Buyer's guide to mountain bike suspension, part 1

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Buyer's guide to mountain bike suspension, part 1
By Paul 'Supersonic' Hayes, Simon Young & Doddy

With a host of terms like four bar, faux bar, virtual pivot, multi link and floating drivetrain to describe different full-suspension frame designs, as well as a raft of manufacturer acronyms to contend with, it's no wonder that many of the biking public are left bemused.

But it’s not really surprising because suspension design is a very complicated arena. Over the course of this three-part buyer's guide we’ll do our best to demystify this murky world with the truth, the whole truth and nothing but the truth.

Suspension theory

Given that most manufacturers claim to achieve the same end result with their full-sus rigs, using baffling phrases to describe their setups such as ‘100 percent neutral’ and ‘fully active’, are there any real differences when you get out on the trail? The simple answer is yes, and what the manufacturers claim is not always true.

It's important to bear in mind that suspension layout isn't simply about bike designers designing around any one factor, because the bike’s geometry, stiffness, weight, instantaneous lever ratio, anti-squat/chain pull effect and shock tuning all have to be considered and designed for as a whole. All these factors work together to bestow the bike the ride it has: there is no one magic ingredient.

Pivot location

The two most common suspension frame designs on the market, with variations, are the single pivot and the four bar linkage. The first is as simple as it gets: the rear wheel is fixed on a cantilever, or swingarm, with a pivot on the front triangle and the shock in between.

This Cannondale Rush has a simple single pivot suspension design

The four bar uses an extra pivot on the chainstay and further linkages to complete the four bars. This creates a virtual or floating pivot point: the geometry of these linkages dictate the precise location of this point, and a point called the instant centre of rotation, which we'll discuss later on.

This Ellsworth Epiphany has a more complicated four bar system with a chainstay pivot

The most well known four bar is the Horst Link, the patent for which is owned by Specialized. We'll cover these suspension designs, and the four other most common systems, in more depth in part two of this guide.

One of the most common misconceptions about suspension is that four bar and other multi-pivot setups will pedal more efficiently than a single pivot, somehow decoupling the suspension from the drivetrain.

This isn't always true; in fact, at times when the floating pivot point of a particular design is located in the same place as that on a single pivot bike, they will behave exactly the same when pedalling and accelerating. The location of this pivot is the key, because it dictates the path of the rear axle and how the suspension will react at any given moment in time.

Certain floating pivot point configurations can place the floating pivot in physically impossible locations, such as within the wheel or several metres in front of the bike. Furthermore, the floating pivot can move as the suspension compresses, allowing the designer more freedom when constructing axle paths or other characteristics.

So where is the best place to put that pivot? That depends on what the designer wants from his setup. The position directly affects the path the rear axle takes; for example, a rearward path can be more sensitive to bumps than a vertical path, but can also introduce some positive and negative effects.

Pedal kickback occurs when the rear axle moves further away from the bottom bracket. The top run of chain wants to get longer (chain growth) so something has to give: the tension pulls at the cranks as if trying to turn them backwards.

You can feel this through the pedals and it is magnified in certain gear combinations. For many people this is an undesirable effect and the simple solution would be to put the pivot very close to the bottom bracket. This does reduce the chain growth, but introduces another factor: pedal bob.

Uncle Bob and Anti Squat

An old fellow named Newton once said “every action has an equal and opposite reaction” and this is true when we press on the pedals and accelerate: the bike moves forward and our weight moves further towards the back of the bike. This constant stop-start effect and weight shifting can compress and extend the shock rhythmically, which we call pedal bob.

There are two solutions to this. One is to introduce compression damping or platform damping/lockouts in the shock to resist the compression; the other is to locate the pivot in such a way that the chain tension and drive forces when pedalling want to extend the suspension.

This balances the tendency for the shock to compress as we pedal and is called anti-squat. 100 percent anti-squat is perfect balance of the forces. Squat itself is how the rear end sinks under acceleration as you pedal. Both methods can be effective, but not without problems.

