Welcome to Shock Week 2023 - Mountain Bike Suspension Terminology Explained
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.
Here's a hypothetical force-travel graph for a 100 lb/inch spring (blue), a 150 lb/inch spring (orange) and a 100 lb/inch spring with one inch of preload (red). The red and orange springs generate the same amount of force at 2 inches of travel, but the orange spring has a 50% higher spring rate (gradient) than the other two.
Spring rate
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.
Image credit: Andrextr.
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.
Volume spacers
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.
RockShox Monarch spring curve with different numbers of volume spacers. Note the spacers only have a large effect on the last 20% 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).
Image uploaded by RMR, Source: "Shocks for Drivers" by Ross Bentley, via www.speedsecrets.com
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.
In this 2015 Fox X2 cutaway, you can see where the low-speed flow path goes up the middle of the valve and out the small holes inside the spring. The spring preload was set by the high-speed adjuster, such that when the oil pressure got high enough, the spring would compress and oil would be allowed to flow around the low-speed port.
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.
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(EXT claims to use turbulent flow, so it's supposed to sound like that. Not the quietest shock.)
Can the + / - be replaced with a image of turtle and rabbit (rebound) and a pillow and rock (compression)?
Ya’ll need to keep it simple for us simple folk. If I had a nickel for ever time I had to go the max range to remember what is what…..WHOOOOOOOWEEEEE.
RockShox (SRAM) used to do turtles and rabbits for low-speed-rebound, maybe they still do on some forks...
Also, even that confuses people, because they think it has to do solely with how fast you plan on riding, not shaft speeds.
www.youtube.com/watch?v=xhnKTZu2AKs
www.youtube.com/watch?v=ylkTWArNX04
Which is true, but omits one crucial part - all top-level rider's shocks are CUSTOM TUNED. So, If you buy a bike with single-tube shock and the bike manufacturer did a good job, you will most probably be OK. If you want to buy single-tube shock aftermarket, better have someone who can tune it right. If you buy any type of shock because someone won an EWS or DH WC on it, then take a real deep breath, because the shock you want to buy is NOT the one that was used.
Btw, I had a chat with an EWS racer who used Ohlins as a privateer and then RS as a racer and he said that RS was as good as Ohlins only AFTER it was custom tuned for him. But on the Ohlins he could just dial that tune with knobs.
It's not as simple as that, though, since the damper tune selected for a production frame is the choice of the product manager. As an example, several years ago most Specialized models used a roughly 3:1 leverage ratio and were spec'd with a particularly light tune, while the leverage ratio of the Devinci Hendrix was 2:1 and it was spec'd with a medium-high tune. The correlation between linkage curves and damper properties isn't rigidly defined; it's essentially a custom tune to the product manager's preference.
Custom tuning shops - whether for pros or recreational riders - usually pick from a catalog of tunes. There are only so many permutations of shims and spacers. It's rare for a tuner to have telemetry data, a mathematical relationship between the data and the damper properties, and the ability to create the desired damper properties from scratch - especially on diverse terrain like mountain biking trails, rather than a motorsports track.
In most cases, it goes more like: "customer needs more support, but sag is already 25%, so go up one compression level", or "customer wants less harshness, but is bottoming out, so maintain average level, but change from digressive to linear or progressive compression stack". Those will all be catalog options. As I said, the lines are blurry between custom tunes and catalog tunes when the catalog is sufficiently large.
Doesn't stop you from picking a different catalog tune if you want drastic changes to a used X2, but fact is you very probably won't need to if you get the frame-specific catalog tune.
If Fox is choosing a "base" tune from an effectively infinite catalog to match the rider and the bike, that's as good as a custom tune - it just transcribes the customization process from the mind of the tuner into the catalog. If the rider doesn't like the tune, Fox can change to another catalog tune. What's the difference between that and custom tuning?
And I tend to believe what comes out of Dialed. They have no reason to lie/fib/bend-the-truth. If Fox corporate didn't want them to share something specific, they'd just leave it out of the video.
Now let's say every reasonable permutation is in the catalog. Same question.
Is Formula's CTS system "custom" tuning?
Cheers
Air is easy to compress at first, but gets much more resistant to compression as the molecules become more tightly packed - i.e. not a linear response to compression. It's like packing people into an elevator: you don't mind a few more people when there's enough space, but you start to mind a lot when it get extremely crowded.
