13 March 1998

Several reasons why fitting bigger can be better

By Andy Collings

FITTING bigger tyres to agricultural vehicles offers three main benefits:

lBetter use of available power and higher work rates through increased traction.

lReduced soil compaction by spreading the weight of tractor and implement over a larger tyre/soil contact area.

lImproved driver comfort where the bigger tyre can be used at lower inflation pressures than the fitment it replaces.

A like-for-like comparison of a 600/65R38 Kleber Super 11L with the 18.4R38 Super 9 emphasises the point.

Fitted on a typical 130hp tractor carrying a five-furrow mounted plough, the smaller tyre needs inflating to 1.65 bar (24psi) to carry the 3380kg (7451lb) per tyre load. The larger tyre, in contrast, needs only 1.17 bar (17psi).

This is partly because of the low-profile tyres structure but more especially because of its larger volume – the 600/65R38 contains 34% more air than the smaller tyre, and it is air more than anything else that supports the tractors weight.

A nominal 28% increase in tread width per tyre tells only part of the story when it comes to assessing the likely improvement in traction and flotation from the upgrade. Thanks to the lower inflation pressure leaving the bigger tyre more "squashy", the contact patch is longer as well as wider.

In area terms, increasing by almost 27.5% from 2632sq cm (408sq in) for the smaller 80-series tyre to 3355sq cm (520sq in) for the bigger volume fitment. &#42

Wider tread is only part of the story – 34% more air volume means this 600/65R38 (right) can carry a typical 130hp tractor and five-furrow plough at 1.17 bar (17psi) when the 18.4R38 alongside needs 1.65 bar (24psi).