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Tyre life and flat spotting.


Brian Kirby

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Guest Brian Kirby
There have been a number of recent postings on the above subjects. The starting point was a rather confusing editorial footnote, to a Caravan Club Magazine reader’s letter that contradicted the Club’s normal advice. Derek Uzzell’s detailed postings at the end of January ’06 pass on the advice he obtained from the tyre industry. While he was busy chasing down that advice, I was also asking around for further guidance, and included two of the industry’s national bodies. I am grateful to the British Rubber (now Tyre) Manufacturer’s Association (BTMA), to the Tyre Industry Council (TIC) and to Michelin UK, for their information. I took up with the Caravan Club their footnote but, despite lengthy exchanges with their technical department, they have not altered their basic stance which, paraphrased, is that ideally tyres should be replaced after 5 years, and must be replaced within 7 years, from their date of manufacture. There is nothing inherently wrong with that advice, but they have not, despite being pressed several times to do so, commented on the apparent conflict between their recommendation as above, and the industry’s recommendation that tyres can be sold / fitted as “new” up to 6 years, and must be replaced within 10 years, from their date of manufacture. I therefore set out to see if the apparent contradiction could be resolved. All of the following comments assume that the tyres in question fully meet at least the minimum UK legal requirements for tread depth, and are otherwise in sound and serviceable condition. The BTMA publishes a booklet, entitled “Tyre Tips for Motorhomers”, which advocates: “never use tyres more than 7 years old and preferably replace any tyre older than 5 years”. A similar booklet from BTMA, entitled “Tyre tips for Caravans and Trailer Tents” reiterates this advice, which is in line with the Caravan Club’s advice. The Club acknowledges the BTMA as one of its advisors. However, Michelin commented that “The BRMA (British Rubber Manufacturers Association) strongly recommend that tyres should be replaced 10 years from the date of manufacture”. Interestingly, this was the line taken by the Caravan Club Magazine’s letter writer. Michelin add the reservation that “A tyre should be examined at 5 years from manufacture by a tyre distributor or tyre manufacturer. Should there be any doubt as regards safety, I suggest that tyres should be sent back to the manufacturer for inspection”. This seems eminently sensible advice. Interestingly, the above BTMA booklet does not refer to the age of a tyre when first fitted, or to a need for periodic inspections. Unfortunately, it is undated, but its cover shows ‘vans on Mk 1 Ducato / Boxer underpinnings. Inside, there is an illustration of typical tyre sidewall markings, carrying the code DOT … 267, which would indicate a tyre manufactured in 1997 – see explanation below. As this seems about contemporary with the ‘vans on the front cover, I assume the date of publication would have been around this time. I next received, via the TIC, an information sheet dated 5 June 2001and entitled “Tyre ageing”, which was on BTMA headed paper. This states that the BTMA “strongly recommend that unused tyres should not be put into service if they are over 6 years old and that all tyres should be replaced 10 years from the date of their manufacture”. This seems to be the authority behind Michelin’s, and the Caravan Club Magazine letter writer’s, advice. From its date, and the inclusion of the statement that “Tyres fitted to spare wheels, caravans and trailers are particularly vulnerable to the “ageing” process”, this seems intended to supersede the advice provided in the booklet. A properly stored tyre, fitted when 6 years old, therefore seems to be deemed by the industry to have a useful service life of up to 4 years before it must be replaced on grounds of age, i.e. irrespective of its tread depth and however good its apparent general condition. The BTMA information sheet adds that there is no legislation restraining traders from offering tyres older than 6 years, as “new”. Your best course, therefore, if offered as new tyres that are older than 6 years, is to point this out to the seller and, if he doesn’t offer fresher replacements with an apology, shop elsewhere. The information sheet then states that, “In ideal conditions, a tyre may have a life expectancy that exceeds 10 years from its date of manufacture. However, such conditions are rare.” You have been warned! However, in reality, unless mileage is exceptionally low, the prospect of 10 year old tyres with legal treads does seem a bit remote. A simple reading of this advice might imply just inspecting the tyres at 5 years and, if OK, keeping them in service for another 5. However, this does seem