Inside this article:
- What do chaser bin tyre ratings mean?
- Common chaser bin tyre sizes
- What is cyclic loading?
- How speed changes carrying capacity
- Tyre pressure and load capacity
- How width and tread affect your soil
- Matching tyre ratings to your total loaded weight
- Tyre checks that protect your rating
What do chaser bin tyre ratings mean?
A chaser bin tyre rating tells you how much weight each tyre can safely carry at a given speed and inflation pressure. The markings on the sidewall follow a standard format, and once you can read them, you can check whether your bin, your crop load and your travel speed sit within what the tyres are actually rated to do.
Take the standard fitment on Davimac 30T single and 45T dual-axle models: the Harvest 900/60-32 20pr HB45 185A8 TL. Broken down:
| Marking | Meaning |
|---|---|
| 900 | Section width in millimetres |
| 60 | Aspect ratio: sidewall height is 60% of the section width |
| 32 | Rim diameter in inches |
| 20pr | Ply rating, an indicator of casing strength |
| 185 | Load index, a standardised code for maximum carrying capacity |
| A8 | Speed symbol: rated to 40 km/h |
| TL | Tubeless |
The load index and speed symbol always work as a pair. A tyre rated 185A8 can carry its indexed load at 40 km/h, and these codes are standardised across manufacturers by bodies such as the Tyre and Rim Association of Australia, whose annually updated Standards Manual is the technical reference for tyre and rim data in Australia. The exact kilogram figures for your tyre at each speed and pressure come from the manufacturer's load and inflation tables, such as the Harvest Tyres load and speed rating tables.
The critical point for chaser bin operators is that the sidewall rating is not a single number. Carrying capacity changes substantially with speed, and chaser bins are assessed under a specific tyre service category called cyclic loading.
Common chaser bin tyre sizes
Chaser bins in Australia overwhelmingly run on 32-inch rims, and the same handful of tyre sizes appear across most brands regardless of who built the bin. The sizes fitted to Davimac chaser bins are these industry-standard sizes, so the guidance in this article applies just as well if you are checking ratings on another machine.
| Size | Construction | Where you'll see it |
|---|---|---|
| 30.5L32 | Bias (cross-ply) | The traditional chaser bin tyre in the older imperial format. Its close metric equivalent is 800/65-32. |
| 800/65-32 or 800/65R32 | Bias or radial | The metric successor to the 30.5L32 footprint, available in both constructions. Radial versions are often offered as flotation tyres. |
| 900/60-32 or 900/60R32 | Bias or radial | The most common size on 30 to 45 tonne bins. Roughly equivalent to the older 35.5L32, with a wider footprint and higher load capacity than the 800 sizes. |
| 1050/50R32 and 1250/50R32 | Radial | Very wide singles seen on large-capacity grain carts, more common in North America than on Australian chaser bins. |
Two things in a size designation matter beyond the dimensions. A dash (900/60-32) indicates bias construction, while an R (900/60R32) indicates radial. Radials have flexible sidewalls that spread the footprint under load, while bias tyres have stiffer casings that ride firmer and resist sidewall damage. Both are legitimate choices on a chaser bin, but they behave differently on the soil, which we cover below.
What is cyclic loading?
Cyclic loading is a tyre service classification for machines whose payload rises and falls repeatedly during work, such as chaser bins, combine harvesters and sprayers. Because the tyre only carries its maximum load for short periods between fills and unloads, it can be rated to carry considerably more weight in cyclic service than it could carry continuously on the road.
The Tyre and Rim Association of Australia defines cyclic loading service for grain cart applications with specific conditions. In plain terms:
- The payload gradually changes from transport load up to the maximum allowable load.
- Unloading must occur before any road transport.
- The minimum load must not exceed 60% of the maximum tyre load, so the load genuinely fluctuates rather than sitting near maximum.
- The maximum load may not be carried more than 1.5 km before unloading begins.
This is why Davimac's maximum towing speeds distinguish between distance travelled when loaded. A loaded bin may travel at up to 20 km/h over distances under 1.5 km, but must not exceed 10 km/h over longer distances, and an empty bin is limited to 30 km/h.
