Our Motorcycle's Telescopic Front Forks
From the outside, the front forks on an early '80s motorcycle don't look so different from those on a 2012 motorcycle.Okay, the new 2012 motorcycle has upside-down forks, but the concept of a pair of sliding telescopic tubes inside sliders, with a spring inside each, mounted on pivoting yokes and a wheel bolted between them, is broadly the same.
What has changed is the nature of the hydraulic damping circuits inside. The old forks are truly basic inside, with no more technology than you'd find inside a fire door closing mechanism. They're half-filled with oil, inside which a simple piston slides up and down as the forks move. Holes in the piston provided a cursory damping function, absorbing energy from the moving wheel.
No clickers, no shims, just holes the oil was forced through – meaning they didn't handle different speeds of movement well at all. If you set them to give the proper support during braking, they would be unfeasibly hard when you hit a bump.
With such a basic mechanism, it's no wonder the '80s saw a host of bizarre setups which aimed to produce 'anti-dive' or 'variable damping' effects. Suzuki and Kawasaki both fitted systems that used a solenoid to close off part of the compression damping circuits when the brake light operated. The aim was to improve handling on the brakes, but the effect was to lock up the already rotten suspension, reducing what performance there was to virtually nothing.
By the '90s, though, manufacturers were beginning to use cartridge-type forks (although they'd been used from the mid-eighties in competition). The best versions of these had two separate, adjustable damping circuits for compression and rebound, and used sprung shim stacks inside a removable cartridge to control the damping oil flow. Careful development has allowed them to provide superb wheel control, with a wide range of adjustability, without resorting to gimmicks like the anti-dive systems of the '80s.
The latest technology in mainstream fork design is the so-called 'Big Piston Fork' (BPF) first seen on the 2009 Suzuki GSX-R1000 and Kawasaki ZX-6R. In some ways, this is similar to the old-school '70s design, since it goes back to a close-fitting piston inside the inner fork tube, moving up and down through the damping oil. But this 'Big Piston' has a much more complex, shim-based damping system that aims to give the benefits of 'normal' cartridge forks, but with finer control, since larger volumes of oil are moved for smaller fork movements.
