© Tony Foale 1987 --
The two main functions of suspension are firstly, to insulate
both the rider and the bulk of the machine from road shocks - the
first for his comfort, and the second for mechanical reliability
and longevity, -- secondly, to keep the wheels in the closest
possible contact with the ground. Many factors contribute to this
including ;springing, damping, suspension geometry and the ratio
of sprung to unsprung mass.
For our purposes the most important characteristic of a spring is its "rate". This is a measure of its stiffness and is determined by measuring the extra force needed to compress (or extend in some cases) the spring by a given small amount. In the imperial system of measurement the rate is usually expressed in terms lbs/inch. So a spring with a 100 lbs/in. rate will need an additional force of 100 lbs. to compress it by a further 1 inch. In some cases this rate does not vary through-out the useful range of movement of the spring, and is termed linear. On the other hand some types of spring exhibit a different rate in various parts of range of movement, this is often known as a progressive rate spring and in motorcycle use this progression is positive, i.e. the rate increases with added load. It is very important to understand the distinction between rate and load. Load is the total force being supported by the spring, whereas rate is the ADDITIONAL force needed to compress the spring by an extra amount.
Springs can take many forms and be made from many different materials, but the practical range is more limited. Coil springs in steel are the most common by a long way. They may be evenly wound (constant pitch) to give a linear rate, or they may be wound with a varying pitch to give a progressive rate. In this case as the spring is gradually compressed, the closer wound coils become "coil-bound" (i.e. touch each other and effectively reduce the number of active coils ) and so the rate rises. Also in steel, leaf springs and torsion bars have been tried occasionally, but have been superseded for various reasons, although in some applications, torsion bars may save valuable space. It is worth noting that a coil spring is really only a torsion bar wound into a helix, though that subjects it to bending stresses and stress concentration that are absent in a straight torsion bar, hence for a given load capacity the coil spring will be subject to higher stress or will have to be heavier to reduce the stress.
As an alterative to steel, titanium is a very attractive spring material. It is twice as flexible as steel and little more than half the weight, size for size. Theoretically, this should lead to a spring of one quarter the weight of a similarly stressed steel one. In practice, the need to have closed coils at the spring ends, to seat properly, means that this saving is not fully available and an actual titanium spring will be closer to 1/2 or 1/3 of the weight of the steel one. There is really only one disadvantage with this material that prevent more wide spread use, and that is one of cost, confining its use to exotic works racers.
Rubber has many properties that make it an interesting spring material, and it can and has been used in a variety ways. It is not suitable as a direct replacement material to make a coil spring, it must used in an approprate design. Greeves used it in the form of large bonded bushes that served as both the pivot bearings and the springing medium in their leading link front forks. The rubber itself was loaded in shear, though the bush as a whole was loaded in torsion. In the Hagon telescopic forks, designed for grass track racing, rubber bands provided the springing medium. Adjustment was made by adding or removing bands as required. Rubber has a natural progressive rate, it also provides some inherent damping, though this generates heat which may become a problem with very hard use in rough terrain. It is a versatile material and its characteristics can be tailored to suit different requirements by varying the composition and/or the mechanical design. Although it would be less adaptable for individual suspension tuning, than coil springs which can easily be obtained in a wide range of rates and lengths, on a mass production basis, rubber springing lends itself to low unit costs. The ubiquitous Morris Mini was suspended thus, and adequately demonstrated the potential, this material merits consideration in any new design, and may be overdue for a revival with the current interest in progressive rate suspension.
To complete our survey of spring materials we must consider air or gas, which automatically provides a progressive rate. This can easily be demonstrated with a bicycle tyre pump. First extend the pump then cover the outlet with your finger to seal it, and compress the pump. You will find the initial movement generates little resistance but as the movement increases the required force increases very rapidly. The load supported by a pnuematic unit depends on its internal pressure, which in turn depends on the initial static pressure and internal volume. This pressure is inversely proportional to the volume. i.e. If the internal volume is halved then the pressure doubles and the unit will be supporting twice the initial load. This relationship between pressure and volume is known as "BOYLE'S LAW" , which you would already know if you had paid more attention in school. The extent of the progression in rate is determined by the compression ratio of the unit ( i.e. the ratio of the gas volume at the two extremes of travel ).
Fig.1 shows this variation between two units that start off by supporting the same load. Apart from this progressive rate, air shocks have the advantage of easy adjustment to compensate for different loads on the bike. If a passenger and luggage for a trip, doubles the load on the back end, then just double the gas pressure. Then at every point in the suspension travel the load supported will double and the rate will be double. This gives perfect compensation for the increase in load, unlike the preload adjustment found on normal coil spring units. This adjustment does not have any effect on the spring rate, only on the initial load capacity. Such suspension will show an increased tendency to bottom out when heavily laden, unlike the adjusted gas shock. However, despite this adjustability the pneumatic unit can be at a severe disadvantage when it comes to tuning the unit for a particular application or to suit a particular rider's needs.
