Suspension basics

Suspension systems are amply applied on off-road machines, but are not used on-road cycling as they rob cycling efficiency with energy sapping flex. For a road bike – where the actual road assumes responsibility for rider-comfort - the stiffer the frame’s set up, the better.

An off-roader requires a suspension system to facilitate a more comfortable ride over harsh terrain enabling control, allowing you to ride faster, further and longer. All suspension systems will relatively increase comfort, but how it will effect pedalling efficiency depends more on available travel, spring and damper tuning, than the overall design. The important differences in suspension design lie in how it reacts to pedalling, braking and acceleration forces, as any design can absorb bumps.

A typical full suspension system splits into two main parts, being the (front) fork section and (rear) swingarm / linkage coupled with the rear shock (usually containing a spring and damper). The linkage / swingarm controls the amount of suspension movement, the spring stiffness and the damper the speed of movement (in each direction).

The ‘spring’ in most rear shocks is either compressed air (pneumatic) or a metal coil (steel or even titanium). Pneumatic shocks are referred to as ‘progressive’ or ‘rising rate’, as they start out softer and get stiffer toward the end (relative to a coil spring). The damper controls the speed of movement (by forcing/restricting the flow of air / oil from one chamber to another).

Suspension rate (spring stiffening) is significant to suspension performance and is determined by the paths that the shock-mounts travel relative to the main triangle.  Shorter travel suspensions tend to have higher rates that drastically increase as the suspension move through its arc (in part due to the fact that many use air shocks these days).  However, most frames mate well with their ‘stock shocks’, and all common suspension types can achieve useful suspension rates (either linear or rising).  Rate is only a real issue for those wishing to swap different shocks in and out of a given frame. Commonly, pneumatic forks and shocks are used for their lighter weight in cross-country, whilst coil spring shocks are more robust and reliable and used in jumping, dropping, trail-riding and downhill (where performance is more important than relative weight). Cheap suspensions are typically un-damped, making them springier & substantially less controllable.

With advances in technology the coil spring is catching up with the progressive nature of pneumatic shock absorbers, as coil springs can be wound to react remarkably progressive (by winding the coils closer together at an end, or by employing tapered wire construction).

A suspension should be ideally designed to use about 25 - 33% of its maximum travel under normal load. With uprights & short wheelbase recumbents, the weight distribution between front and rear wheel is approximately 50/50, whereas with long wheelbase recumbents the distribution comes closer to 30/70 front/rear, and with low racers to 60/40 front/rear.

Suspension is furthermore influenced by:

• chain force generating torque upon the swingarm,
• acceleration effects the centre of gravity causing the wheel-load distribution to be shifted towards the rear wheel,
• a vertically directed vector of the force effects upon the swingarm pivot, induced by the driving force taking effect at the rear-wheel axis,
• the gear ratio inside a hub generates a torque upon the hub axle, which is (in turn ) transmitted unto the swingarm.

Suspension nirvana strives toward three core features:

• pedalling independence,
• brake independence,
• complete suspension activity, having to soak-up the full spectrum of impacts ranging from high-frequency trail vibrations to heavy-hitting compressions (and extension),

and all designs fluctuate within these parameters.

The early days

Early full suspension bikes tended to bounce up and down while a rider pedalled. This movement was called pedal ‘bob’, ‘kickback’ or ‘monkey motion’ which took power out the pedal stroke. Bobbing can either manifest  in compression (squat) or extension (jack) from chain tension.  Hard braking efforts also negatively affected early designs as they lost some ability to absorb bumps in situations where the rear suspension was needed most (known as ‘brake jack’).

One of the first successful full suspension bikes was designed by Mert Lawwill. The Gary Fisher RS1 - released in 1990 - adapted an A-arm suspension design and it was the first 4-bar linkage in mountain biking that could solve the problems of braking and pedalling input to the rear wheel. Unfortunately it couldn't use traditional cantilever brakes and had to use disc brakes, but the lightweight, powerful disc brake wasn't developed until the mid 1990s, and the brakes used on the RS1 were just not good enough.

In 1991, while working for AMP Research (owned and run by Horst Leitner) Karl Nicolai designed a bike that utilized a 4-bar linkage design able to accept a ‘normal’ cantilever brake. It was marketed under the AMP brand, and a version came to the mass market as the Specialized FSR. It became the standard by which all other full suspension designs were judged for the next decade. (Specialized bought several of Leitner's patents (1998) and other manufacturers pay licence fees to Specialized for the use of the 'Horst Link' suspension design.)

