Suspension debate: coil or air?

The 2017 enduro season was remarkably different to those of years past, largely due to the number of riders opting to run a coil shock over traditional air sprung systems. Coil shocks have always been the suspension of choice for downhill riders, but it’s only recently that riders needing to ride up hills have started adopting a coil based system. Coil shocks, for a long time, were heavier, less adjustable and more prone to bottoming out than air shocks. So why are more and more riders opting for them now?

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It’s becoming increasingly common to see coil shocks come standard on high-end enduro bikes

Regardless of whether your shock uses a coil or air to physically suspend you, all suspension systems have the same goal and operate using the same principles. Every shock is made up of two parts: a spring and a damper. Unsurprisingly, this spring can be air or a coil. The spring acts as an energy sink, storing the energy generated during compression and using it to rebound the stanchions once the energy stored exceeds the amount of energy compressing it. The point at which this occurs is determined by your air pressure or your coil size. The spring works in concert with the damper, which does basically what it says; without a damper, your suspension travel would oscillate upon rebound, making for a terrible ride feel. The damper (which is basically pressurised oil running through a series of holes) dissipates the energy in a more controlled fashion, giving the smooth extension and ride feel we experience with our current suspension.

….coil shocks are so much more than heavy, unrefined air shocks….

Besides weight, the most pressing issue concerning coil shocks is their lack of ability to ‘ramp up.’ Ramping up is a shock’s increasing ability to resist compressive forces near the end of its stroke. Because you can add tokens to an air cartridge but not a coil shock, for us mountain bikers, this means air shocks are better at resisting bottoming out. This is known as progressive suspension kinematics. Coil shocks, however, are bound by Hooke’s law, meaning they resist compression at the same rate, regardless of their position through their stroke, a property known as linear suspension kinematics.

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Rockshox’s new coil, the Super Deluxe RT comes with a remote lockout to optimise the coil for climbing

In the past few years, coil shock technology has come a long way. Suspension companies have fixed the major issues, such as weight and adjustability, that prevented enduro riders from opting for a coil based system. But coil shocks are so much more than heavy, unrefined air shocks. The main riding advantages of coil shocks is small bump sensitivity and traction, but more than that, coil shocks are also much more durable than air shocks. They don’t heat up like air shocks do, meaning you get better performance on long descents. Coil shocks also have the major benefit of being much simpler. An air sprung fork requires a number of internal seals, any of which can leak and degrade. A coil shock simply doesn’t have these, and therefore doesn’t require the same level of maintenance and servicing.

The biggest difference between coil and air shocks, which is either a selling point or an insurmountable obstacle, is the level of adjustability. Air shocks, by their very nature, are infinitely adjustable. You can increase the resistance to compression by a single PSI if you’re picky enough (Aaron Gwin famously is). Air shocks require constant tuning and re-tuning, meaning you can optimise your suspension for every ride. Conversely, coil shocks operate on more of a ‘set and forget’ principle. You can still adjust compression and rebound settings on coil shocks, but the actual resistance to compression is set by the spring rate, which is only tunable in 25lb increments. However, once you do find the right spring, you’ll never need to adjust it or re-inflate the shock because the canister has leaked.

So what shock should you get? It’s hard to draw a blanket conclusion, as there are just too many variables, the largest of which is the suspension kinematics and the leverage ratio of your bike’s frame. If you have a bike with a progressive suspension design, like most bikes, a coil will suit your bike perfectly. However, a bike with linear suspension paired with a linear shock will be much more prone to bottoming out. That said, coil shocks are now as light as many air shocks, require much less maintenance, and provide far superior traction and small bump compliance, so if you’re in the market for a new shock, a coil is definitely something you should consider.

About Tim_Davis@Pushys

I'm a Science Graduate and Medical student at the University of Queensland, specialising in anatomy and physiology. More importantly, I'm all for any type of riding; road, mountain, dirt jumping, I love it all! Let me know if there is anything you want to know about nutrition and health, and I'll do my best to help you out!

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