A coil spring is a really long cylinder that's being twisted (a torsion spring). For convenience, we twist it into a coil so it isn't a metre long, but it's still just a rod being twisted. Metals have a linear material response to deformation when flexed in the "elastic" region (when they're not strained so hard as to be permanently deformed). Sometimes the shape changes while it's being flexed, making things more complicated, but that's not the case for a typical coil spring.
We can make a progressive coil spring by allowing some of the coils to collide with each other, thereby removing them from the effective length of the cylinder being twisted, which increases the rate of flex (i.e. the spring constant) in the remaining "free" part of the spring, but it's still nowhere near as progressive as a typical air spring.
Now imagine a balloon filling up the inside of an open soup can that you're going to depress with a potato masher sized just right for the soup can. Lets say in this experiment the amount of force required to BEGIN depressing the balloon with the pototo masher is exactly the same as the amount of force required to BEGIN depressing the small spring.
As you continue to depress the balloon with the potato masher, the force ramps up, just like the coil spring. By the time you've managed to compress the balloon 1" into the soup can, however, the force has ramped up much faster and is way more than what was required to depress the coil spring by the same 1".
Both the coil spring and the balloon took the same amount of force to BEGIN compressing, and both required more force the more you compressed them, but the force required to compress the balloon ramped up faster so at 1" compression you were exerting more force on the balloon than the spring.
Now lets switch to bikes, but in reverse - the air spring and the coil spring both need to require the same amount of force at the END of travel in order to prevent a harsh bottom out. This would mean that at the BEGINNING of travel, the air spring would require more force to start moving than the coil spring would. This is part of the reason why coil springs have better small bump sensitivity. They other factor reducing the small bump sensitivity of air springs is the increased "stiction" (overcoming static friction) due to the larger diameter required for the piston due to the air spring design.
This twisting thing is new knowledge for me and helps answer that question.
But, the size of the air spring is always shrinking. Same mass of air (gas) in a smaller volume at the same temp (effectively/idealized) equals more pressure. More pressure equals more force. So as it compresses, you need enough force to have already shrunk the volume a set amount, along with even more force again to shrink it again the same amount.
Consider a made-up air spring that starts at 100 pounds per inch, and doubles the force each inch. Will take 100 pounds force to compress one inch, but now the volume is smaller and the pressure is higher so it takes 200 pounds for the next inch, making 300 pounds for 2 inches. Then double the force for the next inch, makes 700 pounds for 3 inches.
It's all much finer than this, a nice curve instead of discrete steps but hopefully gets the idea across: 700 pounds for 3 inches is way more than 300 pounds for 3 inches, even though the first inch took about the same force.
Thanks!
A rider wants their suspension to be able to handle a certain amount of energy via some combination of spring and damping. The rider wants a spring rate that provides appropriate ride height. Big hits will require big energy management, i.e. lots of spring support deep in the stroke or lots of damping. Both come with drawbacks:
Spring: You can't just increase the spring rate add spring support at all points in the stroke or the chassis won't sag (inappropriate ride height) or will require heavy preload (inappropriate force at the start of the stroke). A progressive spring can work, but there will be lot of stored energy at the end of the stroke, which is difficult to manage.
Damping: A great way to dissipate unwanted energy without raising the ride height, but the support is excessive for small, fast impacts (ex. your typical rocks and roots), leading to a harsh ride. It's difficult for a damper to differentiate between a rock / root and a cased landing simply by shaft speed.
Another challenge: Ideally, suspension should bottom out softly, not crash to a stop. If so, this means the velocity of the shock shaft decreases to zero as it bottoms out. Low velocity produces low damping force, so the damping support is dropping off when it may be most valuable to control a severe impact.
Ideal solution: Damping that varies with speed and position (how deep it is in the stroke). Typically, this means minimal damping near top-out and heavy damping near the end of the stroke. As you noted, HBC devices are an example of the latter, and RockShox' new Vivid has a simple version of the former. Fancy shocks, such as for trophy trucks, have "bypass" valves that provide additional flow paths to reduce damping force at various points in the travel - these bypass pathways can even have compression and rebound shim stacks to add speed sensitivity to these position-sensitive flow paths!
Since we're on the subject, I'll get on my soapbox: damper manufacturers should quick screwing around with electronic dampers and give us bypass dampers that many of them already make for other applications. Doesn't have to be trophy truck shock architecture that looks like an octopus; a shimmed internal bypass design will do nicely.
cheers
If this shock was mounted on a bike with a constant leverage ratio of 2:1, the bottom-out force at the wheel would be
4 lb ÷ 2 = 2 lb
Edit: I may know my stuff, but not my proofreading! As you probably guessed, the last sentence of the first paragraph should read:
If the shock has 4" of stroke, the bottom-out force (at the shock, not necessarily at the wheel) is 4 lb.
