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07/05/2012
IN BLACK AND WHITE

KEEPING TABS ON TYRES
In the cold season, winter tyres allow us to drive with maximum safety. Having the appropriate tyres for the season is not just a precautionary measure, current laws and the highway code make them obligatory

Duilio Damiani

SPECIFIC TYRES and devices that grip. Translation: winter tyres and snow chains for everyone, or almost. And increasingly everywhere on our peninsula, from November to April.
By now aware of the current laws on tyres (or chains) that are patchily applied by local authorities,  together with Assogomma and its members we did an on-the-spot check of the real potential of winter tyres compared to summer tyres.
Against the prestigious backdrop of Madonna di Campiglio and in the presence of the authorities and trade press, a series of practical tests and demonstrations showed how vehicles fitted with new summer tyres, new winter tyres, worn winter tyres or snow chains behaved differently in a variety of situations. All the main manufacturers were represented - Bridgestone, Continental, Goodyear Dunlop, Marangoni, Michelin, Pirelli and Yokohama – not to make a direct comparison of their products, but to demonstrate the real advantages of having specific tyres.


The facts speak for themselves
The nine tests reproduced the customary conditions in which vehicles circulate and the cars and commercial vehicles (and ambulances) involved drove on pre-established  routes. It was a sunny day with typical winter conditions, low temperatures (below -9°C) and roads that were slick with frost and snow.
The first uphill and downhill braking and stopping test was in three Volvo XC60 2.4D sport utilities fitted with new 235/60 R18 summer tyres, new winter tyres and tyres of both types with about 50% wear. Travelling at 20 km/h, momentum took the car fitted with summer tyres halfway up the 30% gradient ramp, but starting off again was impossible as its wheels span and even though it was in first gear, it began to slide dangerously backwards. It was even worse on the descent, when the ABS kicked in at the first sign of braking and prevented it from slowing down (at least it still had some steering, which would have been impossible with blocked wheels) until it reached the level. The result was totally different for the cars equipped with winter tyres, which restarted immediately on the slope and the stopping distance on the descent was short (almost the same for both).
The second test was only a demonstration as passengers in the two commercial vehicles, Fiat Ducato MultiJet 120s fitted with new 225/70 R15 summer and winter tyres. A short drive on the road allowed us to compare the behaviour of an unloaded vehicle and one loaded with 400 kg. The van with the winter tyres drove the entire snow-covered road without a hitch, but when the one with summer tyres had to stop at a traffic light, it became well and truly stuck and couldn’t even cope with a slight mound of accumulated snow.
The third test of stopping distance was in three 235 HP Alfa Romeo Giulietta 1.7 TBI Turbos fitted, respectively, with new summer tyres, summer tyres and chains on the front, and new winter tyres; all 225/45 R17. At 45 km/h on a snow-covered, slightly downhill stretch the stopping distance for the first configuration was about 63 metres, with chains it was 31 metres and with the winter tyres it was only 12 metres.
The fourth test was an extension of the third and the exercise was repeated this time with two Fiat Freemont 1.9 JTD crossovers with front-wheel drive and summer tyres with chains (with 7mm links on the front wheels) and new winter tyres in the 225/65 R27 size. Carried out at about 45 km/h, it emerged from the test average that, even though authorized by the law in force – and it should be pointed out that supplementary gripping devices permitted under the decree of 13/03/2002 “must comply with the definitive vehicle unification standard  (CUNA) or, alternatively, the equivalent standard in force in the European Union member States” – the stopping distance for a vehicle fitted with chains increases by over 50% compared with one fitted with winter tyres, about 35 metres instead of 14 metres.
The fifth test was braking by emergency vehicles. Travelling on snow at 45 km/h, two AWD Volkswagen Caravelles fitted with 215/65 R16 summer tyres and winter tyres, stopped in about 50 metres and 20 metres, respectively. A difference of over half, despite the all-wheel drive. For an emergency vehicle this could influence the outcome of its intervention.
In the sixth test we compared summer tyres with partly worn winter tyres. The two Volvo XC60 2.4 Ds were fitted with new summer tyres and winter tyres with about 5 mm of tread, half the depth of the tread on new tyres. At 40 km/h the stopping distance was 65 metres for the first and under 20 metres for the second, confirmation of their efficiency, despite 50% wear.
Although it is not forbidden by the current law, the use of a mixed set of two winter tyres on the drive axle and two summer tyres at the other end, it is decidedly counterproductive in terms of consistent behaviour. The confirmation came with the seventh test on a snow-covered track with three Fiat Panda 900 Twinair Turbos, one with a complete set of summer tyres, one with a complete set of winter tyres and the last with a mixed set, winter on the front and summer on the rear. The car with summer tyres skidded on the slippery surface and struggled to gain speed; even when it got going, it was very difficult to maintain any grip and direction, with accentuated under-steering on cornering that ended only when the vehicle came to a stop. With the winter configuration, grip and steering was good in all conditions and genuinely predictable dynamic behaviour gave the driver a feeling of safety. The feeling was completely different with the mixed set: the grip of the two winter tyres was decisive in achieving momentum as far as the corner, but the insufficient grip of the two summer tyres in the rear meant that steering was impossible, which led to immediate loss of control and the inevitable 180° spin.
We were in doubt about whether this behaviour was accentuated by the lightness of the vehicle, but this was immediately settled in the eighth test, when the same mix was used on three Maserati GTs. Needless to say, the summer tyres did not provide enough grip to move forward, despite all the electronics. The winter set gave us good speed and steering on the snow-covered roundabout which we were able to take at a constant 25-30 km/h, even with the traction control switched off, when the sensitivity of the foot on the accelerator must be heightened to surgically dose the powerful eight-cylinder, 395 HP engine. The mixed set in this case was two 245/40 ZR19 summer tyres on the front and two 285/40 ZR19 winter tyres on the rear. The starting speed was impressive, but already at 20 km/h a hint of steering prevented us from staying on track  and led to a drastic drop in speed. Over-steering did not stop until below the 10 km/h threshold, the maximum possible speed on the snow-covered roundabout.
The last test on this intense day put us behind the wheels of two Fiat 500 Abarths on up and down  roads open to traffic. At an average speed of about 30km/h, the feeling of uncertain grip on surfaces that were dry, wet, icy, with compacted or soft snow thrown up by the many passing cars, called for considerable caution when making any manoeuvre and was a constant strain for the driver. A change of car and the feeling of greater grip was evident right from the start. Steering response was prompt, even when speeding up on the corner, braking was also reliable downhill and cornering was stable at a lively average speed of 50 km/h.

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