We've been talking about doing a Shock Week for years, mainly because it's such a fun play on words. What exactly is Shock Week, other than a chance to use Taj Mihelich's glorious illustration as much as possible? Well, for this inaugural round we have a series of air shock reviews lined up with options from Fox, RockShox, DVO, Ohlins, and Marzocchi, where Matt Beer and Dario DiGiulio each spent time riding laps and laps in and out of the Whistler Bike Park in order to assess each shock's performance and adjustability.
To accompany those reviews and videos, Seb Stott put together the primer on shock terminology you'll find below. There's also a wide-ranging Burning Question interview article with multiple suspension product managers, and a couple more bonus articles to finish things off.
And now, it's time to dive into Seb's primer. It starts with the basics and gets more in-depth towards the end, hopefully preparing you for all the suspension-related nerdery that'll be floating around this week. - Mike Kazimer
What actually is a shock anyway? Springs & dampers.
The rear shock (or shock absorber) controls the motion of the rear suspension. It does this with two basic components: the spring and the damper.
The spring stores energy when it's compressed and releases it as it extends, while the damper dissipates the energy as the shock moves, turning it into heat. The spring generates a force that increases with travel (how far the spring is compressed) and holds up the weight of the rider while the suspension is static (not compressing or extending). Meanwhile, the damper generates a force that depends on the shock speed
(the rate at which the shock is compressing or extending). Generally, when the shock moves faster, the damper generates more force opposing this motion. The damper's job is to slow down the suspension's motion and stop the spring from oscillating as fast as possible.
Shock springs: coil vs. air.
Shocks are generally divided into two categories: coil or air. Coil springs are typically a few hundred grams heavier and require a spring swap to change the spring rate (stiffness), but they can also offer improved sensitivity and traction due to the lack of friction and lower spring rate at the start of the travel. Air springs have closed the gap on this last point in recent years, and offer independent tuning of the end-stroke spring rate with volume spacers. Thanks to their lightness and tunability, air is by far the most popular choice.
Spring rate is often confused with spring force, but the spring rate is the amount of additional force required to compress a spring by an additional increment of travel. In other words, if you plot spring force against travel on a graph, the spring rate is the gradient
of that graph. Another word for spring rate is stiffness.
Spring rate is measured in Newtons of force per millimetre of travel, or more commonly, pounds per inch. This is often abbreviated to "pounds", hence the confusion. So for example, a 300 pound-per-inch spring takes 300 lb force to compress by one inch, 600 lb to compress two inches, and so on until the spring is fully compressed.
Coil springs can be preloaded using the threaded collar. This increases the amount of force required to compress the shock from 0% travel, but doesn't increase the spring rate at all. In mountain bikes, it's generally agreed that preloading a spring is bad news because it reduces the sensitivity and predictability of the suspension. Preload collars are really there to accommodate different lengths of spring rather than to adjust the ride height of the bike.
Coil springs generally have the same spring rate throughout the travel, whereas air springs have a spring rate that varies with travel.
How does an air spring work?
An air spring generates force by compressing air on one side of a piston and allowing it to expand on the other. As the shock compresses, the piston slides such that the volume of the positive chamber decreases, and the negative chamber decreases. This creates a pressure difference above and below the piston, and this difference in pressure creates a force that increases the further the shock is compressed.
When the shock is fully extended, the pressure in each chamber is set such that the force on either side of the piston cancels out, meaning the force goes to zero as the shock reaches full extension. Otherwise, the shock would over-extend and top out harshly every time the rear wheel was unweighted.
As the shock is compressed into the first part of its travel, the negative chamber's volume expands severalfold, so the pressure on the negative side of the piston drops rapidly over the first part of the travel. This results in a rapid rise in the spring force in the early travel (aka a high spring rate). As the shock moves into the middle part of the travel, the negative pressure has already dropped so low that it can't get much lower, but the positive pressure is still building relatively gradually, leading to a gradual increase in spring force (a low spring rate). But as the shock moves into the final part of the travel, the air in the positive volume is squeezed into a rapidly shrinking volume, so the spring force ramps up steeply again (leading to a high and increasing spring rate).
