We asked an ex­pert to an­swer some tyre ques­tions

Go! Camp & Drive - - Contents -

What im­por­tan­ceis the of tak­ing tem­per­a­ture into ac­count when mea­sur­ing a tyre’s pres­sure, es­pe­cially at the ex­treme ends of the pres­sure spec­trum?

Com­pressed air con­tains wa­ter vapour. This has a higher vari­a­tion to pres­sure changes due to the ex­pan­sion in vol­ume when it heats up. For this rea­son, it’s ad­vis­able to check your tyre pres­sure when the tyre is cold and has not been sub­jected to the ris­ing am­bi­ent ef­fect of the sun or the heat gen­er­ated by the fric­tion from driv­ing. This is even more crit­i­cal at the top end of the in­fla­tion spec­trum – the tyre man­u­fac­turer has a max in­fla­tion pres­sure rat­ing on the side­wall of the tyre that must be ad­hered to.

Why do tyres lose pres­sure by them­selves and how of­ten should you check your ve­hi­cle’s tyres?

Tyres lose pres­sure due to a num­ber of rea­sons: leak­ing valve stems, slow punc­tures, poor fit­ment, im­pact breaks and os­mo­sis, which is when the air mol­e­cules pass through the in­ner liner of the tyre over time. It’s pos­si­ble to lose 1 to 3 PSI per month de­pend­ing on the type/ qual­ity of the tyre in­ner-liner com­pound and thick­ness. For most types of driv­ing a weekly tyre-pres­sure check is ideal, but ev­ery two weeks is ad­e­quate.

Why is it nec­es­sary to ro­tate your tyres? And if you do, how do you ro­tate them?

The rotation of tyres al­lows for a more con­sis­tent and even wear pat­tern over all four of your car’s tyres. Front tyres tend to have a higher wear rate on the shoul­der (par­tic­u­larly in front-wheeldrive ve­hi­cles) Rotation will cer­tainly ex­tend tyre life. Ideally the tyres should be ro­tated from front to back or vice versa. If the tyre is one di­rec­tional, ie. it can only ro­tate in one di­rec­tion, then it can only be moved for­wards or back­wards and not to the other side of the same axle.

Any other sug­ges­tions to ex­tend your tyre’s life?

An im­por­tant el­e­ment in tyre life ex­ten­sion is to en­sure that your ve­hi­cle’s wheel align­ment is cor­rect – this will pre­vent un­nec­es­sary scrub­bing of the tyres from toe-in or toe-out. This, to­gether with cor­rect in­fla­tion and rotation, will give your tyres the best chance at op­ti­mal life span.

How far can driv­ers safely de­vi­ate from rec­om­mended pres­sures, and for how long a pe­riod or at what speeds (in km/h) be­fore run­ning into prob­lems? Peo­ple of­ten do week-long trips in the Namib at pres­sures of be­tween 0.7 and 1.0 bar. Is this safe?

The rec­om­mended pres­sure is spec­i­fied to sup­port the max­i­mum load-car­ry­ing ca­pac­ity of the ve­hi­cle. A tyre’s pres­sure should only be de­flated for the pe­riod of time that it is op­er­at­ing in that en­vi­ron­ment, i.e. when in sand or mud de­fla­tion is ap­pro­pri­ate to pro­vide bet­ter trac­tion due to a wider foot­print. Once the ve­hi­cle leaves this ter­rain and moves onto a harder sur­face such as graded dirt or tar­mac, the tyres must be in­flated as soon as pos­si­ble to avoid dam­age. As the speed of the ve­hi­cle in­creases, so does heat, and by not in­flat­ing the tyre cor­rectly, pre­ma­ture wear or even fail­ure can oc­cur.

