Mountain biking has been experiencing an incredible renaissance for the past few years, one that should continue throughout the foreseeable future. We are witnessing a vast array of biking styles in the forms of fat bikes, enduro bikes, DH/AM, all trail, cross country, dirt-jumping, city bikes, even the controversial e-bike!
Probably, one of the most critical components in determining the responsiveness/smoothness of a mountain bikes ride, is its suspension system (Yes, frames are numero uno). For the most part, this component has been dominated by air-type suspension systems, but more recently, we’ve witnessed the return of coiled spring systems, which can even be substituted on forks running air systems by companies like PUSH Industries that has been making shocks and coil systems since 2016, though they began in 2003, performing custom tunings of suspension systems. Additional companies like Fox, Cane Creek, Rock Shox have rapidly followed with new offerings.
But are coil shocks superior to air systems, or, is one type better suited to specific riding conditions versus the other? Let’s examine these questions and see if there’s a clear answer to the problem: Coils vs Air Suspension Systems.
Today, these weight differences are: 2018 Fox Van Performance (7.875 x 2.25) with an SLS rear spring, 643 grams, Fox DHX2 Factory (9.5 x 3), 774 grams, RockShox Vivid R2C (7.875 x 2.25”), 918 grams, PUSH Industries’ ElevenSix, 825 grams, Cane Creek DB Coil, 960 grams, and Cane Creek DB Air is a featherweight at merely 600 grams. Today’s air shocks like the Fox DPX2 weighs 496 grams (size dependent), Rock Shox Monarch Plus RC3, 335 grams (size dependent), and Cane Creek’s DB Air, 500 grams (size dependent).
It’s interesting to note that the coil spring systems have gotten lighter while the air-based systems have increased in weight, due to the addition of more sophisticated features like piggyback chambers that keep the gas, usually nitrogen, away from the main body of the shock to dissipate heat more effectively and by offering more adjustment options. Coil spring systems have lightened due to special steel formulations like “super light steel” (SLS) and titanium, plus advanced design engineering that allows for lighter parts throughout the system.
Both systems share a similarity in their utilization of dampening technology to absorb energy generated from impact against anything that lurks along the trail. Unsurprisingly, air shocks use a pressurized chamber, while coil spring systems use metal springs of either steel, SLS, or titanium. But here they depart in terms of compression vectors, which tend to be progressive in air shocks, while linear in (most) coil springs.
In a linear system, if it required 100 N to move/compress the shock 10cm, then it would take 200 N (100 + 100) to compress it 20cm, 300 (100 + 100 + 100) N to compress it to 30cm, and so forth throughout the full stroke of the shock. In other words, it would be consistent and directly proportionate. In progressive systems, the force (N) required to compress/move the shock x distance increases exponentially, therefore, suppose it required 50 (N) to compress the shock 10cm, then it took 150 N (50 + 100) to compress it to 20cm, then 300 N (150 + 150) and so forth throughout the full stroke. As the stroke traveled towards its maximal compression, it would take more energy to compress it, relatively, versus during the initial compression at the beginning of the stroke.
Thus, progressive air springs are less responsive to impact absorption as they approach the end of their strokes as they become progressively harder to compress without imputing additional force, whereas linear coil springs react consistently throughout the full stroke length. Air shocks possess greater adjustability in terms of progression, which can be altered via volume spacers that can be added or removed as well as changing the air pressure, which varies by combined bike and rider weight. Coil spring shocks, due to their linear rates, exhibit better responsiveness to trail surfaces, which results in improved traction for the rear wheel that leads to the feeling you’re glued to the ground. Thus, riding becomes more controlled and confidence-inspiring. Unlike air shocks that possess ready tune-ability, coil spring systems require different sized springs, given varying rider weights. Keep your weight constant, or be faced with buying a different spring for your rear suspension system. An additional caveat is that both shock systems, will not fit every frame, which depends on whether the frame has a progressive, regressive or linear leverage curve ratio. These curves are used as predictive models that help determine, when coupled with an appropriately matched shock, how the bike will handle. One example would apply to a frame with either a linear or regressive leverage curve, wherein one would mate it with an air shock (progressive) to improve its curve, thus overall predicted handling. If one added a linear shock to a frame with a linear leverage curve design, the shock would bottom out towards the end of its stroke. So, be sure you research your frame’s specs thoroughly, before purchasing a different type of rear suspension shock, versus the one supplied with your bike.
Manufacturer’s love to point out the benefits of heat dissipation in terms of its effect on dampening provided by air and coil suspension shocks, with some arguing the superiority of their piggyback air shocks, but in real world testing, this becomes important only if bikes are ridden at extreme speeds, well beyond the realm of mortal mountain bikers whom ride for pure pleasure, sans competition. AND-with the increased tune-ability of air shocks, wherein riders can easily adjust the compression, rebound, and with the addition or subtraction of volume spacers, one can alter the amount of mid-stroke along with bottom-out resistance. Therefore, the ride feel/differences are rapidly decreasing between these two shock types.
Don’t lose track of the fact that with coil spring shocks, you must change the coil spring with fluctuations in rider-bike weight. Coil springs are identified using two numbers. The first represents the weight in pounds required to compress the spring 1”, while the second number represents the spring’s travel length in inches, not the travel length for the bike’s suspension. So, if you were packing a bike for a trip into the wilderness, you’d need a heavier spring for a run down a mountainside, or a cruise along some flat trail. You may have noticed many DH racers switch out rear shock systems depending on day-to-day course layouts and weather conditions, i.e., mud versus dry, hilly, rocky, etc. Again, that’s an extreme situation, hardly related to the everyday rider. In the final analysis, it all boils down to frame design, rider preferences, and trail environment. Try one type, or better yet, try both, then make an educated decision based on personal experience and not an expert racer or professional mountain bike rider-reviewer. Just have fun!