Spray application manual
31 January 2025
Module 15: Weight, balance and tyres
15.1 Introduction
Published 24 January 2025 | Last updated 20 January 2025
Tyre selection for sprayers can be quite different to other pieces of farm machinery. A spray rig can go through a cycle of having either a full spray tank or an empty one, having the boom folded or unfolded, and may be travelling at different spraying speeds or road speeds. All of these can place different demands on tyres, particularly inflation pressures.
The weight and balance of the sprayer must be taken into account when selecting a suitable tyre. If the sprayer’s weight is not correctly balanced, this can make the choice of a suitable tyre and inflation pressure very challenging.
Weight, balance and tyre pressure may affect the performance of functions such as auto-steer and auto height control, and will impact on the operator’s comfort. Weight and balance can also have a significant impact on tyre wear, traction and fuel economy.
When the tyre type is not correctly matched to the sprayer, the tyre pressure may need to be set at the tyre manufacturer’s maximum pressure to be able to carry the loads required of a full sprayer. At the maximum inflation pressure, the tyre wall’s flexing ability can be compromised, and the shape of the tyre tread contacting the soil could be a ‘V’ shape, rather than being flat. Running the tyres at maximum pressure can lead to damage to the wheel tracks.
Introduction sprayer weight, balance and tyres
9 January 2025Importance of matching tyre pressure to weight on each of the spray rig's wheels. Another video from GRDC's Spray Application GROWNOTES™ series.
Graham Betts: Ash, when you purchased your Miller sprayer a few years ago, how long did your first lot of tyres last for?
Ashley Wakefield: Approximately 2 years, I reckon.
Graham Betts: And why did they only last 2 years?
Ashley Wakefield: The sidewalls kept bubbling up and they were just failing, cracking and eventually blowing.
Graham Betts: So what process did you go through to replace the tyres?
Ashley Wakefield: We started looking at the tyre ratings and the numbers on the side of the tyres. We found that the tyres that were on there were only barely specced to do what we needed, so we started looking in the industry to find tyres with a better load rating and better quality.
Graham Betts: With the tyres that you put on, what did you notice?
Ashley Wakefield: Firstly, because we went for a tyre that has more tread on the ground, we noticed less compaction and less cutting up of the soil. Also, since we opted for better quality and better-rated tyres, they were carrying the weight better.
Graham Betts: You had damage on one of the new tyres. What do you think caused that?
Ashley Wakefield: One of the new tyres developed a bubble on the sidewall. We don’t know what caused it, but we had to replace it to avoid the risk of it blowing out when driving down the road at 40 kilometres an hour.
Graham Betts: To select the correct tyre, we need to be able to weigh the sprayer. What have you done to accommodate that?
Ashley Wakefield: A few years ago, I bought a set of scales for weighing grain. We use those scales to weigh the sprayer, allowing us to measure the weight on the front axle, back axle, or individual tyres. This way, we know the exact weight of the machine front-to-rear and on each individual tyre.
Graham Betts: What did you notice physically about the new tyres?
Ashley Wakefield: One tyre had a significant bulge after we installed it. I got the tyre rep to check it, and while he said the pressure was fine, we still had one tyre with a major bulge, another with a minor one, and the other two weren’t too bad.
Graham Betts: Now that we’ve adjusted the pressure in the tyres with the boom folded and the tank full, what have you noticed about their shape?
Ashley Wakefield: Two of them look as though they’re pumped up normally, but the other two still seem flat even though they have the same pressure.
Graham Betts: You added the tyre dog system on your sprayer tyres, which monitors pressure via the valve stem and displays it on a screen in the cab. What have you noticed?
Ashley Wakefield: It shows the accurate pressure in the tyre. The only thing is that you do notice a slight increase in pressure as the tyres go from cold to running temperatures, usually by 2 or 3 psi.
Graham Betts: That’s not uncommon. How did you figure out the issue with the tyre shapes being uneven?
Ashley Wakefield: By chance, we weighed the front and back tyres on the dual pads, then weighed them individually. We found a significant discrepancy in weight between the side-by-side tyres, front and rear—up to 2-3 tonnes.
Graham Betts: That’s a lot. So what are you going to do about the pressure?
Ashley Wakefield: We’ve adjusted the pressures so the tyres look the same. There’s now a 30-psi difference between two of the tyres to balance the load properly.
Graham Betts: Does the shape of the tyres look better now?
Ashley Wakefield: Yes, all four tyres now look as though they’re evenly pumped, even though two of them have a 30-psi difference.
Remote systems for checking tyre pressure
9 January 2025Tip for a visual check of tyre pressure. Another video from GRDC's Spray Application GROWNOTES™ series.
Graham Betts: Just a good little handy tip, a farmer tip. Run an imaginery line down your tyre there, if you've got a bit of dirt up here that means the tyre could be too low a pressure, and if you've got a black line and the dirty line finishes just there that would mean your tyre is too high. It's very important that when you're travelling down the road, or down a bit of bitumen or whatever that that contact point just across the leading edge there.