Tech Stuff Driveline
Motorcycle engines are powerful, lightweight and compact. But motorcycle gearboxes are relatively fragile, being designed to feed power to the road surface via a small contact area. The tremendous torque reversals generated as off-road tyres crash back to the ground while racing cross country as sufficient are simply inadequate for standard transmissions.
The McRae solution was to utilise a CVT (Constantly Variable Transmission) where a big ‘rubber band’ takes power from the crankshaft to the differential via two expanding and contracting drive clutches. It sounds complex, but in principal it’s exactly the same as the Derailleur gear system on a mountain bike. The source was a snowmobile with a relatively torquey, 140hp 3-cylinder 1050cc, Japanese motor.
CVT
The CVT is key to using a compact motorcycle engine. It tolerates mud, water and dust, but fesh-fesh is problematic and slippage can occur. The solution – yet to be perfected in the finest of sand – is to create an over-pressure in the engine bay. This is achieved by sealing most of the bay, allowing a small exit for hot air to vent, while using fans to force in pressurised, filtered air.
The Kevlar-reinforced drive belts cost around £30 and can be replaced in minutes – a good deal cheaper that a traditional gearbox.
Partners
Adaptive Space
The wide but otherwise compact chassis is a difficult shape to dress. But that was only the start for the designers at Adaptive Space. Meeting legal requirements while avoiding frequent damage while racing, only adds to the task. And then there are all the functional requirements such as driver visibility; air to the radiators; deflecting sand and dust from the front wheels; the carrying a spare wheels; access to tools, spare parts; and underbody protection. Working together with Fox Composites, the body design took 8 weeks from pencil to paper to launch at the Goodwood Festival!
Rage World renown for their leisure buggies, there was no better start point for McRae than Rage, when in March 2009 the project received the go ahead – leaving just 7 months before the boat left Le Harvre for the 2010 Dakar.
The single-centre-seat McRae 4x2 is based on the twin-seat Rage. Substantial additions, upgraded steels and a revised build procedure results in a bespoke chassis and safety cage meeting FIA safety regulations, with structural mounts for spare wheels.
4x2 Tech Spec
Single-centre-seat with mid-mounted, dry-sumped, 3-cylinder 1050cc engine Chassis: FIA homologated tube-frame Overall width: 1820 m Weight: 750 Kg (Dakar trim) Power: 140hp / 132 Nm Suspension: double wishbones; optional Reiger dampers Electrics: bespoke, loom, ECU and dash by DC electronics Driveline: Constantly Variable Transmission utilising reinforced Kevlar belt Differential: Manually locking gear-driven Torsen differential, incorporating reverse gear by Quaife Drive shafts: GKN M300 material Top speed: 150 kph (limited for race regulations.) Fuel capacity: 140 litres, 2 ATL fuel cells Fuel system: Bosch Engine management: DTA, including altitude correction Air filter: K&N Tires: ITP 9” Terra 10”Baja Wheels: ITP T7 14” Bead lock Steering: bespoke aluminium rack and pinion (1.7 turns lock to lock) with optional adjustable electric
Two McRae 4x2s reach Buenos Aries to Finish the 2010 Dakar and win the new FIA T1 Lightweight Class.
Read more on our Dakar page
World renown for their leisure buggies, there was no better start point for McRae than Rage, when in March 2009 the project received the go ahead – leaving just 7 months before the boat left Le Harvre for the 2010 Dakar.