For this reason, an air spring force-travel curve looks like a reverse S-shape, with steep increases in force at the start and end of the travel. If you were to plot the spring rate (the gradient of the force-travel curve), it would form a U shape (high spring rate at the start and end of the travel, low spring rate in the middle). This is why air springs are often characterised by harshness at the start of the travel and a lack of support in the mid-stroke.
Importantly, shock manufacturers have been increasing the volume of the negative air chamber, which results in a more linear spring curve because the pressure in the negative chamber decreases more gradually as the shock is compressed. See the chart above, where an air shock's spring curve is calculated with various lengths (volumes) of the negative chamber. Larger negative chambers result in improved beginning-stroke sensitivity and more mid-stroke support. This is one of the biggest factors that distinguish between air shocks; higher-volume air springs tend to create a more predictable and more supportive ride, with better beginning-stroke sensitivity and traction.
Also known as "tokens", these can be used to reduce the volume of the positive air chamber of most modern shocks, thereby increasing the compression ratio and so the force required to reach full travel. Adding spacers can be handy if you want to make it harder to bottom out without having to run more air pressure and less sag. Conversely, removing them can allow access to more travel and help the bike soak up impacts without having to increase sag. It should be noted that adding volume spacers isn't usually a good solution for increasing mid-travel or cornering support, as their effect is most pronounced towards the very end of the travel.
How does a damper work?
Put simply, a damper works by forcing oil to flow through narrow valves as the shock compresses or extends. Like pushing fluid through a syringe, this is easy to do slowly but takes much more force at higher speeds.
A simple hole or port valve produces damping force which is proportional to the shaft speed squared. This is known as a progressive damping curve (see image below) and is generally considered to result in too little damping force at low shaft speeds (not enough pedalling or pumping support and an uncontrolled "bouncy" ride), and at the same time, too much damping force at high speeds (causing harshness and stingy travel use on big impacts).
To compensate, most shocks use some sort of valve that opens up as the damping forces increase, allowing oil to flow through a larger area. This can take the form of a shim stack (which is essentially an array of thin washers which bend to allow more oil to flow around them as pressure increases) or a small coil spring that holds a port shut until enough pressure builds up to compress the spring and open the valve, or a combination of both. Either way, suspension tuners generally design the damping curve (the relationship between shaft speed and damping force) to be roughly linear or digressive, meaning the damping force increases in proportion to shaft speed, or levels off beyond a certain point (see graph above).
The characteristics of a damper are largely determined by the relationship between shaft speed and damping force, which is generally different between compression and rebound and, in many cases, adjustable by the user.
How do low-speed and high-speed damping adjustments work?
Almost all shocks feature low-speed rebound adjustment, which controls how fast the shock extends. Many also feature low-speed compression adjustment, which affects how readily the shock moves into its travel, especially at relatively low shaft speeds such as when pedalling or pumping.
In either case, low-speed adjusters work by setting the size of a port, usually with a conical needle that moves into the port (closing it off) when you turn the adjuster clockwise. Crucially, once you've finished making your adjustment, the size of the port through which the oil can flow is then fixed. If the shock needs to move quickly (for example during a heavy landing), the oil will be forced through a parallel flow path, which may be controlled by a shim stack or poppet valve which opens up when the oil pressure (and therefore the damping force) gets high enough.
High-speed adjusters usually work by adding preload to a small coil spring, which pushes down on a shim covering the high-speed flow path. As the shaft speeds increase, the oil pressure eventually becomes high enough to push on the shim, compress the spring and allow oil to flow through a much larger area than the low-speed port alone, preventing excessive damping force. The more preload the high-speed valve has, the more pressure is required to open it up, and so more damping force is generated at higher speeds.
There's no strict definition of what counts as high-speed suspension movements versus low-speed, but generally, motion controlled by your body (pedalling, pumping and weight shifts) tends to stay within the low-speed range, while motion too fast for your body to react to (landings jumps or hitting bumps) usually push into the high-speed range.
