It is the contact between the tyres and the road surface that actually reduces vehicle speed. 

• The ability of brakes to retard vehicle speed depends on contact between tyres and the road surface. So regardless of legal aspects, ensure that tyre footprints are the best you can get. 

Continental makes the point: ‘The braking distances in the wet depend on the tread depth of the tyres, with or without ABS or ESC. It is a simple rule of thumb: The lower the tread depth, the longer the braking distance. On braking tests from 80 km/h carried out in the framework of the UEFA EURO 2008, more than 1500 of Continental's business partners and representatives from the media found out first-hand the serious differences at a water depth of 15mm: 

• A car equipped with new summer tyres (8mm tread depth) comes to a standstill after just 42m braking. With a sufficient 3mm residual tread depth, the car stops after some 52m. 

• A residual tread depth of 1.6mm is risky for all road users in such a traffic situation: The braking distance increases even more since the water cannot drain off quickly enough. The car doesn't come to a stop until after 61m, which is 45% longer than the braking distance for a new tyre. 

• And when the car equipped with sufficient treads comes to a stop after 42m, the car with 1.6mm residual tread depth is still travelling at a collision speed of 44 km/h in this test. In actual traffic, this can lead to very serious accidents. 

In order to keep the braking distances as short as possible on wet roads, Continental therefore recommends that summer tyres be replaced at 3mm.
 

The braking distances in the wet depend on the tread depth of the tyres... The lower the tread depth, the longer the braking distance.

What about truck tyres in the wet? 
What about truck tyres in the wet? The same legal Regulations (Reg.212) and physical principles apply. The adhesion between tyre and road is the main factor that controls the minimum stopping distance and depends on: 

• Road surface type. 

• Surface condition – wet, dry or greasy. 

• Tyre tread design, type of tread material and most importantly, tread depth. 

Marcus Haw of Bridgestone Firestone SA observes that truck tyres, due to mass and size, displace water on the road. Unlike passenger car tyres, truck tyres do not rely on the grooves – the tread void, a long, patterned furrow molded into the tread – to displace water from the contact between the tyre and road. That’s at 56t gross combination mass (GCM). 
 

Tread depth is serious stuff and should be part of any fleet’s tyre management programme  Marcus Haw Bridgestone Firestone SA

However the majority of trucks – a good 40% – fall into the medium truck category between 3 500kg and 8 500kg gross vehicle mass (GVM) and when running without payload, would behave more like a car on a wet road than an extra-heavy combination. A Toyota Dyna 4-093 is a typical example of a cross-over point where a light commercial becomes a truck but drives like a passenger vehicle in terms of handling – the Dyna 4-093 is not equipped with ABS to rely on rearaxle load-sensing and tyre tread depth for stopping distance. 

But the real measure of tyre success is leaning towards kilometres travelled per tread millimetre. Haw comments that “current benchmarks for long haul truck tyre life run around 18 000km/mm for a steering axle and up to 22 000km/mm for a drive axle tyre.” And then there’s a ‘retreadability’ requirement. An original tyre casing run below 3mm tread depth minimizes the possibility of another two retreads over its economic life.

Night ops & tread depth 
A well-aimed dipped headlight beam reaches 55m. Many vehicles are on the road at night and most of the time, lights will be dipped for oncoming traffic, so 55m rules OK! Whatever happens, drivers must be prepared to stop within the 55m parameter but a 56t heavy vehicle combination at 110kg/h - which is the speed a lot of trucks travel at night - takes up to 150m to crashstop. And the braking tests that Continental carried out on light vehicles are based on 80km/h whereas most South African motorists travel at 120km/h - and more - regardless of night or day, wet or dry. 

Many companies, particularly the courier and express parcel/freight segment, operate in total darkness. This means that tread depth is a critical factor in running costs and vehicle safety in these applications. Do they really worry about this issue? 

Never mind your personal cocoon of ABS, BAS, ESP, EBD and all the other ‘Intel Inside’ acronyms that equip your latest set of wheels. The research from Continental is benchmark stuff and you must ask yourself: When it’s wet, do you or your fleet have enough rubber to meet the road. Tread depth is serious stuff and should be part of any fleet’s tyre management programme. Regulation 212 (j) (ii) is only a legal ‘indicator’ and not really a safety breakpoint.
 

A Toyota Dyna 4-093 is a typical example of a cross-over point where a light commercial becomes a truck but drives like a passenger vehicle in terms of handling.