By 1998 there had been significant...
By 1998 there had been significant improvements in fork design and performance as demonstrated on yamaha's r1-the new benchmark for sportbike performance.
The first of these improvements came with the cartridge fork designs of the late 1980s and early 1990s. These had much more sophisticated damping systems than the damper rod forks. They used so-called damping "cartridges" which use sprung shims to give much more refined damping. These shims, which are like thin, springy washers, are arranged to cover the holes that the oil has to pass through. But if you hit a big bump, the oil has enough force to bend the shims back, opening the holes and letting in more oil. In this way, you can make the fork firm enough to resist dive on brakes, yet when you hit a big bump or pothole, the extra force created at the shims opens more damping holes, giving more movement and letting the fork absorb the bump.
Cartridge forks also added much more adjustability, including separation of the compression and rebound circuits. Aftermarket suspension specialists could remove the cartridges and subtly alter the damping characteristics to suit different riding styles and riders as well.
The 1995 Honda CBR900RR made...
The 1995 Honda CBR900RR made a huge step forward in sportbike suspension with lightweight, fully adjustable USD forks.
When 'upside-down' forks began appearing in the early 1990s, these helped address some of the other problems with fork design. Putting the narrower chrome-plated tube at the bottom made the fork mechanically stiffer, especially under braking, as the thin tube slid inside the stiffer outer stanchion. It also reduced the unsprung weight of the fork. Unsprung weight is, basically, the amount of stuff below the spring in a suspension system. More unsprung weight makes the suspension work harder over bumps since the heavier parts produce greater forces. Similarly, less unsprung weight improves suspension performance since the forces are decreased.
As the 21st century approached, the combination of upside-down forks with cartridge damping units inside was pretty awesome. The front end performance available on supersport bikes like Yamaha's R1, Honda's CBR954RR and Ducati's 999 were good enough for any road rider.
But in the past few years, even more advanced fork designs have been produced, with separate high and low-speed compression damping adjustment. These two separate adjusters look after the two types of damping movement. High-speed damping is important when a bike hits a bump or pothole: the wheel has to move up or down very quickly with a sudden, violent motion. Hit a two-inch square-edged bump at 50 mph, and your fork has to move up those two inches practically instantaneously. This is the sharp motion that pushes open the shims inside your damping cartridge, and that old damper rod forks were so bad at dealing with.
Low-speed damping is important when we're talking about acceleration and braking. Get on the gas hard out of a bend, and the weight is transferred to the back of the bike, compressing the rear suspension. The opposite happens when you brake. This is a much smoother, less sudden motion, but it's important for the handling of a bike to control it properly.
With separate high and low-speed damping adjustments, a suspension tech can set up forks (and shocks) to address handling and ride issues in a much more comprehensive fashion than was possible with older fork designs.