What is twinning
Twinning refers to the mounting of 2 tyres on the same hub of an axle. It enables the vehicle to carry nearly twice as much load on that axle.
Twinned tyres are mainly found at the rear of rigid dumpers and at the front of certain front-loading forklift trucks.
The 2 tyres making up a twinned set should ideally behave like one and the same tyre.
The main advantages of twinned tyres are that they:
- Enable the vehicle to transport heavier loads with tyres having lower individual load capacities;
- Facilitate repairs in the event of a puncture (the vehicle is not completely immobilised on the haul road and can be moved in a safe place).
On the other hand, there are two disadvantages to this system of mounting:
- Possible retention of objects between the 2 tyres; the inner sidewalls of the twinned tyre can be weakened as a consequence;
- Abnormal and fast wears in the case of twinning tyres of different characteristicsl;
- Longer service and maintenance operations.
A few simple measures can minimise these disadvantages; they are outlined below.
What are the best conditions for twinning?
The twinning of tyres is subject to certain strict rules, the most important of which are as follows:
- The tyres must be of the same size.
- Always mount tyres with the same construction (either both diagonal or both radial); never mix different constructions.
- As far as possible, mount tyres of the same brand and type.
- If the 2 tyres in twinned formation have different diameters, both will wear more quickly and in a different way:
- This is because the difference in diameter of twinned tyres will generate abnormal behaviour in the contact patch of each. To simplify, this behaviour can be compared to sliding movement in the opposite direction of each tyre. In consequence, both tyres will wear at a much faster rate.
- As mentioned above, the 2 tyres making up the twinned set should ideally behave like one and the same. However, when the difference in diameter is too great, the behaviour of the twinned pair become incompatible.
- We also strongly advise against twinning tyres with different tread depths (for example an E3 tyre with an E4 tyre).
- The inflation pressure in each of the twinned tyres should be as close as possible, and ideally identical. It is advisable to equip twinned tyres
with rock ejectors to protect the inner sidewalls of the pair. Rock ejectors should be, and remain, in good condition. A bent stonedeflector can puncture the sidewall of a tyre; a worn stone-deflector can turn into a veritable knife-blade and damage the tyre. If they cannot be replaced, a simple solution consists in welding a sphere onto the end of the bars.
- If the twinning of 2 different tyres cannot be avoided, we advise twinning 2 tyres that are over 50% worn.
- Earthmover tyres of twinned pair sometimes present a tapered wear pattern, where the outer edge of each tyre is more worn than the inner edge (the east worn edgesare face to face). To limit this type of wear, it is advisable to interchange the inner and outer tyres of the twinned pair before the difference in the wear on either side of the tyre becomes too pronounced. Moreover, a correctly designed track limits the appearance of this type of wear (for more on this subject, see the paragraph on the construction of tracks in the chapter devoted to the factors affecting tyre life).
Note: this type of conical tread wear is found commonly on industrial tyres whatever the surface conditions. It comes from frequent manoeuvring by machines in restricted areas (small turning circles in relation to the machine size).
The protective measures for operators are described in the chapter on Mounting/Demounting tyres.
Special precaution for demounting twinned tyres
Objects may have been caught between the 2 tyres of a twinned pair. During demounting operations, it is possible for assemblies to be projected from the machine the inflation pressure, causing serious damage or injury.
Furthermore, in the case of vehicles equipped with demountable rims, twinning makes it impossible to check the condition of the inner rim.
If the latter has been damaged (for example a crack around the circumference), only the force exerted by the wheel rim clamps holds the parts in place. When the tyre is demounted, there is a risk that the broken parts will be propelled into the air.
As a result:
It is imperative to fully deflate both tyres of the twinned pair before the demounting.
Failure to comply with this instruction could lead to a serious accident.
MICHELIN has developed a specific training module devoted to mounting and demounting tyres.our usual MICHELIN representative would be happy to present it to you.