Category: Dailies


As used on the 2009 Kawasaki ZX-6R and 2009 Suzuki GSX-R1000, the Showa Big Piston Fork is the next big thing in motorcycle front suspension. Allegedly capable of simultaneously reducing dive under braking, better coping with bumps and delivering a more direct connection to the front wheel for riders, the BPF's method is simplicity even if its effects are complicated.

Traditional forks, like those found on older or contemporary, but basic
motorcycles drive a piston through oil, achieving damping by using a
calculated size of holes in that piston to moderate the rate at which
oil can flow through. This works fine at moderate speeds (the speed of
suspension movement, not road speed); as the fork compresses the oil
flows at an easily calculated pace. Problems arise with extremes. At
very low speeds, there's virtually no damping, the oil just flows
through the holes as it sees fit. This can lead to wallow, as the
damping fails to cancel out the spring's forces. At high speeds --
imagine hitting a large bump while traveling very quickly -- the oil
can't flow through those holes fast enough, meaning the fork can't
adequately absorb the impact. All of this together leads to a bike that
will have trouble keeping its tires in contact with the road, hurting

More advanced cartridge forks, like those found on most sportsbikes and
similar, contain the piston and oil inside that cartridge. Instead of
fixed-size holes, the oil passes through a series of flexible shims, at
slow suspension speeds only a small number of shims flex to allow oil
through, providing effective low-speed damping. At higher speeds, more
shims flex, providing high-speed damping. The result is suspension that
works better everywhere, more effectively keeping tires in contact with the road,
increasing performance.


Showa's BPF design -- characterized by the top-mounted compression and
rebound adjusters, preload is now on the bottom -- effectively turns the
whole fork into the cartridge. Greater volume means lower pressure,
which in turn means the oil travels at lower speeds. The greatest
advantage this delivers is to low-speed damping control, which means
compression under braking can be better controlled, leading to less
dive. The greater damping control also leads to improved feel and
increases the fork's ability to cope with high-speed vibration.

All this together means the BPF delivers more confidence in the front
end while doing a better job of keeping the tire in contact with the
road across a wider range of circumstances. By reducing the number of
parts in the lower fork leg, unsprung weight is reduced, further
enhancing the suspension's ability to control wheel travel.

If you're like us, you think it's the small things that make big motorcycles complete. Read more of our Details articles here.

Showa via Kevin Ash

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