© TONY FOALE April 1986.

Suspension systems have for a long time been a favourite topic of many a motorcyclist, whether he understands their principles or not. Designers are continually coming up with slightly different variants, which the marketing men then go overboard to promote. But, rarely is the question asked --- Just why do we have suspension anyway? The answer seems obvious, to reduce the shock fed back to the rider and machine, but also to keep the wheels in better contact with the ground.

However, this is not without problems, or why else would those who modify street bikes for the drag strip, so often replace the rear shocks with a solid bar. The basic problems stem from the ability of the bike to move on its suspension in response to factors other than road shocks. e.g. Weight transfer under braking or acceleration causes pitching movements, known as dive or squat depending on their direction. These pitching motions lead to steering geometry variations as well as rider discomfort. Another problem is the suspension compression at each end under the action of cornering forces. If we are leaning into a turn at an angle of say 45° then the centrifugal force combines with the machine weight such that the load on the suspension is 41.4% higher than the static figure. This means that when cornering on a smooth road the bike will be sitting lower than when upright. This leaves less of the total movement available to deal with bumps, perhaps necessitating a greater range of movement to be provided in the first place. Because of the lower attitude of the machine it may be impossible to avoid mounting the engine high up to provide sufficient ground clearance. Despite these problems it is usually better to have sprung wheel movement than not, but wouldn't it be much better if this could be offered without the disadvantages.

Well, this is exactly what 'active suspension' promises. The term ACTIVE has been coined because these systems rely on some external power source to respond correctly to the suspension inputs. Whereas, the conventional designs, PASSIVELY allow themselves to be pushed and pulled about under the action of the dynamic forces. Before studying the active designs, let's look at a few basic things first.


The pneumatic tyre is the first line of defence and is the most important of all the suspension mediums. To the extent that, while uncomfortable, it would be quite feasable to ride a bike around the roads,at reasonable speeds with no other form of bump absorbtion. Whereas, regardless of the sopisication of the suspension, it would be quite impractical to use wheels without pneumatic tyres, or some other form of tyre that allowed considerable deflection. The loads fed into the wheels without such tyres would be enormous and continual wheel failure would be the norm. A few figures will illustrate what I mean:--

Assume that a bike, with a normal size front wheel, hits a one inch high, sharp edged bump at 120 mph. With no tyre the wheel would then be subject to an average vertical acceleration of approximately 1000 G. (the peak value would be higher than this). This means than if the wheel assembly weighed 50 lbs. then the point load on the rim would be 50,000 lbs. or about 25 tons. What wheel could stand that? But if the wheel was shod with a normal tyre, then this would have at ground level, a spring rate, to a sharp edge, of approx. 100/200 lbs./inch. The maximum force then transmitted to the wheel would be about 100-200 lbs. i.e less than four thousanths of the previous figure, and this load would be more evenly spread around the rim. So the next time that you hear someone in the pub suggest that tyres should be no more than a strip of rubber around the wheel, you will know what to say! The shock loads passed back to the sprung part of the bike would be much higher too. The vertical wheel velocity would be very much greater, without a supple tyre, and so the damping forces, which depend on wheel velocity, would be tremendous. These high forces would be transmitted directly back to bike and rider. As the tyre is so good at removing most of the road shocks, right at the point of application, perhaps it would be worth while to consider designing it to absorb even more and elimiate the need for other suspension. Unfortunately we would run into problems. We have all seen large construction machinery bouncing down the road on their ballon tyres, sometimes this gets so bad that the wheels actually leave the ground. A pneumatic tyre acts just like an air spring, and the rubber acts as a damper when it flexes, but when the tyre is made bigger the springing effect overwhelms the damping and we then get the uncontrolled bouncing. So there are practical retraints to the amount of cushioning that can be built into a tyre for any given application.


Let's look at the need for this important function. Imagine the wheel running over a small step to a higher level. Firstly the spring will compress somewhat, but then as the wheel ceases to move upward anymore, the spring will begin to return to its normal length, forcing the bulk of the bike upward, without damping the bike will overshoot its normal ride height. In this position the spring force will be insufficient to maintain it in this position, and the bike will begin to drop. This sequence of events will continue and the machine will continue to oscillate up and down. The damping forces which are basically dependent on wheel velocity, absorb the excess energy in the spring and prevent this overshooting effect. The absorbed energy is converted into heat, which is why the dampers get hot with hard use. Dampers also help keep the wheels on the road by preventing the wheel from continuing upward at the crest of a bump. The dampers and springs must be correctly matched to each other, too little damping and we get the bouncing problem, too much and the ride becomes harsh because the damper force will resist the free response of the suspension to any bumps. With all these problems then, it is no wonder that the search for alternative methods have occupied the time of many engineers for a long period. The most promising developement looks to be among the range of 'active' systems.


Whilst the Lotus developements have captured the headlines in recent years, active suspension in other forms has been around for some time. The Citroen being the prime example, instead of just toying with springs and things as the British motor industry did with the hydro-elastic system as fitted to some Austin and Morris cars, the Citroen engineers perservered with their pump powered, self levelling, pneumatic design. Anyone who has ridden in one of these cars will tesify to the wisdom of their choice. Ride comfort is of a very high order.

