Hydraulics are at the heart of the world's first twowheeldrive production motorcycle — the Yamaha WR450F 2-TRAC. The 2WD system developed by ÷hlins Racing AB, Upplands Vsby, Sweden (ohlins.com), part of the Yamaha group, incorporates a piston pump and matching fixed-displacement hydraulic motor to power the front wheel when the back wheel loses traction and slips.
The system is based on standard components from Rexroth Sweden, a unit of Bosch Rexroth, Lohr am Main, Germany (boschrexroth-us.com). 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 significant modification to the frame or front forks, improving traction and stability both on and off-road.
Unlike a car, where a mechanical drive can mount from the side of the wheel hub, a motorcycle is essentially two dimensional and would require a complicated mechanical transmission to drive the front wheel, explains Lars Jansson, R&D manager for ÷hlins. That would mean overly complex front suspension members and frame elements, together with unorthodox styling. In contrast, a hydraulic transmission retains the standard bike layout.
Ohlins engineers chose the Rexroth A2F05 axial-piston pump for its high-efficiency and power-to-weight ratio, says Jansson, and it is suited to the application's speeds to 12,500 rpm and maximum operating pressure of 320 bar. Modifications to the standard products include cast-aluminum casings that reduce weight from 2.5 to 1.4 kg. A relief valve in the pump safeguards against excessive hydraulic pressure should the rear wheel lose all grip and spin excessively. And minor modifications to the piston and bearings improve internal efficiency.
The hydraulic pump fits neatly ahead of the rear suspension, below the carburetor and above the gearbox. It is chain driven by the gear output shaft so rotation speed is directly proportional to that of the back wheel. The corresponding A2FM5 hydraulic motor mounts in the machine's front hub and powers the front wheel by means of a reduction-gear unit. Reinforced flexible hydraulic hose, with steel tubes in concealed sections, connects the two.
Pump and motor are tuned to rotate at the same rate, creating a simple, selfregulating system requiring no rider actuation or adjustment. When the back wheel rotates more quickly than the front, this speed differential causes pump pressure to increase and transfer 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 transfer to the front wheel.
The front wheel actually transmits relatively little power, typically around 5%, but by using some of the drive wasted by rear-wheel spin, the hydrostatic system improves overall transmission efficiency. This results in better traction, especially in sand, mud, and snow, improved corner exit speed and handling, and greater stability at high speeds. On the road, it delivers better traction, more-predictable cornering, 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 or do power slides, but timed on a race course he'll be much quicker."
In fact, the top speed of a Yamaha WR450F 2-TRAC on a sandy track is around 10% better than its conventional counterpart, thanks to improved traction. Tests show faster lap times on wet tracks as well.
The system is delivered preassembled, complete with hydraulic fluid, and ready for installation. It has already been fitted to over 300 production bikes by Yamaha Italy, and other motorcycle manufacturers have expressed interest.
At the moment, 2WD is a limited-volume market, although if consumer preferences change and competition regulations are modified, the system would have a major impact on off-road events, road racing, and even everyday motorcycling.