TECH TORQUE

4 x 4 Australia - - Contents - FRASER STRONACH

SPARE a thought for your 4x4’s tyres, if you will. They de­serve it. At open-road speeds – say 110km/h – the stan­dard tyre fit­ted to some­thing like a new Ford Ranger or Toy­ota Hilux ro­tates at 12.5 times per sec­ond. That’s 753 face-to-face ‘meet­ings’ for any point in the tyre’s contact patch with the road’s sur­face in ev­ery minute, or more than 45,000 ‘con­tacts’ ev­ery hour. Drive your 4x4 for just six hours and that’s more than 250,000 ‘wear events’ your tyre has to cope with.

If that’s not bad enough, much of the time your 4x4 is not just cruis­ing along in a straight line but is ac­cel­er­at­ing, brak­ing or cor­ner­ing, which puts lon­gi­tu­di­nal and lat­eral loads on the tyres, cre­at­ing even more fric­tion and wear. The fact 4x4s are gen­er­ally big­ger and heav­ier than nor­mal pas­sen­ger cars doesn’t help ei­ther, nor does the fact 4x4s are of­ten heav­ily loaded, which gives the tyres even more work to do.

On top of that, 4x4 tyres have to cope with road-sur­face tem­per­a­tures in high sum­mer in ex­cess of 60°C with­out melt­ing, and tem­per­a­tures in win­ter of be­low zero in many parts with­out be­com­ing hard and pos­si­bly brit­tle. All this is, of course, be­fore you head off-road, where 4x4 tyres then face a whole new world of tor­ture as they bat­tle with sharp rocks and tree roots all wait­ing to pierce a hole in the tread or, worse still, tear open a side­wall.

Most peo­ple prob­a­bly think en­gine com­po­nents that ro­tate or re­cip­ro­cate at high speeds, most no­tably pis­tons and big ends, are the hard­est-work­ing parts of a 4x4, but it’s a close con­test be­tween these en­gine com­po­nents and tyres. And while tyres may look pretty sim­ple and ba­sic com­pared to mod­ern pow­er­train com­po­nents, they hide an amaz­ing ar­ray of very so­phis­ti­cated and ever-evolv­ing tech­nolo­gies.

Given the vast ar­ray of tyres on the mar­ket, de­cid­ing on re­place­ment tyres for your 4x4 can be a daunt­ing task. While most peo­ple seem to be most con­cerned with tread pat­tern, it’s just one of a num­ber of fac­tors to con­sider and one that tends to sort it­self out any­way. Af­ter all, a HT (High­way Ter­rain) tyre isn’t much use on a 4x4 if you want to go off-road, and MT (Mud Ter­rain) tyres aren’t prac­ti­cal if you spend a lot of time on the road; given they are noisy, wear quickly and of­ten don’t pro­vide much grip on wet bi­tu­men. All of which leaves AT (All Ter­rain) tyres as the only prac­ti­cal choice for most do-itall ap­pli­ca­tions. It’s worth not­ing that most tyre man­u­fac­tur­ers will rate AT tyres in terms of the bal­ance be­tween on- and off-road per­for­mance (gen­er­ally as a per­cent­age), so you can match this to your us­age pat­tern.

Be­fore you buy new tyres for your 4x4, it’s worth check­ing to see if you can fit smaller di­am­e­ter wheels from a low­er­spec model in the same range. A smaller wheel means a taller tyre side­wall (so as to main­tain the same over­all rolling di­am­e­ter) and a taller side­wall brings a num­ber of sig­nif­i­cant ben­e­fits in­clud­ing a more com­fort­able ride, bet­ter dam­age re­sis­tance off-road, bet­ter air-down per­for­mance, and a gen­er­ally cheaper pur­chase price. What you lose com­pared to a lower pro­file tyre (bet­ter on-road steer­ing re­sponse and turn-in) is not worth wor­ry­ing about.

It’s also worth look­ing for a tyre with a lower speed rat­ing than the orig­i­nal tyre. While it might seem il­log­i­cal, tyres with a higher speed rat­ing are more lightly built than tyres with a lower speed rat­ing. That’s be­cause a lighter tyre doesn’t build up as much heat at higher speeds as a heav­ier tyre, and heat is a tyre killer (see: ‘Speed and Load Rat­ings’ break­out).

For road-go­ing and non-lifted 4x4s you’ll need to stick rea­son­ably close to the OEM tyre size (for le­gal and clear­ance/foul­ing is­sues), but note that tyres with dif­fer­ent size des­ig­na­tions can still have the same rolling di­am­e­ter. A 235/65R17, for ex­am­ple, has the same rolling di­am­e­ter as a 255/60R17, even if the lat­ter is nom­i­nally 20mm wider. Just re­mem­ber that with any given tyre size there will in­vari­ably be slight vari­a­tions in the ac­tual tyre size from one man­u­fac­turer to an­other.

Last but cer­tainly not least is the is­sue of the con­struc­tion of the tyre’s car­cass, which can ei­ther be des­ig­nated as ei­ther Light Truck or Pas­sen­ger. Light Truck tyres will al­ways have an LT in the size des­ig­na­tion, while Pas­sen­ger tyres will have ei­ther a P or noth­ing. All Ter­rain tyres are avail­able in both these types. The dif­fer­ence is that Light Truck tyres have a more ro­bust car­cass and are more prac­ti­cal for off-road use and re­mote-area tour­ing. The only down­side is that Light Truck tyres gen­er­ally don’t ride, steer or han­dle as well as Pas­sen­ger tyres.

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