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A QUESTION OF PRESSURE One of the few areas in which the disparate sectors of the tyre industry, as well as safety organisations, are in total agreement is the importance of regularly checking tyre pressures. Exactly how often this should be done is open to debate – some say monthly, others fortnightly or weekly – but all agree that it should be carried out regularly. As well as stressing the safety benefits of driving with tyres at the correct pressures, there is an economic aspect as well, as incorrectly-inflated tyres can adversely affect fuel consumption; a major consideration in these days of spiralling fuel prices. There is also the green issue, as correctlyinflated tyres will reduce CO2emissions. If you type the words "checking tyre pressures" into Google, you get 446,000 results, the vast majority of which specify that pressures should be checked when the tyres are cold. Here are a few random selections from these websites, covering a variety of organisations both inside and outside the tyre industry: Note: You should always check your tyre pressures when the tyres are cold. Under driving conditions the tyres will warm up and give you a false reading, so if you have driven further than roughly a mile, leave your car for an hour or so for the tyres to cool down. DIY Motoring

means the car should not have driven for hours prior to checking. AA Experts say that you must check your tyre pressure at least once a month and before going on a long trip. Also, make it a habit to check your tyres every time you fill up with fuel. Ideally, tyre pressures should be measured when tyres are cold – that is, when you have driven less than a mile, otherwise your tyres may have heated up, increasing the air pressure inside them by several pounds. Cooper Tires So, the overwhelming advice is "check them regularly and check them when cold". But this could cause a problem, as the following comments from the message board section of the Honest John website illustrate: How cold is cold? If tyres warm up in use anyway, regardless of the ambient temperature, then what is the point of checking the pressures when cold? Wouldn't it be better and more convenient to specify the normal running pressure? Answer 1 When it says 'cold tyre pressures' read 'tyre pressure taken before you start driving' and the car has not been parked in the sun or a greenhouse(!)

Remember to only check your tyres when they are cold; checking the pressures after a journey will give inaccurate results due to the heat in the tyres. Revolution Driving Tuition

Answer 2 This has always intrigued me. How do I get the car to the petrol station to check the tyres without driving it? Realistically, how long does it take for the tyres to warm up?

Always check the tyre pressure with a tyre pressure gauge when your tyres are cold. If you are using your vehicle to carry additional load or weight, always consult your vehicle handbook for the correct loaded tyre pressure. Kwik- Fit

Although it sounds frivolous, answer 2 above does raise a point, as it is hardly practical to drive to a petrol station and wait for the tyres to cool down. And what about those roadside pressure checks which used to give results that the majority of tyres were incorrectly inflated – how was this measured? In short, how can you correctly measure pressure when tyres are hot?

Check pressures every two weeks with the tyres cold, using a reliable/accurate tyre pressure gauge.

Michelin’s view ….. Pressures should only be checked when cold, which


“Correct tyre pressures are critical for the safe operation of your vehicle, which means that regular tyre pressure maintenance is essential. It is recommended that your tyre pressures (including the spare) are checked and adjusted as necessary monthly and before any long trip. Ideally, check pressures when the tyres are cold; this means that they have not been used in the last 2 hours, or they have covered less than 2 miles at low speeds. Any tyre not in this “cold” condition is considered to be “hot”. If the tyres are “hot” when they are checked: Add 4 to 5 psi (0.3 bar) to the pressures recommended by the vehicle manufacturer.

Never reduce the pressure in a “hot” tyre, even if the pressure is above the recommended level

Re-check the pressures when the tyres are cold.

In cases of unusual pressure loss, have the internal and external condition of the tyre, the condition of the wheel and the valve checked by a tyre specialist. Inflation with nitrogen does not dispense with the need for regular tyre pressure maintenance as specified above. Recommended tyre inflation pressures for your vehicle can normally be found in the vehicle handbook or on a label fixed on the vehicle, for example on the door frame or the fuel filler cap. Use the pressures relating to your tyre sizes and vehicle load/speed conditions “ - Michelin So, not only does Michelin give a comprehensive answer to the "what does 'cold tyres' mean?" but also tells how to check pressures on 'hot' tyres. In fact, it goes even further than that, as a question often asked on motoring message boards, and presumably in tyre depots, is whether, if your tyres are inflated with nitrogen, you need to check pressures so often?

WHAT ABOUT REPAIR I NG SST? For the tyre dealer, this is a potentially tricky problem. Faced by a customer who has presumably paid a premium for his runflat tyre, said customer will not be too thrilled to hear that the tyre cannot be repaired. On the other hand, should the dealer go ahead and repair the tyre and it subsequently fails, he is in trouble. The situation is not helped by the fact that different manufacturers had, or have, conflicting views on the subject. In its 2006 statement on self supporting runflat (SST) tyres, the British Tyre Manufacturers' Association made the following comment:

of the reinforced sidewall structure of an SST tyre, these symptoms are likely to be masked and not be visible even if they are present. Even a fully qualified tyre repairer may not be able to detect runflat damage on SST tyres”.

"For these reasons alone, and in the interests of safety, BTMA does not recommend the repairing of SSTtyres."

"SST tyres have specially-reinforced sidewalls to enable them to perform when deflated, for a limited distance and at a restricted speed. In the course of running in a totally-deflated, or significantly under-inflated condition, the tyre structure is subjected to high stresses and hence may become weakened and permanently damaged, rendering the tyre both unsuitable and unsafe to repair.

