In this technical feature from 1988, the great John Robinson explains all you need to know about suspension.
volution is a wonderful process. Once upon a time getting the best from your suspension system was easy – it either worked or it didn’t. Rear shock absorbers might be adjustable for spring pre-load if you were lucky, but that was about it. There was no knob twiddling or air pressure gauge required to set it up. The suitability of a bike’s suspension for a whole range of potential riders and uses was dictated by the preference of the design department. Tough luck if you (or they!) didn’t conform to the British Standard Human size and riding habits.
Now, however, as suspension systems have been improved to deal with the ever increasing weights and power of the modern machines, you cannot move without being bombarded with a plethora of gauges, C rings, screws, knobs, balance pipes and reservoirs, especially on the top of the (Japanese) range models.
But what does this current fashion which involves the fanatical application of anti-dive and rising rate on even 50cc machines give us other than more parts to wear and a higher replacement cost because few aftermarket alternatives are available? How can you optimise the seemingly infinite number of options they provide?
We need to consider why we have suspension. There are two main reasons – the first is comfort! To achieve this we require soft springs, matching damping, and lots of travel.
But the second requirement the suspension has to fulfil is to keep both wheels on the ground for as much of the time as possible and to combat the changes in attitude that occur under braking and acceleration. These opposing requirements, coupled with the physical limitations of how long we can make the suspension stroke result in a system which is a compromise designed for use over a given load and speed range.
If you ride across road undulations the wheels will have to move upwards the height of each bump in order to travel forward. Now if you don’t want the chassis (the sprung mass) to notice these bumps the system will have to allow the wheels to move independently of the chassis. This is the function of the swing arm and front forks.
To support the sprung mass we introduce springs into the system between the sprung and unsprung (wheels, fork sliders, swing arm, etc) masses. They will deform and allow wheel movement but, at the same time, will transmit a force out of one end equal to the force put into the other.
A gauge of ride potential is the ratio between sprung and unsprung mass. The lower the unsprung mass for a given chassis weight (sprung mass), the better the wheels will be able to follow the road – because the shock absorber has less work to do. As P&M proprietor Richard Peckett reminded me, the unsprung weight of most race bikes has increased over the last few years due to bigger tyres and brakes, which has in turn caused problem in the suspension. And because this ratio is worse for a motorcycle than a car, a car is smoother to ride in than on a bike.
All these points become important when we study the way our suspension system works (or doesn’t as the case may be!).
Looking at the spring part of a shock absorber, the first concept to appreciate is that of spring rate. This is the amount of force required to compress the spring a given distance – a constant rate spring compresses the same distance with each unit of load. However, as suspension travel is limited the better set up is to have a progressive rise in rate as the spring is compressed, to retain softness over small bumps but prevent bottoming over large ones. This is achieved by either a combination of two constant rate springs, one softer than the other, or a spring which is wound in a coil with a progressive change in the distance (pitch) between coils.
Of course, a rising rate suspension system achieves a rise in effective spring rate by a system of levers which gradually decrease the leverage of the rear wheel as the suspension is compressed.
Varying rider and luggage weights can be covered by increasing the preload on the spring, If you add 200lb weight, then to produce the same restoring force on the chassis you will need to increase the preload by 200lb! Easy. On a twin shock machine with 100lb constant rate springs this will require 1 inch of preload. Single shock machines naturally tend to have springs with double or more spring rate than each of those on a twin shock.
This type of preload is called mechanical preload. Static preload is the mechanical preload plus the additional amount of spring compresses when it is supporting the ciassis at rest. If there is so much mechanical that it exceeds the static preload, the suspension will seem hard because it will not work until a bump large enough to overcome it is encountered – a condition to be avoided. As a general rule of thumb, the suspension should compress by about 1-1½in, front and rear (road bikes), when rolled off its centre stand, and a further half-inch or so with the addition of the rider.
Air assistance to suspension makes its spring rate more progressive as well as providing a mechanical preload effect. This progression is due to the fact that the pressure exerted by an enclosed gas doubles if the volume is halved (Boyles Law, or for ’O’ level physicists). So if we take 200cc of air at 10psi and compress it to 100cc, the pressure it exerts on its surroundings will rise to 20psi. Compress it only another 50cc and the pressure rises to 40psi.
Think about this in terms of fork travel for example and you can see how the spring rate rises. It’s a good idea to fit a balance pipe to equalise the air pressure in each leg and make filling less fiddly. The only drawback is that air pressure varies with temperature so always check with the suspension “cold” and if possible with no load (i.e. on its centre stand).
Let’s look at the more controversial part of a suspension system, the damping. If suspension consisted only of springs, a motorcycle would continue to oscilate up and down after riding over a bump until the energy of the bump imparted to the spring was dissipated by the effort required to bounce the sprung mass up and down. The shock absorber does just that, it absorbs the energy of the spring, it does so during and after the initial compression.
Shock absorbers are usually hydraulic (oil-filled) devices which burn up the energy of suspension movement by pumping oil through a system of valves and orifices to create a resistance. This resistance to movement controls the spring and converts the “stored” spring energy into heat in the shock oil. The heat is then dissipated into the surrounding air.
The key to a successful shock absorber is to match its resistance to the forces generated by the spring. So different springs require different rate shock absorbers. A shock suitable for a heavy spring would overpower a lighter spring and therefore limit suspension travel. There are varying schools of thought on how much damping should be applied under the compression part of the stroke and how much on extension.
