Pressure and ballast maketh the tyre

They’re expensive and a right pain in the proverbial when they get a puncture.
The relationship most people have with their tyres, on a tractor or a car, is at best indifferent. But it’s time to love your tyres.

The importance of correct tyre and sufficient ballasting cannot be underestimated. If you take the time to accurately set the two there is potential to save thousands of dollars in operating costs such as reduced fuel consumption, less tyre wear and ultimately less time in completing jobs.

Firstly there’s the question of ballast. This has been argued over at many demo days and rural watering holes for many years. Do you go for water? Or add some cast iron? 

Water may be cheap and convenient, but liquids prevent tyres from flexing, so they can’t do the job they were designed to do. It also makes it difficult to accurately set tyre pressures as the volume of air in the tyre is greatly reduced. 

Cast iron makes tractors difficult to handle and once added is left on for good, although front linkages with large front weights are more easily removed when not needed.

The most important aspect of ballasting to understand is how much the tractive weight of the tractor/implement combination needs to be to do the job. For many years the standard figure for cultivation work has been about 45-55kg/hp. So a 200hp tractor should be set up to have an operating weight of about 10 tonnes. Less weight might be needed for higher speeds. And, indeed, an increase for slower speeds causes high draught loadings. Some tweaking may be needed for variable conditions such as implement weight, soil conditions or topography.

With the optimal operating weight determined, we then need to accurately weigh each axle. This will allow us to understand where, and how much, ballast we might need to apply. In general, most manufacturers will work on a split of 60% rear and 40% front static loadings, which will shift nearer to a 50/50 ratio as the draught loadings are applied.

Once the operating weight is determined the hard work is over. We know what the load will be on each axle and how many tyres are being used on each axle – either singles or duals. 

So now the manufacturers’ data will tell us what tyre pressure is required to carry the operating load. Tyre makers’ websites are a great resource for determining load capacities and recommended pressures for their products.

We will need to adjust tyre pressures for different jobs – maybe decrease them for cultivation, or increase them for road work. The most important tool will be an accurate tyre gauge.

In the end, it all adds up to money. A recent test in Europe showed some interesting results. Two identical 190hp tractors were fitted with 7-leg sub-soilers with a 3m operating width. 

Time and fuel consumption were measured over a 200m straight line course.

Tractor 1 ran with the recommended tyre pressure of 0.9 bar (13 psi); tractor 2 ran with a tyre pressure of 1.4 bar (20.5 psi).

Tractor 1 completed the course in 2mins 28 seconds and used 1.2L fuel; tractor 2 completed the course in 2mins 36 seconds and used 1.4L fuel.

The 200m run represented an area of about .058ha, so taken over this area, tractor 2 would use about 3.4L/ha more fuel – at today’s price ($1.22/L) costing $4.15/ha.

The other key point noted was that tractor 1 showed about 10% wheel-slip, whereas tractor 2 showed nearer 20%.

The modern tractor tyre is high-tech, developed over many years by manufacturers who have spent millions on R&D. The correct set-up and utilisation will directly affect your bottom line, so it might be time to fall in love with your tyre rep.

Inflation, deflation take a toll

As a general rule, over/under inflation can result in a number of problems

Over-inflation 

• excessive wheel-slip                   
• greater compaction                       
• reduced performance                        
• accelerated tyre wear

Under-inflation 

• reduced performance       
• increased temperatures at higher speeds                      
• irreparable damage to the tyre carcase                              
• increased fuel costs