In the 60s and 70s, most bikes used a simple damper rod in the forks, with a basic piston moving through oil to control the return (rebound) of the front-end. As bikes and tyres improved, more precision was needed. Enter cartridge forks. These give separate control of the damping as the fork compresses and rebounds, by moving oil within a chamber (the cartridge). A piston on the end of the sprung fork rod displaces oil through a valve in the base of the cartridge under compression. On its return (rebound), the oil is governed by a valve built into the fork rod’s piston.
During slow-speed damping (gentle movement of the bike’s suspension), a needle restricts the flow of oil through a hole in the centre of these valves, just like a pilot jet in a carburettor. In high-speed damping (over harsher bumps), a sprung stack of shims are pushed out of the way and the oil flows through carefully designed ports in the perimeter of the same valve. The spring and the compression damping both work to support your bike – reduce one, and you can balance it with an increase to the other, but then you have to consider the rebound damping that works against the spring when the suspension return
Your shock absorber is generally connected to the swingarm through a linkage (except on twin-shock bikes like the XJR) – which translates the travel of your rear wheel to the movement of the shock by a certain ratio: sometimes by as much as 3:1. There are also rising-rate and linear linkages to consider: A rising-rate (or progressive) linkage moves the shock by a greater ratio as the swingarm rises further. On a smooth track, a progressive linkage helps the rear get firmer as the suspension is loaded, making it easier to get the back wheel to spin up, and drift controllably out of corners. At the Isle of Man TT, for instance, there are large bumps, and fast changes of direction, so linear suspension is best: If the rear shock ‘locks up’, that force has to go somewhere, so the weight is thrown to the front of the bike over bumps.
On road bikes, while a linear setup would generally be the best bet, you’ll tend to find progressive linkages. Relatively cheap shocks are used on most of our machines, so the linkage is helping the shock to support the motorcycle, especially where pillions and luggage have to be taken into consideration.
It can be very difficult to obtain that perfect suspension set-up. On a stock road motorcycle, built to a specific budget yet designed for a huge range of rider weights (with the option to carry a pillion and luggage) getting it spot-on for everyone is, understandably, a near impossible task. A bike manufacturer has a huge window when designing a standard road machine – from the physically smaller, lighter Asian market, to the often larger US owners. Will the rider be carrying luggage? What kind of road surface will they be on? A bike with too wide adjustment could, in the wrong hands, be set up to handle dangerously.
Due to the litigious times we live in, this could prove costly for manufacturers; hence an average is accommodated for. It’s this lack of real adjustability that contributes to many riders not believing they understand suspension – until you can actually feel what’s happening as you tweak it, it will always seem something of a dark art.
To be continued....
To be continued....
Fast Bike magazine May 2017
Twist the throttle and enjoy the ride