Ever wondered how your mountain bike’s suspension actually works?
Wonder no more, our guest suspension expert Paul Mackie is here to break it down for you in easy to digest chunks.
Part 1: Rear suspension
photos by Ian Lean / Roo Fowler / Pete Scullion / Jacob Gibbins / Red Bull
In this series, our man Paul helps break down the science behind suspension, putting the much bounded about jargon that fills bike chat into layman’s terms. Part 1 covers a fundamental part of rear suspension, why do we want it?
Who is Paul you ask? He’s a suspension nerd, engineer, not-too-shabby enduro racer and designer for a mountain bike brand.
Is it just black magic?
VPP, instant centre, rising-rate, single-pivot, anti-squat, brake jack, progressive, kinematics, four-bar, faux-bar, gay-bar.
These are all terms that get banded around when talking about mountain bike rear suspension (well, most of them anyway). But how many of us really understand what these mean?
Is suspension design black magic or just simple physics?
I’m going to de-bunk suspension related jargon and help you to understand how suspension works in everyday language. As rear suspension systems are a challenging topic with many inter-related aspects, this article is broken down into seven bite size parts.
Take a deep breath, hold my hand, here goes…
What goes up must come down, right?
The holy grail for mountain bike frame designers is arguably the lightweight ‘all-mountain’ bike which must not only gobble up terrain like a downhill bike but must also be energy efficient to be pedalled back up the hill.
Balancing these two opposing objectives is possibly unique to mountain bikes as other vehicle suspension systems don’t have to deal with the issue of the rider vastly outweighing the vehicle. This big lump on the vehicle also shifts position during riding and bobs around when delivering power (pedalling).
On top of that, the rider has a finite and precious energy source which must be preserved. Most other types of vehicle suspension systems are powered by mechanical engines which can be re-fuelled easily, and therefore energy efficiency in the suspension system is less important for these.
Let’s talk about rear suspension
Front suspension is a given. Rigid forks are pretty much obsolete on mountain bikes now for the reasons of comfort and control of the bike in motion. It’s also pretty easy to understand how a telescopic fork works (from the outside at least) as the linear movement is obvious.
Rear suspension is different. The way that the motion of the rear wheel is handled and the force transferred into the shock absorber can become complicated by many different pivot, linkage and swingarm arrangements. Hardtails are still popular, but rear suspension is seen to give better performance.
However, how this rear suspension performance is delivered has produced a wide variety of configurations which have been promoted with much marketing spiel and hype which has further added to the confusion.
The 5 Fundamental Rear Suspension Elements.
Having analysed suspension design over several years and having knocked up several working prototypes myself, I consider there to be 5 fundamental elements that come together to create a good rear suspension ‘platform’ for a mountain bike. These 5 elements directly influence performance, functionality and ride characteristics.
These are all interrelated, but to understand how the whole works, it is necessary to look at them in detail individually. Before we do that, we need to go back to basics and understand why we have rear suspension and what are the forces acting on the rear wheel…
Why even have rear suspension, my hardtail is great?
The purpose is to absorb impact forces as the rear wheel comes in contact with terrain obstacles.
It might seem like simple, obvious stuff, but it is worth reminding ourselves what the benefits are and why we want rear suspension in our lives. The purpose is to absorb impact forces as the rear wheel comes in contact with terrain obstacles.
By absorbing the terrain, the rear tyre also stays in more contact with the ground which aids traction for cornering, braking and acceleration. This means that the rider is in more control of the vehicle and is able to ride faster. In addition, bump absorption can reduce rider fatigue and can be helpful when landing jumps or drops.
The big thing for most of us though is speed. A rear suspension system will absorb bumps and maintain forward momentum better than a wheel that hooks up on terrain obstructions.
What are the downsides?
Full suspension systems tend to weigh more because there is linkage, pivots, shocks and just simply a more complex set of components needed. Because of this added complexity and moving parts, the durability of a full suspension is always going to be less than a hardtail because there is more to go wrong. The additional cost and weight of full suspension are also downsides.
Arguably though, the main downside of a rear suspension system is energy efficiency, meaning that we don’t want to waste pedalling energy. For other vehicle suspension systems, this isn’t such a problem because there is typically an engine, however on a bicycle the power comes from the rider, who has a finite amount of energy which we want to conserve and reduce fatigue.
An inefficient suspension system would wallow and bob as the rider inputs pedal strokes essentially using his or her energy supply to activate the suspension, rather than just propelling the bike forward.
Monster trucks vs F1 cars
Alongside efficiency, the rear suspension should provide the right amount of ‘support’ so that good handling of the bike is maintained through the suspension travel. In this regard it is like trying to blend the terrain gobbling qualities of a monster truck with the composure of a Formula 1 car. Not an easy task.
Balancing the requirements of bump absorption, energy efficiency and support is why mountain bike rear suspension design is unique and also so tricky. Throw into the mix the different type of riding we do now, and it’s almost impossible to design a suspension system that will cover all riding styles. The obvious comparison is a downhill bike vs. an XC bike: The downhill bike is nearly all about bump absorption, whereas an XC bike leans much further in the direction of energy efficiency.
Hardtails are great because they tend to be simpler in construction (less to go wrong), lighter, cheaper and can teach/re-learn fundamental rider skills that might be lost with a full suspension. But for most applications, a rear suspension platform will offer better performance and essentially just be quicker. That’s not to say hardtails don’t have their place though.
For ease, we are going to focus on a typical 140-160mm travel ‘trail/enduro/ all-mountain’ type bike as this is the bike that most of us can relate to. It also needs to best blend up-hill efficiency with downhill terrain-smashing qualities.
The dark side and light side of the force.
The rear wheel in a suspension system is acted upon by 6 forces:
1. Bump impact (moving the wheel upwards and, ideally, slightly backwards)
2. Drivechain (chain tension wants to pull the rear wheel forwards when pedalling)
3. Squat (the mass of the vehicle, including the rider, moves backwards and downwards over the rear wheel under acceleration typically compressing the suspension)
4. Bob (the mass of the rider cyclically goes upwards and downwards during pedalling, loading and unloading the rear suspension system)
5. Braking (under braking the terrain is trying to rotate the wheel, which the brakes are obviously trying to prevent)
6. Body position (the mass of the rider will change depending on the terrain being ridden resulting in more or less weight on the rear wheel).
The diagram above main forces acting on the rear wheel of a mountain bike
In the simplest terms, the point of a rear suspension system is to absorb bumps, therefore this aspect is considered the beneficial. The other aspects, in round about terms, are negative and must be carefully designed for to avoid detrimental effects.
The joker in the pack however is drive-chain forces which can be both beneficial and negative depending on how they are applied. More of that later…
