Differential calculus

A new transmission system will benefit drivers and engineers in the challenging world of extreme rally racing, its UK developer has claimed.


A new transmission system will benefit drivers and engineers in the challenging world of extreme rally racing, its UK developer has claimed.

Xtrac produced its new six-speed gearbox and rear differential, called ‘532’, in response to rule changes by motorsports governing body the FIA.

The changes allowed additional approved manufacturers to provide transmissions for cars entering Super 2000 rallies.

Previously, SWR Motorsport, with its SADEV gearbox, was the only authorised supplier of transmissions for Super 2000 cars. But there are now three FIA-approved suppliers, including Xtrac and Ricardo.

The FIA introduced the new regulations in an effort to keep down the cost of taking part in motorsport by increasing competition.

‘To meet FIA requirements we have put a lot of effort into achieving the lowest possible costs, while ensuring that technically we have a reliable and lightweight transmission,’ said Adrian Moore, Xtrac technical director.

The gearbox was designed at the company’s technical centre at Thatcham in Berkshire, where the product will also be manufactured and assembled.

Xtrac designed ‘532’ to comply with regulations for four- wheel-drive Super 2000 rally cars, which usually need power and torque of around 280bhp and 250Nm respectively.

This meant that strict quality control at all stages of the manufacturing process was necessary to ensure the production of a transmission system that could withstand the extreme conditions of the sport.

‘This latest gearbox from Xtrac is strongly underpinned by major investments in state-of-the-art manufacturing processes,’ said Moore.

‘Our advanced research and development facilities have given us a real edge with this new transmission in its design, sub- system behaviour, gear technology, rotating dynamics and metallurgy,’ he claimed.

Xtrac said the 63kg ‘532’ should have benefits for both drivers and rally engineers. Drivers should notice a more efficient differential and a lighter sequential gearbox.

Rally engineers should find the gear clusters easier to remove to make ratio changes. They will also be able to adjust the front, centre and rear differentials from the exterior of the car.

In the ‘532’, the gearbox has a range of up to 25 gear ratios (the ratio of the speed of rotation of the powered gear of a gear train to that of the final or driven gear). The drive is directly connected from the clutch to the cluster of six forward gears, the maximum number authorised by the FIA.

The force produced powers the bevel gear ‘Salisbury’ type centre differential with a 50/50 torque split. Salisbury gears are either of a pair of gears with teeth surfaces cut so that they can connect unparallel gear shafts.

The differential also contains a Belleville spring to provide a positive pre-load that is externally adjustable.

The centre differential creates an output directed to the front differential and the rear drive bevel gear set. The rear differential contains a hydraulically operated rear drive disconnect unit operated without additional complex and costly pumps.

The locking features of the transmission can be adjusted using a range of differential ramp and cross pin angles.

To make the ‘532’, Xtrac coated the friction discs with molydenum steel, and used the Klingelnberg cyclo-palloid hard cutting process to manufacture the gearbox output and rear differential final drive bevel gears.

Xtrac said all the components fulfil the required FIA life of a minimum of 2,000km of extreme use.

With more than 100 Super 2000 rally cars operating in the field, the newly opened market should provide ample opportunities for the company.