Monday, July 13, 2009

Wheel meat again

The trouble with tyres is that they’re expensive and they wear out. In the harsh Arabian climate, tyres don’t last until the tread is down to its last 1.6mm. Heat and abuse see to it that the sidewalls crack or the tread separates from a tyre’s structure well before then. This is one reason why a vehicle presenting for registration with tyres more than four years old will automatically fail. It is, apparently, illegal to drive on the UAE’s roads with tyres more than five years old, so there has to be at least one year’s life remaining on the inspection day. The date of manufacture is embossed on the sidewall, along with other stuff including the size, temperature and speed ratings, plus lots of additional guff about not fitting the tyre on the wrong size of wheel rim.

Gulf News ran this story about the dangers of cheap, low-quality, expired or part-worn tyres. It is probably true that your average punter can’t tell the difference between a decent tyre with partly-worn tread and one that’s about to fall to pieces having been repeatedly run flat or bounced against kerbs. But where do you draw the line? If buying or using pre-owned tyres is illegal or at least inadvisable, then every rental car and certainly every second-hand car must have all its tyres replaced immediately there is a change of driver. And I can see that happening only in my wildest fantasies.

The biggest tyre scandal in my opinion is dodgy operators allegedly renting out a set of decent tyres just so that some villain, heedless of his own mortality, can get a vehicle past inspection, after which the old knackers are put back on. Do goods fleet operators keep a spare set of wheels for their trucks solely for registration purposes? Next time you’re at the traffic lights behind an artic, take a look at the treadless racing slicks with canvas or wires poking through, and draw your own conclusions.

“ ’Ello, ’ello, ’ello. Is this your vehicle, sir?”

I don’t appreciate unexpected blow-outs, so I recently replaced all five of the Goatmobile’s boots. After 46,000km of asphalt, gravel tracks and dune driving, there was plenty of tread left on the Cooper Discoverer H/Ts, but running deflated for off-road use had helped to crack the sidewalls. Several puncture repairs in each tyre finally drove me to buy replacements. The previous set, Yokohama Geolandar G901s lasted around 60,000km, but these really were knackered when I finally replaced them. The new boots are a set of Cooper Discoverer ATRs. The tread is a bit more aggressive than the H/Ts, but I didn’t really want anything too gnarly and knobbly. My friend over at the Yellow Box of Doom went out and bought STT tyres, which have so much tread they’d not look amiss on the back of the Mississippi Queen. But aggressive tread is extremely noisy on asphalt.

The standard tyre size for the Goatmobile is 265/65R17. I’ve been using slightly oversize 275/65R17, but I’ve now decided to try 265/70R17. The additional height of the latest tyres increases the circumference by about 3.4%, the practical consequence of which is the speedometer is now accurate instead of being optimistic. I was briefly tempted by the 285/70R17 but I was put off by the prodigious cost.

Speaking of cost, Renaissance Tyres (N25°16'26" E055°20'47") is the Cooper distributor and offers good service and the best price I’ve found. It’s cash only, and fitting and balancing is done for no additional charge at Radial Tyres (N25°16'19" E055°20'00"). About five years ago I gave up on trying to save a couple of additional dirhams by patronising the dodgy Sharjah tyre emporia. I was sold a set of old-stock that were ‘impossible to balance’, and it took a trip to Tyre Express to get the balancing done so that my old Landy didn’t shake to pieces at 80kph.

Concerning the fashion for nitrogen fills, I think the idea of inflating tyres with an inert gas instead of air has its merits, especially for high-performance vehicles like Airbus A340s, but I’m sceptical about the value for a normal, bog-standard street car. The benefits are supposed to include reduced oxidation and destruction of the tyre from the inside, no corrosion of the rim, and reduced temperature effects. Of course, if there’s no oxygen present, it can’t attack the tyre or the rim. Frankly, I suspect most of the benefits come from using processed, dry gas instead of ordinary boring old air, sucked from the humid atmosphere and blown into the tyre along with water vapour, carbon dioxide, bus drivers’ farts and small insects. And as for supposedly not expanding when it gets hot, I wonder which part of the gas law (PV/T = k) does not apply to nitrogen?

I was flicking through a motoring magazine while waiting for my new tyres to be fitted. Hilariously, some luminary stated in writing that, whilst 100% nitrogen wasn’t warranted for most vehicles, 30% nitrogen was recommended. Does he not know that air is already 80% nitrogen? How is he proposing to remove the additional 50%, and what is he going to insert in its place? Does he also not comprehend that the benefits of a nitrogen fill are only realised if oxygen and water are completely eliminated from the fill?

One final thought. In A View to a Kill, James Bond (Roger Moore) evades the Legions of Darkness by hiding underwater in a car wreck, breathing the contents of the spare tyre. I’d not recommend trying this with a nitrogen fill.



the real nick said...

What a coincidence. I just replaced the tyres on the Cherokee, with Coopers, because of the impending re-registration. Otherwise I'm sure the wife could have safely driven the 500 metres to Spinneys and back for another five years with the same old rubbers...
And then I needed a new battery too, because it seems the climate here kills them off after about two years. Just like *click* that.

R Bailey said...

30 % Nitrogen

10/60 will do nicely, however not sure what the helium would do.

Maybe the same "expert" who recomends 30%N2 will claim it reduces unsprung mass !


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