If you want to buy tyres for your 4x4 that are more suited for off-road­ing than the stan­dard HT tyres, what should make the dif­fer­ence when de­cid­ing be­tween Al­lTer­rains and Mud-Ter­rains? When de­cid­ing on what tyre to pur­chase, one must weigh up the pros and cons that are rel­e­vant to the ap­pli­ca­tion of the tyre. Let’s look at some spe­cific per­for­mance cri­te­ria. For AT com­pared to MT: • Tread blocks are more closely spaced and more rub­ber is in con­tact with the road sur­face. • The AT tyre has more sipes which lets the blocks flex with ease for im­proved grip. • AT tyres will pro­vide bet­ter high­way/ road per­for­mance due to its 50:50 char­ac­ter­is­tics vs a MT tyre which is typ­i­cally an 80:20 tyre (80% of­froad us­age) • The MT has big­ger gaps be­tween the tread blocks and a smaller solid to void ra­tio (solid = rub­ber and void = gaps) which aids in claw­ing on loose sur­faces and dig­ging in mud. • The con­struc­tion of the MT, which is typ­i­cally a 3-ply side­wall, makes for a less com­pli­ant/harder ride – this will be more no­tice­able on the tar when your pres­sures are higher. Is one spare wheel enough when go­ing on an over­land trip? Any sug­ges­tions on re­pair equip­ment to get by un­til you reach “civil­i­sa­tion” again? Apart from plugs, what about gaiters? What do you rec­om­mend keep­ing in mind from a safety point of view when hav­ing to re­sort to such emer­gency re­pairs? For rel­a­tively short over­land trips, one spare tyre is usu­ally suf­fi­cient when used in com­bi­na­tion with a good qual­ity tyre sealant, com­pres­sor, and tyre re­pair kit. How­ever, if the ter­rain is known to be par­tic­u­larly tor­tu­ous and you’re rea­son­ably far from civil­i­sa­tion, then its best to carry an ad­di­tional spare tyre. Prac­ti­cally, plugs and gaiters are some­times nec­es­sary in emer­gency sit­u­a­tions, but we do not rec­om­mend them for ex­tended use, i.e. if the cut is on the shoul­der or side­wall, the tyre should be re­placed at the ear­li­est pos­si­ble op­por­tu­nity. What is the most im­por­tant char­ac­ter­is­tic to you: grip or tyre life? And why is it that these two are of­ten mu­tu­ally ex­clu­sive? One needs to un­der­stand the ap­pli­ca­tion and ve­hi­cle type prior to mak­ing this de­ci­sion. Tyres with ex­cel­lent grip char­ac­ter­is­tics gen­er­ally wear quicker due to mi­cro­scopic par­ti­cles that get torn off un­der ac­cel­er­a­tion/brak­ing/cor­ner­ing. Con­versely, tyres with ex­cel­lent tread wear char­ac­ter­is­tics min­imise rub­ber be­ing re­moved. Bear in mind that op­er­at­ing con­di­tions are also im­por­tant, in other words does your ve­hi­cle spend most of its time driv­ing in hot/dry con­di­tions or wet/cold? These are also im­por­tant de­ter­min­ing fac­tors for which type of tyre would suit you best. If All-Ter­rain­sy­our ve­hi­cle (not is to fit­ted men­tion with Mud-Ter­rains), how should you change your driv­ing style or what do you need to keep in mind when driv­ing on tar, es­pe­cially in wet weather? A 4x4 SUV or bakkie fit­ted with HT­type tyres is the ideal so­lu­tion for good road-go­ing abil­i­ties. Once you’ve changed from HT to AT or MT-pat­tern tyres, you should be aware that you’re com­pro­mis­ing the road abil­ity of your ve­hi­cle. To en­sure that this com­pro­mise is less no­tice­able, you should do the fol­low­ing: • In­crease your fol­low­ing dis­tances – your brak­ing dis­tances will in­crease in the dry, but even more so in the wet. • En­sure the speed rat­ing is equal or higher than the orig­i­nal tyre and that the tyre is suited to max­i­mum ve­hi­cle load. • Lower your speed – your ve­hi­cle will not cor­ner as quickly or pre­dictably as with the HT-pat­tern.

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