In brief the operation is as follows;- each wheel is suspended by a pneumatic spring, this takes the form of a pressurized steel sphere containing a sealing rubber diaphram. The unloaded static gas pressure is between about 400 and 800 psi. Under wheel load the gas is compressed by oil on the other side of the diaphram, this oil is displaced by a piston connected to the wheels. Between this piston and the gas sphere is located the damper valve. Even if this was all there was to it, this would be a good basis for a very simple and elegant design of suspension unit. But the clever bit is that all four (one to each wheel ) such units are connected to a high pressure ( up to 2500 psi.), engine driven hydraulic pump. Each unit has between itself and the pump, a ride-height sensing control valve. These valves allow fluid to flow into the units when they sense that the ride-height is too low, say due to an increase in vechicle load, an extra passenger for example. When the load is reduced the valves allow excess fluid to flow back to the supply tank. The pump is of small capacity and is also used to supply the brakes with their working fluid, as a result the response time for height correction is designed to be quite long, and so is only useful for long term load changes. For normal bump absorbtion and inertia generated loads ( braking or acceleration pitch and cornering roll ) the system operates as if the suspension was only provided by the pneumatic units, the 'active' part only coming into play to correct height errors due to changes in static loading. This will also keep the suspension frequencies constant.

In the early '70s. there was a motorcycle use for the Citroen hardware. John Renwick at that time, lead the world in racing sidecar design with his wishbone suspensions. To provide the springing he modified these units so that they could stand alone and work without the pump. Another mid 70s. British developement took the Citroen type system to its logical conclusion. Automotive Products developed a very fast acting design, which was capable of reacting with such rapidity that braking dive and cornering roll could be eliminated. This then enabled very soft spring rates to be used for the betterment of ride comfort. The sketch shows the basic layout for one wheel.

The pendulus mass, damper and support spring fitted inside the control valve mimics the action of the full size components controlling the wheel. This enables the valve to assess whether wheel movement is due to bump response, which is permitted to take place, or to inertia generated effects, which are prevented. The price paid for this sophistication was the large amount of power required to drive the pump, up to 15 bhp. when taking a fast corner. When fitted to a test Rover the increase in road holding due to the lack of roll was quite significant.

As we know this type of roll is not a consideration on a bike, but a similar system could be used to limit or remove pitching and the ride height change caused by cornering centrifugal forces and/or changes in load. The power required would obviously be less than for the car, we only have two wheels to control but equally important is the lower weight of a bike. However, I believe work ceased on the AP. system, but whether this was for technical or commercial reasons, I know not.

A form of active suspension has been available on bikes for some time, namely the self levelling Nivomats as fitted to some BMWs. These have been cleverly designed so that the power required to pump them up is supplied by the wheel movement due to road shocks. As this energy would otherwise need to be dissipated as heat in the damper, there is no drain on engine power as with the previous two designs. The small price to pay for this is that the correct ride height is only achieved after the bike has been ridden for a short distance.

Now to the Lotus scheme, which has put the car world of late, into a state of excited anticipation. 'MOTOR' magazine went as far as suggesting that history will view it as being a developement landmark of similar importance to pneumatic tyres and disc brakes. The Lotus design sets out to achieve the same ends as the AP device but there are two main differences. One is the inevitable use a micro-processor to evaluate the signals from various sensors in order to decide the most appropriate corrective action. The second is that there is no need for any type of spring to support the car weight, this can be done entirely by the pump supplied pressure. The drawing shows a simplified layout.

As the wheel moves relative to the body the transducers sensing load, wheel position, acceleration and velocity, send their signals to the computer together with the output of pitch and roll accelerometers mounted in the body. These signals are processed to determine whether the wheel movement is due to bumps, cornering roll or to acceleration/braking induced pitch. Signals are then passed to the control valves to either maintain, increase or decrease the amount of fluid in each actuator. The result is a system which can eliminate pitch, roll and ride height variations while at the same time giving a very soft and comfortable ride. Normally the requirements for a smooth ride are in conflict with those other needs, and so as usual a compromise must be struck. Active suspension can greatly reduce this need for compromise. All this clever technology is fine for the cars but how can we benefit from it. Well, if we program the computer to provide the necessary responses to the wheel movement and load sensing transducers, then we could have a pitch free, exceptionally comfortable ride with self levelling that would compensate for load changes and cornering forces. What more could we ask for, except an ability to deal with landing after a jump or a wheelie.

The following is how I see such a system operating. Let's divide up the suspension requirements into different cases, and look at the optimum response needed from the system for each.


The first job of a suspension system is to support the weight of the machine, rider/s and luggage at the desired ride height. This is the simplist task and all that is needed is a signal from the wheel position sensor to notify the micro-processor, any departure from the desired ride height and the control valve would release or feed fluid into the actuator to correct the situation.


