Big Piston Forks
By The Merchant

Big Piston Forks (BPF) have been around for a few years now, first introduced by Showa on the 2009 Kawasaki ZX-6R and advertised as giving better fork response in both long, sweeping corners and over harsh bumps. So how do they work?

There are a few good resources for understanding these forks, principally Kevin Ash's article here, and a YouTube clip demonstrating their operation here, and maintenance here. I'll embed all of these videos in this article but you can also open the links by clicking on each references above. Below I'll try and break it all down as best I can from a laymen's point of view.

Traditional cartridge forks use a sealed cartridge inside the fork tube that contains a piston and either shims or holes which control oil movement as the fork compresses and rebounds. Essentially the oil is forced around or through the piston inside the cartridge and past a series the shims or through the holes, which are sized to regulate flow based on the speed of compression or rebound desired in the fork.

Newer cartridge forks almost always use shims. The oil is passed by the piston inside of the sealed cartridge before going through a stack of shims of various sizes which bend as the oil flows past, providing the damping adjustment through the various speeds of compression. The faster the fork compresses, the higher number of shims move to allow oil to flow by. "Tuning" a fork in the past has meant replacing the entire cartridge with an Ohlins unit (or some other aftermarket unit) or disassembling the forks to adjust the shims according to a manual provided by hard experience or a company like Race Tech, which also provided aftermarket valves to improve flow. More effective than older, cheaper forks, which simply had a series of holes in them to allow oil through but were prone to hydraulic lock, shim forks were a step forward. But even these share the problem inherent in all sealed cartridge forks, namely space.

Inside of a 20 or even 30mm cartridge there's only so much room for the piston surface area and the resultant passages through which oil can flow. The problem is that oil is a fluid first and fluids don't always react as people expect them too. In some ways fluids are like petulant children: the harder you push them to do something the more they resist. According to Ash's article, fluids resist flow in proportion to the square of the speed at which they're being asked to move. In simpler terms, fluids become "harder" the harder they are pressed. The smaller the piston, the higher the speed range the oil has to flow to get through its passages.

Enter the new Big Piston Fork, which like its name suggests, has a big piston. Showa created a bigger piston by getting ride of the cylinder inside of the fork tube that traditionally held the cartridge assembly and turning the fork tube into the "cartridge." Basically they got rid of the tube within the tube and simply have one larger tube now. That allows for the bigger piston with larger passages which allows the oil to flow slower. Slower is better according to Ash and Kawasaki.

So there you have it: Big Piston Forks require fewer parts, offer better flow and work with the inherent dynamics of fork oil rather than against them.