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BFGoodrich released a new evolution tyre for the Acropolis Rally, the 2007 WRC season's first hard gravel fixture – which featured new stages, a longer total stage distance, even more abrasive surfaces.  

But just how does the g-Force Gravel tyre transmit power to the ground on hard-wearing, rock-strewn surfaces? To an extent, the mission of gravel rally tyres is identical to that faced by asphalt rain tyres. Their first job is clearance (of water in the case of the latter, of the top coating of soft dirt or loose gravel in the former) before going on to provide grip.  

The g-Force Gravel features 84 rubber tread blocks which are laid out either longitudinally (for straight-line traction) or obliquely (traction through corners). These blocks – the height of which varies between 11 and 15mm – function in much the same way as a bevel gear as they pass over the irregularities of the surface.  

If you take the example of a tyre passing over a protruding buried stone, its work can be divided into three phases:  

1) Clearance: the first row of blocks clears the loose top coating preceding the stone.  

2) Indentation and adhesion: the following blocks grip the stone thanks to the phenomena of indentation and adhesion.  

3) Deformation: the next blocks deform as they pass over the stone.  

Specific qualities are demanded of tyres for each of these three phases.

Clearance – this phase accounts for 40% of total traction. Theoretically, the bigger the gaps (or grooves) between the tread blocks, the higher the pattern's clearance capacity. However, the wider the groove, the less the block is able to resist wrenching and tearing.  

Indentation & adhesion – accounts for 60% of total traction. As the rubber block comes into contact with the stone, it begins to deform – indentation – and produces grip as the molecules of rubber bond with those of the stone. For maximum grip, the rubber must be highly flexible and have high hysteresis*. * Hysteresis: the rubber's ability to recover its initial shape after deformation.  

However, the softer the compound, the less resilient and durable it is.  

Deformation – this phase is specific to hard gravel which features surface irregularities of between 20 and 40mm and not mere microns as is the case with asphalt.  

The important thing here is the rubber's 'squashability' and resistance to tearing. The blocks mustn't be too rigid so that they can hug the stone's forms without perforating the plies that make up the tyre's construction. Yet they need to be sufficiently rigid so as not to tear as they come into contact with the stone, and also sufficiently elastic to be able to recover their initial shape.  

That, in a nutshell, is how gravel rally tyres perform on hard surfaces and it should be noted that each tread block will strike the ground some 19,000 times in the course of a 30km stage!  

As ever, it is a question of finding the ideal compromise when specifying the gap between each tread block, the positioning of the blocks themselves, the flexibility, rigidity and elasticity of the compound, its 'squashability' and its resistance to tearing.  

Not to mention the concealed part of the iceberg: the tyre's construction, namely the positioning and the number of casing plies and the density, diameter, etc. of the aramid, Kevlar or metal cords.  

External factors also come into play to complicate the life of BFGoodrich's developers, including the tyre's working temperature, which can vary from 40 to 100°C. The combination of heat and the mechanical constraints to which tyres are exposed causes tread blocks to deform and stretch until they are practically touching each other.  

Indeed, the g-Force Gravel's pattern when the car is in movement is not at all the same as when it is at a standstill! In this rudimentary approach to the work of gravel tyres on hard ground, BF Goodrich have looked at the example of a protruding buried stone which is in fact the simplest situation to describe.  

The equations required to design tyres capable of propelling 1,400kg of rally car and crew at speed over a carpet of loose gravel and stones are, in reality, far more complex and numerous!  

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