The amount of travel on full suspension bikes has steadily increased, and:

• 100mm is considered as acceptable for cross-country application,
• 150mm for trails bikes,
• freeride and downhill machines feature suspension travel of up to 225mm.

In 2003, Specialized introduced the ‘Brain’, an external inertia valve that effectively eliminated suspension bob. Situated atop the drive-side chainstay (near the rear dropout, connected to the shock directly or through a hose) the Brain  was simply  a brass weight suspension valve that deactivated the rear shock. (Upward force from rough terrain displaces the weight, opening the valve and engaging the suspension. Even terrain allows the weight to return to its lockout position, deactivating the suspension. Downward pedaling force – the traditional problem in rear suspension - has no effect on the brass valve.) The Brain equipped Specialized Epic became the first full-suspension mountain bike to win a cross-country world championship, and changed mountainbiking forever.

Over time, and in addition to the above, several different designs have become well established in the marketplace.

Suspension Types

The Soft Tail relies on the flexing of the rear triangle and a rear shock or elastomer placed in line with the seat stays - being a variation of original Amp Research ‘Mac-Strut’ design-  and it is really a 3 bar suspension design. Soft tails have no moving parts, besides the shock/elastomer - making it extremely simple – and it maintains pedalling efficiency via solid chainstays. They are light compared to other designs, but suffer limited rear axle travel of around 25mm.

Various manufacturers have used soft tail technology on their road bicycles. One example is Trek’s SPA (Suspension Performance Advantage) rear suspension available on some of their Pilot models. It consists of a sliding linkage in the mono seatstay encased in an elastomer. All of the rear travel comes from flexing of the chain stays.

The Single/Mono Pivot is also a simple design, and consists of one swingarm / solid chainstay connecting the rear axle to the main pivot around the bottom bracket area, allowing the rear axle to rotate around the pivot point. The axle path is a perfect arc about the main pivot, and they react to bump force, acceleration and chain tension in exactly the same way as a seatstay 4 bar pivot linkage bike does. Some variations:

• use linkages to attach the rear triangle to the rear shock for a progressive spring rate (see Faux Bar) such as the Trek Fuel which has a ‘solid’ chainstay, yet uses a rocker link to activate the shock.
• directly attach the rear triangle to the rear shock for a more linear rate (Gary Fisher's “Cake”)

It's a simple design with few moving parts, featuring good small bump ability, however, the design suffers from bob, brake jacking and chain growth, but many  manufacturers use the design (Morewood, Orange, Cannondale, Santa Cruz and those department-store bikes). Monopivot bikes cannot be as neutral under acceleration as a Horst link bike, and all the forces encountered in the rear suspension must be handled by one pivot. Accordingly they are large to be able to handle the forces, and the swingarm is also built stronger  / heavier to avoid failure or excessive flex that will wear out the shock. Monopivots are often heavier than 4 bar linkage bikes.  For downhill riding with a single front chainring excellent performance can be realised from a well-designed monopivot with a floating disc brake calliper.

A Unified Rear Triangle (“URT") keeps the bottom braket and rear axle connected. The pivot is placed between the rear - and front triangle allowing the rear axle and bottom bracket to move in relation to the rest of the frameset. URT's constitute ‘floating drivetrains’ that have the bottom bracket (& pedals) mounted on the suspension-swingarm. The design uses only one pivot with the advantages of having a hardtail-like drivetrain (no chain growth), consistent front shifting (because of a known chain angle) and less chain-suck. It's a suspension design that can be turned into a single speed, but URT’s design has mostly fallen out of favour as the saddle-to-pedal distance changes. The design tries to allow a standing rider to shift weight from a high leverage position (the seat) to a low leverage position (the pedals) relative to the suspension, to stiffen up when pedal power is applied.  In reality these designs bob rather badly, suffer movement between the seat and pedals, and in trying to remove bob it brings about a set of suspension travel compromises.