illogical, and would take no account of age at the date of fitting. To make sense of this anomaly, it seems preferable that the tyres should be inspected by a specialist 5 years from date of fitting (in preference to date of manufacture, as above), again after a further 2 years, and ideally, every year thereafter, up to 10 years following manufacture, when they must be replaced. A tyre fitted to an unused spare wheel since new must be inspected at the same intervals as above, and must also be replaced when 10 years old. If the age of the tyres when fitted cannot be verified, assume they were fitted six years following manufacture, with four years life remaining, as the safe option. If this assumption results in a date before the tyre’s marked year of manufacture, or after the date the vehicle was acquired, assume they were fitted immediately before acquisition of the vehicle, and proceed with inspections accordingly, as the safe fall back. In either event, if tyres were not properly inspected when the vehicle was purchased, now is the time! Such a regime would seem to cater as well for tyres that were freshly manufactured as for those that were not, (but still less than 6 years old) when fitted. The date of manufacture of virtually all tyres can be found coded into the sidewall. Look for the letters “DOT” followed by three, or four, numbers. If three numbers, the tyre was manufactured before 2000 and the last number represents the year. As it is currently 2006, tyres marked 7 or lower should immediately be replaced. If there are four numbers, the first two are the week and the last two are the year i.e. 2604 = week 26 of 2004. Flat spotting is the term applied to the distortion a tyre may suffer if left parked for an extended period. This can result in vibration and noise during subsequent driving and, in extreme cases, tyre failure. The following seems to be the consensus on avoidance. As a preface: if, in the interests of better ride quality, you have obtained the tyre manufacturer’s recommendation (based upon actual axle loads) to use tyre pressures lower than those recommended by the base vehicle / converter, Michelin recommends you revert to the base vehicle / converter’s (higher) recommended pressures whenever the vehicle will be parked during an extended period. They recommend maintaining these higher pressures until a more normal pattern of use is resumed. This reduces the sidewall bulge, which forms just above the tread contact area, which otherwise tends to become permanently “set”. However, it also results in a reduced area of tread in contact with the ground, and hence increased forces within that area of tread. This, unfortunately, increases the likelihood of the tread blocks themselves flattening. To counter these distortions, the vehicle should be driven periodically, preferably for several miles, to “work” the tyres and warm them up. Warming the tyres in this way allows them to regain their normal shape, and negates any incipient distortion. There is no need to reduce the tyre pressures back down for these relatively short trips: the resulting harsher ride can instead just be accepted. Such trips will also beneficially work the brakes to remove any rust from drums/discs, warm and circulate engine oil and coolant, exercise the clutch, the gearbox, the gearchange linkages, charge the starter battery and flush next door’s cat from under the bonnet! Oh, and you can check the tyre pressures while out, as well. The normally recommended interval for these trips is monthly; however, this is not a hard and fast rule. After all, there is little point in driving in foggy, snowy, icy, wet or, especially, salty conditions, if this can be avoided just by waiting a few days. The object of the exercise is to benefit the vehicle, not to accelerate its rate of decay or subject one’s self to avoidable risks! Then, if the vehicle has stood a week or so longer, perhaps due to bad weather or illness, just extend the usual trip by a few miles to give everything, but especially the tyres, a more extended work out. You may immediately think of a winter lay-up period in connection with these precautions against flat spotting. However, it seems the distortions form more quickly at higher ambient temperatures, when the tyre rubber is softer. So, higher tyre pressures and periodic trips out should really be adopted during any stay that lasts more than three to four weeks, even - perhaps especially - during a summer seaside break. I hope you may find this useful. Regards Brian
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I am surprised Brian, that there has been no response to your posting. You are to be congratulated for the amount of trouble you have taken to research the subject. Your posting usefully consolidates other, earlier contributions, with some valuable , new to me, additional information. Thanks John S
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