I appreciate that your point is that gas springs can be lighter and the mass can be positioned more favourably. However gas springs are, in fact, not as easily tuned as I had expected. I had thought that pneumatic suspension was a really versatile system where, simply by changing the gas pressure, you could easily tailor the spring rate to whatever suited you. I read the following quite long extract in
‘Motorcycle handling and Chassis Design, the art and science.’ by Tony Foale. He too had been an enthusiast of the system until he built a bike using Fournales gas spring units.
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 that the initial movement generates little resistance but as the movement increases the required force increases very rapidly. The load supported by a pneumatic 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".
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 6.3 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 initial gas pressure. Then at every point in the suspension travel the load supported will double and the rate will be double. This gives perfect spring compensation for the increase in load. Of course to achieve perfect compensation we would also need to double the damping.
The preload/ride-height adjustment found on normal coil spring units, 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 adjustable 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 trouble. 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 effective spring rate, but the degree of progression will be changed also, because the compression ratio will have been altered.
If a suitable setting cannot 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 will now move 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, that cannot be optimum throughout the full range of loading conditions. Do not expect to match any gas shock to your bike just by changing the pressure, it doesn't work like that.
To provide easy load compensation for large touring machines the American firm, S&W, used to offer air shocks with an on-board air-compressor. It was a simple matter of pumping the bike back up to the desired ride height after the addition of a passenger or heavy luggage.