Extra shock damping and platforms can stifle the suspension performance over smaller bumps, while high levels of anti-squat bring back our friend pedal kickback, due to the pivot position required. Most designers have to trade off pedal kickback and bob with their pivot locations and resulting axle paths.

If four bar machines behave the same as single pivots when pedalling then, all else being equal, it would seem to make sense to go for the less complicated single pivot design. However, having the shock driven by a linkage, as in a four bar setup, allows the designer to tune how the shock is compressed through the stroke and resulting suspension rate.

This gives more flexibility to the design. Some single pivots have linkages added for this same reason – think Commencal Meta or Kona Dawg – to give a rocker-activated single pivot, also known as a faux bar or complex single pivot.

Commencal's Meta series bikes use a rocker activated single pivot design

They're still technically single pivots because the wheel can only arc around the main pivot – if you look closely at most Konas you'll see the pivot on the seatstay, not chainstay. Four bar and single pivot machines can behave very differently to each other under braking, though.

Instant centre

Every part of the floating fourth bar of a four bar system has a unique virtual pivot point, but shares a common second point called the instant centre of rotation, or IC. The IC can be used to calculate the levels of anti-squat and how the system reacts under braking, as well as the actual floating pivot location and axle path.

It's simple to calculate: draw a line through the upper linkage pivots, draw a second line through the lower linkage pivots, and where these intersect is the IC. It varies as the suspension compresses.

It can be said that all parts of the fourth bar are travelling at 90 degrees to a line drawn from it to the IC, or revolving about it for that instant. The actual floating pivot point will lie on this line too, and calculating the many IC points as the suspension compresses allows the floating pivot point to be found.

Braking effects

When we brake, the forces try to rotate the tyre contact patch around the IC, its position determining how braking forces influence the suspension. Certain positions can compress the suspension (brake squat) or extend it (brake jack).

Braking causes our weight to move forward, extending the shock. So, squat can be useful to maintain even geometry to counterbalance this effect, but it can also make the suspension feel harsh and lose traction, while a net extension may upset the geometry but increases the available traction.

The designer can place the IC in such a way as to give a desirable balance of forces. If you take two designs, they may both have the same virtual pivot point for the rear axle, and the same axle path, but different ICs.

The designer can tune how the suspension behaves under braking independently from how it behaves under acceleration with a four bar. This is not possible with a single pivot bike, as the IC is always where the main pivot is, but a four bar allows the designer further flexibility, and was the original reason the Horst Link was created.

Design variations

There are many rear suspension designs, and they all have slightly different traits. We'll cover the six most common frame layouts in part two of this guide.

There are many suspension types out there and each has its pros and cons, yet the rider is still the most important factor

Jargon buster

Axle path: This is the virtual course that the rear axle, and hence wheel, moves along as the suspension is compressed (ie. when it hits an obstacle ). There are certain handling characteristics that are defined by the axle path.

Brake jack: Brake jack is extension of the rear suspension. It can improve tracking of the wheel, but upsets geometry due to the rear extending and the front compressing, causing the fork to feel harsh.

Brake squat: Brake squat is compression of the rear suspension under braking. This can cause the suspension to feel harsher, but can actually balance the geometry of the bike as both ends compress together. Most bikes tend to squat to some degree.

Chain growth: Compressing the suspension usually causes the wheel to initially move away from the bottom bracket, causing the chain to lengthen (chain growth) which is taken up by the rear mech’s spring.

Instant centre (IC): The instant centre of rotation, or IC, is a virtual point in space: for a split moment in time it can be said that all other points are revolving around the IC for that instant. The IC migrates as the bike moves through its travel, and will be at a different spot at different points in the travel.

Falling rate suspension: See 'Suspension rates'.

