Posted on September 23 2022
It's no secret that coil springs are back in a big way. Whether it's the amazing value of the Marzocchi Bomber CR, the world cup winning performance of the Fox DHX2 or Rockshox Super Deluxe Ultimate, or the ultra premium, custom built Push ElevenSix R, there's now a coil shock to suit almost any rider. Are you wondering if a coil shock is right for you? Are you struggling to figure out what spring rate you need? Well Cyclinic have developed a spring rate calculator (check it here) to assist you in this, so read on while we lay some information down to make your decision a little easier. In this blog we're going to discuss spring rate curves, frame kinematics and leverage ratios, and other factors that influence the optimal spring rate selection. This is a basic overview and by no means an exhaustive list of every single aspect of coil springs.
So what's all the hype about? Why might you prefer a coil sprung shock over an air sprung shock?
With many of the options from the large manufacturers, the damper circuits are the same for both the air shock and the coil shock, such as the Fox DHX and Float X, DHX2 and Float X2, or the Rockshox Super Deluxe Ultimate range, so you'll generally have the same external adjustments for compression and rebound damping. The difference really lies in the spring they use and the inherent differences between air springs and coil springs. At the very core, air springs are progressive, and coil springs are linear (yes we know there are companies that claim to make progressive coil springs, but for simplicity sake, let's keep coils linear). We'll discuss how this affects things shortly. The other difference between the two spring types comes in the form of sensitivity, which can basically be put down to the extra seals of an air shock creating extra friction that needs to be overcome before the shock will start compressing. While modern air shocks have come a long way with their large negative volumes improving sensitivity, they're still not at the level of a coil spring.
Spring Curves
Overall, these differences result in a coil shock feeling more supple, planted, and glued to the ground when compared to an air shock. Some riders will report a coil shock feeling plusher on big hits, but this likely is a result of the progressive nature of air springs that we mentioned earlier, so let's dive into that as it can be a big factor in whether a coil spring shock is right for you and your bike. Coil springs are made with a certain spring rate that's typically listed in pounds per inch, so a 400lb/in spring requires 400 pounds of force on it to compress it one inch. As they're linear in nature, that means that to compress it another inch you'd need another 400 pounds of force, totalling 800 pounds of force to compress the spring two inches. This linear spring rate gives coil springs a consistent feel throughout the travel that is very predictable.
Air springs, on the other hand, behave a little differently. Often referred to as a progressive spring rate, and air springs effective rate will change throughout the stroke of the shock as a function of the volume of the shock (thanks physics!), so while the equivalent spring rate might be 400lb/in for the first inch of stroke, the second inch of stroke might have an average spring rate of 450lb/in, and the next inch of stroke might be 525lb/in. So for our 400lb/in coil spring to compress 3 inches, you'd need a total of 1200lb of force, whereas the air spring might need 1375lb of force to compress the same 3 inches. Not only does the effective spring rate of the air spring increase as you move through the travel, but the rate at which this increase occurs also increases, leading to the spring rate ramping up towards the end of the shock's stroke. This natural ramp up that air springs have means they provide great bottom out resistance, and most air shocks offer some tuning of this with volume spacers to match the bottom out resistance with the rider's requirements.
Leverage Ratios
When selecting a spring rate the linear nature of the coil spring needs to be taken into consideration, especially on bikes that were only ever designed with air shocks in mind, as they may not have much progression in the suspension linkage design of the frame to compensate. Progression in the frame, what's that all about? Glad you asked! Dual suspension mountain bike frames are designed to use a certain size shock, with a certain amount of stroke, to deliver a certain amount of rear wheel travel. The ratio of this rear wheel travel to shock stroke is known as a leverage ratio and gives you the amount the rear wheel travels for each millimetre the shock compresses. As an example, the Yeti SB130 Lunch Ride uses a shock with 55mm of stroke to deliver 137mm of rear wheel travel, resulting in an average leverage ratio of 2.499:1 (137mm divided by 55mm). The Yeti SB165 on the other hand, uses a 65mm stroke shock to deliver 165mm of rear wheel travel, with an average leverage ratio of 2.538:1, so pretty similar as an average to the SB130LR, and the vast majority of bikes on the market will have an average leverage ratio somewhere between 2.4:1 and 2.8:1. This average leverage ratio and the rider's weight (with all the usual riding gear on) forms the basis of what spring rate to start with. Bikes with higher leverage ratios will compress a shock more easily, which is great for small bump sensitivity, but not so good for bottom out resistance, so frame designers have to balance out a number of factors when designing their bikes. These differences in leverage ratios also mean that a 450lb spring working well for you on one frame does not mean that you'll need a 450lb spring on a different frame.
