Factors that Affect Tyre Life.
Ambient temperature (seasonal and climatic conditions).
The road surface.
The route (winding roads, steep inclines and down-hills).
The journey or nature of work.
The ambient temperature does not affect the heat generated by the tyre but it does affect the way that heat is dissipated. Heat, we will learn, is the one great enemy of tyres and in this situation, the hotter a tyre runs, the greater the tread wear.
Water acts as a lubricant to rubber and so tyre wear will decrease in wet conditions but the likelihood of punctures increases because rubber cuts more easily when it is wet.
The Road Surface.
Generally, we travel on three basic types of road surfaces:-
Tarred roads are made with varying sizes of stone and are generally less abrasive than concrete.
Concrete roads are generally more abrasive than tarred roads but do not tend to develop potholes which can lead to shock ruptures, which we will study closer later.
Gravel roads vary greatly from being extremely smooth to extremely rough and will affect wear accordingly.
Certain older roads are severely cambered and this can also cause tyre wear because it is necessary to steer against the natural pull of the vehicle. This in turn can change suspension angles, particularly the cambers which are tyre wearing angles.
On straight level roads far fewer demands are made on the tyres when compared to winding mountain passes, for instance.
Tyres will wear much faster when they are asked to brake, change direction suddenly and accelerate continuously, even when speeds are relatively low.
The Journey or Nature of Work.
Long, high-speed journeys give tyres little or no time to cool down and so running permanently hot will decrease tyre life.
Town driving, with its constant braking and accelerating will also decrease tyre life.
Inflation pressures and load.
Tyre and vehicle maintenance.
Some drivers are able to conserve their tyres much better than others. Factors such as high speed driving, rapid cornering, sudden braking and rapid acceleration are all major contributors to increasing tyre wear.
Inflation Pressures and Load.
These two factors are one and the same thing to a tyre. Both affect the flexing or deflection of the sidewall. An overloaded tyre causes the sidewall to deflect in the same way that it would if it were under-inflated. As the speed of the vehicle increases, so does the frequency of the sidewall deflection, which in turn causes an increase in the heat generated by that deflection. Tyre failure will almost always result if this is allowed to continue for too long. The time it takes for the tyre to fail depends on the amount of deflection, speed and distance travelled.
The inflation pressure also influences the way in which the tread makes contact with the ground, and the relationship between the tyre and the rim. Both these factors can increase uneven (and therefore, accelerated) tyre wear due to the tread not meeting the surface of the road across its full area.
It is important to fit the tyre best suited to the work being performed by the vehicle. This becomes more important in the trucking industry but applies equally to L.D.V.’s and S.U.V.’s. Tyres with lug-type tread patterns will wear more quickly on tarred roads at high speeds than tyres with tread patterns better suited to “on-road” work.
Fitting a tyre that is either too wide or too narrow for a particular rim will cause the tyre to behave in a way in which it was not designed. A phenomenon known as “rollover” occurs when the tyre is too wide for the rim. Weight transfer, particularly in cornering, causes the tyre to roll over onto its shoulder, causing increased wear in that area.
Rollover wear was commonly found on the older cross-ply and textile-belted radial tyres where it was often mistaken as “under-inflation” wear. There are still many tyre charts showing the classic, “if the outside shoulders are worn it means under-inflation and, if the centre is worn it means over-inflation” Modern, steel-belted tyres are affected differently by inflation and this will be looked at more closely under that section.
Checking inflation pressures, at least twice a month, is recommended and cannot be avoided by the use of nitrogen or sealants.
Wheel alignment and suspension settings should be checked between ten and twenty thousand kilometres or when one suspects that a pothole or bad bump might have caused a change to these settings.
Abnormal Rapid Tyre Wear .
We have seen how outside factors can influence tyre wear, some unavoidable and some, avoidable. Let us take a closer look at identifying and rectifying avoidable rapid tyre wear.
Mechanical irregularities such as incorrect or excessive suspension settings, excessive wear in suspension components and a faulty steering or brake system are all likely to cause uneven or rapid tread wear.
Of these factors, those that cause incorrect toe angles are responsible for the most accelerated mechanical wear.
A vehicle that has a total front toe reading of + 4mm means that each front wheel is “toeing-in” by 2mm. With every revolution, each tyre is "dragged" inwards or across the direction in which the vehicle is travelling by 2mm.
This might not sound too impressive until one realises that a 205/55R16 tyre rotates 513 times per kilometre. On a journey of 100 kilometres the following would occur:-
||revs per kilometre
||revs per 100 kilometres
|| sideways drag
|| 102 600
|| mm = 102.6 m sideways drag.
The effect of a 2mm excess individual toe is the same as dragging the vehicle 102.6 metres sideways for every 100 kilometres travelled! The resultant "feathering" is easy to understand when looked at this way.
Toe wear is generally characterised by one tyre being more affected than the other. In countries where traffic drives on the left it has been noted that excessive toe-in causes the front left tyre to wear more rapidly on the outside shoulder. The opposite applies to countries where traffic drives on the right-hand side of the road.
Too much toe-out causes the right front, inside shoulder to be particularly affected.
Advice: Check and correct the alignment. The adjustment should be made in accordance with the vehicle manufacturer's recommendations and the type of tyre wear.
Front and Rear Tyres.
Sloped wear is commonly caused by excessive positive or negative wheel camber. This condition is often exaggerated by the tyre being under-inflated or the fitment of incorrectly rated tyres with less strength or "stiffness" in their sidewalls.
It is defined as positive when "the top of the tyre leans away from the centre of the vehicle". This means that the outside shoulder of the tyre carries a bigger load than the inside shoulder. Excessive wear will take place on the outside shoulder.