The job of the suspension here, is to transfer the minimum possible vertical acceleration back into the sprung part of the machine. This could be achieved if the actuator force remained constant throughout the period of the disturbance. This requires constant fluid pressure, and to achieve this means that fluid is bled away under the control of the valve, the electronics signalling the correct amount necessary to keep the output from the load or pressure transducer at the appropriate level. After the crest of the bump the valve would then be instructed to allow fluid back into the actuator at a rate which maintained the correct pressure. Because there is no change in force with wheel displacement the equivalent spring rate is zero, soft springing indeed! Simple eh? Well it would be, if that was all there was to it, but there are a couple of complicating factors. The above cycle of operation would be fine provided that the bump height was less than the available suspension movement. It would be daft if the reaction to a large bump was unnoticeable initially only to be followed by a severe jolt as we run out of movement. Perhaps the answer to this would be to allow just some of the movement to be at zero effective rate followed by a progressively increasing rate. In other words a glorified progressive spring. Another potential problem might occur as the wheel approaches the crest of the bump. During the initial phase of the disturbance the wheel is accelerated upward by the bump itself but as it nears the top the suspension must provide the force necessary to deccelerate the wheel and keep it in contact with the ground. This is the job of the damper with a conventional system. If the ratio of sprung to unsprung mass is unfavourable then the problem is made worse and the pressure in the actuator may need to be increased to maintain road contact. This would then pass some shock into the main chassis.


As mentioned earlier, the suspension is subjected to increased loading because of our need to lean whilst turning. Both ends are subject to the same proportional increase. If we allow this extra load to reduce the ride height as with a normal spring system then the engine would have to be mounted higher in the chassis, to avoid ground clearance problems, than otherwise required. So when the computer detects that the increased load is due to cornering it will order the control valve to maintain the correct level. The difficult bit is distinquishing between cornering forces and those due to both wheels hitting similar size bumps simultaneously, not such an unusual occurrence, as anyone who has ridden on some Australian corrogated dirt roads can tell you. However, there are some detectable differences which can be used to separate the two actions. a). Reaction time:-- Bump loads are applied very quickly, whereas the extra forces due to cornering are applied gradually as we bank over into the turn. b). Wheel acceleration:-- When we lean into a corner on a smooth road the greater suspension forces do not cause a wheel acceleration in the plane of the machine, but encountering a bump does. So the wheel accelerometers can send the appropriate signals to the black box. c). Magnitude of load:-- Except for ice-racers the increased forces are unlikely to exceed about 50% of the static load in the cornering mode, but a bump of any significance would have a much greater effect. These factors could all be monitored by the microprocessor, and the relavent instructions issued to the control valve.


Pitch changes due to, for example, acceleration and braking, could be eliminated by simply leaving the amount of fluid in the units constant. But we still have the problem of deciding between dive and squat, and the two wheels hitting a bump and hollow at the same time. This could be dealt with as above by utilising the same parameters with perhaps additional information from a chassis mounted longitudinal accelerometer. Then it would be possible to progamme in a little dive if it was necessary to provide braking feedback.


The aftermath of hump-back bridges and high wheelies is coming back to earth with a bang. The zero equivalent spring rate suggested earlier would be incapable of handing this situation. The ideal solution would be that the pressure and fluid flow rate from the actuator be adjusted to just absorb all the vertical kinetic energy of the machine within the available stroke of the unit, but with some movement left in reserve to cope with road shocks encountered. Perhaps we could programme the device to prepare itself when full rebound is detected by the position sensors. On touch down the initial relative velocity between the sprung and unsprung parts could be calculated and used to estimate the energy to be absorbed.

All the above sounds very complicated and hence perhaps prone to trouble. Actually the hardware is no more complex than that used on the Citroen cars, all the complex calculations and decisions are made by the ubiquitous silicon chip. Properly designed and programmed these devices can reliably process many times the amount of information needed for our machines. I think that the improvement in ride comfort and handling possible would be worth the complexity, but another price to pay would be the drain on engine power to drive the pump. I have not done any estimates on this, but it would probably be several BHP. which may be unacceptable on all but the largest machines.

It may be possible to use a simpler system with smaller power demands and still derieve most of the benefits. The response time needed to deal with the problems of pitch, cornering and weight change is much slower than that for handling bumps and landing. So perhaps we could have a fast acting Citroen type system, controlled by a much simpler computer, to maintain the required ride height. The response should be quick enough to compensate for the slower acting functions but not so quick as to inhibit the bump response. The basic suspension functions would be performed by hydro-pneumatic springs and dampers as per the Citroen, but reaction to the inertia generated loads would come from the rapid action of the pump powered hydraulics.

The Suzuki Falcorustyco is claimed to have electronically controlled suspension, but whether it is along any of the lines suggested here is anybody's quess. If so let's hope they get on with it, then we can all look forward to more comfort and better handling. Longer tyre life too, because the load on the tyres will be kept more even. My own feeling is that most of the claims made for the "Rusty Falcon" were just headline grabbing hype.

( 1997 note ) The problem with side loading the wheels and chassis, on today's racing machines, due to bumps at extreme lean angles could be helped considerably with properly designed active suspension ( but more of this in another article ).

It has long been my opinion that the developement of active suspension for bikes would be the next big step forward after the adoption of FFEs. ( Funny Front Ends ).