With few exceptions tyre size markings are standardized throughout the world. Although millimeters and inches are used (even combined in the case of radials) they no longer relate exactly to the dimensions of the tyre, but serve rather to identify the type of tyre involved and to give an approximation of its dimensions. The tyre markings enable one to know the type of tyre involved. This is important because it is detrimental to mix types i.e. SR steel radial with HR steel radial.
Taking the 175 SR 14, the R indicates a radial ply tyre, S the speed rating, the 175 its cross section in millimeters, and the 14 is the rim diameter in inches. As no reference is made to aspect ratio it will be an 80 service tyre.
91 – is the LOAD INDEX which is an internationally recognized numerical code associated with the maximum load a tyre can carry at the speed indicated by its SPEED SYMBOL under service conditions specified by the tyre manufacturer (see Load Index Chart on Page 8).
H – is the SPEED SYMBOL i.e. tyre is capable of speeds up to and including 210 km/h.
HEAT IS THE KILLER
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| Mounted on a test machine, this tyre running at only 30% underinflation at 200km/h dramatically demonstrates the stress caused by the deflection ripple or standing wave. |
Motorists must remember that a correctly inflated tyre will generate heat while traveling, but this temperature will eventually level off well within the critical design temperatures of the tyre.
An underinflated tyre, with its sidewall deflection greatly increased will, as it flexes, generate more heat than normal in the rubber. The temperature will begin to creep up until it passes the critical running temperature of the components. As the heat level continues to rise, the physical properties of the compounds start to deteriorate. Runaway temperature generation can cause a vicious circle of destruction.
Underinflation also increases shoulder wear. In cornering, the slip angles is increased and heavier steering results. Under braking, acceleration and cornering the car will be unstable, while grip is radically lowered.
Check your tyre pressure when tyres are cold. Do this every time you fill up your tank and ensure that at all times they are inflated to the pressures recommended by the tyre manufacturer for the vehicle to which they are fitted and for the loads and speeds contemplated.
Never let air out of hot tyre as a pressure build-up of around 20% is normal on a long journey.
Apart from the very real dangers of running with underinflated tyres, it is a costly mistake.
A tyre underinflated by 20% will lose 10 to 15% of its tread life, while a tyre underinflated by 30% will more than likely fail through component separation before it has completed 50% of its life. A tyre which is losing air through a puncture or leaking valve, and has deflated to around 20% of its correct pressure (that is approximately 50kPa), will fail within 20 kilometres at normal speeds.
A final point on underinflation. Apart from ruining a good set of tyre, extended underinflated running can increase your car’s fuel consumption by up to 10%.
When setting off on a long journey, especially when going on holiday, increase your pressure by 30 kPa above normal to counter the extra load and speed which increases tyre deflection, resulting in rapid wear through excessive heat build up.
When Replacing Tyres
POINTS TO CONSIDER
1. MIXING VARIOUS SPEED RATED TYRES
With the proliferation of car tyre sizes and makes of tyre in the Southern African market, the same size tyre with various speed ratings is readily available.
Many people are of the opinion that with our open road speed restrictions, they do not require a tyre capable of 210 km/h (H rated) or higher fitted as origin equipment to the vehicle. Purely from a speed point of view, this may be so, and a ‘T’ rated tyre (190 km/h) may be more than adequate, but other aspects such as the tyre’s construction and rubber compounding must be taken into consideration.
The higher the speed rating, the more emphasis is placed on road holding, steering response, stability and high speed safety. It is for these reasons that when tyres with a lower speed rating or a budget equivalent are fitted, the driver sometimes feels that the car no longer handles as it did previously.
Note: if for some reason 2 tyres of a lower speed rating are fitted, they must be fitted to the steering axle. The same reasons apply, i.e. TR tyres have a higher slip angle than HR tyres.
2. DO NOT BUY ‘DOWN’
From a safety point of view and to comply with insurance requirements, high speed steel radial tyres (HR, VR, ZR) fitted as original equipment must not be replaced with lower speed rated steel tyres (TR, SR) and definitely not with textile radial tyres.
3. DON’T FORGET THE SPARE TYRE
Particular attention must be paid to spare tyre tyres. Old or aged spare tyres should be used with caution, and replaced as soon as possible.
When replacing tyres, remember to fit the spare tyre onto the car. This ensures that the tyre will be used within a few years of manufacture.
It must be borne in mind that a tyre that has not been on the road and has spent 5 years or more in the boot of the car may have the appearance of new tyre. In actual fact, from a manufacturing point of view, the tyre is 5 years old. Anti-oxidants and anti-zonants which tyre manufactures build into tyres to slow down ageing, gradually migrate to the tyre’s surface. This finally results in the rubber compound oxidizing, thereby losing its protective capabilities. The same applies to the various compounds used in the process of bonding the rubber to the steelcord. In both cases degradation takes place, irrespective of whether the tyre is being used or not.
Obviously, using the spare tyre when replacing tyres does not apply to vehicles which have space-saver tyre spares, but it must be borne in mind that space-saver tyres have a restrictive speed which must be adhered to (see the sidewall of the tyre for details).
4. NEW TYRES TO THE REAR
Many motorists and tyre outlets are of the opinion that, when only 2 tyres are replaced on a car, they should be fitted to the front axle. This is not correct.
The following extract from a recent statement published by the British Rubber Manufacturers Association (BRMA) clarifies this matter.
An analysis of technical enquiries received by BRMA members has confirmed the belief that many customers – dealers and users alike – are of the opinion that, when only two tyres are replaced on a car, they should be fitted to the front axle. This was undoubtedly the case a number of years age when it was felt that the retention of vehicle control was more critical with front wheel deflations and, therefore, that the new, and thus less vulnerable tyres, should be fitted to the front.
