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NOT JUST KNOBB1Y, ROUND AND BLACK

22nd July 1993, Page 91
22nd July 1993
Page 91
Page 91, 22nd July 1993 — NOT JUST KNOBB1Y, ROUND AND BLACK
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Understanding tyre design helps figure out one of transport's highest costs and know what duty any tyre is suitable for.

There is a single principle that you should always keep in mind when looking at tyres: resistance to slippage is greatest across a groove not along it. Obvious you may say but with so many tread patterns available, it is easy to loose sight of that fact. It follows, therefore, that all steer axle tyres should have grooves around the circumference and all drive axle patterns will go across the tread.

But while those traits are discernable from tread pattern design, other factors like wear rates, adhesion, noise creation and water removal all have to be considered, Unfortunately the cures for these ills are often opposites. A hard rubber compound will wear more slowly but gives less adhesion to the road. Slick tyres create minimum noise but cannot cut through surface water.

Designing tyres is like playing threedimensional chess; alter one detail here and another feature suffers elsewhere. In fact most tread patterns are designed on supercomputers like the one at Michelin. Its Cray supercomputer can do millions of calculations a second and still has to spend several hours thinking about the ramifications of each design change. After the computer has made its deliberations, prototype tyres are made and tested. One such test is to drive round a wet concrete circle of a set diameter as fast as possible. This allows tyres to be compared objectively; the faster you can go, the better the tyre's wet grip capabilities.

Braking performance is tested using a system that automatically applies full braking at a preset point allowing stopping distances to be measured and compared. Worksh0 witnessed a test on a loaded but solo tractive unit with Michelin XZAs all round. Fitting XZA on the front and drive axle tyres (XDA) on the rear showed a 14% shorter stopping distance on the second run. From 50km/h the all-XZA shod vehicle took 50.52m to stop on a wet track, with XDAs on the rear this came down to 43.28m.

This result is due to the grooves across the tread in the drive axle tyres tread pattern designed to transmit the torque which also provide the grip for braking. This reverse torque requirement obviously applies to steered axle tyres and is why their patterns do not consist solely of longitudinal grooves. So why don't all tyres have grooves longitudinally and across the tread in a sort of giant 'noughts and crosses' game you may ask? There are two reasons—firstly, the wear rates and secondly, noise.

Both factors are related to 'tread shuffle', the name given to the relative movement of the tread blocks as they move in and out of contact with the road, This not only dictates the rate the rubber wears away but also noise created. In order to minimise the noise tread patterns are now designed with different block sizes to varies the pitch of the sound produced. While the tyre may not produce any less noise, it is spread over a wider frequency and so is less obtrusive.

The spare tyre has all but vanished this side of the Channel with UK hauliers preferring the payload and arshflow achieved by not carrying one. Fine until you get a puncture.

To reduce downtime caused by a puncture Michelin's subsidiary ATS has set up a central breakdown point in Leeds. With nearly 7,000 operators and 126,000 vehicles signed up to the breakdown system, it handles around 400 calls a week via a fi-eefone number. Each breakdown location is logged into the computer which displays ATS locations near the incident and their distance from the stranded vehicle. With 456 ATS depots dealing with CVs there are usually several close to the incident and one will be a specialist stock holder carrying an enlarged range of CV tyres.

Processing a call takes about four minutes as most vehicles' tyre requirements are already logged into the operator files and a fitter is usually on site within an hour. For the service, a flat fee of £6.75 per call is charged on top of the repair costs.

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Locations: Leeds

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