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Static motorhomes - tyre flat spots?


Guest Derek Uzzell

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Guest Derek Uzzell
In Brian Kirby's "Tyres" posting of 31/12/2005 the subject of tyres developing 'flat spots' was discussed. As there seemed to be contradictory opinions about this I sought advice from a leading tyre manufacturer. I asked the following question: "Can the weight of a static vehicle over a prolonged period of time harm radial tyres? (Motorhomes are often unused during the winter and consequently may be unmoving for 4 months or more. Traditional wisdom has been that, unless a vehicle is moved regularly (say once a fortnight), the tyres may develop permanent 'flats'. However, a letter in the January 2006 Caravan Club magazine from a "tyre consultant" advises that radial tyres do not distort permanently under prolonged static load and the 'flats warning' applies only to cross-ply tyres. Let's assume that, during the static period, the vehicle's tyres are inflated to correct running pressures.)" This was the reply I received from the tyre manufacturer: "Flat-spotting is a known phenomenon, where prolonged pressure from the static vehicle's weight upon the tyre 'footprint' produces a depression in the shape of the tyre carcass and tread pattern. The effect will be worse in hotter climates, with lower tyre pressures and increased weight over the same tyre (higher pressure). The effects of 'flat-spotting' may be noticed when the vehicle is driven for the first time, following storage - vibration and noise can be felt/heard, due to the literally 'flat' portion of the tyre rotating at speed against the road surface. The effects will normally/eventually disappear after long distance driving, regaining the normal tyre shape after use at 'running' temperatures. Vehicle manufacturers quite often raise tyre pressures quite high, whilst the vehicle is originally being stored/transported. Raising the pressures, however, whilst the vehicle remains stationary for several months, is only a partial solution. The raised pressure will alleviate any effect on the tyre carcass, but will have little effect on distortion of the tread pattern. Rolling the vehicle a short distance, from time to time, will help remove 'flat-spotting'. Jacking-up the vehicle onto axle-stands will solve this phenomenon completely and tyre pressures can actually be reduced to relieve stress on the unladen tyre(s)."
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Guest Brian Kirby
Derek Very interesting. What they haven't clarified is whether they tyre suffers any more than this temporary distortion i.e. whether some of the tread / carcass distortion effects amount to permanent damage. The implication seems to me to be that the phenominon is temporary, and will be rectified through normal driving, but I'm a bit dubious that it's quite that simple! Incidentally, following that letter, I wrote to the Caravan Club querying why they had apparently accepted without clarification the "consultant's" opinion, when that seems to be in conflict with their own advice i.e. that tyres will develop flats if not moved from time to time. Hasn't been published yet, but it'll be interesting to see - if it does get an airing - whether they get the same answer as you. I do hope so! Many thanks Brian
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Guest Derek Uzzell
Seems to me you are looking for black or white answers to grey questions. How would you define 'temporary distortion' - something that lasts 5 minutes, 5 hours, 5 days, etc? How long can one store a vehicle before flatting becomes significant - one week, one month, one year, etc? Can't say I care about the fine detail in this instance. The guidance is that the vehicle be moved every so often, which was what everyone accepted as sensible practice before the CC letter muddied the waters.
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Guest Brian Kirby
Derek Well, yes, and no. General guidance, yes; but I'd agree with you there's no point in over refinement. In terms of moving vehicles periodically, I think this leaves us straight back with the CC guidelines, in which case it is just a shame they appeared to endorse the contradictory advice in the letter. However, see penultimate para below. There was more, though. The CC letter went on to advise that, provided they were otherwise legal, tyres really only need changing after 10 years (not 7 as often stated: for example, that an unused spare should be safe for use for up to 10 years) and then - and this really surprised me - that the Tyre Industry Council would support fitting a 6 year old tyre as new. If the advice to changetyres after 7 years were followed, you'd only get 1 year's use out of the new boot. That seems totally unreasonable. It is probably fairly unlikely that such an old tyre would be fitted but, given an unusual size and an unpopular (expensive) make? However, if tyres really are good for 10 years, perhaps not so unreasonable. What surprised me most, however, in the light of all these contradictions, was the Club's apparent endorsement of the writer's advice. However, I had a e-mail from the club yesterday, saying they'd had a rather large mailbag as a consequence of the letter, and are to publish a series of clarifications in the March magazine. May beinteresting! I wonder if they'll get that same advice as you had? I do hope so! I'll post a synopsis when I've seen it. Regards Brian
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Guest Derek Uzzell