Negative camber is described as “when the top of the tyre leans towards the inside of the vehicle". In this instance the inner shoulder is working harder than the outside and will therefore wear at a faster rate.
A negative camber of between one quarter and half a degree does not cause excessive wear but enhances road holding on most modern vehicles with independent suspensions.
Camber wear leaves the tyre surface “smooth” and is noticeably different from the distinctive rough or "feathered" wear caused by excessive toe.
Advice. Adjust the cambers in accordance with the vehicle manufacturer's recommendations and the way the tyre is wearing.
Fit the correctly rated tyres.
Replace bent parts.
This wear pattern is particularly common on the rear axle of front wheel drive vehicles.
The study, “Diagonal Wear Predicted by A Simple Model”, undertaken by W.K. Shepard, the senior scientist at Michelin Americas Research & Development Corporation in June 1985, states the following:-
“‘Diagonal Wear’...is characterised by a series of flat spots on the tread, running diagonally from one shoulder to the other. Diagonal wear is often found on both sides of the affected car with the diagonal pattern running in opposite sides on the left and right of the car.
Data from the field indicate that the alignment of the rear suspension is a major factor in the generation of diagonal wear. Toe and camber settings on vehicles with diagonal wear are often found to be excessive. The orientation of the diagonals also changes depending on whether the suspension is set to toe-in or toe-out. In addition, there appears to be a correlation between diagonal wear and driving habits. Freeway driving at constant speeds tends to worsen the problem”.
The study found that readings in excess of 3 mm total toe, and one-and-a-half degrees camber were enough to initiate diagonal wear.
Advice. Adjust alignment where possible. Interchange "front-to-back" and "back-to-front" at regular intervals.
Heel and Toe Wear.
This type of wear occurs mainly on tyres with block-type tread patterns. The leading edge of each tread block is sharply defined with the trailing edge excessively worn. It is most commonly found on the undriven axle.
Advice. Change tyres between driven and undriven axles, or turn the tyre on the rim.
(Shoulder wear). It is no longer acceptable to simply ascribe this type of wear to under inflation. There are a number of different factors to take into account when inspecting a tyre showing domed/shoulder wear.
- Construction type. Domed wear is particularly common on textile-belted tyres. This is due to the inability of the bracing to keep the tyre's footprint firmly in contact with the road surface during cornering. The tyre "rolls" onto its shoulder, even during low speed cornering. This phenomenon is known as "rollover".
- Apart from abnormal rapid wear taking place, it should also be noted that cornering ability and therefore safety, is adversely affected.
- On steel-belted tyres, domed wear is fairly commonly found on Minibus Taxis, the effect of overloading being the same as under- inflation. Most vehicle owners are aware of correct inflation pressures and domed wear caused by under- inflation is no longer very common.
- Mechanical wear cannot cause a tyre to wear on both shoulders simultaneously. However, in a case where tyre positions have been changed, the combined factors of both positions could cause domed wear.
- A rare instance of domed wear could be caused by excessive castor angles. When a vehicle with large castor angles turns, the tyres lean towards the side of the turn - this is known as "camber roll". It would require a vehicle with large castor angles, a Mercedes Benz for instance, to travel an extremely windy road frequently, to cause this type of wear.
Advice. Recommend the fitment of steel-belted tyres, avoid overloading/under- inflation and correct misalignment.
The most common cause of centre wear is not overinflation, particularly in steel-belted tyres. Centre wear is found mainly on the rear axle of powerful rear wheel drive vehicles. Although not conclusive, it appears that a factor such as centrifugal force plays a role in the cause of centre wear. Centre wear is very common on powerful sports cars with wide tyres.
Advice. Move tyres from "front-to-back" and "back-to-front" at regular intervals, shorter intervals for powerful, heavy vehicles (where front and rear sizes are the same).
Wavy, lumpy or scooped wear can be caused by "floating" of the wheels caused by faulty adjustment or play in the suspension or steering. Imbalances in the rotating assembly (wheel, tyre or hub) might also cause irregular wear.
Advice. Have the suspension, steering, brakes and wheel balance checked. Switch the tyres around to avoid aggravating the wear.
This area of wear appears smooth or the rubber may be rasped and torn, also there may be a slight flat with scratch marks in the direction of rotation.
Brake flatting can be caused either by an irregularity in the braking system such as an oval brake drum or by the wheels locking under heavy or emergency braking.
Advice. Check and adjust the braking system and avoid locking the brakes.
Tread Hacking.(“Peppering” )
This is caused by running on stony surfaces, roads or tracks in poor condition and off-road conditions. This type of wear is very common in the rural areas of South Africa. Wet conditions and over-inflation make tyres more susceptible to cuts and “multi-penetrations” resulting in the elements affecting the belts adversely. The rusting of the steel belt often leads to tread separations.
Advice. Use inflation pressures adapted to the conditions of use by avoiding over-inflation. Use the tyre which is suited to the conditions of use and always remove embedded objects in the tread to avoid worsening the damage. Have the tyre repaired if necessary.
Circumferential Tread Scoring. (Obstruction cut).
This may be caused by the fact that the tread is rubbing against or is periodically in contact with a fixed part of the vehicle.
Contact may only occur under conditions of heavy load, on bumpy roads or when cornering. This might be due to:-
- the fitment of pirate alloy wheels with incorrect offsets;
- bolts, screws or fittings which are too long or have become displaced;
- bent mudguards;
- insufficient clearance between the vehicle and the tyre under a heavy load;
- weak or sagging springs;
- wheelspin on a sharp object.
Advice. Check clearances under a full load and on full lock. Ensure that after market rims have approved offsets and other dimensions.