Which tyre, which axle?.
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With inflated tyre costs in mind Tim Blakemore looks at the case for specialpurpose tyres 4's Tables of Operating Costs ows that tyre costs for maxiJrn-weight articulated combitions rose on average by 28 r cent last year. As a percenge of total runnings costs they 3 relatively small at 11.2 per nt compared to fuel. 35 per nt, and maintenance, 29 per nt. Most operators do not ve serious problems with .es, but every transport maner or fleet engineer would like ffertread life.
Are tyre manufacturers at all mplacent? As competition in.3ases in a shrinking market, 3y cannot afford to be.
So far as I can discover, delopment continues unabated. ,cently, changes have been 3de, particularly to tread patms. Brigestone, Firestone, and Uniroyal have introced broad tread tyres similar concept to Michelin's XZA. )odyear's G191 and Dunlop's '311 have buttresses in their aad grooves to encourage D re even tread wear by resisttread squirm, and the Duna tyre also has a single wide ntre groove to put more rubr in contact with the road.
One operational difficulty is at of uneven wear on tyres fitted to the front axles of commercial vehicles, particularly tractive units. Road camber is accepted as the major cause of outer shoulder wear on rearside front tyres, but front-fitted tyres also have to cope with many forces generated as a result of any vehicle's steering geometry. Manufacturers are trying to overcome or at least minimise this problem.
Avon Tyres, of Melksham, Wiltshire, was convinced by market research that the problem of uneven wear on steered axle tyres warrants the development of a tyre specifically for steered axles. The result is the Avon Steermaster. I believe that this is the first commercial vehicle tyre with an asymmetric tread pattern.
However, the two wide centre grooves are symmetrical, with a continuous centre rib between them, while the outer groove nearest the outer wall of the tyre is narrower than the one near the inner wall. Both of these grooves are narrower than the centre ones. The outer sidewall is clearly marked, "Fit this side outwards" in English, German and French, but even if this engraving should be scuffed off,
the outer wall can be distinguished by its three kerbing ribs.
Avon's research into irregular wear of steel radial tyres began several years ago, and Brian Clapson, the company's technical director, told me he had been surprised by the high values of the longitudinal forces acting on the shoulder ribs of tread, particularly the nearside front one because of road camber.
Avon wanted to find out how the forces existing in the contact patch between tyre and road would be affected by changes in tyre design. It was discovered that the forces vary almost completely independently of each other depending on the tyre shape and pattern. Once shoulder wear sets in, further wear is accelerated —shoulder wear causes shoulder wear.
Avon's way of off setting the longitudinal forces and consequently wear, was to change the relative tread and breaker radii. For tyres that would be fitted all round a vehicle the wear performance of a five-rib tread pattern was found to be better than that of a three-rib design. So the
five-rib pattern was used for th. multi-purpose Extra Tree' range. But for steered axles th, three-rib tread design was -bum to be marginally better — henc, its use on the Steermaster.
The broad shoulder ribs resis wear by putting a lot of rubbe on the road, but if both inner an, outer ribs were the same then would still be the problem a uneven wear. So the rib in th, fastest wearing position, th, outside shoulder, is more wea resistant than the inner one, ant the result should be a tread tha stays flat and wears evenly a long as possible.
The narrow outer groov, closes up in the area of th, contact patch between tyre ant road when a vertical downwan force is applied to the tyre thu giving a greater area of rubbe contact. Avon's conclusions rE sult from tests on its own teE vehicles, two Bedford TM 170 rigids, and from the experienc of hauliers who have tried Stee masters in service. The compan is now satisfied that the asyrr metric tread pattern does we down evenly on steered wheels
The Steermasters that I saw Melksham that had covere around 32,000km (20,000 mile: on the front axles of the TV showed no signs of uneve wear. The computer-predicted mileages at which these tyres tread depths would be worn to 1mm were between 60,000 and 70,000 miles.
Steermasters are available in two sizes, 11 R 22.5 tubeless or 10.00 R20 tubed, but other popular sizes will be added to the range.
Steermasters may be fitted to other axles apart from steered axles and perform well, says Avon, but, of course, the best wear rate will be obtained by fitting them the right way round to steered wheels.
