Easy conversion to 4WD

UK Automotive expert ProDrive has developed a transmission system that it says will make it easier to convert vehicles to four wheel drive.

Most front-wheel drive vehicles use a transverse engine with an end-on gearbox. The efficient packaging of this system can cause problems when finding space for a four-wheel-drive system.

In many vehicles the problem is exacerbated by the trend towards narrower chassis rails and shorter and steeper bonnets. A popular solution is to switch to a longitudinal gearbox, but until now this has required significant reengineering of the vehicle and a complicated power transmission path.

In Prodrive’s new Compact Diagonal Transmission (CDT) system, power is taken from the transverse engine to the clutch in the conventional way, then back through a hollow shaft to a bevel gear. From here it is transmitted via a diagonal transfer shaft, at 20° to the longitudinal axis of the car, to an in-line gearbox.

’The biggest challenge is providing an efficient and reliable link between the diagonal transfer shaft and the in-line gearbox,’ says Roland Broadbent, Prodrive’s chief engineer for powertrain. ’Skew gears rely on the teeth sliding across each other, so they get very hot and suffer severe wear problems. To ensure an efficient, reliable powertrain, we had to find a way to completely eliminate this effect.’

Prodrive’s solution was to create a new type of compound gear using a new gear cutting technique to achieve the required angles and tolerances. From this compound gear, drive goes to the integrated centre differential, which sends torque back to the rear differential and forwards to the front differential. The CDT can be integrated with the vehicle manufacturer’s gearbox or with a wide range of Prodrive gearbox technologies, including a proven auto-manual six speed that provides a gearchange time of less than 50ms.

At the front differential, Prodrive has introduced another innovative design, by using a supplementary pair of in-board constant velocity (CV) joints to allow an angled drive shaft to connect, via the main CVs, to the front wheels.

This reduces the size of the differential pack, allowing the engine to be moved rearwards, if required, without changing the original drive shaft positions, thus improving the weight distribution of the vehicle.

Jim Hey, Prodrive’s business development manager, claims that the transmission could be fitted to prototype vehicles within three months. ’It’s ready for testing’ he says, ’all we need is a customer’s car to attach it to.’