If you're on an E-Bike you need to factor the extra weight of the motor and battery into the equation. Our calculator has an E-Bike option for your full powered models like the Norco VLT range, and an E-Bike Light option for lighter e-bike models like the Specialized Levo SL, Orbea Rise and Trek Fuel EXe.
Frame Kinematics
Where things get interesting is that the SB130 is only supplied by Yeti with an air shock, whereas the SB165 is supplied only with a coil shock. Yeti state you can run an air shock or a coil shock on both bikes, but there's a reason they spec the bikes with the shocks they do, and that's where the frame's progression comes into play. With modern suspension linkage designs, bike manufacturers can place pivot points in different locations to achieve different things and one of the variables they can manipulate is the leverage ratio throughout the travel. A larger leverage ratio at the start of the travel needs less force being required to compress the shock, resulting in improved small bump sensitivity, and is particularly beneficial with air shocks that have a bit more friction to overcome, but this large ratio would be detrimental at the end of the travel, where you're trying to resist the shock bottoming out. As such, frame designers will generally try to have a lower leverage ratio at the end of the stroke which requires more force to compress the shock and thus helps resist bottoming out. This decrease in leverage ratio at the end of the frame's travel when compared to the start of the travel is referred to as progression and frame designer's take into account the type of shock and its spring rate characteristics when determining how much progression they need their frame to have. If we take out two Yeti examples, the SB130 has roughly 13% progression, where as the SB165 has almost 33% so at the end of the stroke the SB165's leverage ratio is 33% smaller than it was at the beginning, meaning the force acting on the rear wheel is being applied through a 33% smaller lever and therefore, irrespective of spring rate, more force is required to compress the shock at the end of the stroke.
Transition bikes illustrate their fairly progressive frame leverage curves very nicely, shown below is the Transition Sentinel with roughly 24% progression.
Putting it all together
What does this mean for you and deciding on a coil shock and what spring rate to use? Well, if your frame doesn't have much progression in the linkage design, you'll want to go up at least a spring rate to ensure you have enough support from the coil spring to not bottom out too easily. If you're a particularly aggressive rider riding more challenging terrain with larger features, you may even need to go up two spring rates. On bikes with less progression, the frame designer's rely more on the progression of the air spring in the shock to provide that bottom out resistance, so you'll need to take that into account when selecting a spring rate. In general, we've found bikes with 20% progression or more to be suitable for coil shocks, bikes with more than 30% progression to work really well with coil shock, and bikes less than 20% progression often having to compromise on spring rate by running a firmer spring to avoid bottoming out too easily, resulting in less than optimal traction and comfort. If your frame has less than 10% progression, such as the Giant Trance, we don't recommend a coil shock at all as it really needs the progressive nature of an air spring. More and more brands are publishing their frame kinematics and stating if they recommend a coil shock or not, but unfortunately not all brands do, so if you're not sure, get in touch and we can point you in the right direction.
The final piece of the puzzle is your riding style and the terrain. High speeds and big drops will require more support and a firmer spring might be more optimal than slower, more technical terrain might require. Similarly, riders who are gentle on their gear and thread their way through rock gardens might get away with a softer spring rate than a wheel destroyer who just points, shoots, and sends. It's not uncommon for riders to have one spring rate for their day to day trails, and then a firmer spring rate for bike park style terrain. Combining the fundamentals of your riding weight, frame travel and shock travel will provide a range of spring rate options that you can then narrow down by factoring in the characteristics of your frame's suspension design, riding style and terrain. This then gives you a solid starting point to work with because after all, there's also personal preference and the whole point of all the dials and adjusters on suspension is to set it up to work for you.