Leverage ratio: This is the ratio of wheel travel to the shock compression. A 2:1 ratio with 4in wheel travel at the axle gives 2in of shock shaft travel. With many designs the figures vary through the travel. Values between 2:1 and 3:1 are used currently. Higher leverage ratios equal higher stress on the shock and less sensitivity to small bumps.

Linear suspension: See 'Suspension rates'.

Pedal bob: Bob is the feeling of bobbing the rider experiences due to the pedalling/drivetrain influence on the rear suspension, and how our mass reacts when we accelerate. It’s exacerbated by a choppy pedalling style and dynamic rider weight shifts. Certain pivot locations use the chain tension and drive forces to counteract the tendency to bob.

Pedal kickback: With too much chain growth, if you hit a bump while pedalling the pedals want to kick back. When this force is opposed by you pedalling, it acts to stiffen your suspension. Most designs behave like this to some extent, however certain setups may actually compress the suspension further. This happens when the axle path gets closer to the bottom bracket, and can be seen on some floating pivot designs, such as the beginning part of the Santa Cruz VPP and at the latter stages of vertical axle paths. It's called pro squat.

Platform shocks: Platform shocks are the saviours of certain single pivot and floating pivot designs that have a natural tendency to bob, ie. those with pivots near to the bottom bracket. These shocks use slow-speed built-in compression damping to overcome the very active nature of these designs. They work at the expense of some small-bump sensitivity. Other shocks can be fully locked out so they don’t move at all (for steady climbing), while some platform shocks – such as the Fox RP23 – have three-stage platform settings (light, medium and full) so you can tailor their feel to suit.

Progressive suspension: See 'Suspension rates'.

Sag: The suspension travel caused by the weight of the rider sitting on the bike when it is stationary. You usually set this to 20-30 percent of the available travel.

Squat: Squat refers to how the rear end of a suspension bike sinks under acceleration in response to rider pedalling.

Suspension rates: There are three types of suspension rate: progressive (also known as rising rate), linear and, less commonly, falling rate. Progressive suspension stiffens up at the end of the shock travel; this is a typical cross-country bike setup. Falling rate suspension feels super-plush at the end and is much easier to blow through the travel (ie. bottom-out), while a linear setup feels the same throughout the travel.

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Buyer's guide to mountain bike suspension, part 2
By Paul 'Supersonic' Hayes, Simon Young & Doddy

In the first part of this buyer's guide we explained some of the theory behind mountain bike suspension. This time we're looking at six of the most common frame designs and discussing which is best. Next week we'll look at suspension forks and rear shocks.

Suspension designs

There are many rear suspension designs, and they all have slightly different traits. Some feel plusher than others, but the plush feel can desensitise your riding. Frames that feel firmer often feel faster and livelier. Some designs, such as single pivots, can have more flex than other bikes, but a little flex can be good as the bike can contort slightly through rough terrain.

Designs using cartridge bearings may be ultra plush, but they carry a weight penalty – bushings can be as efficient and weigh less, so they have their place. It can be tricky finding the right bike to suit you – the best way is to test ride as many different types as you can. Here's a quick guide to the six most common frame layouts:

1 Single pivot

This classic design places the pivot above and in front of the bottom bracket and is very simple and light in weight. A lower pivot gives a more active feel and less pedalling-related bob. A higher pivot suffers from more pedal kickback due to excessive chain lengthening.

Examples: Orange 5, Santa Cruz Heckler, Santa Cruz Bullit, Diamondback XSL Comp.
Pros: Simple, minimal pivots, light weight, reduced pedal bob.
Cons: Prone to pedal feedback and interference from braking forces.
Buy if: You're worried more about long-term durability than a more refined suspension feel. This is the least technically demanding suspension platform so it's ideal for the tool shy.

2 Faux bar

Also called a complex or rocker-activated single pivot, this is a variation of the single pivot but with the pivot placed much closer to the bottom bracket, usually just to the rear of it. The conventional looking rear triangle has a pivot on the seatstay and a rocker linkage to drive the shock to produce a strong and torsionally stiff design.

