Should I get an air shock or a coil shock? Is a question that we commonly hear from customers looking to upgrade the suspension on their bike with some new pieces. There are pros and cons to each setup and I wanted to make a bit of a guide to how each works. It is important to note that any high end suspension item on the market today is most likely excellent and choosing any of them will get you a good product.
Spring rate refers to how much force it takes to compress a spring (coil or air) and they are typically stated in different ways depending on what type of spring we are talking about. Air springs you would refer to the air pressure (in Pound pre Square Inch or PSI) when the spring is fully extended. On coil shocks the spring will have a number printed on it, for example 400x65mm, which means that the spring takes 400 pounds of force to compress it one inch and the spring is for a 65mm stroke shock. Fox measures their springs differently and the second number will be in inches, for example 2.45, and that number will refer to how far the spring can compress before it binds on itself. For all other brands the second number will be the stroke length of shock it is for. It is recommended that you use the same brand of coil spring as your shock so that their are no fitment issues.
Air springs on forks typically have two chambers (positive and negative) and in most cases, the two chambers transfer air between each other at a set point so that the pressure is slightly higher in the negative spring when the fork is fully extended. This pulls the fork into the travel slightly to overcome the static friction (stiction) of the seals. It is uncommon but some brands use a coil negative spring which is simpler but is not usually adjustable. This works well if you are in the weight range that the spring will work for, but is sub optimal if you are heavier or lighter as the coil spring will compress the fork too much or not enough leading to a low ride height or poor small bump sensitivity.
Coil suspension has a lot of positives and a few drawbacks that are not always obvious. The main benefits of a coil spring over an air spring is the reduced number of seals leading to a higher level of sensitivity (less stiction) which can be very noticeable on trail. This generally leads to better traction and stability over small bumps. Another benefit of coil suspension is that due to the decreased friction, you can often run a slightly higher spring rate with a coil spring to get a higher dynamic ride height and more support in the mid stroke (middle of the travel). One downside to coil suspension is that there are often fewer ways of controlling the bottom out force as the coil spring does not ramp up the spring rate as quickly at the end of the stroke as the air spring does. While on rear shocks you will often find a large rubber bumper or a hydraulic circuit to control the bottom out force, on coil sprung forks there are fewer options and with limited exceptions you need to rely on your damping circuits to resist bottoming out on large or very fast impacts. The other main drawbacks of coil sprung suspension are weight (coil suspension is generally heavier than air, but not always by much) and that to do a spring rate change you need to have additional coil springs and you have to take some stuff apart to change them.
Air
Pros Cons
Low weight Lower sensitivity (often over-stated however)
Highly adjustable More maintenance
Better bottom out control Cannot match coils consistent feel
spring rate in small increments Affected more by heat
Coil
Pros Cons
Very supple Ramps up a fixed rate
Lower maintenance Extra springs can be expensive
Not affected by heat Heavier
Consistent ramp up Few options especially for forks
Can be impossible to get spring rate perfect
Coil suspension is best for someone that values sensitivity and stability over all else and is willing to spend the time to try different springs to nail exactly the feeling they are looking for. Air springs are easier to fine tune and offer more ways to tune the exact spring rate at any point in the travel. Many people mix and match; it is very common to see bikes with a coil rear shock and an air fork. What is your favorite?