Technically, the difference between high-speed and low-speed adjusters is how they work. Low-speed adjusters control the size of a port that doesn't change in size while riding, whereas high-speed adjusters control how easily the valving opens up as the shaft speed (and oil pressure) increase. I recommend the above video for a more thorough explanation of this, including how the low-speed adjuster can affect high-speed damping, and the high-speed setting limits how much force can be generated at low speeds.
But here's a simplified, practicable summary of how these adjusters can be used in the real world. If you have low-speed and high-speed compression adjusters, the low-speed adjustment can be used to trade off pedalling/pumping support against small-bump sensitivity, while the high-speed adjuster controls how much the shock resists impacts coming up from the ground.
Some shocks offer both high- and low-speed rebound adjustment too. These work in a similar way to compression adjusters but, of course, they control oil flow in the opposite direction. Rebound speed is primarily affected by the force from the spring, which of course is higher deeper in the travel. So generally, the high-speed adjuster has more of an effect when the shock is rebounding from deep in its travel (for example after a heavy landing) while the low-speed adjuster is more effective in the early part of the travel. So, if you're getting bucked during heavy landings or big holes, you might want to slow down the high-speed rebound; if you feel the bike is too fidgety and uncontrolled over smaller undulations, you might want to increase low-speed rebound damping.
If you have a single rebound dial, it will technically be a low-speed adjuster controlling the size of a port, but the high-speed valving is often designed so that this adjuster has an effect on the whole damping curve (low- and high-speed), so these single adjusters can be thought of as an all-round rebound adjuster, rather than an adjuster that only significantly affects the low-speed range.
Climb switches & lockouts
A mild climb switch or pedal platform usually works by completely closing the low-speed compression port. This means the oil has to force open the high-speed valve in order to flow at all. Ideally, this means the force acting to compress the suspension while pedalling is not enough to open up the preloaded valves (meaning no suspension movement while pedalling) but when you hit a bump, the valves can open up and allow the suspension to compress. This damping threshold has a significant effect on bump sensitivity as the shock can't react as quickly when the wheel hits a bump (which is why you'll want to turn it off for descending), but the suspension can still absorb larger impacts without too much fuss.
This kind of pedal platform may not be enough for people who want maximum efficiency when stamping on the pedals. This is why firmer lockouts often close off both the low- and high-speed valving and instead use a separate blow-off valve with high preload, so the shock will only compress under heavy loads. Some shocks are offered with different blowoff thresholds for a firmer or lighter lockout effect.
Whether you really need a climb switch is obviously a matter of debate. They can certainly make a bike feel more efficient and spritely, but measuring the effect
they have on efficiency isn't straightforward.
A cutaway of a single tube shock on the left and a twin tube damper on the right. Single tube vs. twin-tube shocks
Some brands, such as Cane Creek and Ohlins, primarily offer twin-tube dampers; others, such as EXT, only make single-tube shocks, and others still use either design depending on the application. Essentially, a single-tube damper generates damping force by forcing oil to either flow through the main piston on the end of the damper shaft, or the oil that's displaced by the damper shaft is forced through valves in the head of the shock as it flows into the piggyback reservoir. In a twin-tube shock, the damper piston may be solid (with little to no oil allowed to flow through it); instead, the column of oil is pushed by the piston, through a set of valves at the head of the shock, then through a second outer tube which is concentric around the first, before the oil filters back into the first tube behind the main piston.
It's not the case that one design is inherently superior to the other, at least not in the mountain bike world. Twin tube shocks can offer a wider range of damping adjustment, especially if all the oil is forced to flow through the user-adjustable valves at the head of the shock. This can be useful if one shock is designed to work with a wide range of bikes. But many single-tube shocks offer an appropriate range of adjustment, especially if the (non-adjustable) valves on the main piston are tuned to suit the bike in question (remember that most shocks are sold to bike manufacturers and tuned to suit a particular bike). Plenty of top-level enduro and downhill races are won with either design, and it doesn't make sense to pick an MTB shock simply based on whether it's twin-tube or single-tube.