I did in fact ask the tyre manufacturer about the longevity of tyres in use and stored (including the spare tyre). As the advice did not conflict with the CC letter I chose not to mention it. If it's of interest, here are the relevant questions and answers: 1. Does a new tyre have a recommended 'shelf life' and, if so, how long is it? (What I'm asking is, following a tyre being manufactured in the factory, is there a maximum period of time (1 year, 2 years, 5 years, etc.) after which you would advise that the tyre should not be fitted to the wheel of a vehicle? Let's assume that the tyre is being stored carefully, out of direct sunlight, in a warehouse or tyre-fitting depot.) 2. If there is a recommended shelf life for a new tyre, does this same period apply when the tyre is fitted to a vehicle's spare wheel? (As many motorhomes cover a very low annual mileage the spare wheel may not be brought into play for several years. So, is there a recommended period of time (commencing with a tyre's date of manufacture) after which an unused tyre on a spare wheel should be discarded?) 3. Is there a maximum period of time after which you would recommend that 'in use' tyres should be replaced irrespective of their visual condition? (This continues the motorhome low annual mileage theme. It is not uncommon for the tyres on motorhomes to be UK MOT-test legal as far as tread depth and appearance are concerned, yet be 10 or more years old.) This was the response:- "Only 2 recommendations exist throughout the tyre industry regarding purely the age of a tyre, with legal rulings only coming into play when an inspection shows the tyre casing/carcass visible at the base of any cuts/splits:- 1. 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. 2. The BRMA (British Rubber Manufacturers Association) strongly recommend that tyres should be replaced 10 years from the date of manufacture One wonders at the general quality of rubber, beyond 10 years of age, tending to have become too hard beyond this time limit. I would recommend, therefore, that this 'spare' tyre be discarded and replaced. Various elements can alter the condition of the tyre - the main one being 'sunlight' attacking the outer components leading to visible crazing of the surface. The use of detergents to wash them leads to further deterioration and especially contact with solvents, fuels or oils, along with salty air if used near to the coastline. Tyres stored in cool, dark and dry conditions should have a very long life. Relating these storage conditions to tyres fitted onto a vehicle, washing off the road-salt regularly with water only and garaging/shielding the tyres wherever possible should minimise degradation. Shelf life: In the absence of legislative controls governing tyre shelf-life or the age at which tyres should be removed, BRMA (British Rubber Manufacturers Association) members strongly recommend that unused tyres should not be put into service if they are already over 6 years from the date of tyre manufacture. However, this recommendation takes into account the short period of use available to the customer (4 years gap, between fitting at 6 yrs and removing at 10 yrs) - any tyres stored in correct conditions and exhibiting no indication of deterioration should perform well. Spare tyre: The above recommendations apply to an unused spare tyre, in respect of age and tyre condition."
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Guest Brian Kirby
Thank you for this, Derek. This part of your very comprehensive reply goes very much along the lines of the CC letter writer's view. This is one of those issues where one needs to proceed with a degree of caution, not unlike the tyre pressures issue, where the converter's / base vehicle recommendations are generally (although clearly not always) foolproof, albeit with some penalty as regards comfort. To safely diverge from these one needs a bit more information: i.e. the actual axle weights and the tyre makers corresponding recommendation for pressures. So, in a way, with tyre life. If one changes at 7 years, the tyres will probably still be in serviceable condition at the time they're discarded. Another 3 year's useful life may be possible, but this is dependent upon very careful checking of all aspects of their condition as they progress towards finality at around 10 years. It is not, therefore, sensible to talk generally in terms of 10 years tyre life. One should expect to get 7 years (wear/damage excepted), but with care (and a bit of luck) one should get more, possibly up to 10 years. Still, the cost of a replacment set of tyres is substantial. Replacing after 10 years probably means a 'van would end at the scrapheap on the last legs of its second set. Changing after 7 years, however, would have necessitated buying a whole new set of tyres in the interim. Since this cost often falls upon a second (or third etc.) owner, the cost of tyre replacement proportionate to purchase price of the 'van will be that much more onerous. It seems quite a lot of motorhomers (and tuggers) will be very interested in the outcome of this discussion. Given our generally low mileages, this does seem to have quite significant implications for the costs of owning and running motorhomes in the longer term. I await March with interest, to see if the advice you recieved is repeated. By the way, were you intending to publicise your advice any further? Regards and thanks Brian
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Guest Derek Uzzell
No, to your question of further publicity. I've had enough of tyres for a while and I'm satisfied with guidelines of a 10 year tyre life expectancy and discard-if-unused after 6 years. Moving a static vehicle occasionally to minimise the risk of flat-spotting is traditional advice that most motorcaravanners will be aware of. Like you, I await the CC's response with interest.
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