At the most basic level the rear suspension on a motorcycle, like the front, has actually changed very little in design over many years, save for the obvious when we switched from twin shock to a single, mono-shock absorber. That’s not to say nothing has changed of course, the internal workings of a modern shock absorber are incredibly highly developed compared to even just a few short years ago. Like the forks on the front of your bike development filters down from GP level through Superbikes to even the most basic of road bikes. The starting point for most of us is our road or sportsbike shock. On most modern bikes you will find a monoshock which is basically a high pressure tube type design with an external spring and a load of oil flowing internally for damping. The role of the shock absorber quite obviously is to support the weight of the rider and bike as they ride over bumps, brake and accelerate. It does that by holding you up with a spring and controlling the up and down movements of that spring with oil and that damping piston. Adjusting the weight of the spring and how freely that oil can flow within the damper are the common ways of adjusting your suspension. The difference with a shock over a fork is the gas pressurisation of that oil (with nitrogen). The oil and gas are kept apart by a separating piston which can be fitted internally or via an external reservoir, sometimes connected by a hose or fixed directly on top of the shock absorber (which you might see as a ‘piggy-back’ reservoir). Basic suspension design controls two rates of oil movement within its tube – through valves at a low rate of flow and through a number of holes in the piston itself at a high rate of flow. So as you ride and it causes small compressions of the shock, the fluid flows through the valve which you can usually control via a screw or clicker externally on the unit (commonly both rebound and compression, though not always). At higher rates of compression, or bigger bumps, oil has to move quicker at which point the needle valve isn’t big or capable enough to cope so it calls upon a few mates, in the form of shims underneath the piston, to help out and open to allow for a greater rate of flow. These two ways of controlling oil flow are commonly called high and low speed damping. On all but the most basic of shock absorbers we can control the damping movement by adjusting either a needle valve from the outside with a screw or by altering the size of the shim-stack (i.e. number, thickness and diameter). This is a generalisation of course as some bikes come with adjustable high and low speed damping as standard and certainly many of you will have an aftermarket shock fitted, which is likely to have this control externally already. Typically standard shock absorbers cater for a broad range of riders weight and riding conditions, which is why we look to aftermarket, race-bred products to tailor our bikes to our needs and for better performance. Damping works both ways of course, both under compression and rebound. So when the spring forces the shock absorber to extend again after a bump, the fluid flows back in a circuit forced by the pressure of the gas back into the shock absorber, usually via a separate non-return valve. If the piston velocity is high, the shims on top of the piston will also open to allow the fluid to flow through. Again, the details in design of different shocks depends on the manufacturer and level of sophistication. Which brings us neatly to the twisted part of a shock absorber, the spring. As with forks the spring is important because it holds the shock extended and the weight of bike and rider – without them the bike would simply collapse. So having the right spring (commonly called spring rate) makes the difference between too hard and sitting fully extended and not moving, or too soft and forever hitting the bump-stop. As with damping control, standard road bike spring rate is a compromise between every possible load (thin rider, fat rider, pillion, luggage, etc) and road or track condition. While that is clever of the manufacturer it does mean they often don’t work too well on track where more force is applied. Which is why opting for an upgrade to an aftermarket product like an Öhlins makes sense (unless you are prone to dramatic changes in body weight and regularly ride to trackdays with a pillion via the Alps with a load of luggage). The other thing to understand which we haven’t mentioned yet is preload. Spring preload is normally adjustable by two large locking nuts in a thread at the top of your shock’s spring or via a manual adjuster, or occasionally via an external adjuster – depending on which type of shock you have. Preloading a spring simply means compressing it before any load is applied. So if we preload the springs in our suspension it thinks it’s carrying a weight and pushes back with the equivalent amount of force. Put together preload and spring rate are what determine the amount of sag on your bike. Sag is how much suspension travel is used when supporting the bike’s weight (without the rider). Preload is the way we adjust sag so in the case of an 8kg/mm spring with 12.5mm sag, for example, if we only wanted 10mm of sag we need to pre-load the spring to reduce the sag, hence the term. Apart from the settings on your shock, there isn’t much you can change at the rear end to affect the geometry so it’s important to bear in mind adjusting your shock does affect it. Adjusting preload and shock length, or indeed any shock movement at all, affects the wheelbase and steering geometry of your bike. By adjusting the length of the shock the steering geometry and chain force changes – a shorter shock makes your bike lower and vice-versa. If you make it shorter the fork angle as well as the trail increases, and normally the chain force will be reduced with less anti-squat as a result. A longer shock therefore decreases the fork angle, trail and normally the chain force will increase with more anti-squat as a result. A slight change of centre of gravity is another result of changing the length of the shock absorber too. But it’s not all as complex as that might sound – put simply, a longer shock quickens steering and a shorter one slows it down (considering the difference between a chopper and a trials bike might help here). Ride height adjustment isn’t always common on standard bikes but it’s our other form of ‘adjustment’ on our rear suspension. Typically you’ll find it at the top of the rear shock where it bolts to the chassis. In some cases it is simply a matter of introducing a washer (and raising the ride height by the width of the spacer or large washer) or by a bracket bolted to the chassis which can be moved up and down using spacers. Some bikes are fitted with a threaded version of the same thing and can be moved by adjusting the locking nuts. The other form of ride height adjustment is found on the shock itself. Öhlins shocks for example have a pair of locking nuts near the bottom shock eyelet. Undoing these allows you to screw the bottom eyelet in and out of the shock body, changing the overall shock length and therefore the bike’s ride height too. It’s also possible to have your shock pre-built to a longer length via a specialist. One further note to point out for all you looking at this with a view to track use is that shock absorbers with external rebound adjustment, as is common on many Öhlins shocks, have what they term an “integrated temperature compensation” too which basically mean the shock maintains constant performance and feel to you on the bike whether you’re just setting off on a warm-up lap or twenty laps into a race at Cadwell Park. Finally it would be tardy to talk about shock absorbers and rear suspension without also mentioning linkages. While they could easily be a Guide For Dummies all to themselves (which we don’t have room for here) it’s as well to mention briefly how they work just so you understand what’s happening under you. A more typical type of linkage for road bikes is a rising rate linkage. When the shock is extended (0mm travel) the linkage ratio is softer (i.e. is takes 3mm of wheel movement to squash the shock by 1mm). Then as you use more travel the rate rises to something like 1.8:1. So by the time you’ve used 70mm of shock travel, it only takes another 1.8mm of wheel movement to compress the shock by 1mm. The benefit of a rising-rate linkage (often called by a slightly different name depending on manufacturer) is flexibility. If your bike was fitted with a linear rate linkage you’d expect problems when you applied weight say, carrying a pillion, because the rear end wouldn’t offer enough support and would have sunk so low the bike would be unmanageable. Or if it did offer the right amount of support it would then be too hard when that pillion got off. With a rising-rate linkage you have the option of fitting a spring firm enough to support both pillion and rider, or just rider and it will operate progressively across different riding conditions. Like all standard road bike suspension components this is a compromise manufacturers have to make but for most sportsbike models the linkage rates are perfectly adequate for race track application, it is the shock settings which we need to alter. (Lone Wolf - Bikesport News)