For example, if the rider determines that for his use a softer spring rate would be desirable, then with a coil-over shock he need only obtain and fit an appropriate softer spring. On the other hand, the man with the gas unit is in a bit of bother. His first thought may be to simply release some gas. But, as a given pressure is needed in the unit to support the static weight of the machine, all that happens is that the ride height is reduced to a level which compresses the gas back up to the required pressure. However, at this new ride height the volume in the unit will have been reduced and hence the spring rate at this position will in fact be higher, so not only has the ride height and hence available wheel travel been reduced but the rate has been increased too. If he took the opposite approach and added some gas, then the rate might well be decreased as desired but ride height will be increased. If too much gas is added then the preload may become so high that a bump is needed to even begin to compress the suspension, so negating the desired effect. Other than buying another unit with different characteristics, there is little that can be done.
Some units allow for the addition or removal of small quantities of oil, this alters the internal volume and hence the spring rate, but the degree of progression will be changed also, because the compression ratio will have been altered. If a suitable setting can not be obtained, then the only option is to get out the welding torch and change the leverage ratio. (This is not applicable to telescopic front forks, but these are not usually supplied as totally pneumatic.) For example, suppose that we move the suspension mount on the swing arm from the wheel spindle area to half way along, assuming that the frame mounting was moved also to compensate, the leverage on the suspension unit will now have been doubled. i.e. the static load on the unit needed to just support the weight of the bike will be twice the previous value. To achieve this the pressure in the unit will also need to be doubled, which in turn will increase the rate by a factor of two. The change of leverage will also have the effect of halving the movement of the unit compared to the wheel displacement. Now, the reduced movement and the increased rate of the unit combine to give the effect of halving the wheel rate, and the degree of progression will remain the same in terms of unit movement. But as the wheel now moves through twice the range as before we may be in trouble with not being able to accommodate this increase. Another problem with this approach is that although the increased pressure in the unit increased its spring rate it did nothing to increase the damping rate, which as a result will now be too small, assuming that it was OK. previously.
In my experience, most complaints levelled at after-market gas
shocks are due to the rider's expectations of the adjustability
benefits being raised excessively by the manufacturer's
advertising hype. Once a suitable unit is matched to a particular
application, then the ability to perfectly compensate for load
differences by changing the pressure is a valuable benefit,
although unless the unit also has adjustable damping, then that
can not be optimum throughout the full range of loading
conditions. ---Do not expect to match any old gas shock to your
bike just by changing the pressure, it doesn't work like that.
This is necessary to prevent uncontrolled oscillations in the suspension. Imagine that a large bump has fully compressed a suspension strut; at that instant, energy is stored in the spring as potential energy. As the spring returns to its static length it gives up this energy, which if there were no damping, would be transferred entirly to the mass of the bike in the form of kinetic energy ( energy of motion ). This would cause the suspension to extend well beyond its normal position. This will have transferred the kinetic energy back into stored energy in the spring, which will then repeat the whole process again in the opposite direction. Thus after any disturbance, we would proceed down the road as if on a pogo stick. The introduction of damping will absorb some or all of the energy imparted to the suspension by the bump and hence the oscillation will be reduced or eliminated, depending on the degree of damping. The energy absorbed by the damper will be changed into heat, and this is why hard worked suspensions, such as in motocross, sometimes overheat. A damper is an energy absorber and should be matched to the rate of the springing medium, and the type of use of the machine.
Before the introduction of hydraulics, dampers were of the friction variety and their characteristics were precisely the opposite to those required. The STatic frICTIONal force ( often called STICTION ) was high, but once the damper moved the friction dropped somewhat. But since a damper can only absorb energy when moving, obtaining adequate damping involved excessively high stiction, hence a large force was necessary to start the suspension moving. This meant that the ride was harsh and insensitive to small bumps. Any form of stiction is detrimental to suspension performance.
By contrast, hydraulic dampers begin to move with a minimum of force, so providing sensitivity at low rates of motion, while high damping forces are available as the speed of the damper rises. The seals and rod support bushes introduce some stiction and the manufacturers reduce this as much as possible, sometimes seals are made of PTFE., a low friction -- non-stick material.
