Hydraulic system creates 2WD for bikes

Rexroth hydraulic components are at the heart of a revolutionary two-wheel drive system featured on the world's first production motorcycle of its kind, the Yamaha WR450F 2-TRAC.

Rexroth hydraulic components are at the heart of a revolutionary two-wheel drive system featured on the world's first production motorcycle of its kind, the Yamaha WR450F 2-TRAC.

The 2WD system developed by Ohlins Racing of Sweden, part of the Yamaha group, incorporates a Rexroth A2FO5 piston pump and matching A2FM5 fixed displacement motor, supplied via Rexroth Sweden.

Regarded as a major technological breakthrough for the motorcycle industry, the reliable, compact and relatively inexpensive 2WD system can be installed on virtually any bike without any significant modification to the frame or front forks, offering traction, stability and performance benefits both on- and off-road.

Like many great engineering ideas, the system is simple and efficient.

A Rexroth hydraulic pump driven from the machine's gear output shaft delivers hydraulic pressure to a compact motor built into the bike's front wheel hub.

Totally automatic in operation and requiring no rider actuation or adjustment, it applies power to the front wheel when the rear wheel starts to slip, the amount of traction delivered increasing and decreasing in accordance with the amount of spin at the rear.

Rexroth, which has it UK headquarters in St Neots, Cambridgeshire, was the only manufacturer able to meet Ohlins' technical requirements in a standard product, although modifications were subsequently made to reduce weight and tune efficiency to this high speed application, and also willing to support the long-term project, for around 12 years to date.

The 2WD system, which is delivered sealed and ready assembled, has already been fitted to over 300 production bikes by Yamaha Italy and a similar number is expected in 2005.

Interest has also been shown in the Yamaha-patented innovation by other motorcycle manufacturers and it is hoped the system will have the same impact as 4WD rally cars some years ago.

Ohlins shock absorbers are an integral part of motor sport and Valentino Rossi's recent MotoGP series win is among more than 100 world championship and other major titles gained with the Swedish firm's equipment.

Ohlins Racing has been majority owned by the Yamaha Motor Company since 1987, although it continues to operate as a dynamic independent company within the group, listing Ducati, Aprilia and Gas Gas among its motorcycle customers, as well as Audi Le Mans LMP 900 and DTM cars and various Champ Car and IRL teams in the USA.

Development of its 2WD system for motorcycles was carried out at the purpose-built Ohlins Research Centre in Upplands Vasby, under the direction of Lars Jansson, R and D Manager for Future Projects.

Unlike a car, where it is easy and efficient to fit a mechanical drive from the side of the wheel hub, a motorcycle is essentially two-dimensional and would require complicated mechanical transmission to drive the front wheel, resulting in overly complex front suspension members and frame elements, together with unorthodox styling.

In contrast, hydraulic transmission retains the standard bike layout and allows the use of proven components.

Ohlins Research identified the technical characteristics required of an hydraulic transmission system and found that the Rexroth axial piston units were almost a perfect match.

"The Rexroth A2F axial piston unit is one of the most efficient pumps on the market", says Jansson.

"The axial piston configuration is far more efficient than external gear units".

"It is also more suited to this application's high speeds, up to 12,500rev/min, and maximum operating pressure of 320bar".

The first prototype was built in 1993, initially using machined aluminium housings for the axial piston units, although once the first preproduction batch was required, Rexroth cast the aluminium casings specially, reducing weight from a standard 2.5kg to a lightweight 1.4kg.

It also incorporated a pressure valve into the pump, to safeguard it against the enormous hydraulic pressure generated when the rear wheel loses all grip, and slightly improved internal efficiency by making minor modifications to the piston and bearings.

Otherwise, the Rexroth A2F pump/motor units offer the compact dimensions, proven functionality, prolonged service life and favourable power/weight ratio of the standard components.

Fitting very neatly ahead of the rear suspension, below the carburettor and above the bike's gearbox, the hydraulic pump is chain driven by the gear output shaft, so that the speed of its rotation is directly proportional to that of the back wheel.

The corresponding hydraulic motor is mounted in the machine's front hub and powers the front wheel by means of a reduction gear unit, produced by Ohlins.

The pump and motor are linked mainly by reinforced flexible hydraulic hose, with steel tubes used for concealed sections.

Normal engine oil is used to cope with high temperatures and the entire system is delivered pre-assembled, complete with hydraulic fluid and pressure tested, ready for fitting within minutes at the Yamaha Italy factory.

In action, the pump and motor are tuned so that they always rotate at the same rate, creating a system that is simple and self-regulating.

When the bike transfers more power to the ground under acceleration, the back wheel rotates more quickly than the front, due to deformation of the rear tyre; this slight difference in rotation speed causes the pump to supply more pressure, therefore transferring traction to the front wheel, through the geared hydraulic motor.

The more the back wheel spins, such as on wet and slippery surfaces or when negotiating curves, the greater the power transferred to the front wheel.

The pump's pressure relief valve also prevents dangerously high pressure in the system, should excessive rear wheel spin occur.

The front wheel actually transmits relatively little power, typically around 5%, but by utilising some of the drive wasted by rear wheel spin, the hydrostatic system improves overall transmission efficiency.

Off-road, this results in better traction, especially in sand, mud and snow, improved corner exit speed and handling, and greater stability at high speed and during wheelies.

On the road, it delivers better traction in the wet, improved predictability in corners and better straight-line acceleration out of bends.

"The bigger and heavier the bike and the more inexperienced the rider, the more you benefit from 2WD", says Jansson.

"An experienced rider will think he's down on power, because he cannot spin the rear wheel as normal or do power slides, but timed on a racecourse he'll be much quicker than before".

In fact, the top speed of a Yamaha WR450F 2-TRAC on a sandy track is around 10% higher than its conventional counterpart, thanks to improved traction.

Ohlins Research also tested a powerful road bike and found that lap times in wet conditions were 5s faster over a 1min 20s circuit.

Most impressive of all, French professional endurance rider David Fretigne won the first three slippery and soft sand stages in the 2004 Paris-Dakar, eventually finishing just a minute behind the winner, despite a heavy crash on day four.

Moreover, the Rexroth hydraulic system proved its extreme reliability by running without fault.

"The Rexroth components are perfect for this application and thoroughly reliable", concluded Lars.

"We've enjoyed tremendous support from the company over the complete development project".

"At the moment, 2WD is a limited volume market, although if customer attitudes change and competition regulations are modified to accept it, the system would have a major impact on off-road events and road racing, even MotoGPs where they really have to handle too much power, as well as everyday motorcycling".

"Essentially, there are no drawbacks".

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