Rear-Wheel Steering - Rear Steer? - Up To Speed

Drawing The Line

Rear-wheel steering is nothing new. During the 1990s, auto manufacturers experimented with and even introduced such systems to improve handling. While most of these expensive and complex active systems are no longer around, most high-end cars have what are called multi-link rear suspensions, which use intricate geometry and tuned elastomeric bushings to achieve the same effects. Basically, these suspensions use cornering forces to change the toe-in or toe-out angle of the rear wheels to improve turn-in, grip, stability and even ride quality. The systems in effect steer the rear wheels.

Over on the two-wheeled side, engineers have made changes to reduce the lateral stiffness of frames and swingarms to improve the cornering performance of race- and sportbikes. Some analysts have speculated that the purpose here is to provide a measure of "lateral suspension" when the motorcycle is radically leaned over.

I've been skeptical about this explanation, mostly because flex like this isn't going to provide good suspension. There will be very little travel and no damping. At least at the rear, motorcycle suspensions still work OK when leaned over, as swingarm pivot bearings have provisions for thrust loads and don't bind appreciably. Effective spring rates do increase when a bike is leaned over, as the suspension is working at an angle while trying to accommodate an essentially vertical change in road surface-not the perfect solution, but the suspension is still working.

I didn't really see what was going on here until I did a simple experiment, demonstrated in the photo at lower right. If you bend a structure, the ends don't remain parallel. Simple, really, but the "lateral suspension" believers don't realize that if the swingarm and frame have lateral flex, then the wheels are no longer in line. You might think that a swingarm would form a parallelogram with the axle and pivot remaining parallel, but for various reasons that won't happen. And, of course, we also have to consider the frame.

Let's look at Yamaha's MotoGP challenger, the YZR-M1. Figure 1 (above) is a side view showing the basic layout. Long, long swingarm, rear engine mounts just above and below the swingarm pivot and long thin "hangers" descending from the frame to connect the front engine mount. Figure 2 (left) is a top view. We will hold the front part of the frame (steering head area) steady and put a side load to the left against the end of the swingarm. What happens? The swingarm is a pretty long lever, connected to the engine quite rigidly (in terms of lateral forces) because the rear engine mounts are close to the pivot. The swingarm tries to move the engine. If the frame on each side can twist in the area above the swingarm pivot (and the M1 frame is quite thin here), a pivot is created approximately where I've drawn the quartered circle. The rear of the swingarm goes to the left, while the front of the engine tries to go to the right. The front engine hangers, being thin and long, can provide little resistance, and bend to the right. The overall result is the rear wheel, as shown by the dotted lines, moves to the left, steering to the right.

The Ducati Desmosedici is interesting here because its swingarm is attached directly to the engine, not to the frame at all, potentially simplifying the motions.

So, does lateral chassis flex provide lateral suspension or rear steer? Kawasaki muddied the waters further by designing long front engine-mount hangers much like those on the M1 into its 2007 ZX-6R, claiming improved turn-in characteristics from the reduced stiffness. No mention of lateral suspension. Of course, if reduced stiffness is to have an effect, something's got to move. If the front of the engine is moving side-to-side, what is the swingarm doing? Ditto the rear wheel?

I'll discuss what may be going on here in a subsequent column. One of the most fascinating aspects of motorcycle development is how complex and convoluted it can be. If you decrease chassis stiffness in an automobile, the results are predictable. But decrease chassis stiffness in a motorcycle and you're looking at a mystery wrapped in an enigma.