Big Piston Forks
By Kevin Ash
Pictures: Kawasaki
We’re used to big pistons making us go faster, but on Kawasaki’s 2009 ZX-6R it’s big pistons which are designed to slow the bike down more effectively. The Showa front forks are appended BPF, which stands simply for Big Piston Forks, and Kawasaki claims improved stability and better damping because of it, especially under braking when the forks are working their hardest.
Click on image for galleryMost other sports bikes are fitted with cartridge forks, where a piston moves up and down through the damping oil within a sealed damper cartridge. As it does so, the oil is forced to pass through orifices into a stack of flexible shims. When the pressure is low, which happens in gentle fork movement such as over long undulations in the road, just one or two shims bend apart to let the oil through. At very high pressures, such as when the front wheel hits the edge of a pothole and the forks compress very quickly, all of the shims bend apart and more oil passes through more quickly.
This system is superior to simpler forks where the piston, instead of having a shim stack, has a series of fixed size holes in it. As it moves along the fork leg the oil passes from one side to the other through the holes. It’s cheap to make and easy to understand, but the problem here is that fluids resist this sort of thing in proportion to the square of the speed they’re being asked to flow. So at low damper speeds there’s very little resistance, but at high damper speeds (hitting a kerb is the extreme, but bumps and ridges are normal) the fluid refuses to flow through fast enough, so you get hydraulic lock and the forks don’t compress at all, just when you need them to the most. the consequence is a very harsh ride and poor grip as the tyre stops following the road surface accurately. You get reasonable damping in the mid ranges of the suspension’s speed (not the same as the bike’s speed, we’re talking about the speed of suspension compression and extension) but it’s inadequate at low speeds and too strong at high speeds.
BPF forks gain advantage by reducing the speed range over which the damping fluid has to flow. Instead of having an internal cartridge, the whole of the fork leg effectively becomes the cartridge, so the piston can be much bigger: 37mm diameter on the new ZX-6R instead of 20mm diameter on the old model. This increases the area of the piston by almost three and a half times, and in turn that means it can flow more oil at lower pressures. Effectively the whole operating range of damping oil speed is reduced, and its rate of increase is decreased too, meaning the damping control is substantially improved in particular at lower suspension speeds, such as when the front end is beginning to compress under braking. There’s also a reduced danger of hydraulic lock, which reduces harshness on poor surfaces.
Expect to see more BPF forks, with their characteristic compression and rebound (also known as tension - see the image above right and in the gallery) adjusted from the top of the forks.
