Tyre Pressures

[Guest Jack]

Can anyone offer any advice as to what pressure I should be running my tyres at? The motorhome is a Auto Trail Apache 640 with Michelin XC Camping Tyres (215/70 R 15C) (109/1070). On the tyre it has @80psi. The dealer states between 50 and 60 psi, Auto Trail state 5 bar (what is the equivalent psi?) as per the Fiat handbook.The reason for asking is that the van has just come back from its first service and on the way back home I got a puncture and the tyre was a rightoff, when the tyre fitter checked the other tyres they varied between 65psi and 88 psi not one of them the same pressure! Not sure if inappropriate pressure caused the puncture but at £150 a tyre I'm not keen on a second happening. Many thanks.

[Guest Clive]

I doubt if excessive tyre pressures caused the puncture. Very low pressures are more likely to damage the side walls which could fail prematurely. 5 Bar equates to about 73 PSI in old money and I would suggest you run the rear tyres at that. If the weight on the drive wheels is the same (and I doubt this) then use the same pressure. What pressure does the Autotrail book recommend for the front axle? In a camper application the centre of gravity is a bit higher than a van so a bit more tyre pressure helps stability. 1 BAR is close to 14.5 PSI. (I use 15 for simple calculations) Oh yes, where did you get your replacement Michelin Camping tyres from? They tend to be a bit scarce.

[Guest Derek Uzzell]

Michelin's XC Camping pattern is a specialist tyre and much has been written about it on this forum (SEARCH on "XC Camping" (omitting the quotes) for more previous postings than you could shake a stick at). The optimum inflation-pressure for each of a motorhome's tyres will depend on the specific load being placed on that tyre. This load will, in turn, depend on the basic 'empty' weight of the motorhome itself, plus whatever extra weight the vehicle carries relating to people, luggage, water, gas, accessories, etc. And, of course, the load on each tyre will also be affected by the configuration of the motorhome (body type, wheelbase, etc.) that will apportion that 'overall' weight to the vehicle's axles. The tyre manufacturer assigns each of his tyre patterns a load-index. This index is given as a set of axle-weights with the inflation-pressure suitable for each of those weights. The load-index (axle-load followed by pressure) for a pair of 215/70 R15C XC Camping tyres fitted to a 'single-wheel' axle (ie. a 'normal' axle with just one wheel at each end) is as follows: 2060kg - 65psi 1880kg - 58psi 1660kg - 51psi 1460kg - 43psi The load-index figures permit a graph to be drawn that allows intermediate axle-load/pressure relationships to be found. Now, if you read through earlier XC Camping forum-postings, you'll see that this tyre is design-capable of being inflated to 80psi (around 5.5 bar). At 80psi XC Camping can carry a higher weight than the load-index's 2060kg stated axle-maximum but, unless the motorhome is loaded to match (which in the vast majority of cases will mean it being overloaded), the vehicle's ride-quality will suffer as a result. Assuming your Apache 640 is a recent model, it will be based on Fiat's Ducato 15 chassis with a 3400kg maximum authorised weight. For this Fiat chassis Michelin suggest inflation-pressures of 59psi (4bar) and 65psi (4.5bar) for front and rear tyres respectively. (These pressures are, in fact, given for Michelin's 215/70 R15C Agilis 81 pattern not XC Camping, but the two patterns' load-indices are virtually the same so the inflation-pressures will be too.) However (as you rightly state) Fiat's recommendation for this chassis in "camping-car" form with 215/70 R15C tyres is 5bar (73psi) for front and rear tyres. My personal opinion is that only significant overloading of a 3400kg Ducato 15 chassis fitted with XC Camping tyres would justify using inflation-pressures beyond 65psi. But the only sure way to confirm/deny whether that's the situation with your Auto-Trail would be to weigh it in fully loaded state. Taking the maximum authorised axle-loads given on your Apache's VIN plate and using the load-index data I've given above, you should have no difficulty working out the inflation-pressures that relate to those load-maxima. If you choose to obtain real-world axle-loads by taking your Apache in fully loaded condition to a weigh-bridge (www.chrishodgetrucks.co.uk is a useful website for weigh-bridge info), a similar calculation can be performed. Note that Michelin normally advise adding 10% to front-tyre inflation pressures derived in this way to allow for the extra load produced by weight transferring under braking towards the front of the vehicle. Perceived wisdom is that motorhome owners use the tyre inflation-pressures given in the base-vehicle manufacturer's or motorhome converter's handbooks. This is sensible advice as, if you are ever stopped for a roadworthiness check, it's these figures that you will be expected to have adhered to. However, it's most unlikely that 'crude' handbook pressures will provide a motorhome with the optimum combination of comfort + good road-behaviour + safety. Unfortunately, there have also been many examples (regular forum-readers may recall the Auto-Sleepers Nuevo saga) where the tyre-pressures provided are totally inappropriate for the motorhome model. For some strange reason the idea of a proportional relationship between a tyre's inflation-pressure and the load being placed on that tyre seems to be heretical for many motorcaravanners (see the piece on page 263 of MMM April 2005 and GC's response) and an apparent mystery to some motorhome converters. If contacted via their website, Michelin will be very happy to provide advice on tyre-pressures as long as the enquiry includes details of the tyres themselves and either VIN-plate maximum axle-load data or (preferably) exact axle-loads obtained from a weigh-bridge. Lastly, I agree with Clive. The inflation-pressures you've mentioned (65psi to 88psi) are within XC Campings design-envelope even if they may not be ideal for your Apache. There is a lot of junk on today's roads and punctures are the result. The snag is that even a minor puncture occurring while travelling can gradually reduce the pressure within a tyre until it overheats and suddenly fails. More serious perforations are usually noticeable immediately after the event but, because of a motorhome's weight, the punctured tyre may well be damaged beyond repair by the time the vehicle has been stopped. Lots of other tyre deflation causes - old age (the tyre's not the driver's!), construction defects, valve defects, vandalism, 'kerbing' - but stuff lying in the road is the usual culprit.