The linkage is used to tune the leverage ratios under compression; by using a longer stroke shock an extremely compliant and plush action can be achieved. The faux bar is often mistaken for four bar designs that use a pivot between the main pivot and wheel axle (ie. on the chainstay).

Examples: Kona Dawg, Ventana El Ciclon, Scott Spark.
Pros: Fairly light, low pedal kickback.
Cons: Prone to bobbing and braking forces.
Buy if: You like to ride hard and fast. Ideal on hard-hitting trail bikes where absolute subtlety isn't a major concern – what it lacks in sensitivity it makes up for in direct drive feel. More pivots means it needs more regular checking.

3 Standard four bar

The standard or Horst Link four bar is visually similar to the faux bar except the rear triangle pivot is on the chainstay, just in front of and below the rear axle. It gives a plush ride, its action minimally affected by braking. Specialized own the patent on the Horst Link, but many brands pay for the licence to use it.

Examples: Specialized FSR Stumpjumper, Boardman FS, Ellsworth Epiphany.
Pros: Fairly light, limited pedal kickback, improved suspension under braking.
Cons: Pedal bob. Requires regular pivot checks.
Buy if: You want better suspension action under braking than single pivot and faux bar bikes; harder, faster trail riders like this design as it allows you to attack the trail without losing rear wheel traction.

4 Twin link four bar

A recent concept, and variation of the four bar, connects the rear triangle to the front by two short links. This generally allows the floating pivot point to vary quite a lot through the travel. There are various – and subtly different – versions, including:

VPP: The Virtual Pivot Point was developed by Outland Cycles in the early Nineties but has been refined by Santa Cruz and Intense Cycles. Twin linkages separate the rear triangle from the front, and allow a certain wheel path that uses chain tension to tame rear end bobbing. Examples: Santa Cruz Blur, Intense Spider 2, Santa Cruz Nomad.

dw-link: Like the VPP system, Dave Weagle's dw-link design uses twin links to isolate the rear end, but the axle path and characteristics differ from VPP. The heart and soul of the dw-link is the anti-squat curve. The Weagle design specifically alters the amount of anti-squat through the travel from where he thinks you need it most, at the start, to where you need it least, deep in the travel where you are less likely to be pedalling.

This upshot of this is that pedal kickback can be reduced while still maintaining a good pedalling performance. This is coupled with precisely tuned leverage ratios in an attempt to make the bike remain fully active in the majority of conditions and gears. A few manufacturers have their own take on this setup with varying degrees of performance, and even the dw-link itself alters according to the type of bike it is on. Examples: Turner Flux, Ibis Mojo, Pivot Mach 5.

Other: Both Giant with their Maestro system and Marin and Whyte with their Quad Link systems use variations on the twin-link four bar theme. Examples: Giant Anthem and Trance, Marin Mount Vision, Whyte E120.

Pros: Can offer good braking, pedalling and pedal kickback performance.
Cons: Above pros depend on precise setup, is relatively heavy.
Buy if: You want a supple ride with a strong climbing capability. Riders of mid- and long-travel trail/all-mountain bikes love the clean feel of the stop/go action for its terrain-eating ability, but it needs careful shock setup to work optimally.

5 Active Braking Pivot (ABP)

Trek’s Active Braking Pivot design mimics the four and faux bar setups, but places the rear pivot concentrically to the rear axle. Technically it’s a faux bar, but it has the braking advantages of the four bar. The shock is isolated from the front triangle and is actuated by both the chainstay- and the seatstay-activated rockers. This offers an incredibly plush ride in all conditions and, combined with the Active Braking Pivot, has the least brake jack of all designs.

Examples: Trek Fuel EX, Trek Remedy, Gary Fisher Roscoe.
Pros: Low pedal kickback, very good under braking.
Cons: Pedal bob, needs unique rear skewer for axle.
Buy if: You like how faux bars and single pivots ride but want the benefit the ABP gives to retain a supple suspension action even with the rear brake fully on.