Those limits exist to keep the bin inside the tyre manufacturer's cyclic rating conditions, not just as general caution.
Cyclic ratings are an industry-wide concept rather than something unique to one brand. Tyre manufacturers such as Michelin explain in their agricultural load rating guidance that tyres for harvesters and similar machines are rated with these cyclical load patterns built in.
How speed changes carrying capacity
Slower speeds generate less heat and casing stress, so the same tyre at the same pressure can carry significantly more weight at 10 km/h than at 40 km/h. On the tyres fitted to Davimac chaser bins, the difference between road speed and unloading speed is close to double the carrying capacity.
Harvest grain cart tyre ratings by speed
Harvest Tyres publishes the following maximum loads per tyre for chaser bin (grain cart) application, at each tyre's maximum inflation pressure. These loads are covered by Harvest's 7-year guarantee for grain cart application.
| Max load per tyre | 30.5L32 18pr HB45 167A8 | 800/65-32 20pr HB45 179A8 | 900/60-32 20pr HB45 185A8 | 800/65R32 Trojan 185A8/181D |
|---|---|---|---|---|
| Up to 40 km/h | 7,200 kg | 9,300 kg | 11,100 kg | 10,082 kg |
| Cyclic, up to 25 km/h | 8,496 kg | 10,974 kg | 13,098 kg | 10,638 kg |
| Cyclic, at 15 km/h | 12,249 kg | 15,810 kg | 18,870 kg | 14,338 kg |
| Cyclic, at 10 km/h | 13,465 kg | 17,390 kg | 20,757 kg | 15,725 kg |
| Max inflation pressure | 45 psi (310 kPa) | 60 psi (414 kPa) | 60 psi (414 kPa) | 64 psi (441 kPa) |
Source: Harvest Tyres grain cart application ratings, as supplied in the Davimac chaser bin operators manual. Cyclic loads at 15 km/h and up to 25 km/h apply for a distance of up to 1.5 km before unloading must begin. Download the Harvest Grain Cart Tyres info sheet for the full conditions.
Worked example: checking a loaded bin against its tyre rating
A Davimac 35T dual-axle bin fully loaded with wheat has a total loaded weight of approximately 49.3 tonnes (14.3 tonnes empty machine plus 35 tonnes of wheat). Not all of that rides on the bin tyres: 2,200 kg sits on the tractor drawbar as tongue weight, leaving around 47.1 tonnes across the four Harvest 900/60-32 tyres, or roughly 11,800 kg per tyre. Against the table above, the bin sits comfortably inside the 20,757 kg per-tyre rating at 10 km/h, and still inside the 13,098 kg rating at 25 km/h for short runs to the truck.
These figures assume wheat. Other crops fill the same bin volume at very different weights: a 35T bin holds over 37 tonnes of lentils but only 23 tonnes of oats, so the load your tyres actually carry depends on what's in the bin, not the badge on the side. Use our grain weight and density chart to work out what your crop actually weighs before checking it against the tyre rating.
Run the same numbers on a fully loaded 45T at 62.8 tonnes total, less its 2,950 kg tongue weight, and each of its four tyres carries around 15,000 kg. That is well within the 10 km/h and 15 km/h cyclic ratings, but above the 25 km/h rating. This is exactly why the loaded speed limits matter: the tyres are generously rated for how a chaser bin is meant to be operated, and the margin disappears if a loaded bin is towed at road speeds.
Even distribution across the tyres is an assumption, too. Uneven filling and slopes shift weight between tyres, which is one reason the manual instructs operators to fill the bin evenly. Loaded tongue weights for every model are published on our specifications page.
Tyre pressure and load capacity
Every load figure in the table above assumes the tyre is inflated to its maximum rated pressure, and an underinflated tyre cannot carry its rated load regardless of what the sidewall says. Harvest Tyres covers this principle and the parameters behind working pressures in its tyre pressure guidance.
Air carries the load, not the rubber.
Running below maximum pressure is legitimate and often desirable in the paddock, since lower pressures spread the footprint and reduce ground pressure. The trade-off is reduced carrying capacity, so the correct pressure for your operation depends on your actual per-tyre load, travel speed and distances. The tyre OEM's load and inflation tables are the authority here, and it is worth a call to the manufacturer if your setup does not match a published scenario.