Why does it need to differentiate? If the shaft speed is high, the damper needs to dissipate the forces the same no matter if you're slamming into a rock or slamming into a knuckle. Let the wheel move out of the way in a fast yet controlled manner.
Also, if a bottom-out is happening softly, why does the subsequent low velocity and thus low damping force matter? It's already bottoming softly and not crashing down.
Also also, the shaft speed always goes to zero when it bottoms-out, not just when it's soft. The important thing is to make that deceleration take longer (be soft) than the instant decel of a hard mechanical stop (crash to a stop).
Let's say we have a rock that's 3" high. Minimal compression damping is needed, since this rock isn't large enough to bottom out a long-travel chassis and there won't be enough stored energy to be difficult to control on rebound. Better for the suspension to be as compliant as possible to minimize the chassis deceleration and the force transmitted to the rider. The shaft speed is high, though, so the suspension can't help but provide a lot of damping force, even though it's not ideal to do so. In this case, the ideal suspension response is not to reach a gradual transition to reversal, but for the wheel to track the ground, which likely requires abrupt wheel accelerations. This is the kind of support the spring provides.
Now imagine the cased landing. A huge amount of energy needs to be dissipated and the object to be absorbed is the entire planet, so the only end point for the amount of travel to be used is the amount of travel available. This is the kind of impact that - as you described - favours a gradual deceleration of the wheel. Shaft speed is high and, in this case, the large damping force is a good thing - likely not enough, since the damping was probably tuned to strike a balance between this kind of event and the previous rock / root example.
As you can see, similar shaft speeds and damping forces, but different needs. One provided more than ideal damping force and the other less than ideal. A damper that is only speed-sensitive cannot distinguish these needs. Adding position-sensitive elements can allow less compression damping in the early part of the stroke, where rocks and roots are usually handled, and more compression damping late in the stroke to help manage big hits - or to resist bottoming out when rocks / roots are encountered when the suspension is already deep in its travel.
I have a feeling servicing an externally adjustable internal bypass shock would be a nightmare.
And yes, servicing won't be easy, but it's already being done in motorsports and it's never cheap to squeeze out the last drops of performance.
Small bumps don't come up from the ground?
Whats the point of a "piggyback"? Why do some shocks have them and others don't?
Piggyback architecture doesn't do anything that can't be done with inline architecture. You could put all the same valves into the end of an inline shock, it would just be really long; piggyback architecture allows some of that length to be exchanged for width, which often helps to package it into a frame.
Piggyback shocks often have fancier valving than an inline shock - that's the reason why the designer bothered with the more complex shock body - but it doesn't have to be that way.
Edit negative *increases*...or vice versa. Inverse relationships
pretty big typo there. the neg chamber increase as the positive decrease...
@Fat4242: To reiterate what justinfoil said more succinctly: the absence of adjusters does not necessarily mean the damping mechanisms are absent.
For example a simple fork like RockShox 35 Turn Key, does it have the 2 stages?
Another question: What about a normal car shock absorber, like a Civic or Corolla?
One more question please. I can't get my thick head wrapped around bottomless tokens/spacers. I understand PV=PV, what I can't make myself understand is why more tokens doesn't increase pressure, more so than normal, throughout the stroke, not just last 10% aprox of the stroke before bottom out.
If the user inserts a reducer and uses the same starting pressure, the pressure will be greater throughout the stroke. The increase is small for most of the stroke; the difference becomes significant only near the end, which is why we only talk about the effects near the end.
Another way to set up the spring would be to maintain the same force at the end of the stroke, as this may be the maximum force the rider can endure. Adding a reducer would require a lower starting pressure to reach the same final pressure, so the pressure would be lower at all points until the end.
Yet another way to set up the spring would be to maintain the same "area under the curve", which maintains the same energy input into the spring over the full stroke and uses a starting pressure between the previous two examples. Compared to the original set-up without the reducer, the pressure is a little lower for most of the stroke, becoming greater near the end.
Piggyback shocks often have fancier valving than an inline shock - that's the reason why the designer bothered with the more complex shock body - but it doesn't have to be that way.
Elastomer walks in.