Tractor Tyres on Front Wheel Assist Tractors
The introduction of front wheel assist tractors probably caused more heartache to rural tyre services than any other recent innovation.
The traditional 2 wheel drive tractor had small front tyres, with just enough load on them to steer with, and around 75 to 80% of the weight over the rear axle. This was obtained by filling the tyres with water. Weight over the drive axle translates directly into pull at the drawbar, which means that more work can be done by dragging more, or bigger implements. However, the extra weight chewed up fuel, which got more and more expensive.
Enter the front wheel assist tractor. These have all wheels driven, and the front tyres are around 2/3rds the size of the rear. The advantage is that all tyres pull. So water is not necessary for good traction, and fuel economy is improved.
A gearbox to transfer drive to the front axle as well as the rear, is incorporated. If the tyres are 2/3rds the size of the rear, then driving them 3/2 times faster means that they lay down the same track speed. But there’s a little more to it than that!
The front tyres are generally driven around 2% faster ( 2% lead) than the matching speed over the ground of the rears. This makes the tractor easier to steer, but more importantly, more efficient. The front wheels when working in tilled soil, operate more efficiently at higher slip than the rears, which are tracking in the soil consolidated by the front wheels.
Higher slip means higher wear, particularly on hard surfaces, so it is recommended that the front wheel drive is disconnected when on hard surfaces e.g. roads, where soil slip cannot take place. There is generally a switch on the dash to permit this easily. If the driven track speeds do not match, then the axles fight each other- the transmission “winds up”, and may break, or the tractor “bucks” in the field, and it uses more fuel because it’s not pulling as well as it should.
The introduction of these tractors has boosted the popularity of radial tractor tyres. The old design bias ply varied their rolling circumference with changes in pressure. Radials don’t- they roll around the circumference of the belts built into the casing, which stretch very little – as intended. This means the axle ratio chosen on the transfer case can be selected by the manufacturer to very tight tolerances, since the rolling circumferences of the front and rear tyres stay constant.
This does not mean that bias tyres cannot be used. Indeed, it may be that the imported radials are not available at all, so that you have to adopt another fitment. A matching tyre size can be chosen, and the inflation pressure varied within limits, both tyre casing limits, and ride comfort, to achieve the desired “lead” of 2% or thereabouts (consult your manual) on the tractor. A later article tells you how to test your tractor on a farm road to determine the “lead”.
Be aware though, that because they are smaller, rotate more, and run at higher slip angles, that you will become aware of the need for replacements faster than you think. Be sure to consult your tyre expert. There have been some very expensive disasters.