Hydraulic dampers can be endowed with various characteristics by their internal design --- e.g. one way damping only, two way damping with different rates for bump and rebound, dead spots in the movement and so forth. Early on, one-way damping only was quite common, on rebound only. The reasoning for this, was to present the minimum resistance to wheel movement after hitting a bump, transmitting the minimum force to the sprung part of the machine for the rider's comfort. Rebound was considered less important from the comfort point of view, and so all the damping was applied in this phase, requiring approximately double the damping forces that would have been necessary had equal damping be applied in each direction. This approach has several disadvantages, the reasoning may have been sound from the comfort aspect when negotiating a single bump but could give rise to a serious problem when crossing a corrugated surface. Each bump compresses the suspension quickly (because of the absence of damping) while the subsequent recoil stroke is slowed by the heavy damping. This may prevent the suspension from returning to its static position before the next bump. The rapid repetition of this action may soon ratchet the suspension into a fully compressed state, so giving the effect of a solid frame. Even single bumps can cause trouble with one way damping. At high speed the heavier rebound damping may prevent the wheel from maintaining contact with the ground as the wheel passes the crest of the bump, in a straight line this reduces traction and braking, which is bad enough but if this happens when cornering the result may be more serious.
It is now almost universal to have two-way damping, but not equal amounts for the two directions. This would cause ride harshness and so a compromise must be struck. The ratio of bump to rebound damping used varies with the intended use of the machine, and some expensive racing units have provision for the independent adjustment in the two directions. For non-adjustable units the ratio may be fixed typically around the 1:4 to 1:2 region.
In a normal hydraulic unit there are two types of damping -- viscous and hydrodynamic. Viscous damping arises from the shearing action of the fluid and the force produced is proportional to the speed of damper movement. Hydrodynamic damping is proportional to the square of the damper velocity, and is due to the mass transfer of fluid within the strut. This is the characteristic obtained by forcing the fluid rapidly through an orifice.
Viscous damping is a mathematical nicety and can be matched precisely to a single rate spring to give "critical" damping (or a desired percentage of critical) over a range of operating conditions. Critical damping is that amount which just prevents any oscillation or overshoot after hitting a bump.
Hydrodynamic damping is by far the highest proportion present in a normal damper. However, this in unmodified form can give undesirable effects. Because of the "squared" effect the damping forces rise very rapidly with damper speed, and give little resistance at low speed. This means, that at low road speeds damping may be inadequate over small bumps, but grossly excessive on larger disturbances at high road speed. In order to make a satisfactory damper the manufacturers must modify the basic idea of just forcing the fluid through small simple holes. To reduce the excess high speed force, the orifices need to be enlarged and then controlled by a blow off valve which opens only at high speed. This valve prevents the mid range damping from being reduced but will probably raise the low speed to excess. This can be overcome by introducing bleed holes not controlled by the blow off valve.
By juggling around with these techniques the manufacturer can straighten out the damping curve and match it to the application. Basically the response is brought closer to the nature of viscous damping. It would therefore seem sensible to design a damper as a viscous one in the first place, however, simple as this would be in an electrical context, the realities of practical hydraulics make this none too easy. Thick fluid would need to be sheared between moving plates or cylinders, and the damping would be very dependent on the fluid viscousity, which is of course affected by temperature.
As if the problems of obtaining a satisfactory damping curve were not enough the designer also has the problem of displacement to contend with. In other words, as the damper is compressed the volume of the rod entering the cylinder reduces the space available for the fluid. Hence, a compressible medium such as air, must be introduced to compensate. But as the damper is shaken on bumps the air and oil mix drastically reduces the damping forces. Many ingenious solutions to this problem have been tried over the years. One simple way to bypass the difficultity is to extend the rod through both ends of the cylinder so that there is no change in internal volume with piston movement. This does nothing to compensate for volume alterations due to temperature change, or just plain old fashioned fluid leakage, and this method is not currently used for suspension damping but is almost universally used for steering dampers.
Many years ago, Girling adopted a clever solution on their twin tube strut. In this the free air was replaced with a sealed nylon bag containing freon gas (under the trade name of Arcton). This particular gas was used because it is composed of large molecules and they did not permiate through the bag. Production of this unit was short lived because of the production difficulties, although the idea has been used by other manufacturers. A later Girling solution was to revert to free gas but to subsitute nitrogen at 100 psi. for air. An emulsion was formed between the oil and gas, the resultant fluid giving constant damping characteristics.
Current design trends have been influenced by the DeCARBON principal, where the oil and gas are separated by a floating piston, either in the unit body or in a separate remote chamber. The gas pressure (up to 300 psi.) keeps all the seals pressurized to reduce leakage and to stabilize the damping and prevent cavitation, however rapid the recoil stroke. On some dampers of this type there is provision for adjusting the pressure as a fine tuning aid.
There are many factors that determine ride quality and
roadholding other than the spring and damper parameters. Next
time we can look into some of these, ---- like rising rate
linkages, sprung and unsprung mass and how to determine the
spring and damping rates necessary.