**Suspension Systems**

Suspension allows the wheels to displace when they encounter a bump. This helps them maintain traction in rough terrain and improves rider comfort. A suspension fork is used for the front wheel and a shock absorber (known as a "shock") is used for the rear. Suspension seatposts and stems are also available for rider comfort, but they don't substantially assist with traction.

On road and most gravel bikes, the tires deliver sufficient suspension for the terrain these bikes traverse. In contrast, mountain bikes are designed to excel on rougher terrain. Almost all MTBs have at least front suspension, aside from those who elect to run rigid MTBs. The image below shows a sample of suspension forks.

There are three main categories of bikes based off their suspension layouts:

 1. Rigid - the bike does not feature a discrete suspension system.
 2. Hardtail - only the front wheel has a suspension system.
 3. Full suspension (colloquially known as "full squish") - both wheels are suspended.

Previously, some bikes had a soft-tail suspension, with a shock unit attached between the seat stays and the seat tube. These gradually fell out of favor.

Some frames feature built-in suspension features such as curved seatstays, decoupled frame tubes (such as Trek's IsoSpeed) and elastomeric inserts (such as Specialized's Zertz system). These are generally not considered suspension systems in the context of the above classification due to their very limited range of travel and the absence of dedicated suspension components.


**Front Suspension**
![Fox Front Forks][1]

Typically, in Cross Country (XC), All Mountain (AM), Enduro or Aggressive Hard Tail frames (ie, those that have no rear suspension), front fork travel can range from 100mm to 160mm travel dependent on the frame geometry.

Many modern forks offer a lockout (either a lever on the fork itself or a remote lockout mounted to the handlebar) which allows the rider to quickly disable the fork travel. This is useful when riding on road to provide a rigid platform so as not to unduly waste rider effort.

**Rear Suspension**

[![Rear shock][2]][2]<br/><sub>(source: <a href="https://bouncecycles.co.uk/wp-content/uploads/2014/11/Fox-Forx-DHX-Air-5.0-e1428489119195.jpg">bouncecycles.co.uk</a>)</sub><br/>

Typical rear wheel travel for mountain bikes ranges from 100mm to 200mm depending on the frame discipline with Down Hill (DH) bikes offering the most travel (roughly 200mm front and rear). On the other side of the spectrum, race-oriented XC bikes may feature much less than 100mm of rear suspension, and a few gravel bike models have just a few mm of suspension (most are rigid).

Note that while front suspension travel is fairly easy to measure and visualize, the rear suspension is a bit more complicated. Due to the need to maintain frame rigidity and give the bike desirable ride characteristics, rear suspension is generally based on a linkage system where a small shock movement is multiplied into a larger wheel movement. Hence, it is important to distinguish between wheel travel and shock travel.

Linkages are carefully designed arrangements of rigid links and flexible pivots. There exist many different layouts, each with their own set of tradeoffs and ride characteristics. Choosing one you like may be an important consideration when purchasing a full suspension bike. Note that with carbon fiber frames, pivots with a small range of motion may be implemented via intentional frame flex, eliminating the metal hardware that would have otherwise needed to be there.

Similarly to front suspension, some rear shocks offer a travel lock out which limits the rear shock travel, useful for riding on tarmac or more predictable surfaces.

**Some key concepts**

At their core, good suspension consists of an energy-storing device (normally some sort of spring) and an energy-dissipating device (called a damper). I say "good" because low-cost suspension found on BSOs often lacks the damper and therefore behaves more like a pogo stick than a suspension system.

The primary options for springs are:

 - Coil-sprung: the suspension uses a metal spring – normally steel, but sometimes titanium on more exotic bikes. Spring stiffness is changed by installing different springs.

 - Air-sprung: the suspension meanwhile features a cylinder containing compressed air and one or more pistons. Spring stiffness is changed by changing the air pressure.

 - Elastomer-based systems rely on rubber or another elastic material to compress. These typically offer less travel than the other two types but are lighter and easier to produce (and were hence often found on the earliest suspension forks). The elastomer is prone to degradation due to oxidation and wear and tear. Spring stiffness is changed by installing different elastomers.

Some key terms are:

 - Travel: how much range of movement the wheel has. Very roughly speaking, more enables you to handle rougher terrain.

 - Bottom out and top out: when you hit a bump, if the suspension system reaches the limit of its travel, it has bottomed out. If you have too many harsh bottom outs, you need more suspension and you also risk damaging the suspension. Conversely, a suspension system will *rebound* after taking a hit, and top outs occur when it hits its travel limit on the rebound. Suspensions may have rubber bumpers inside the suspension chambers to prevent damage if it bottoms out.

 - Sag: when you sit on the bike, your weight compresses the suspension to some extent. This is called sag. Notably, suspension manufacturers may recommend that you set the sag to be a certain percent (usually 15-30%) of the total travel. This is done by tuning the spring stiffness (which is also how you accommodate different rider body weights).

 - Compression damping and rebound damping: There are two main types of damping: compression (when the suspension retracts) and rebound (when it extends). These are usually adjustable using knobs on the fork or shock. More compression damping means more resistance as the fork compresses, which feels firmer. You can similarly adjust the amount of rebound damping. 

 - Suspension tuning: this refers to the act of adjusting all the suspension's parameters: spring stiffness, compression and rebound damping, air volume, damper shim stack, etc. The goal is to obtain a suspension system that works best for a given rider's weight, riding style, local trails, and preferences. Defining "best" is usually a mix of riding faster and more comfortably.

**Maintenance**

Suspension systems require periodic maintenance. Riders should gently wipe the stanchions (the metal sliding surfaces) after each ride, and inspect for scratches. Periodic higher-level maintenance may involve replacing the fork seals, which keep dirt out of the suspension chamber, lubrication, possibly rebuilding springs and replacing bushings. Manufacturers will publish recommended maintenance schedules.

Newer cyclists riding only on paved paths should consider foregoing a suspension system. The prime reason is that suspension is not necessary for this use case, and furthermore a suspension fork requires maintenance and will eventually seize without maintenance - at which point it is just dead weight that you paid extra for. 

[1]:https://keyassets.timeincuk.net/inspirewp/live/wp-content/uploads/sites/11/2016/04/Fox-32-2017-featured-630x400.png "Front Suspension"

[2]:https://i.sstatic.net/PrlJe.jpg "Rear Shock" 
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