Air cartridge or the classic coil? The difference between suspension types


You may have asked yourself at one point if an air cartridge suspension fork works better than a coil one or is it the other way around. Found more abundently are infos about the air cartridge forks, meaning that less data about the coil ones gets around. So, we’re going to explain the latter in this article.

Elicoidal cylindrical coils

The ancestor of the modern suspension coil has been around from the 15th century, the 1850’s producing its best version, the one made from steel, that has never been changed since then, for the simple reason it works very well. Some other iterations appeared in recent years, mainly wraped around using other metals, but steel remains king.

How it works

When pressed or pulled, a coil compresses or expands, leaving the initial position. This in turn triggers an opposite reaction force, proportional with the distance of the pressing or pulling. However, you don’t simply mount a coil on a bike, since it needs integration in the fork or the frame. The closest thing to this simple motion of pressing happens inside the suspension fork, while the frame shock acts after the impact force is diverted through several segments. Also, worthy to know is that when pressed, the coil stores energy.

air-cartridge-or-coil-bicycles-suspension-2Writtings on the coil

The figure 225 x 3.5 means that you have to put in 225 ounces of force to make the coil compress 1 inch. The second figure is expressed in inches as well, and it denominates the full travel of the shock’s rod. Doing some math, we find out that fully compressing the shock (3,5 inches) requires 787,5 ounces of force. As a corollary, every inch of compression stores 225 ounces of energy.

air-cartridge-or-coil-bicycles-suspension-3The only thing interfering with the travel is the bottom-out cap, made out of rubber or sponge.

air-cartridge-or-coil-bicycles-suspension-4The mechanical features of a coil remain the same regardless of the material chose for building it (steel mixed with carbon, high alloy steel etc.), but differences are made by volume, mass and resistance to corosion. However, not all models keep to this line and an example for this is Fox’s SLS steel-made coil, having a shorter and less thicker spiral. Still, operating features and performance is the same. Also, it’s good to know that every manufacturer has it’s own type of coils, so you cannot interchange them.

Various reasons convinced the manufacturers, at one point, to use elastomers in building-up the coil, mainly for keeping the functioning features of the second half of the travel similar to the ones in the first half. The outcome was less than pleasing, so only entry-level forks feature nowadays this type of material.

SrSuntour fork.

As a brief FYI, you should know that racing car mechanics and engineers fit the cars with axially overlapped coils, of which one is harder to compress, while the other is softer.

air-cartridge-or-coil-bicycles-suspension-6Air chambers

air-cartridge-or-coil-bicycles-suspension-7Gases also act like a coil given that at normal temperature and pressure the molecules are squeezed closer together and the natural tendency is to return to their natural distance. Fueling the air chamber is done using a regular pump, which is connected to the valve situated at the end of the cartridge. The amount of air goes hand in hand with the weight of the rider.

The negative chamber exerts an opposite force to the one from the positive chamber, thus making the cylinder slide smoother in the first part of the travel. However, the volume of the negative chamber is smaller that the one of the positive chamber, which makes negative pressure drop faster than the positive one. So, the more gas volume is found inside the negative chamber, the more the suspension works like a coil in the first part of the travel.

This results in what you could call an elastic gas coil and if the chambers are adequately sized, then the air cartridge shock behaves like a coil one.

If it wasn’t for this negative chamber, the force required to get the shock moving should be very big, and the speed of rebound couldn’t be controlled by the hydraulic system. This would end up in a disaster as far as shock functioning goes, cancelling all smoothness.

The chart above depicts the evolution of ratio between applied force and deformation of the elastic element. In the case of coil springs, the line is smooth, while in the case of air cartridges, the line is set by different sets of ecuations that generate uneven behaviour. The two lines meet in two points, and they mark the beggining or the end of the segments that provide proper functioning.

Elasticity of the gas chamber and of the coil

From the chart we can notice that coil elasticity is constant, while gas chamber elasticty varies, with the aforementioned two points of intersection.

In the first leg of compression one can notice diminishment that drops below the one of the coil, after which it reaches a low and starts growing again, eventually overpassing the coil’s value. Put together, it turnos out that the air cartridge and the coil together provide efficient shock absorbption during riding.

Pros and cons

If you’re searching for higher sensitivity over small bumps, the coil suspension works best. Also servicing such a fork is less demanding, since less parts are subject to wear. Coil weight can prove a downside and you will spend more time setting up the proper tension according to your weight. Another perk is that functioning generates less heat and therefore doesn’t influence the viscosity of the oil inside.

Air cartridges allow a wider range of settings compared to coil spring suspensions. A simple pump is all you need to insert air into them and adjust, while low weight is also a plus, the system saving something around 350 to 400 grams. However, you’ll visit the technical service more often in order to inspect the chambers and you’ll have to keep in mind that this system generates more heat during riding, which will influence the functioning on the spot. In this case, it isn’t about only the oil, but the gas inside as well. Shortly put, the oil becomes more fluid, so compression and rebound speed up.

Recent years have brought improvement to both air cartridge and coil spring shocks, so deciding between them becomes a difficult choice, since performance output reaches an almost similar level.

Author: Cezar Bajenaru for