Honda's RC211V gave Repsol Honda rider Valentino Rossi a distinct edge over the competition, an edge he used to win the 2002 MotoGP world championship. There was one particular advantage, though, he used to devastating effect: The ability to open the throttle earlier when exiting a corner. That advantage is directly attributable to Honda's new Unit Pro-Link rear suspension, as used on the RC211V--and now on the CBR600RR.
The first generation of Honda's Pro-Link rear suspension (which made its production debut on the 1981 CR250R) was unique because of its lower link pivot. That characteristic allowed the linkage design to create the desired degree of manually produced progressive spring rate. Unit Pro-Link takes the same concept to the next level.
Conventional rear suspensions attach the top eye of the shock unit to an upper rear frame cross-member. Unit Pro-Link, though, attaches the top eye to the top of the braced swingarm structure itself. Below the shock is a joined pair of arms at roughly right angles to each other, pivoting on a shaft attached to the bottom of the swingarm. One of these arms connects to the bottom shock eye, while the other is joined to the bottom of the frame by a pair of tension links.
As the rear wheel rises in passing over a bump, the pull of the tension links rotates the arms, compressing the shock from the bottom. The fact that the top of the shock moves upward with the swingarm is compensated for by increasing the rate at which the bellcrank compresses it from the bottom. Depending on the geometry of the linkage, the bottom of the shock moves more, and more rapidly, than the top of the shock. Compared to the shock used on the F4i, the RR's shock has increased damper volume, and no hose connecting to the piggyback reservoir. The heavily braced swingarm features a press-forged right-side arm and a box-section extrusion left-side arm, both welded to a cast crossmember/pivot.
The Unit Pro-Link design provides a wealth of benefits. To begin with, the shock is contained entirely within the swingarm, and the shock is positioned lower than in a conventional design. Both contribute to mass centralization, in part by giving the centrally mounted fuel tank room to extend downward to the engine crankcases.
More importantly, with the top eye mounted to the top of the swingarm, shock absorber loads don't get fed into the frame, so engineers were free to tune the frame to the best stiffness for superior handling. A new construction technique--Hollow Fine Die-Casting--also aided engineers in tuning the frame. Compared to the F4i, the CBR600RR's frame features greater torsional stiffness at the steering head, decreased lateral stiffness via thinner rear frame rails, and vertical stiffness uncompromised by a beefy structure for the top shock mount. All together, those qualities reduce wheelspin when exiting corners, which allows the rider to start accelerating earlier. The result: Earlier throttle application translates into quicker corner exits, and the additional speed can be carried all the way down the next straight.
It's a benefit that made the difference in the RC211V's winning the MotoGP championship in its first year. It's also an entirely new, groundbreaking technology that had been confined to the race track--until now, and the CBR600RR.