SUSPENSION 104: Air vs. Coil

Words: Hayden Wright

Photos: Gerard Lagana

Through the first three parts of our series on mountain bike suspension we have delved into the worlds of servicing, eMTB setup and shock tuning with some of Australia’s perennial experts in their respective fields. In this fourth and final part of the series, we catch up with Joshua Mclnerney who is a suspension technician at NSDYNAMICS. Josh, a mechanical engineer by training loves all things suspension and given his technical background we thought who better to quiz on the age-old question that is air versus coil. While we appreciate that you could easily fill this entire magazine from cover to cover discussing this inflamatory subject, we’d prefer to let someone like Josh who has both the theoretical knowledge and practical experience designing, servicing and riding both spring types do the talking. 

“Many modern bikes give riders the option of choosing between a coil sprung or air sprung suspension system, each have their own advantages and draw backs,” stated Josh when we pulled up at his workbench. “While they might have the same adjustments, they can feel very different when riding. Coil shocks are easily identified by their wound steel or titanium springs, mounted over the damper body (sometimes also referred to as a coil-over-damper or coilover). These springs typically have a set stiffness for the entirety of their stroke, although some brands, such as Cane Creek and DVO offer progressive rate springs.” 

“Progressive springs give an option to have one stiffness at the start of the stroke that gradually increases as they reach the end of their stroke. Coil shocks, by their linear nature, do require a frame designed with a progressive leverage curve, so some bikes may not suit a coil shock as well as others.” Typically, air shocks have a naturally rising rate, and many frames have been designed for one type of shock or another – but that is changing. 

“The disadvantage of a coil shock is that to change the spring rate a different spring is typically required. Sprindex have linear rate adjustable springs, allowing for approximately a 10% change in stiffness through tool free adjustment.” 

Coil shocks are often known to have minimal stiction, and Josh explains it's all to do with the seals involved. 

“Coil shocks tend to have only a small number of dynamic seals inside the damper, resulting in reduced seal drag friction. By reducing the friction, combined with the low loads required to begin depressing the coil spring, coil shock tend to track the ground better and are more able to absorb small fast bumps. Coil shocks area often described with words like “supple”, “ground hugging”, “predictable”, and “composed” thanks to how the reduced stiction feels.”

“In contrast, air shocks are much simpler to get the correct spring rate, as all that is required is a shock pump to change the spring pressure. With greater freedom to adjust the suspension's firmness, as well as the option to tune the shocks spring characteristics with volume reducers, air shocks represent a more adjustable and tuneable option.” 

Ease of adjustment is on of the best featyures of an air shock – and also spring rate range. Some big eMTBs may need a spring rate that doesn't readily exist, depending on the rider aboard the bike.Another important factor is the role the shock plays in the frame design.

“Air shocks also prove as a stiffer option than their coil counterparts, thanks to the much larger main shaft making up the shock body. With this larger shaft diameter and the extra sealing requirements of the air spring comes increased seal friction and as such air shocks tend to lack the ultimate sensitivity of coil shocks. To combat this and reduce the initial spring rate of the air spring a negative chamber is included in the shock with the pressure acting to compress the shock particularly at the beginning of the stroke.”

One of the big factors between both options is weight. Unsurprisingly, air shocks are lighter – but times are changing according to Josh.

“Modern metallurgical techniques in producing low weight steel springs are bridging the weight gap. While it may not be noticeable in all conditions it is worth noting that the environment that an air shock is operating in can have an impact on the air spring, in particular as the shock is compressed multiple times the damper converts some of this motion into heat to dissipate the impact, this heat, if not managed properly can raise the pressure of the air, increasing spring rate as it heats up. For most users this effect may be negligible, but if you are someone who often rides in a bike park, or a racer who sets their suspension on a cool morning before riding during a warmer part of the day this may be noticeable.”

There is no right or wrong answer when it comes to the choice between air and coil springs. They both have their distinct advantages and disadvantages. In summary, riders who are looking for a damper which prioritises weight over performance, particularly at the beginning of the stroke, will go with an air spring. Similar assumptions can be made to riders who prioritise traciton and heat management over all alse tending to prefer coil springs. That is not to say that it’s uncommon to see a long-legged downhill bike with an air spring, or a 120mm trail bike with a coil unit. If you are looking to replace or upgrade the suspension on your ride, the best thing to do is call your local bike shop or suspension tuning store and ask their opinion. They will be able to help you pick the component that best suits your specific needs. 

Suspension Glossary

Damper: A device used to oppose motion, typically paired to a spring (may be a coil or an air spring) to provide increased control of the motion of a system.

Compression: Reduction of available travel due to cornering loads, bumps, and body inputs.

Compression Damping: Compression damping refers to the slowing of the shock as it moves through the compression stroke. This is used to stabilise the system. Some dampers have high and low speed controls, this refers to the speed the damper is compressing, not the speed the bike is traveling.

Rebound: Extension of the suspension as load is removed from the bike.

Rebound Damping: Rebound damping controls the rate that the suspension can return to its resting position after a bump. Some dampers have high and low speed controls, this refers to the speed the damper is returning, not the speed the bike is traveling.

Bottom Out: Reaching the end of the available travel due to compression, the shock is unable to be compressed any further.

Top Out: Reaching the end of the available travel when rebounding, the shock is unable to be extended any further. 

Packing: An increase in the suspension's stiffness resulting from multiple compressions that the suspension can not return from in time for the next impact (overdamped), the suspension becomes too stiff to function effectively, the correct spring rate, and reducing rebound damping will help mitigate this.

Bucking/Kicking: The tendency of a rear shock that has too little rebound damping (underdamped) to control the return of the shock, resulting on weight being transferred to the front of the bike and a potential loss of control.

Preload: Refers to the loading of a coil spring when at rest, increasing the force that needs to be applied to it before it begins to move. Note; a common misconception is that preload increases the spring rate, this is not correct.

Shim Stack: inside the damper the shim stack is an array of thin washers (shims) that restrict the flow of the damping fluid, creating the damping forces that work in opposition to the spring.

Spacers: Used in air shocks to adjust the volume of the shock, adding spacers reduces the volume and increases the force required to bottom out the shock. Removing spacers has the opposite effect.

Shock Length / Eye to Eye Length: The uncompressed distance between the shock eyelets, measured from the centre of the eyelets.

Shock Stroke: The distance that the shock can compress, this can be calculated by measuring the shocks eye to eye fully extended, then subtracting the measurement made at full compression.

Air volume: For air shocks the volume of the air can has a significant effect on the feel of the shock, with a higher volume typically feeling more linear and coil like, while a smaller volume will increase the ramp of the air spring to help resist bottom out.

Sag: The measurement of the compression of the suspension under the rider’s weight when static on the bike. This is most commonly used as the baseline for setting up spring rate on a bike.

Lockout: A device for closing the compression damping circuit, resulting in a firming up of the suspension. Most shocks will still allow some movement when “locked out” to prevent damage to the shim stack or other components due to the large forces generated by impacts when the shock is in the locked position.