How to Choose the Right Rear Shock
Linear vs Progressive...Shocks:
As a mountain biker, these words are a necessary part of your suspension vocabulary - so let's get them committed to memory. They help to determine first, the type of shock (coil or air) that matches properly to your bike.
Linear means that the same amount of force is required to compress the shock through its full stroke. A coil shock does exactly that. The spring rate (labeled on the spring) is the constant force. We'll write another article soon all about preload and spring rate. Until then, to determine your proper spring rate, use our Spring Calculator.
Progressive means that an increasing force is required to compress the shock through its stroke - an air shock has this action. The air chamber of an air shock naturally gains pressure as the volume inside the air can decreases during a stroke compression (less volume for same amount of air = increased pressure).
Knowing the traits of these two types of dampers is necessary for the next phase of determining the right shock for your bike.
Linear vs Progressive...Bikes:
Once you've wrapped your head around the differences between progressive shocks and linear shocks. It's time to discuss the differences in bicycle frame design that cater to these two types of rear suspension. When using the linear vs progressive vocabulary on frame design, we are discussing the "Leverage Curve Ratio" of the frame, not the stroke of the shock.
The leverage curve ratio is a calculation done on the bike to determine if a specific frame has a linear, progressive, or regressive leverage curve ratio.
1. Linear leverage curve ratio = requires the same amount of force all the way through its stroke.
2. Progressive leverage curve ratio = a general increase in force is required to move the frame through its stroke.
3. Regressive leverage curve ratio = a general decrease in force is required to move the frame through its stroke.
So what type of leverage curve ratio does your bike have? It is often difficult to find this information about your bicycle. As this type of information is only for the bike nerds! There are however, some great third party websites that provide the leverage curve data of several frames. For example http://linkagedesign.blogspot.com/ has several different frame leverage curves displayed on their site. But information on a specific frames can be found by digging online if not displayed on Linkage Design.
Pairing the Two Together
Understanding the differences between a linear coil shock and a progressive air shock is not rocket science. However, the next step, leverage curve ratio analysis of the bicycle frame, can be tricky.
For example, a bike could have a regressive curve during the first 0-30mm of stroke, followed by a progressive curve from 30mm to full travel. Or a progressive curve for the first 0-15mm of stroke, then linear curve from 15-40mm , then progressive again from 40mm of stroke to full travel.
All this to say that the lines can vary and allow for different types of shocks to be applied to the same bike. The most important aspect of this analysis to understand is the way a linear/progressive shock matches with the regressive/linear/progressive leverage curve ratio of a bicycle frame. Try saying that three times fast.
Most commonly, the ideal result for matching shocks to bikes is to create a required force that is progressive during the shock/bike's stroke to allow for "ramp up" to reduce bottom out occurances to hard hits and landings only. If the general leverage curve ratio of the subject bicycle frame is regressive (the bicycle's suspension linkage has a general decrease in force that is required to move the frame through its stroke) or linear (the same amount of force is required to compress the shock through its stroke), it is recommended to be paired with an air shock to improve the curve of the bike with a progressive damper.
The same "balance" technique is applied if the general leverage curve ratio of the subject bicycle frame is progressive (the bicycle's suspension linkage has a general increase in force that is required to move the frame through its stroke), it is recommended to be paired with a coil shock to allow the already-progressively curved bicycle frame to do its business with a linear spring rate damper.
How Do You Choose?
So if leverage curves changes throughout the stroke, then what shock do you get? It comes down to preference of feel sometimes, and other times it is mandatory that you use a specific damper type (for example an extremely progressive frame, like an Intense 951 EVO, is so progressive that a standard air shock is not recommended), so it really depends on the frame. So obiviously frame designers will select either an air shock or a coil shock that best complements the frame's leverage curve ratio and provide this type of damper in the spec of a complete build, but on the frames that are closer to neutral/slightly progessive or slightly linear, then the frame could be used with either a coil shock or an air shock. Some people really like the smooth and sensitive top end of a coil shock, and others prefer the progressive bottom end of an air shock. The preference is totally justifiable for each of these example riders if the bikes' leverage curve ratio caters to their selected dampers.