Pressure should be checked weekly during the season, and both over-inflation and under-inflation carry real costs: under-inflation causes bead slippage and sidewall damage, while over-inflation increases soil compaction and machine bounce. Our guide on how to check your chaser bin tyre pressure covers the checking procedure and the safety steps for inflating large agricultural tyres.
Maximum inflation pressures
Maximum inflation pressures for the standard Davimac tyre options are set out below.
| Tyre size | psi (max) | kPa (max) |
|---|---|---|
| Harvest 30.5L32 18pr HB45 167A8 TL | 45 | 310 |
| Harvest 800/65-32 20pr HB45 179A8 TL | 60 | 414 |
| Harvest 900/60-32 20pr HB45 185A8 TL | 60 | 414 |
| Harvest 800/65 R32 Trojan R3 Flotation | 64 | 441 |
How width and tread affect your soil
A fully loaded chaser bin is one of the heaviest machines that will cross your paddock during harvest, so the tyres are not just a load-carrying decision. They determine how much of that weight the soil feels, and where.
Tyre width
Width is the biggest factor. Ground pressure is load divided by contact area, so at the same load and inflation pressure, a wider tyre spreads the weight over a larger footprint and pushes less force into the soil profile. Moving from an 800-section to a 900-section tyre adds roughly 100 mm of width per tyre, and the difference compounds across four tyres on a dual-axle bin. This is why grain cart tyre suppliers publish footprint comparisons: NTS Tire Supply's grain cart tyre comparison shows how total footprint area changes with tyre size and construction, and how that maps to estimated compaction.
Tyre Tread
Tread pattern and construction matter too. Traction (R-1) lug treads bite into the soil for grip, which also means more surface disturbance. Flotation (R-3) treads are shallower and more closely spaced, designed to ride over the soil rather than dig into it, trading traction for reduced disturbance. On a towed machine like a chaser bin, which does not need to generate its own drive traction, a flotation tread gives up little in practice. This is the reasoning behind the Harvest Trojan R3 flotation option on Davimac bins. Radial construction adds to the effect, since the flexible sidewall lengthens the contact patch under load compared with a stiffer bias casing at the same pressure.
Controlled Traffic Farming
None of this replaces good traffic management. Compaction from harvest machinery persists well below the surface, which is why CTF-adjustable wheel centres are standard on Davimac 25-35T models, letting the bin run on the same wheel tracks as the rest of your gear.
Matching tyre ratings to your total loaded weight
The tyre rating is one layer of a chaser bin's overall load capacity, alongside the axle rating and the empty machine weight. Davimac single-axle models carry a 40-tonne axle rating and dual-axle models an 80-tonne rating, but the actual undercarriage capacity is always subject to the fitted tyre specification. In practice, the tyres are usually the governing limit, which is why they deserve more attention than they typically get.
The working method is straightforward. Calculate your total loaded weight (empty machine weight plus crop load), subtract the tongue weight carried on the tractor drawbar, divide the remainder across the number of tyres, and check the result against the OEM's cyclic rating for the speed you intend to travel. Crop load varies significantly with bulk density, so a bin that is at capacity with lentils behaves differently to the same bin with oats.
Our grain weight and density chart walks you through the full calculation with per-crop capacities for every model.
Tyre ratings are also worth checking whenever you change tyres, since all Davimac chaser bins can be fitted with a range of tyre options and any replacement should meet or exceed the load and speed ratings specified by the OEM. On dual-axle bins, all four tyres must be matching sizes: mismatches shift load unevenly between tyres and cause overloading and premature failure.
Tyre checks that protect your rating
A tyre's rated capacity assumes it is in sound condition. Inspect tyres regularly for cuts, cracks, abnormal wear, bulges or embedded objects, and replace any tyre showing damage or deterioration. Check wheel nut torque daily for the first three days after fitting and weekly after that, and keep tyres clear of oils, fuels and solvents that degrade the rubber. Tyre changing carries serious risks and should only be performed by trained personnel with the correct equipment.
Full tyre safety procedures, torque specifications and the OEM tyre information sheet are included in the chaser bin operators manual, available on our manuals page.