They can be regrooved to a maximum recommended depth of 4mm and remoulded. Avon does not remould casings itself and acknowledges that established remoulders will have specialised equipment designed around the largest volume tyre producers, and the West Country company does not claim to be among these. Buffed casings for remoulding have to be within fairly tight tolerances for tread platform width and sometimes for overall diameter, so Avon has ensured that in these critical dimensions its tyres are compatible with those of the volume producers.
The biggest of these volume producers in the UK is Michelin and it too has recently introduced a new truck tyre. But Michelin's XZT is quite unlike Avon's Steermaster, except that both tyres are steel braced radials.
The XZT is designed specifically for drive axles. It has a bold tread pattern with an unusually deep original tread depth of 171/2mm. Michelin says the XZT was designed to cope with the high torque developed by modern turbocharged diesel-engined lorries.
Traditionally, Michelin, is secretive about its production processes. Security is strict and it is rumoured, though I can't confirm this, that gauges in plants are marked in Ms (for Michelin) rather than degrees or psi. Employees therefore can not pass on to outsiders information about manufacturing temperatures and pressures, even if they want to. It came as no surprise then when my request to visit the Stoke factory with a photographer to see ZT tyres being made was politely declined. Very few of Michelin's own employees are allowed access to all parts of the factory.
Apart from its tread pattern and depth the ZT appears to be the same as the XZA, but the changes to the tread are significant, claims Michelin, in improving grip and tread mileage on drive axles.
Bob Thurston Michelin's technical advisor, told me that since 1978 there have been 'close to 200" sets of ZTs running on survey in selected fleets in the UK. All have been fitted to the drive axles. "Of those removed so far," said Bob, "the average original tread life has been 141,000km (88,000 miles). The average life of all previous fitments, covering many makes and types of tyres, was 89,000km (55,000 miles)."
One of the biggest fleets involved in the trials is British Industrial Sand which has had the new tyres fitted to the drive axles of eight-wheeled ERFs and Seddon Atkinson 400-Series tractive units, all powered by Cummins NHC 250 or NT 240 engines.
Records kept by Roy Ashton, BIS's western region transport manager, show that the the ZT's original tread life (before being regrooved at 3mm) is between 102,000 and 165,000krn (63,000 to 103,000 miles). He believed the main reason for the fairly wide variation between the worst and the best case was the different driving techniques of BIS's drivers who generally do not change from vehicle to vehicle.
Compared to the previously used XZY tyres, the XZTs' trek .life in the BIS fleet is around : per cent longer. Drivers report considerably better grip fro the XZT compared to the XZ but then the XZY is an on/o road tyre whereas the ZT is d signed purely for on-road use.
The ZT's bold tread patte with its larger contact patch responsible for the tyre's bett adhesion, but Michelin was u able to tell us how much bigg the contact patch is compared ZA, ZY or ZZ.
Roy Ashton at first treat with some suspicion the idea a tyre intended for drive axl only, because of the extra ca that would be needed in ty monitoring. But BIS has run . most exclusively on Miche since 1968 so there was no coi plication of different tyre mak as well as tyres.
Moreover, the company ready kept detailed tyre recorc on individual tyre record car supplied by Michelin, and t extra tread mileage of the XZmore than compensated for a additional man/hours spent them. Like the rest of 1 Michelin truck tyre range, t XZT can be regrooved and moulded (or Remixed if SE back to Michelin).
Initially, the only size availat is 11-22.5 tubeless with 12-2: to follow soon. BIS has us tubeless tyres exclusively sin 1968, and like many fleets h had its share of problems w wheels cracking. "But this iE much more rare occurren now," Roy Ashton told me.
Avon and Michelin have ea identified a need for a speci purpose tyre for commerc vehicles, Avon's for steered les and Michelin's for drive les, and there is no reason doubt that in each case the ni tyres will perform well. But best use such tyres must be ET nitored to ensure that they fitted in the correct position.
How many operators c afford the time and manpov to keep such a close check tyres? Or are they prepared sacrifice a few thousand mile: tread life for a tyre that could fitted anywhere?
Even the most carefully ma tamed tyre policy can disrupted both by a puncture ( in the country and a tyre dis butor who can't (or won't) fit particular type of tyre request,
Many operators will prepared to try a few Steerm ters and XZTs in their fleetE and keep a close check on thE If they perform as well as manufacturers claim, perN there will be a more gene acceptance of the idea of cert tyres for certain axles.