However, the current recommendation on the part of the Tyre Industry is, as a general rule: NEW TYRES TO THE REAR. This applies to both front and rear wheel drive cars.
Primarily, the justification is increased safety, particularly in the wet, where it has been demonstrated that, with the partly worn tyres fitted to the rear, their diminished water dispersal capability leads to a greater tendency towards oversteer and thus loss of control. Similarly, straight line braking in the wet can be adversely affected. It is also arguable that used tyres are more prone to punctures and since it has now been established that rear deflations are more likely to cause loss of control, this is another reason for fitting new tyres to the rear.
As a secondary benefit for the owners of low annual mileage front wheel drive cars, the switching of the partly worn rear tyres to the front, enabling the new tyres to be fitted to the rear, creates a cycle which helps prevent their deterioration due to ageing/prolonged exposure. The rear tyres wear relatively slowly and leaving them in their original positions for a prolonged period, can result in the need to replace them before they are significantly worn.
Note: There are certain circumstances where the principle of NEW TYRES TO THE REAR does not or may not apply. For example:
- Where the front and rear tyre sizes are different.
- Where the two new tyres are of a lower speed rating.
- Where the characteristic rear concave wear pattern has developed to an extent that moving rear tyres to the front would affect the stability of the car.
- Where the “system” concept of directional front and asymmetric rear tyres is applicable.
- With certain combinations of winter and summer tyres.
- On certain four wheel drive vehicles where it would result in significant differences in tread depth.
If in doubt please contact the tyre manufacturer.
5. FIT NEW TUBELESS VALVES WITH NEW TYRES
One of the main components of the tubeless valve is rubber, which will, as in the case of tyres, deteriorate (perish) over a period of time. This is due to exposure to the sun, ozone, solvents etc. for added safety; replace the tubeless valve when fitting new tyres. Remember, the main cause of sudden tyre failure is dangerously low inflation pressure.
6. VALVE CAPS
These very important items keep dirt and moisture out of the valve and prevent air leaking from the valve if the valve core is faulty.
7. BALANCING TYRE/RIM
The tyre/rim assembly is usually balanced when tyres are being replaced.
This procedure is vital to avoid unnecessary vibration and irregular wear on tyres fitted to cars, bakkies, caravans and trailers.
Wheel Alignment
IMPORTANT POINTS
1. EXTEND TYRE LIFE EXTEND TYRE LIFE
It has happened to almost everyone:
You buy a set of tyres, and before long one or two tyres are wearing out before the others. On today’s cars, this applies to both rear and front types. The most common cause of unusual tyre wear is incorrect alignment.
2.SPOT TYRE PROBLEMS EARLY
Regular (once a month) inspection of the tyres will increase the chances of spotting any alignment or mechanical wear early. By doing regular checks, small problems can be resolved before they become big, costly ones (see Rotation on page 27).
3. ENSURE SAFE DRIVING
Should the rear axle be misaligned (see figure on page 18) it will try to take over the steering function, causing the driver to correct with left hand or right hand down. This causes irregular and rapid tread wear on the front tyres and can also lead to vibration. A periodic four-wheel alignment check makes sure your car handles properly and, more importantly, provides an opportunity for inspecting the suspension system for defective parts. The safe driving aspects may be the most important benefit of regular wheel alignment checks.
4. STRETCH FUEL MILEAGE
Fuel efficiency increases as rolling resistance decreases. Proper wheel alignment sets all four wheels parallel which, in turn, ensures minimum rolling resistance. This plus proper inflation provides top efficiency for maximum kilometers.
5. IMPROVE HANDLING
Does your car pull to one side? Do you constantly have to adjust the steering wheel to keep your car traveling straight ahead? These and other handing problems can generally be corrected by a four-wheel alignment check.
6. GET A BETTER RIDE
Correct alignment will result in easier driving, enhanced comfort and stability as well as a greater degree of safety.
7. FREQUENCY
Follow the vehicle manufacturer’s recommendation in your owner’s manual. As a general rule your vehicle’s alignment should be checked every 20,000 kilometres or at least once a year.
You should have the alignment checked if you notice any of the following:
- Odd tyre wear, front or rear
- Steering wheel pulls to left or right
- Feeling of looseness or wandering
- Steering wheel vibration or shimmy
- Steering wheel hard to turn
Vehicle Vibrations
VEHICLE VIBRATIONS AND RIDE DEFECTS
An imbalance in any rotating component of a car can cause vibration. In many cases it can be easily rectified by normal tyre balancing.
If, after accurate balancing, the vibration continues, then the problem will in all probability be centred in any of the following components – tyres, rims, prop shaft, shock absorbers, engine mountings or half shafts.
Garages and tyre specialists are equipped to deal with this problem.
It is recommended that all the above factors be checked. The procedure for inspecting tyres and rims is as follows:
1.
Check that tyres are inflated to recommended pressure.
2. Ensure the tyres are correctly fitted (se tyre fitting).
3. Warm the tyres by driving the car for a minimum of five kilometers.
4. Immediately jack up all four wheels and check tyres for radial and lateral run out.
5.
When the run-out exceeds 1,5 millimetres, removes the tyre and check the rim bead seats for radial and lateral run-out. If this exceeds one millimeter, the rim should be rejected. If the rim is within limits, refit the tyre and by progressively moving it around the rim, establish the position of minimum radial run-out. If it if not possible to achieve less than 1,5 millimetres, refer the problem to the tyre manufacturer.
6.
If after completing step 5, the run-out has been brought to less than 1,5 millimetres, the tyre should be re-balanced and returned to its position on the car.
