A common question that we get asked is – what alternative tyre or wheel size will help me increase traction or reduce pressure on the ground? Responsible resellers will first ascertain that the tractor is set up correctly for the task on hand. You may not need to change your wheels at all!
1) Work out whether you have the correct level of ballast for your application. You should consider:
- What implement you are using
- Your forward speed
- The horsepower of your machine
- The amount of draft force (heavy draft is application such as ploughing or subsoiling, transport and PTO operation will typically be low draft)
As a rule of thumb, you should work on these suggested levels:
- Heavy draft applications: 55kg/hp
- Light till and PTO work: 45kg/hp
- Transport and PTO work: 35kg/hp
2) Consider how you will adjust the overall weight requirement. Reducing weight is difficult, but tractors with a high amount of weight for their horsepower are typically best suited to heavy draft work. To add weight to your tractor consider liquid ballast or cast weights:
- Liquid ballast can be a cheap and effective solution, but adding ballast will make the tyre stiffer and some of the advantage of radial tyre technology will be lost. Your tyre data book will give typically give you the number of litres for a 75% fill, but to retain the best use of your tyre try not to use more than 40% of the volume with liquid ballast.
- Cast weights are a preferable solution but the cost can be prohibitive.
3) Now think about your weight distribution.
- Articulated and equal wheeled tractors should have approximately 51-55% of their weight on the front axle when using trailed implement, or 55-60% when using mounted implements. This accounts for the amount of weight transfer from your implement when in work.
- Standard tractors differ depending on whether front suspension is fitted. Typically a tractor without front suspension should have 35% of its weight on the front axle, increasing to 40% if suspension is fitted. Mounted equipment and trailed equipment with the drawbar positioned high on the tractor can require up to 55% of the weight on the front axle. Take care though, as when a mounted machine is in work the power distribution can move the front axle leading to ‘power hop’.
Achieving correct weight distribution will enable you to reduce tyre pressures, achieve correct wheel slip, decrease fuel consumption and increase traction.
4) Now that you have the machine set up, you can consider the tyre load in your application – a weighbridge or weight cells may be required here! Various machines affect tyre load in different ways:
- Trailed implements with depth/transport wheels that carry the implement weight. These have little effect on the tractor weight distribution, but you should consider the machine overall weight in road transport – weight transfer to the tractor means an increased pressure requirement, but if there is little transfer you should ensure the tractor has adequate weight to maintain stability.
- Trailed machine with high drawbar load (wheel at rear of machine). Take care to measure the front axle load when the implement is on the ground, and the rear axle load when the implement is raised
- Mounted implements. Again, measure front axle load with the machine on the ground, and rear axle load with the machine raised
- Front and rear combinations. Measure front and rear axle loads with both implements raised
- Trailers. Measure front axle load with an empty trailer, and front and rear axle load with a fully loaded trailer. Ensure there is enough weight on the front axle to keep adequate control of your tractor/trailer combination.
- Front loaders and forklifts- measure front axle load with the machine under load. Pay particular attention to shovelling from heaps- the load can be greatly increased, but very difficult to measure
5) Having established the correct amount of weight and distribution for your tractor, you should now consider the correct tyre pressure for your application. For this, you should find your tyre technical data which will look much like this:
Your application is important here and you will find it difficult to obtain clear guidance. The following is an effective rule of thumb:
- Low torque work, e.g. fertiliser spreader or trailer, or PTO work. Here, you can work with the speed of your operation compared to the charts, or reduce pressures to the lowest LT setting
- Higher torque work – sometimes the chart will provide details for this specifically, but where this is not the case the figures for 30kmh are generally acceptable
- Tractors with dual wheels – dual wheels will allow for a pressure reduction – consult both your tyre technical data book and your tractor’s operators manual
- In furrow ploughing – higher pressures will be required here as two of the tyres will be doing most of the work.
- Hedgecutting – again – consider the tyre load with the hedgecutter extended to one side.
Be sure to read the small print here – some manufacturers will recommend additional pressure for sustained road usage. There may also be guidance given for cyclic applications where the load increases or decreases periodically, e.g. a loader or combine harvester. There will be further guidance for dual wheels where the pressure can be further reduced.
Be sure to work with a pressure that works for your maximum speed and maximum load. Be safe!
6) Having set up your tractor correctly, you should be able to assess whether you have the correct wheel equipment for your applications. There are some further points to consider here:
- Wheelslip. You should aim for 10-12% wheel slip in a typical agricultural application with a wheeled tractor. Running significantly out of this zone could indicate incorrect set up or tyre wear.
- Horsepower. Each of your tyres will have a load rating, and a professional tyre consultant will be able to advise whether your tyre’s load rating, in combination, is sufficient for your tractor’s’ horsepower.
- Pressure operating range. Tyre will work at their best when you are using pressures away from the high and low points of the pressure table.
- Fuel usage/wear rate. Consider the balance between road and field usage. Tyre technology has advance massively resulting in substantial pressure and fuel saving in the field – however, these gains can sometimes be offset by premature wear on the road. However, some manufacturers also provide tyres with a bias towards road usage with impressive increase in tyre longevity. The key here is focus on your main application – typically a tyre designed for field usage will still give adequate life on the road if correctly inflated. Similarly, a tyre designed for the road can be used effectively in the field. As a general rule, the best focus is to pick one of the best technologies- often, the tyre marketed as ’best of both worlds’ has no compelling advantage. A consultant will be able to advise you here.
- Tyre wear. Tyres provide their most effective performance when there is more than 50% of tread remaining.
- Tyre width. Are your tyres suitable for your crop’s row spacing? Would the row take a wider tyre and reduce pressure? Is it too wide and suffering damage to the tyre sidewall or the crop? There any many alternative sizes available that may not have been options on your tractor originally, and a responsible consultant will guide you here.
BWT is a professional and trusted advisor for tractor wheel and tyre solutions and can be contacted on 01371 832310 or info@bwt.uk.com
