Among the many improvements bestowed upon the 2013 CBR600RR is one that’s a little more difficult to spot until you know what you’re looking for: the new inverted 41mm Big Piston Fork (BPF). The giveaway is the distinctive fork cap with adjusters for compression and rebound (tension) damping settings. As you may remember, the CBR1000RR introduced the Big Piston Fork to the Honda lineup just last year, and while it is a 43mm inverted BPF, the design, operation and advantages all carry over to the new CBR600RR Big Piston Fork.
More traditional forks use dampers that have a set series of holes/orifices that pass fork oil back and forth to create damping action. The more sophisticated cartridge fork incorporates self-contained cartridges within the fork legs to regulate fork oil flow. Typically, a piston moves up and down within a self-contained damper cartridge that uses a set of flexible shims to regulate oil flow in place of the fixed orifices found in more conventional designs.
In a cartridge fork, when the fork moves at lower speeds, such as when the front wheel encounters a gentle dip or bump, the internal pressures are low and only a few of the shims are displaced, passing a flow of oil to control the movement. When higher forces cause more rapid fork motion, as when encountering a sudden big bump or hole in the road, pressures rise quickly and more shims come into play, allowing more oil to flow quickly, resulting in less damping resistance so the fork can travel smoothly over a greater stretch of travel to absorb the bigger shock. Thus the fork action is stiffer when responding to small bumps but will soften as larger ones are encountered.
While both systems have their merits and applications, the Big Piston Fork provides much superior fork action, thanks to its ingenious design. In place of a conventional damper with orifices or a cartridge-type damper, a much larger damper piston—one with about 3.5 times the pressure-bearing surface area of a cartridge-type damper—is used to create the damping force. Thanks to this 3.5-times increase in surface area, the same amount of damping force generates only about one-third the usual internal pressure. As a result, internal pressures of the fork oil flow are greatly reduced, and more oil can flow, more easily. This results in more sophisticated fork action with a distinctly smoother fork stroke, especially during initial travel and during directional transitions, i.e. from the compression stroke to the rebound stroke. During sudden compression, which would happen over a larger, sharper-edged bump, fork response is significantly quicker, absorbing the bump more readily and providing smoother damping action.
From the rider’s perspective, the Big Piston Fork delivers more supple action and a more comfortable ride during street use, and when ridden hard during track use there is a feeling of increased front wheel connection with the ground for a more solid feel with enhanced front-end feedback, especially during hard braking. The overall result is smoother suspension action, more accurate handling, improved rider feel, and increased grip and traction. Bottom line: Whether it’s the Big Piston Fork found on the CBR600RR or the CBR1000RR, sport riders will thoroughly enjoy the benefits of this next-generation front suspension system.