Tyre Pressures

[Tomo3090]

I've noticed a few threads and letters in mags. lately where people are suggesting that they run their tyres on a lower pressure than reccommended by the manufacturers. I am not 100% certain of the legal position about this. When I was in the fire service we attended quite a few crashes and one of the first things the police accident ivestigators did was check the tyre pressure. If it was below, I think, 10% of the recommended pressure they reported this as a possible contributory factor in the crash.

 

I have no idea what happened then from a legal or insurance point of view.

 

Has anyone got the answer?

[Brambles]

The problom is many Motorhome Manufactures habe the figures fior teh Tyres max pressure of 80PSI. This is far too high in the majority of cases.

The only real way t be sure what your pressures should be are to weigh your axles ,when loaded up, on a Public weighbridge and contact the tyre manufacturer for the correct pressure.

If you actually look at the max permissable axle wieghts for a Ducato, you will find the front amd rear repectively is no more than about 60 and 65, so immediately you can see running at 80 is way over the top. I don't know what the Alko chassis rear is.

Rest assured though 80psi is far too high on the majority of Motorhomes if not all.

Cannot repeat enough, weigh your axles and ask the tyre manufacturer.

Some Motorhome manufactures give better guidelines as t tre pressure in typical running order but many don't as they havbe so many models of different weights and do not know what weight owners will be carrying, so take the easy option of specifying the max tyre pressure. This to me is wrong as it totally misleads owners.

[Shaun]

Agreed - contact the manufacturer. With my Adria Twin, all I had to go on was Fiat's figures for the panel van, not as a motorhome. I was surprised that other owners were also in the dark and there were some very different figures being given out. So, I rang Continental after having had my van weighed, including each axle separately. They said 54 psi front and 62 rear, when fully laden (3,300 kg) and slightly less if not. However, as motorhomes carry a lot even when not loaded up, I've left mine at the 54/62. These figures are much less that what Fiat stated in their handbook.

 

Shaun

[Derek Uzzell]

Tomo3090:

 

I remember that a question similar to yours was published in MMM's "Interchange" pages a while back. From what I recall, Interchange's Editor, George Collings, (himself ex-police) suggested that it was unlikely that a motorhome owner who had taken the trouble to have his vehicle weighed, and then had sought advice from the tyre manufacturer regarding suitable inflation pressures, would be prosecuted for using pressures that differed from 'handbook recommendations'.

 

I very much doubt if there's a simple legal position on this. What, for example, if one were driving a car with a punctured 'run flat' tyre designed to be operable even with no air-pressure inside? Clearly over-driving a vehicle in that state could easily lead to an accident for which the driver could be held liable, but would the nil pressure in the run-flat tyre be considered significant if (say) the car was stationary at traffic-lights when an accident involving the vehicle occurred?

 

The fact that the majority of motorhomes are based on light commercial vehicle chassis but are generally operated differently loading-wise, plus the added complication brought about by Michelin XC Camping tyres having been fitted to many motorhomes, has considerably muddied the waters over what tyre pressures are 'correct' or 'incorrect'.

 

My Ford-based Hobby motorhome has a Ford data-plate on it that recommends pressures that are very definitely wrong, the Hobby handbook advises 'generic' (high) pressures that have no link to whether the motorhome is Ford- or Fiat-based, and the Ford handbook does not include pressure advice for the chassis on which the Hobby is built. I've weighed the vehicle, taken advice from the tyre manufacturer and then 'twiddled' that advice upwards a bit to suit my personal preferences regarding ride/handling. I don't know where that leaves me legally, but I can't say I'm worried about not knowing.

 

I suppose it's worth emphasising (again) that, if a motorcaravanner dislikes the ride and/or handling of his/her motorhome and believes that adjusting the tyre-pressures away from handbook recommendations might prove beneficial, then informed advice based on real data should be sought first. As has been advised many times on this forum a proper procedure needs to be followed involving obtaining axle-weights when the motorhome is in 'normal' loaded state and then consulting the manufacturer of the vehicle's tyres. All documentation (weigh-bridge read-outs, correspondence with tyre manufacturer) should be carefully retained just in case it might be needed in future. Just picking nice-sounding tyre-pressure figures out of the air, or trusting 'this is what I use' figures confidently offered on on-line forums is a bad plan.

[tonyishuk]

I would suggest that an email or similar confirmation from the tire manufacturer giving tyre pressure for actual axel loads for specific tyre types would be sufficient. At least very difficult to argue against in a court of law.

 

Michelin give a response when they iare asked for such information and keep the advice for future reference.

 

Still as we all know, there is not much common sense or morality in the law, so who knows ?

 

rgds

 

[Brambles]

Copy of very recent reply from Michelin on my tyre pressures - I have deleted the values. It appears to be a proforma standard reply.

 

Dear xxxxxx

 

Thank you for your recent e-mail.

 

We suggest that individual axle weights from weigh scale readings are used, in order to calculate pressure recommendations for each quoted load condition. These would be preferred, due to readings being taken in the actual vehicle 'running' condition - that is to say, with all water/fuel/baggage & passengers etc. onboard - and may result in better comfort if the vehicle is running way below maximum capacity.

 

The results of roadside surveys indicate that some motor homes have been found to be over laden, especially the rear axle, with potential for subsequent tyre related problems.

 

The overall tyre industry, therefore, tries to build-in a safety factor by fitting strong tyres, inflated to high pressure, in order to remove the potential for consequences of overloading &/or under-inflating the tyres.

 

The drawback of this safety factor, at the corresponding high pressure, may be an extremely hard ride. The contents of the motor home may rattle, but the tyres will cope nicely.

 

One 'overall' weight is of no use in calculating tyre inflation pressure levels, since the vehicle may appear to be within the maximum permitted, but in reality may have the rear axle severely over laden & the front axle very lightly laden.

 

As a guideline from the axle weights that you have supplied the pressures for those weights are as follows:-

 

Normal Pressures

Front axle weight of 1280kg the pressure should be xxxx (pounds per square inch)

Rear axle weight of 1750kg the pressure should be xxxx (pounds per square inch)

 

Laden Pressures

Front axle weight of 1750kg the pressure should be xxxx (pounds per square inch)

Rear axle weight of 1900kg the pressure should be xxxx (pounds per square inch)

 

You would be advised to follow values indicated on the fuel cap / door-pillar / vehicle handbook in relation to the tyre size fitted – specifications can change without our knowledge

 

 

We have allocated your message the reference number indicated above. If you need to contact us again regarding your message, we would be grateful if you could include the reference number.

 

Once again thank you for your interest in Michelin.

 

Yours sincerely

Michelin Tyre Public Limited Company