Spray application manual
31 January 2025
Module 15: Weight, balance and tyres
15.3 Establishing the sprayer's weight per axle
Published 24 January 2025 | Last updated 20 January 2025
Before attempting to establish the sprayer’s weight per axle (load per tyre), the spray operator should measure and record the current tyre pressures using a calibrated tyre air pressure gauge.
Tip
Make sure the gauge is accurate. Take your tyre air pressure gauge to a tyre outlet and check it against one of their calibrated gauges. If it is not accurate, replace the gauge.
Importance of matching tyre pressure to weight on each wheelon each
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.
Weighing the sprayer
Weighing the sprayer (self-propelled, tractor with linkage, or tractor with trailing sprayer) to establish the weight per wheel, or weight per axle, should be done when the sprayer has the fuel, oil, main spray and flush tanks full.
The weight should only be checked when the sprayer is level, by using a weighbridge or single weigh pad.
It is ideal if the sprayer can be weighed per tyre, to see if there is any difference between left and right-hand-side tyres on the sprayer.
Record the following weights per tyre:
main tank empty with boom folded, front and rear axle;
main tank empty with boom unfolded, front and rear axle;
main tank full with boom folded, front and rear axle; and
main tank full with boom unfolded, front and rear axle.
Sprayer tyre on a weigh pad
Tip
When purchasing tyres, you need to tell the tyre dealer what loads the tyre is expected to carry, the rim numbering and the size of the tyre required.
Case study of weights per wheel for a self-propelled sprayer
The information in Table 6 was obtained from a self-propelled sprayer. An analysis of the data and implications for tyre selection and operating pressure are discussed below.
The recorded weights of a self-propelled sprayer
Tyres fitted to the sprayer
The above self-propelled sprayer had Michelin, SprayBib, VF380/90 R46, R-1, 173D tyres fitted onto rim number DW13A.
Michelin recommended information for the above tyre: approved rim DW13A, rolling circumference 5530mm, maximum load 6500kg, maximum pressure of 4.4 bar (64 psi) at 65km/h.
Before weighing the above sprayer, the suggested cold tyre pressure was 2.1 bar (30 psi) for the tyres on the front axle, and 2.4 bar (35 psi) for the tyres on the rear axle.
After weighing the sprayer and using an appropriate tyre pressure calculator, such as the www. tirepressrecalculator.com, the suggested cold tyre pressure was 1.2 bar (17 psi) in the tyres on the front axle, and 3.6 bar (52 psi) in the tyres in the rear axle. (Note: in this calculator the weight needs to be entered in pounds, and not kilograms).
If the sprayer was balanced e.g. 15,760kg ÷ 2 axles = 7880kg per axle, the front and rear cold tyre pressure could be 1.8 bar (26 psi) with the tyres fitted.
As a guide, whatever the cold tyre pressure is, the operator should add about 2.0 psi to determine the pressure that is being put on the ground.
Working the tyre at its maximum load capacity and cold pressure rating could have an impact on the soil.
Find out more
Tyre pressure calculator: https://www.tirepressurecalculator.com/
Calculating tyre pressure using web based tools
9 January 2025Calculating tyre pressure using web based tools. Another video from GRDC's Spray Application GROWNOTES™ series.
Graham Betts: This exercise could be for the tyres you've got on the sprayer at the moment and the potential tyres you're looking at purchasing. So we've got all the information—weights with the boom folded and unfolded, tank full. Now we need to do something with that information.
We recorded the brand and model of the sprayer: obviously the Miller Nitro, model 4135. It’s got a 105-foot boom, a 6,000-litre tank, and 128-inch tyre centres. We recorded the weights with the boom folded and tank full: the front axle was 6,670 kg, and the rear axle was 9,070 kg.
The important weight, however, was with the boom unfolded and the tank full. The front axle was 7,110 kg, which is 15,642 pounds. The rear axle was 8,530 kg, or 18,760 pounds.
Why do we need the weights in pounds? That’s the unit required for entering into the tyre pressure calculator website.
The tyre size is 520, the ratio is 80, and the rim size is 38. The load rating is 172, and the front axle weight was 7,110 kg or 15,642 pounds. The tyres are R3, R4 type tyres, so that’s fine.
Select the application: we’re spraying at about 15 miles per hour. We’re using singles. After calculating, we see that the front tyres need 17 psi.
Next, enter the rear axle weight, which was 18,760 pounds. For the rear tyres, we need 23 psi. So, with the boom unfolded and the tank full, the tyre pressures are 17 psi on the front two tyres and 23 psi on the back two.
The main goal of this exercise is to ensure the tyres are correctly set up to reduce tyre pressure for less impact on the soil, better boom stability, improved handling, and reduced wear. From experience, Ash and others have noticed significant improvements by going through this process.
You might hit a few blank walls when you ask questions, but if you work through it, the effort will be rewarding.