Coil Shock Or Air Shock

Coil Shock or Air Shock Which One is Better?

Coil shocks have made a resurgence on modern trail bikes within the last few years, and with that comes a question we often hear: should I get a coil shock or an air shock for my full suspension mountain bike? There will always be exceptions or outliers to these general concepts discussed below- your own personal riding style or preference will also affect the answer to this question.  It’s not the easiest question to answer, but we’ve broken it down into simple, general steps that will hopefully better inform you to make a more educated decision.

First, let’s understand the difference between coil shocks and air shocks.  A coil shock is linear by nature – it has one constant spring rate – the amount of force required to compress the spring will stay the same as the spring compresses through its stroke. For example, with a 450 lb spring, the force applied is 450 lbs/inch through the entire stroke of the spring – there is no change in spring rate.  Coil shocks are generally more sensitive (easier for it to compress and rebound) than their respective air shocks because there are fewer seals in the system, therefore there is less force required to get the shock moving. Because of this, coil shocks tend to provide more traction and a unique feel.

An air shock is progressive by nature- the amount of force required to compress the spring continues to increase as the shock compresses.  As an air shock moves through it’s stroke, the volume in the air spring will decrease, subsequently increasing the shock’s spring rate. Air shocks also have the ability to reduce air volume inside the shock by installing volume reducers. This is separate from simply adding air to the shock, a volume reducer only affects the mid-end of the stroke, and will not affect the spring rate at sag. Because of this, air shocks ramp-up more and provide more tuning options than coil shocks. 

Knowing the differences between the two shocks is most of the battle. But the bicycle’s linkage design also plays a big role. Essentially there are 3 different types of frame designs / leverage ratios that you should be aware of. 

A leverage ratio is the relationship between the movement of the frame’s rear suspension and the required force at any given point throughout the bike’s travel.  You can evaluate the bike’s leverage ratio curve by plotting those points on a graph. (In most cases, you can find your bike’s specific leverage ratio curve online). Generally there are three different leverage ratio curves found on modern mountain bikes: Linear, Progressive, and Regressive. A bike with that has an average leverage ratio of 3:1 means that every inch of shock movement equates to three inches of rear wheel movement.  As well, this ratio indicates the magnitude of force needed to compress the rear shock. A 150lb rider would need a 450lb spring to achieve proper sag on a bike that has a 3:1 leverage ratio.  However, leverage ratios commonly change through the travel of the bike, meaning that our clean 3:1 example will most likely not be 3:1 everywhere in travel, it is only an average based on the overall leverage ratio curve.  

Progressive Leverage Ratio Curve

progressive spring curve graph

  When the leverage ratio curve trends down on a graph, it means that the amount of force required to move the rear wheel is increasing. Terms like “ramp-up”, “bottom out resistance”, and “progression” are used to describe bikes that have a progressive leverage ratio curve. Let’s go back to our example of a 150lb rider using a 450lb spring. If the leverage ratio curve decreases from 3:1 to 2:1, the 450lb spring will continue to feel stiffer throughout the rear wheel travel.  By building a progressive leverage ratio curve into the frame itself, it can be better paired with a shock that has linear qualities because the frame is progressing through travel.

Linear Leverage Ratio Curve

Linear leverage ratio curve graph

The “Linear” example above shows a leverage ratio curve that stays the same throughout the entire rear wheel travel.  For a bike like this, the frame doesn’t have any progression built in, so if you pair this type of frame with a linear coil shock, the frame’s suspension will not have any progression.  In a scenario like this, you could risk a lack of support and harsh bottom outs. This is why you often find progressive air shocks paired with linear frame designs.

Regressive Leverage Ratio Curve

regressive leverage ratio curve graph

When the leverage ratio curve trends up on the graph, it means that the amount of force required to move the rear wheel decreases. For this example, the frame’s leverage ratio is 2:1 at the start of travel but increases to 3:1 at the end of travel. These types of frames are commonly paired with progressive air shocks because the rear suspension design requires the shock to provide ALL of the progressiveness in order to resist bottoming out. Bikes with regressive leverage ratio curves from beginning to end of stroke are uncommon.  It is important to note that most bike’s leverage ratio curves will change throughout rear wheel travel- we have provided these simple graphs to help illustrate these concepts.

Bikes are generally intended to have bottom out support, or ramp-up (progressiveness) integrated into the bike’s suspension kinematics so the travel of your bike feels more dynamic and “bottomless.” So bikes that have progressive leverage ratio curves perform well with coil shocks – because the frame has progression built in.  Bikes that have very linear leverage ratio curves perform well with air shocks – because the shock has progressive qualities. Bikes that have very linear leverage ratios paired with coil shocks is not always ideal because neither the frame nor the shock have progressive qualities. Bikes with progressive leverage ratio curves paired with air shocks can sometimes be too progressive, or provide too much ramp-up which could limit the use of travel.

Progressive Frame + Coil Shock = Bike with ramp-up

Linear Frame + Air Shock = Bike with ramp-up

Regressive Frame + Air Shock with Volume Reducers = Bike with ramp-up

These are simple examples of why certain bikes are built with a coil shock or an air shock. But this is where our individual riding styles take over. If you are a rider that really likes to stay on the ground but easily glide over rough terrain, then bottom out resistance may not be important to you. But if you are constantly looking for opportunities to hit the biggest gap or highest drop, bottom out resistance is important. In these cases, using an air shock on the three types of frames is going to be more ideal. BUT WAIT. What if you could get the same progressive benefits that air shocks have while still maintaining the sensitive off the top feel of coil shocks?

     Our new Progressive-Rate VALT spring can offer riders an option to use a coil shock on a bike that might not have been ideal for coil shocks in the past.  Because of the lack of ramp up a traditional coil spring has, certain frames bottom out easily. We have worked to bridge this gap with our new Progressive-Rate VALT springs which are designed and tuned by our engineering team to provide riders with a true rise-in-rate spring curve.  Our Progressive-Rate VALT springs maintain the off-the-top sensitivity of a traditional coil set up, and add the progressiveness and ramp-up usually found in an air spring.  Below is a graph comparison between a Linear VALT spring and a Progressive-Rate VALT spring.

 

     There’s no excuse now, coil shocks for the masses!  To celebrate this liberation from the status quo, Cane Creek is now manufacturing the DB Coil IL in black for a limited time.

 

Black Coil IL with White Progressive Spring