[Guest Mike C]

Hello Jack, Derek and Clive have covered your request well. May I just add a couple of other bits of information that are not covered. Firstly the puncture. Was it through the tread or sidewall ? Under inflation will damage the tyre structure seriously as Clive says. Part of the problem of having tyres over inflated or to near maximum pressure is that they become more prone to getting punctures and the deflation is more violent and subsequently does more damage. Think kids balloon. If it is soft it resists puncturing by squirming, stretching or flexing. Pump the balloon up to high pressure and it punctures very easily and the deflation is more violent. Just a note about tyre pressures quoted in the vehicle manufacturers handbook and compared to pressures specified in the coach builders handbook. On my previous Compass Avantgarde 100 the Peugeot book specified "see the sticker on the vehicle B pillar" this was 58 psi (pressure for maximum load). The coachbuilder specified 44 psi in their manual but did not specify any information on weight or even the type of tyre fitted. Following the recommendation mentioned by Derek, there is a correct pressure according to the tyre and axle weight. According to this the correct tyre pressures, confirmed by Goodyear were 51 psi on Goodyear Cargo G26. When I spoke to Compass they eventually told me that the pressures (44 psi) they specified were for Goodyear GT2. The best bet, if you can afford the time, is to get the vehicle in running trim and then get the axle weights measured at a weighbridge and keep the weighbridge tickets. Apply these weights to the recommendation of the tyre manufacturer. You can then demonstrate to the Police or Vehicle Inspectorate, if stopped for a roadside check, that you have correct tyre pressures according to the tyre and axle weight. I am sure that we would all appreciate information on where you sourced the Michelin XC Camping tyre. Regards, Mike C.

[Guest Derek Uzzell]

A brief postscript - I notice that Auto-Trail's 2005 brochure gives (for Apache 640 on a 3400kg Ducato chassis) maximum axle-loadings of 1750kg (front axle) and 1900kg (rear axle). For 215/70 R15C XC Camping tyres the inflation-pressures equating to these axle-loadings would be around 55psi (front axle) and 59psi (rear axle). But you should consult Michelin to obtain 'official' confirmation of this. Having revisited Auto-Trail's brochure, I also noted that the stated payloads of 460kg for Apache 640 (Hi Line), and 550kg for the Lo-Line version, take no account of the weight of passengers, fresh water or gas. Motorhome converters tend to provide 'Unladen Mass' figures +/-5% and there is circumstantial evidence that the unladen weight of Auto-Trail vehicles is at least as heavy as specified in the brochure. (I'm attempting to be diplomatic here!) So, besides providing you with data that will allow you to obtain best advice on tyre-pressures, weighing your Apache would also permit you to find out how much available payload you really have.

[Guest Simon B]

Iwould be concerned that after the service why were the tyre pressures so wildly different. This happened to me with my old A/S hightop. I went back to the garage and got my money back for the service!

[Guest Jack]

Many thanks to everyone for their time and effort in replying to my post. Looking at the info from Derek, the dealers advice was the nearest by stating between 50&60 psi.I have not as yet taken the van to a weighbridge,but it does seem a sensible precaution (especialy if stopped!)I purchased the Michelin tyre from a small local firm, Autotreads Fast Fit 01207571110, who got one from their supplier, unfortunately I don't know this number.I mentioned that this type of tyre was difficult to get hold of and the chap said he could get one ok but was staggered at the price! Further to the comment about the servicing I have subsequently taken a reading of the oil level and find that it is overfilled by about half an inch up from the max mark. I am taking the van back for this and the sevice manager "thanked me for bringing my concerns to his attention!!"