Compression damping stops the suspension from floating higher than the bump, thus minimising suspension travel. It also has the effect of increasing spring rate which means you can get away with a softer spring. This means the shock will have a lower spring force to deal with on the return stroke.
It is general practical then to have some compression damping but a greater amount on the return part of the stroke. Unfortunately, not all shock absorbers are created equal, and beyond the basic requirements on two way damping, the more extensively developed shocks will also control the velocity characteristics of the damping force, achieved by the use of various combinations of orifice and spring valves.
So that’s the theory, now how can we apply it to a “real” bike. It’s pointless wasting time setting up your suspension if the rest of the chassis is in disrepair. It goes without saying that all bearings should be in good condition. This means wheel bearings and fork slider bushes as well as swing arm and steering head. Wheel alignment and tyre pressures should not be forgotten. Soft tyres can mask suspension harshness and of course the seat of a machine can have considerable effect on rider comfort. How many times have you seen us complain on these pages about the bone hard ride on a race replica aggravated by a mere ½ inch of foam as a concession to rider comfort?
Change your fork oil, you probably never have (like me), and you might be surprised by the transformation that this along can achieve (like I was!). If the fork seals are leaking you’ll have to replace them too. Go for a light fork oil at first – it’s best to run the thinnest oil possible. Too thick an oil can ask the presence of springs that are too soft. Secondly, a thinner oil will tend to suffer less for aeration. Castrol’s “light grade” fork oil has a viscosity index of 10/20w which is a good starting point. Thicker oils (with higher viscosity numbers) give greater damping force and vice versa.
It’s important to know how much oil you’ve put in. Start with the recommended quantity which will at least ensure there is sufficient to keep the damping components covered. With air assisted forks, the volume of oil can affect the “spring” rate rise and you may be able to turn this to your advantage.
If you’ve got the option to vary your suspension air pressure, avail yourself with a good pressure gauge of the dial variety, and a suitable pump. Be on guard not to over-pressurise as this can lead to topping out. As to suspension settings and knob twiddling, where the variable is numbered, the higher the number, the more of it you get. With letter systems, C gives you more than B.
Find out as much as you can from your owners manual about the characteristics of the suspension fitted to your bike, and find a bit of test road that produces the effect you’re trying to eradicate. Remember also that suspensions operate best over a part of the sped range, or to put it another way, few are good at coping with all conditions. Riding style can be very important, for example you are unlikely to get a softly sprung touring bike to behave well if driven like a sports iron.
If, after all this you’re still weaving like a dazed camel in corners you may need to look deeper. Is the frame rigid enough? do the shock absorbers have any compression damping by design (some don’t) or are they knackered? Harsh action from the front forks under braking can be caused by stiction.
One reason Honda’s CB1100RC handled so much better than the standard 900s could be the adoption of bushes (like British bikes!) on the slider of the 1100 rather than the plain stanchion and slider on the 900. Bushes are less likely to lock the leg as it bends because they offer a degree of self alignment. Dampers can be replaced or reworked – Moto-X Developments are one company who can overhaul and uprate Japanese monoshock suspension units. And P&M even rework the damping parts of the hallowed Marzzochis to improve their action. Just don’t tell the Italians!
GLOSSARY OF TERMS
AERATION: Air bubble present in a liquid (called an emulsion).
AIR SPRING: Device using air’s natural ability to be compressed and provide a progressive rate spring.
ANTI-DIVE: System to resist a vehicle’s nose-down pitch when accelerating.
ANTI-SQUAT: System to resist a vehicle’s nose-up pitch – when accelerating.
BOTTOMING OUT: When all suspension travel has been used.
COIL BIND: When a spring coil bears against the next coil.
COMPRESSION DAMPING: Resistance offered by a shock absorber to being pushed together.
DAMPING FORCE: Force required to move a shock absorber at any given speed.
DECELERATION: Rate of slowing down.
EXTENSION DAMPING: (rebound or return damping). The resistance of a shock absorber to being pulled open.
FADE: Reduction in damping ability due to heat, either from heavy use or an external source eg the engine.
FREE LENGTH: Length of a spring when no load is applied.
LEVERAGE RATIO: (Of a rear suspension). Rear wheel travel divided by change in length of the shock absorber (fully extended minus fully compressed).
MOMENTUM: Mass multiplied by velocity. (Weight x speed).
ORIFICE: Passage of exact and predetermined size for metering shock absorber fluid.
PROGRESSIVE RATE SPRING: One piece spring with a rate that smoothly increases as it is compressed.
REAR WHEEL HOP: Tendency of the rear wheel to lock under brakes, leading to a jump/skid action. It is provoked by engine braking and or a lightening of the rear wheel due to weight transfer to the front wheel.
RISING RATE SUSPENSION: System with geometry, (usually achieved through a system of levers) producing an increasing spring rate by means of decreasing the leverage ratio.
SHOCK ABSORBER: Hydraulic device used to resist and absorb the energy of movement by pumping oil through orifices.
SPRING RATE: Amount of force required to deflect a spring a given distance.
TOPPING OUT: Condition when all suspension travel is available, when the wheel is probably off the ground!
UNSPRUNG MASS: Combined weight of wheel, tyre, brakes and suspension parts etc.
WEIGHT TRANSFER: When braking this is the amount of load subtracted from the rear wheel and added to the front.
Words John Robinson