The 4-bar design has the rear axle mounted on a floating rear link (and the variant ‘Faux bar’) uses several linkage points to activate the shock. A 4-bar will have a pivot behind the bottom bracket, one in front of the rear wheel drop-out (the venerated "Horst Link") and one at the top of the seatstays. A Faux Bar is similar, but will have a pivot above instead of in front of the drop-out ( circumventing  Horst Link licensing fees). Having the pivot in front of the drop out (i.e. on the chain stay) allows the linkage components to affect the path of the rear axle, allowing for more vertical travel. Having the pivot on the seat stay (above the drop out) effectively makes the rear axle travel path very similar to that of a single pivot bike, since the chain stay is the only component that affects the rear axle's arc. Mounting the axle on the rear floating link means the axle doesn't pivot about either of the frame pivots, which can be used to produce a system that bobs less, offers better braking traction while climbing and accelerating better than a single pivot system.  Some examples of 4-bar designs include Norco's "VPS" bikes, almost all Specialized bikes, Ellsworth, KHS and Kona (who use it on their entire line-up).

4-bar designs with closely spaced pivots near the frame centre can achieve significantly variable, wider path curvature then possible in a mono-pivot.  However, closely spaced 4-bar pivot locations near the highly stressed bottom bracket come with a weight trade-off, as the links and pivots in this area must be heavily built.

The term "Faux bar" is used because they look similar to 4-bars. Faux bar suspension designs are in fact Seatstay single pivots and the rear axle rotates around a single fixed point (near the bottom bracket) being connected with a single link. The seatstays and rocker activate the shock, but has no effect on the path of travel that the rear axle. All the other links in the rear suspension serve to add a little more lateral stiffness and to drive the shock. Functionally a seatstay pivot bike is a monopivot and the axle path is a perfect arc about the main pivot.  Seatstay pivot bikes perform exactly like similarly placed monopivots under acceleration and chain forces, which means they aren't as neutral under acceleration as Horst link bikes, but when the brakes are mounted on the seatstays they do have an advantage while braking over rough ground. In general they do not pedal, climb, accelerate or brake as well as a Horst Link.

The Virtual Pivot Point (or VPP) is a linkage - currently owned by Santa Cruz (who licenses the design to Intense) -  designed to activate suspension depending on the input it receives. Yeti Cycles have created a unique rail system to eliminate pedal jacking . The DW-Link is another design (also licensed to Iron Horse & Independent Fabrications) in addition to Giant's Maestro. Maestro utilizes four strategically positioned pivot points (eight total bearings) and two linkages to create a floating pivot point that improves pedalling efficiency by counteracting forces that would otherwise create suspension compression (pedal-bob). Maestro also features suspension sensitivity even under harsh braking, with no loss in braking or traction.

To define a VPP/DW-link/Maestro suspension design, imagine a line that follows the path the rear axle takes as it moves through its travel and continue it into a complete circle. The center of the circle is the pivot point, being a virtual point (and not an actual pivot on the bicycle). It can either be a fixed point, or one that moves around as the suspension compresses, depending on the design.

The VPP family of suspension systems are in fact 4-bar designs with relatively shorter links than a conventional 4-bar.

The VPP system has reduced the problem of pedal bob, and Progressive Suspension’s 5th Element rear shock allowed riders to suspensionize  almost any frame - regardless of design - to be pedalled without the pedal bob. Other companies have followed Progressive's lead, but these 'intelligent' shocks always have to compromise between their resistance to bob versus performance with small bumps.

Maverick’s "Monolink" uses 3 pivot points and place the bottom bracket on a floating linkage between the front and rear triangle. It was designed by Paul Turner (of RockShox fame). The monolink design uses a shock body  integrated into the rear triangle, and that the saddle to bottom bracket distance changes as the suspension is compressed, although not as much as a URT design. The suspension is more reactive when in the saddle, as pressure on the cranks actively works against the suspension with less bob in out-of-saddle sprints. The design has rearward axle path similar to the angle of attack of the front suspension, but – unfortunately - the monolink suffers from horrible front shifting and long chainstays.

Spring & Sag

When a rider mounts his ride the suspension ‘sits in’ or ‘sags’, which sag depends on the rider’s weight, position, the suspension’s firmness and amount of preload.  Sag is necessary for good suspension performance in order to allow the suspension absorb inequality of the terrain, to have the rider coast over the ground. Too much sag loses the travel that the suspension needs for bump control, too little gives a harsh ride negating suspension, and incorrect damping often ruins suspension performance.

Albeit that the ideal amount of sag varies from rider to rider, strangely many race-snakes run bikes with no sag. A good starting point is to have 10-25% of your total travel on each end for cross-country use and up to 33% each end for downhill use. Aggressive riders typically prefer a firmer fork. 

