How to optimize the Mechanical Lead of a tractor

Within the agricultural field the “Mechanical Lead” is the ratio between the theoretical distance covered by the front tyres and the one covered by the rear ones for each tyre revolution. In terms of percentage it is an important parameter in order to optimise the fuel consumption

During the road transfers of a tractor every revolution of the rear tyres is theoretically equal to their rolling circumference. In theory this distance should be equal to the one of the front tyres when the 4 wheel drive is enabled, with the difference that whereas the rear tyres make only one revolution the front ones need to roll more because of their smaller rolling circumferences. In this situation the mechanical gear between the two axles should help in optimising the covered distances. Considering real market situations things are different. In fact, the distance covered by the front tyres with each complete revolution of the rear ones has to be always slightly greater to the rolling circumferences of the latter. This is necessary in order to gain a traction which is slightly higher to the one of the rear tyres. If the mechanical lead is not correct some problems could arise. For instance, an improved fuel consumption, braking problems, high wear of the tyre tread, potential breaking of the mechanical parts.

A 0.5-5% range

The ratio between the distance covered by the front tyres in comparison to the one of the rear tyres when they make a complete revolution is called “Mechanical Lead” and it normally varies between a 0.5% – 5% range according to the equipped tyres, to the applied loads and to the inflation pressures. These parameters are indeed the ones which influence the effective rolling circumferences of tyres the most when compared to the theorical ones and taking into account the fact that they vary greatly according to each tractor, these are parameters that influence in a direct way the hourly consumption of the machineries and the management costs linked to them.

The tests which have been organised among the premises of “Provana Group Innovation” in Izano, close to the Italian city of Cremona, show that a correct Mechanical Lead reduces the fuel consumption and the tyre wear, with the latter that is being considered for both road surfaces and hard soils. In fact, the soft soils/soils with low grip act as a “natural differential” by allowing a slight slippage of the front tyres which does not negatively affect their improved traction and influences the tyre tread wear in a minimal way. On the other side if we consider the soils with a high grip the slippage is not allowed so the “mechanical stress” caused by mechanical leads which are too high affects both the tyres and the transmission parts. If we want to work in optimal conditions, so with the right mechanical leads, it is necessary to optimise the tyre pairing between the front tyres and the rear tyres in terms of dimensions and then optimise the inflation pressures in order to maintain a constant ratio between the static and the effective rolling circumferences. We also need to consider the idea that tyres with the same sizes but manufactured by different brands do not present the same rolling circumferences.

We are talking about complex set-ups that can be managed only by experts in the industry but it is possible, for instance, to rely on the mechanical lead software created by the Provana Group to simplify the procedures. The software is the result of hundreds of organised tests both on the road and in the field with the use of tractors equipped with sensors, devices and data analysis software. It is available at the following link (Italian version).

The expert’s opinion

“The idea of Mechanical Lead” says Agostino Provana, the manager of Provana Group Innovation, “is based on the ratio between the circumferences of the front tyres and the rear tyres when the 4 wheel drive is enabled. This ratio is expressed in percentage terms and the values usually range from 0.5% to 5%. The percentage varies according to the tractor brand. An optimal mechanical lead gives the opportunity to avoid a premature wear of both tyres and mechanical parts and to optimise the fuel consumption. A wrong mechanical lead causes a premature wear of the tyres and affects negatively the traction of the machinery as well. The mechanical lead should be taken into account from the beginning, when the tyres are mounted, through an accurate size check in order to guarantee the right tyre pairing. Other factors which need to be considered are represented by both pressures and loads since they influence the rolling circumferences in a positive or in a negative way: the result is a consumption optimisation or, on the contrary, an excessive waste of resources. It should be noted that the various tyre brands which are available on the market offer products which present for the same size different technical characteristics: these specifications, such as the lug height, influence the Mechanical Lead too.”

(translation of the article on the following website: Macchine Trattori – Link – Page 88)

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