The rims are mechanically fixed to the axle by various bolts to the hub, at the centre of the wheel disc.
However, this is not the case for the tyre, which is only held in place by the internal pressure on the bead applied against the rim seat.
In these conditions it is possible in certain cases that the tyre will slip on the rim. This is a real problem because it leads to the progressive destruction of the bead and means that neither the airtightness nor the adhesion necessary for normal use of your tyre are guaranteed.
You have to be extremely careful with lubricants because even if this aspect may seem neutral or trivial, use of the wrong lubricant can result in the need to replace your new tyre, with major financial consequences.
All hydrocarbon based products, all mineral oils used in engines must be avoided. These are too lubricating and will remain oily permanently, systematically leading to the rotation of the tyre on the rim. In addition, hydrocarbons and similar products are incompatible with the natural rubber in the tyres, attacking the chemical structure of the polymer.
To ensure correct mounting or dismounting, you must use a specific lubricant designed exclusively for mounting agricultural tyres. The main advantage of this type of lubricant is that it dries and evaporates and therefore doesn’t leave any residue either on the rim or on the tyre. It prevents the tyre from sticking to the rim and preserves it from corrosion.
The knurling or striae on the rim where the bead is placed have an important role to play. They make is possible to maintain and stabilise the bead, by preventing it from rotating.
If there are slight traces of rust on the knurling, this can help the tyre catch onto the rim properly.
When you change tyres or mount a specific tyre, make sure that you clean this area carefully with a metal brush to remove all the loose rust particles. If there are no lumps in the traces of rust, the tyre bead will grip correctly.
The natural reflex is to protect the iron from corrosion by using anti-rust paint and this can be beneficial to protect the rim installation points on the hub abutment face of the axle.
However, you must avoid painting the striae on which the bead lies because the paint may get into the grooves of the knurling, flattening out the surface, which will reduce the grip capacity in this area. Painting is likely to smooth over this part of the wheel, causing the tyre to rotate on the rim whenever the machine has need of torque.
During pulling competitions when torque is very high, the tyre is generally bolted through the rim seat to the tyre bead to avoid rotation.
If the tyre begins to turn on the rim, the striae, which had a positive gripping role, will have a negative role, because they will destroy the rubber in the tyre’s bead, exposing the layers of fibre making up the structure or worse even, the bead wires, necessitating immediate replacement.
The tyre bead must fit perfectly in the right spot on the rim seat and over the full circumference of the tyre. The slightest deformation of the rim may modify the position of the bead slightly, leading to a loss of airtightness or, in certain conditions, when the torque is higher, to rotation of the tyre on the rim.
The deformation may be linked to a low quality rim whose form is slightly ovoid, or a rim that has become deformed under the weight of an excess load, or use in the wrong conditions, or too much accumulation of rust particles on the rim seat which increase its diameter in places.
The rim must correspond perfectly to the size of your tyre, it is the sidewalls and beads which maintain the tyre in place through force of pressure.
If a wide tyre is mounted on a rim that is too narrow, the position of the beads will not be adapted. This incorrect positioning and the resulting deformation will lead to erosion of the rubber of the bead and therefore a premature fragility in this area of the tyre. The risk of the beads coming off the rim increases and can lead to a sudden loss of pressure if you drive with a heavy load, with overinflated tyres or too fast.
Reducing pressure is obviously effective in avoiding soil compaction and preserving the productivity of your fields. However, working with a low inflation pressure is not recommended with standard tyres, which have not been designed for this use, especially if you require a large load capacity or torque.
Pressure that is too low results in deformation of the sidewalls and mechanically reduces the contact patch between the bead and the rim.
The main consequences will be the risk of the tyre rotating on the rim, as well as the risk of loss of grip and airtightness. This loss of additional pressure will amplify the deformation further, leading to breaks in the sidewall, on the rim flange, just above the bead turn-up, obliging you to replace the tyre.
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