However, the statement adds: "Individual tyre manufacturer's views may differ from the above regarding their own brand of SSTtyres and specific enquiries should be directed to the relevant manufacturer."

"When a standard tyre is run deflated, there are visible symptoms, which indicate that damage to the tyre's structure has occurred. However, because

Since this statement was released, runflats have evolved considerably, so what do the manufacturers have to say about repairing them? A


quick trawl through the major manufacturers' websites revealed the following comments: Bridgestone (FAQs) Q. Is it possible to repair a Run- Flat tyre that has been punctured? A. It depends on how far and at what speed the car was driven after the puncture was sustained. Repair is possible only if deemed so by the tyre sales store. Preconditions include a puncture of less than 6mm for both side-reinforced type and support-ring type Run-Flat tyres, plus minimal damage to the support ring in caseof the latter. However, it is strongly recommended that the tyre is replaced as its durability will have been weakened after being repaired Michelin A Michelin ZP (Zero Pressure) may be repaired once and once only by a tyre professional following the normal rules and procedures for the inspection, preparation and repair of standard tyres. ZP tyres which carry sidewall markings indicating that repairs should not be carried out are still considered non-repairable. What about a slow puncture? The damage to the sidewall could occur if you continued to drive on the flat tyre for more than the recommended distance (approx. 100 miles) or over

the car had been sat on your driveway for a few days and happened to deflate as a result of a slow puncture. Surely this tyre would be repairable? Despite there being no damage to the sidewall you won't find anyone that will repair this RFTtyre, the fact is they only have your word that the tyre had not been driven on at zero pressure and they could run into liability problems if the repaired tyre failed. Continental: SSR(self-supporting runflat) tyres which have been used in a deflated condition are subsequently not suitable for repair. Verdict So are Run Flat Tyres safe to repair? In theory yes they are, however as cars running on RFT don't have a spare wheel, unless the tyre went down whilst the vehicle was parked, this would be impractical. In practice you won't find anyone that will repair a RFT because both legally and financially it doesn't make sense for tyre centres to repair Run Flat Tyres. I find the most telling of these statements is that from Michelin, telling the motorist that no tyre professional is going to repair a runflat because it cannot be proved that it has not been abused. The customer may be always right, but is he always honest? One suspects that a customer swearing on a stack of bibles that his tyre deflated overnight and has not been driven on carries about as much conviction as the 28 mph which everyone claims was the speed they were travelling at when filling in insurance claim forms. By repairing a runflat, you are accepting what the customer has told you and, should it subsequently fail, then the buck stops firmly with you. Little wonder then that Michelin says that legally and financially it makes no senseto repair a runflat.

the maximum speed of 50 mph. However what if you'd not driven the tyre at zero pressure, what if

The customer will probably be upset and unhappy, but better that than a possible lawsuit, surely?

NTDA RESPONDS TO TYRE ADVICE QUESTION


Fleet News is a leading publication, aimed at car and

van fleet decision makers, with a weekly circulation of over 18,000. In April, a reader's letter asked "Is tyre advice to be believed?" The letter read as follows: Surely the restrictions imposed on tyre outlets when changing tyres need to be considered. Advice from tyre manufacturers, vehicle manufacturers and road safety groups tells us to always change tyres in pairs, never fit different brands/models of tyre on the same axle and to rotate our tyres to put the new rubber where it gives most benefit. Yet the tyre service providers cannot replace tyres in pairs, nor rotate tyres, meaning in my case I am soon to have tyres from three brands on my car, all at differing levels of wear. I am left wondering whether my vehicle is safe or is all the advice not to be believed?

The letter was noticed by NTDA Director Richard Edy, who responded on behalf of the Association. This is Richard's reply: As it stands, there is nothing in the existing UK Construction and Use legislation relating to car and light van tyres that rules out the mixing of different tyre patterns, speed ratings or brands. It is, however, recommended that matching sets of tyres are fitted to give optimised performance, especially on vehicles fitted with tyres rated 'H' and above, where the handling characteristics could be affected due to the random mixing of patterns or brands. This also applies for high performance cars where the manufacturer only recommends the fitment of matching sets of approved tyres.

The following points must also be considered when replacing tyres: • • • • • •

• •

Tyre size markings across an axle must be identical. Tyre construction across an axle must be the same (ie both radial or both cross-ply construction). It is dangerous to have radial tyres fitted to the front axle and cross-ply tyres on the rear. The load index must be equal to, or higher than, the manufacturer-approved tyre for the vehicle. It is recommended that the speed rating is either equal to, or greater than, that of the original approved tyre. M+S (Mud and snow, or winter) tyres should ideally be fitted in complete sets and should not be mixed with other tyre types across an axle. It is not recommended that a new tyre be fitted across an axle with a tyre that is well worn. It is recommended that, when fitting two new tyres, they are fitted to the rear axle, unless the rear tyres are of a different size to the front tyres, or the vehicle manufacturer's handbook states differently. There may be special dimensional requirements preventing the mixing of tyres on vehicles such as four-wheel drive.

It is important that the vehicle manufacturer's handbook is consulted before fitting tyres, for any special recommendations.

If there are any topics that you would like to see included in future service bulletins please contact the Editor Peter Gardner at petergardner5@btinternet.com or NTDA Direct at info@ntda.co.uk or call 0889 67 07 07 Issue 5/10


Service Bulletin May 2010