## LESSON 20. Off road traction

20.1. Introduction

The primary purpose of agricultural tractors, especially those in the middle to high power ranges, is to perform drawbar work. The value of a tractor is measured by the amount of work accomplished relative to the cost incurred in getting the work done. Drawbar work is defined by pull and travel speed. Therefore, the ideal tractor converts all the energy from the fuel into useful work at the drawbar. In practice, most of the potential energy is lost in the conversion of chemical energy to mechanical energy, along with losses from the engine through the drivetrain and finally through the tractive device. Research shows that about 20% to 55% of the available tractor energy is wasted at the tractive device/soil interface. This energy wears the tires and compacts the soil to a degree that may cause detrimental crop production (Burt et al., 1982).

Efficient operation of farm tractors includes:

(1) maximizing the fuel efficiency of the engine and drive train,

(2) maximizing the tractive advantage of the traction devices, and

(3) selecting an optimum travel speed for a given tractor-implement system.

Throughout the years, official tractor performance drawbar tests have been conducted on hard surfaces and in recent years (30+ years) on concrete. While this provides a valid comparison between tractors, the data does not provide much information about performance under field conditions. The primary difference between official tests and field conditions is the performance of the tires or other tractive devices.

20.2. Solid Wheel on a Hard Surface

An understanding of traction mechanics is fundamental to understanding differences between tractive performance and tractor performance. The basic forces involved in a powered wheel are shown in figure 1 for the simple case of a solid wheel on a hard surface. The torque input (T) develops a gross traction (GT) acting at the contact surface. Part of the gross traction is required to overcome motion resistance (MR), which is the resistance to the motion of the wheel, including internal and external forces. The remainder is equal to the net traction (NT) that the wheel develops, given by NT = GT - MR.

Fig. 3.1. Basic wheel forces for a solid wheel on a hard surface