Pneumatic shocks often run less sag than springs (due to the high preload inherent in air springs). Get as close to ideal as you can by swapping springs or air pressure, then use the preload knob or collar to wind out excess sag on a soft spring. Beware that excess preload can cause top-out problems and spring failure (more than 7 turns of preload means you should be looking for the next spring on a rear shock!)

Rebound damping

Albeit that suspension uses energy, damping robs even more as it counters unwanted suspension motion into heat, through friction. 

• Air damping is found in lightweight forks and shocks. The problem here is air-damped components are usually also air sprung, and with damping the air inside the shock also heats up that causes the pressure to rise and spring rate to increase.
• Oil damping is perhaps the best way to control suspension, as it features a large heat capacity, is available in a multitude of viscosities, and lubricates.  Many oil damped systems have an adjustable free bleed / rebound adjuster, that controls the amount of oil which is allowed to bypass the main damping piston on both compression and rebound. (Closing the free bleed (knob clockwise) increases the damping force on both compression and rebound, but substantially slows rebound speed.) The ‘correct amount’ of rebound adjustment - being the least amount possible - depends on the rider’s preference, terrain, riding speed and the suspension type.

Too much rebound damping will make the fork feel dead as not enough air/oil is let through / returned from the damping chamber.  Too little rebound damping can kick the front wheel off the ground after compression, not being able to control the release. To properly set rebound damping one can start with the adjustment all the way out, find a bump which can compress the suspension most of the way and ride at it repeatedly. Every time the fork ‘kicks back’ (like a mule) increase the damping a tad, until the kickback becomes manageable.

Compression damping

Too much compression damping feels harsh on all bumps and doesn't let the suspension use its travel. Too little compression damping lets the suspension ‘dive’ through its travel and bottoms out harshly. Compression damping is similar in function to rebound damping but is handled by the shims and piston inside the shock. It is generally factory set and not easily changed.  Higher-priced shocks and forks do feature externally adjustable compression damping, having a throttle (on a second piston or orifice) through which oil must travel through after passing the main damping piston. Like all damping the correct amount of compression damping, is the least amount possible. When you can get full travel from your fork and shock (while only bottoming out once or twice a ride) your compression damping is about perfect.

Oil Viscosity

Shock oil is available in a range of different thickness (viscosities). The common term is oil weight and numbers of suspension oil weight range from 2.5 on and up to around 30. Most forks and rear shocks use stock oil ranging from 5wt to 10wt. This gives the tuner a range of oils both thicker and thinner to choose from. Changing to thicker oil increases both compression and rebound damping; thinner oil decreases both compression and rebound damping. Beware though that shim stack type dampers don't rely as heavily on oil weight as orifice type dampers, and changing to a lighter weight oil in a shim stack damper will have a substantial decrease in low speed damping / only a small decrease in high speed damping. Conversely changing to a thicker oil in a shimstack damper will increase low speed damping and may increase high speed damping to the point of spiking (damping so much that the fork stops dead on compression).

Oil Height

Many forks these days are ‘open-bath’ design, having a bath of oil inside each leg of the fork that rises as the fork is compressed. This traps a volume of air against the top-caps of the fork that compresses and adds to the spring rate as the fork nears the end of its travel. The amount of air present in the fork determines how progressive it gets at the end of the stroke, being governed by the height of the oil surface in the fork.  Adding more oil to the fork raises the level in each leg, increasing end-of-travel pressure / progressiveness of the fork.  Too much oil in the fork will prevent full travel being achieved; too little oil will let the fork bottom out harshly. These forks often do not have bottom-out bumpers; the metal on metal impact would kill a chicken. In experimenting with oil height just a handful of mm’s will make a big difference, make only small changes and document each one!

Balanced settings

On a full suspension bike it is vitally important to get the feel of the front and back suspension balanced for it to be able to handle bumps in a similar fashion. In equalising the suspension rates it is perhaps easier to ‘back off’ the end that feels harsher, rather than to ‘wind up’ the end that feels soft. 

Irony

The tried and trusted technology of the 4-bar linkage (with a comparatively simple shock) still offers all-round best performance.However - ironically - as shock manufacturers compete to develop 'pedal platform' technology, suspension riders are opting for expensive dampers on which it is possible to switch off the platform completely!