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Tyres for all uses: there is a huge number of vehicles that need tyres... from scale models to the Space Shuttle

Nicodemo Angì

When we say "tyre" what we mean is an impressive variety of methods and uses. To all appearances the same, but in fact very different, our much-loved tyres are found on a range of vehicles that go on roads, below the ground and even into space!
Modern tyres have achieved such a high level of perfection that they can be used in practically any environment and in any working conditions. Our journey through extreme tyres begins on the ground or, rather, under the ground. Mines are in fact full of vehicles - drills for laying explosives, mechanical shovels for removing debris, dumpers that take it to surface, and so on.


Closer to the centre of the Earth
The tyres for similar uses must have very special characteristics, such as strong resistance to slashing (the tracks are strewn with sharp rocks) and the very high temperatures that are common in underground environments. Another special characteristic that is shared by both smooth and sculpted tyres is a tread thickness that can be more than ten centimetres. It is not unusual for the big dumpers used on construction sites to have tyres of over four metres in diameter with treads that seem to be very slim in proportion to a thickness of 10-11 cm. But we can see that these very special tyres are related to car tyres because their casing is radial.
We'll move away from the centre of the Earth, but remain below ground to take a trip on an underground railway: perhaps we will be in a carriage that either doesn't rattle or rattles very little. This would be an unexpected experience as we always associate this efficient means of transport with rails, but in reality some carriages run on rubber wheels. In Paris, for example, one or two lines have adopted this solution, which was originally conceived - in the early Fifties - not only to increase comfort but also the frequency of the trains. The large torque developed by electric engines made it very difficult for the metal wheels to grip and this limited acceleration and braking performance. The greater grip given by tyres made the trains more dynamic, so the hourly service could be increased, which in turn led to an increase in the number of lines. After various experiments (the first trains had many wheels and the tyres were in direct contact with the traditional type of rail), they came up with the solution that is still used today.


Tyres and rails
These special tyres run along their own track on the outside of the conventional rails. The metal wheel has been kept for safety reasons: it intervenes if the tyres deflates and it is more reliable when crossing the points. In addition to these main wheels, which support the weight of the train and transfer traction and braking forces to ground, there are also "idlers", small-diameter wheels that guide the direction of the train. They have a vertical axle so that they can run along guides that are perpendicular to the ground and follow the railway track. In the first versions, the axles of the main wheels also had brushes to collect the power and return it to the rails; if a metal wheel lowers because of a puncture in the tyre, an alarm is activated by special sensors.
Technical improvements - such as the introduction of electronically-controlled drive and braking torque and motors on each axle to improve train performance - have slowed down the spread of this solution because greater maintenance has outweighed the requirement for comfort.
We'll now go up to the surface or, rather, up into the sky. Although reusable spacecraft like the Shuttle are launched vertically, until such time as there are "objects" that can move vertically like helicopters, their return to Earth must be as simple and conventional as an aircraft landing. Use of this kind is very tough on tyres because in addition to the usual stresses caused by contacting the ground at high speed with a not insignificant load (even aircraft tyres feel it), they must also cope with being in outer space.


Space tyres
Extreme temperatures and zero external pressure are not very healthy conditions for tyres. And in fact the biggest headache is the temperature (zero external pressure is equivalent to one additional atmosphere compared to inflation on the ground) and perhaps it is for this reason that the rear tyres, the ones subjected to the most stress, are replaced for every launch; the front tyres last for two missions. These are some technical data about the tyres: we realize that these are not ordinary tyres from the fact that the manufacturer even uses X-rays to check each one and that the tyres on each main gear could support three times the load of a Jumbo Jet. The front tyres measure 32x8.8 and are 20-ply radials, the rear tyres measure 44.5x16.0-21 and are 34-ply radials; they are inflated with nitrogen to 300 and 340 psi, respectively, the equivalent of about 20 and 23 bar.
We'll close this overview on a playful note by talking about the tyres on static or dynamic models. There are numerous types: on-road, off-road, with inner tube, tubeless, and even with mousse like the ones used on the Paris-Dakar, for example. In this case, technology has little to do with it... it's all about having fun!

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