How a rally car is tweaked to fly on gravel

This week’s Rally San Marino marks a return to gravel for the Intercontinental Rally Challenge regulars, which presents another test, particularly for the engineers and mechanics preparing the competing cars.

Nicolas Roux, Andreas Mikkelsen’s engineer at ŠKODA UK Motorsport, explains some of the changes that have been made to his Fabia Super 2000 to make it as competitive on loose surfaces as it is on Tarmac.

Ride height: “We raise the ride height mainly so we don’t destroy the car. The gravel roads are much more rough than Tarmac roads with stones and bumps everywhere so we have to go higher with the car. Typically the car is 30 millimetres higher than it would be on a Tarmac rally.”

Suspension travel: “The dampers are a different length, longer, on gravel than they are on Tarmac to allow for more travel over the bumps.”

Brakes: “On gravel the regulation states we have to use 16-inch rims and 18-inch rims on Tarmac. Because of this you can’t fit a Tarmac brake into a gravel rim so you have to use a smaller disc. But this is not a problem on gravel because the energy you spend on the brake is smaller so you can use a smaller disc. Although the rims are different in size, outside the rolling radius of the tyre is very close between Tarmac and gravel but the gravel tyre is much higher, which helps a lot with the absorption of bumps.”

Differential settings: “With the exception of suspension, springs and rollbar stiffness, which is much softer on Tarmac because of the lower grip, the main thing we change is the differential settings. They are quite different between Tarmac and gravel because of the tyre behaviour. On every car the crucial point is the tyre because it’s the one thing that transfers the forces and all the settings of the car are done to make the tyre work properly. The tyre is always slipping the more you increase the torque and the more the tyre slips the more force you transfer. After a certain point the tyre reaches maximum. It slips more and it makes the tyre wear but there is not the force any more. This is why the differential settings are important.”