Farm Ireland

Sunday 25 June 2017

Obey the markings to prevent the overloading of machinery

If you have ever taken your car for an NCT test, or the van/jeep for a DOE test, you may be aware of some of the criteria that motor vehicle tyres have to comply with in relation to speed and load carrying capacity. Having the incorrect tyres with no speed markings or incorrect load capacities will result in the vehicle failing the test.

There is no such test -- yet -- for an agricultural tractor but there are maximum load and speed markings which are extremely important for an agricultural tyre because they dictate its capacity.


An older, second-hand tyre for example may not be suitable for a 50kph tractor, or its load-carrying capacity could be less than the tractor's lift capability.

Tyre markings and sizes are a bit odd -- a combination of metric and imperial sizes with percentages thrown in for good measure.

Take the example (above) of a 650/65R38 157D tyre:

• 650: Tyre's section width 650mm;

• 65: Aspect ratio, ie tyre side wall height is 65pc of section width 650mm = 422.5mm;

• R38: Rim diameter = 38 inches;

• 157: From the Michelin tyre load index chart, 4.125t is the maximum load for the tyre;

• D: From the Michelin load chart index, 65kph (40mph) is the maximum speed capability of the tyre carrying a maximum load.

These tyre specifications were on the rear of the Valtra N142 Direct, 160hp tractor which we had out on test last year.

This tractor was capable of 50kph and had a lift capacity of up to 8.1t, so the tyres were well matched for this tractor.

Indo Farming