6 Floating drivetrain

One of the earliest types of full-suspension bike design was the Unified Rear Triangle. This incorporated the whole drivetrain on the swingarm, including bottom bracket, which was then attached to the main frame by a pivot. With no pedal kickback due to no chain extension, it was assumed this would offer the best of all worlds. But alas, with your weight on the swingarm when stood up, the performance varied significantly depending on whether you were seated or standing and the design is little used nowadays.

However, GT and Mongoose with their i-Drive and Freedrive systems, and Maverick with the Monolink, have modified the concept to produce a range of floating drivetrain bikes where the bottom bracket is fixed to a link between the front and rear triangles. The goal here is to try to combine the positive characteristics of more than one design.

While the axle path may be a simple arc like on a single pivot design, the bottom bracket itself moves as the suspension compresses. The relationship of the axle to the bottom bracket, and how the bottom bracket moves, produces different pedalling characteristics depending on the linkage setup.

Examples: GT Sensor, Mongoose Teocali, Maverick ML8.
Pros: Low pedal kickback, low bob, rearward axle path.
Cons: Suspension efficiency can lessen when standing, heavy, affected by braking.
Buy if: You want your suspension to be active across a wide range of terrain types and value traction sensitivity above all else. Again, careful shock setup and pivot care are needed to ensure performance.

So, which is the best?

None of them! Quite simply there's no one design that's better than another in all departments. And we have to remember the second part of the equation – the rider. Personal preference really is the key when it comes down to what works best for you.

Sure, many poor designs have faded from the scene, but what we're left with nowadays is a variety of high performing machines that allow us to choose the very best for our needs. And given that the winners on the racing scene use a mix of machinery from the most basic to the very complex, it reinforces that the rider is the most important part of the picture.

Steve Peat has won World Cup downhill races on a GT Lobo (Horst Link), GT i-Drive (floating drivetrain), Orange 222 (single pivot) and of course the Santa Cruz V10 (VPP). However, small tweaks to the setups to suit your personal riding style can make quite a big difference to how each design type rides.

Maybe you don’t really care about weight? Or a few extra pivots? Possibly you've never noticed pedal feedback, or bobbing isn’t something that concerns you. Perhaps you hate the way some bikes seem to be affected when you brake. Many people simply don’t notice some of these effects, while others are more sensitive. Whatever your cup of tea, there are enough designs out there for you to pick one that suits. Get out there and try ’em.

One of the reasons it’s so hard to say which full sus design is ‘the best’ is that many of them work in different ways in different gear combinations. Typically, a lot of systems are designed to work at their best in the most frequently used gears — middle ring up front, middle sprocket out back — and this is why you see so many main swingarm pivots located in line with the middle chainring.

Related articles
Buyer's guide to mountain bike suspension, part 1
If you test ride a bike, try it in all possible gear combinations. Many suspension systems tend to respond in a very different way when subjected to steep climbs with excessive granny ring pedal pressure. You’ll almost certainly experience different amounts of pedal feedback in different gears.

Your choice of rear shock will also have some bearing on the way your suspension system feels. Air shocks have become dominant on all but the hardest hitting downhill bikes and the cheapest superstore bikes, where steel coil shocks are still popular. But air shocks vary a lot too.

Cheaper full-suspension bikes will often come with preset compression and rebound damping, while more costly bikes have adjustable damping so that you can fine-tune the responses of the shock, as well as tuning the basic weight-bearing feel according to how much air you add (you’ll need a proper shock pump for this, a tyre pump won’t do).

Most air shocks these days also come with some form of platform damping system. This is often adjustable on the better shocks and helps to stifle unwanted shock movement caused by pedal forces and weight shifts, but may reduce the compliancy of the suspension over small bumps. A lot of shocks come with a lockout function too.