My Tech Notes regarding tyre overloading (The Wanderer, January 2003) resulted in a deluge of emails from Members seeking more information. Many Members also ask how to find out the age of their tyres.

Tyre age is vital to know. Michelin’s ‘Recreational Vehicle Tyre Guide’ states ‘The life of an RV tyre cannot be measured by miles alone. Among other factors, tyre life depends on driving habits, driving and storage conditions, geography, atmospheric conditions, loads carried, weight distribution as well as the age of your tyres’. Michelin also advise that [because of the low mileage often covered] ‘It is very likely that tyres on an RV will need to be removed from service before they are worn out’.

Establishing Tyre Age

Most tyres have a small patch (often low down close to the rim) that has an apparently obscure code, which when interpreted, discloses the approximate date of manufacture. The most commonly used such code is prefaced ‘DOT’ and is likely to look like the example shown in this article. Other codes are generally similar but may not be prefaced ‘DOT”.

For some tyres made since January 2000, the last digit on the right of the code sequence is the last digit of the year of manufacture. Thus a ‘2’ indicates 2002.

Eventually all tyres will have a group of four numbers in place of that last digit (some tyres already have). The first two such digits show the week of the year of manufacturer; the last two digits represent the last two digits of the year of manufacture. Thus the code 4402 indicates a tyre made in the 44th week of 2002.

Tyres made prior to January 2000 are likely to have an additional mark (often a small triangle) following the last number. This denotes that the last digit represents the last digit of the date. Thus the example shown was made in 1998. If the tyre is clearly old yet there is no triangle or other mark, it’s odds on the tyre was made in the 1980s.

Particularly if buying tyres of a relatively obscure size, check the date of manufacture. It is not unknown for tyres several years old to be sold by small vendors.

Tyre age is commonly shown by a code that looks something like this. A triangular or similar mark indicates alternate decades. Its presence always shows the tyre was made in the 1990s. This mark is not however always used so its absence could still mean the tyre is of that age. Note that this code is only marked on one side of the tyre wall, you may need to crawl underneath to check.

For recently made tyres, the last four digits indicate the week and year. Thus 4202 indicates the 42nd week of 2002.

Survey Data

Most enquirers seek more information about the US survey mentioned in the original feature. – that showed that most (US) motorhomes are grossly overweight.

This survey I referred to is an ongoing project undertaken by the (US) ‘Recreation Vehicle Safety Education Foundation’ (RVSEF). Leading tyre makers, insurance companies, RV organisations, etc support this organisation. The RVSEF has been weighing motorhomes since 1993 – mostly at rallies. During 2002 the organisation measured the loading on (individual wheels) of 2472 vehicles, bringing the total to date to 17,967 vehicles measured. Of these about two-thirds were motorhomes. The remainder were mostly 5th-wheelers and their towing vehicles. There were also 719 caravans.

Of last year’s (2002 results) an extraordinary 62% of all motorhomes exceeded maximum on-road legal weight or had at least one tyre that exceeded maximum permitted loading.

Exceeding such weights is illegal but, as the RVSEF points out, ‘does not necessarily mean something will break (or at least not straight away)’. But it will shorten tyre life, and may well cause the tyre casing to fail, or tread to separate before that tread is worn out. Exceeding vehicle gross weight inevitably compromises braking (particularly increasing the propensity to brake fade). It is also likely to result in a harsh ride through axles hitting bump stops, poor or dangerous handling, and premature suspension wear.

Legal Loadings Defined

In Australia, the legal maximum weight is called the ‘Gross Vehicle Weight’ (GVM). This is the unladen (Tare) weight, which usually includes some fuel, plus everything you are carrying including driver and passengers, the remaining fuel and water, gas and gas cylinders. If it’s in or on the vehicle it’s included in the GVM – even your dog is included if it’s onboard (serious for us – we’ve just bought a Great Dane puppy - her mum weighs 72 kg).

The GVM is defined as whichever is the lowest of (a) permitted individual tyre loading, (b) individual axle loading, (c) manufacturers’ stipulated maximum weight. Thus, even if one tyre is overloaded, the GVM is, by definition, exceeded.

The Nature of the Problem

According to the RVSEF, a contributing problem is that the ‘unladen’ weight is as delivered from the manufacturer. This may not necessarily include anything that’s not firmly screwed down, nor does it include anything installed by the dealer. New US and Canadian legislation now requires the unladen weight to include full fuel and water tanks, full gas cylinders, and a 70 kg person in each seat provided, and that the true ‘Cargo-Carrying Capacity’ be disclosed.

Further, vehicle manufacturers have little control over the distribution of cargo. If your hobbies include weight lifting, locating 200 kg of barbells (as seen recently) over an already clearly overloaded left front tyre will not help one bit.

Asymmetric Loading

The RVSEF survey also shows that it is common for a motorhome to be within its legal maximum weight and for axle loadings likewise to be within legal limits. Despite this, one or more tyres can be grossly overloaded because of asymmetric placement of batteries or fuel and/or water tanks.

A slide-out placed close to or over an axle can add 300-500 kg to a tyre that is not invariably rated accordingly. The present US trend to multiple and bigger slide-outs, often on one side of the vehicle only, is of obvious concern (unless the weight is balanced on the other side).

Where there is asymmetric loading, tyre pressure for both wheels on the same axle concerned must be set for the higher loading. Overladen light truck tyres can be inflated 10%-20% above the maximum recommended, but this does not increase their legal carrying capacity.

It is strongly recommended that individual tyre weight be checked and internal weight readjusted if necessary. Transport authorities make spot roadside weight checks (in Australia and other countries) by weighing each wheel (or pair if dual wheels) individually. If tyre loading exceeds the legal limit by an appreciable (usually 50 kg) the vehicle stays right where it is until the overload is reduced – or the vehicle lifted and taken away on a big truck. Whilst Australia’s motorhomes are currently rarely checked, the realisation that motorhome overloading seems to be worldwide has not escaped the attention of various authorities. Roadside checking is increasingly common in the USA and Canada.

Air Suspension

It is particularly important to check individual tyre loadings with chassis that have air suspension. One of the primary tasks of such suspension is to keep the vehicle level. It does this by varying the pressure in each air bag. This can severely overload tyres and may not show up except by weighing each wheel separately.

The RVSEF says ‘we have examples of air suspended motorhomes with more than 2000 lbs (>910 kg) difference between each front tyre, and the same (but diagonally opposite) difference at the rear’ [caused by the air system levelling the vehicle – author]. ‘Quite often the result of this characteristic is severely overloaded tyres, which may result in tyre failure and loss of control.’

This feature is based primarily on American, Canadian and South African data and experience. The author acknowledges and thanks the Michelin Tyre Company, the Californian Dept of Motor Vehicles, the (US) Family Motor Coach Association; and, particularly, John Anderson, Executive Director of the US Recreational Vehicle Safety Education Foundation, for his generous assistance.