Christini Two-Wheel Drive Motorcycle | The Necessity of Complexity

A chain and two sprockets—that’s the mechanical connection between the engine/transmission and rear wheel of a traditional motorcycle. It’s as simple as could be.

Three chains. Six sprockets. Four bevel-drive gearsets. One fixed-length driveshaft. Two telescoping driveshafts. And a Sprague clutch—that’s what connects the engine/tranny to the front wheel on a Christini all-wheel-drive motorcycle. Is this Rube Goldberg on a bad day? Does it really take all this to transform a one-wheel-drive motorcycle to two-wheel-drive? The short answer is yes.

Steve Christini has been building AWD motorcycles since 2002, and is now selling complete, ready-to-ride machines. It's been dirtbikes until now, but Christini (www.christini.com) has just introduced a supermoto-style streetbike, the AWD 450SM.

Based on a Chinese copy of a Honda CRF450X, Christini starts by adding a second countershaft sprocket. A chain runs up the side of the frame to above the engine, where a sprocket on a bevel-drive gearset connects to a short driveshaft running to the back of the steering head. Inside the steering head is a second bevel-drive set driving counter-rotating elements concentric with the steering stem.

Sprockets at the bottom of the stem drive small chains enclosed in a hollow lower triple-clamp, running to sprockets on shafts just forward of each fork leg. The shafts counter-rotate to eliminate torque effect, and telescope using ball-spline linear bearings to accommodate the fork’s suspension movement.

At the front axle on each side are bevel-drive gearsets to turn the shafts’ rotation 90 degrees into hub rotation. Inside the front wheel hub is a one-way Sprague clutch to allow the axle to freewheel when not being driven.

All this hardware is surprisingly unobtrusive. Aside from the shafts in front of the fork legs and a vertical chain-cover on the left side of the frame, the bike looks conventional. There are no ergonomic intrusions on the rider. Particularly impressive is the lightness of the additions; just under 15 lbs. is added to the base bike’s weight.

But are there simpler ways to drive a motorcycle’s front wheel? The Öhlins/Yamaha 2-Trac system used a hydraulic pump on the engine connected to a hydraulic motor on the front wheel hub. The system is certainly simple conceptually, but not necessarily in detail, and the relative inefficiency of hydraulic drives would be hard-pressed to complete with Christini’s claim of a 1 percent efficiency loss in his mechanical system.

Electric drive to the front wheel would be analogous to hydraulic drive, with wires replacing hoses. The 2-Trac system’s hydraulic motor added about 7 lbs. of unsprung weight to the front wheel, and an electric motor in the hub would almost certainly add more.

The Christini system is here, and it works. The front wheel is driven at about 80 percent of the speed of the rear wheel. This means that if the rear wheel has full traction and is not spinning, the front wheel is freewheeling. As the rear begins to spin, at the point when the relative wheel speeds equalize, the front wheel begins driving. The opposite situation also holds: If the front wheel slows significantly relative to the rear, as in a situation of low traction when the front “washes” and wants to stop turning, the wheel is kept turning by the freewheel clutch engaging. It’s an elegant mechanical “compensation” device for poor traction conditions in which wheel slip at either front or rear is addressed as needed by the other wheel

Christini set himself a really challenging design problem when he set about creating a two-wheel drive motorcycle. It’s a complex problem with a complex solution, but it’s hard to see a simpler way to do it. Added to the tough design brief has been an equally tough manufacturing development process that’s been done with care and attention to detail. We may see further developments in hydraulic and electric 2WD systems, or combination systems using electric, hydraulic and/or mechanical elements. But as for purely mechanical 2WD, Christini has set the standard with an impressively well-integrated package. MC