Part 1: The Purpose of Suspension

Part 1: The Purpose of Suspension

July 02, 2018

Welcome to our new Suspension Tech Series where the aim is to break down the mechanics behind suspension, translating the tongue-twisting jargon that fills most bike websites and forums into layman’s terms.
In Part 1 we answer a fundamental question: what is the purpose of rear suspension?

Image: Pete Fogden

Virtual Pivot Point, anti-squat, horst link, falling rate, mid stroke support, brake jack, kinematics, four-bar, faux-bar, the list goes on. It’s pretty common to read these terms in blog posts, forums etc, but how many of us really understand what these mean? And more importantly how do they impact the enjoyment and performance of our ride? Is suspension design marketing mumbo jumbo, chatroom engineering or is it a science?

We're wanting to help clarify suspension related jargon and help you to understand why it’s there, how it works, and what makes it good (or bad). Yes, it’s a pretty complex topic and can seem daunting, but work with us and we will break it down in bite size parts that are easy to understand.

AN APPLE FALLS FROM A TREE

Every bicycle brand worth its salt refers to their latest model as the new “one bike quiver” that is the holy grail for mountain bike frame designs. Each new model is lighter, stronger, stiffer, and features a whole string of acronyms to make these arguments credible. The most important communication by the marketing department is always the bike's ability to tackle the roughest and most demanding downhills as well as give you e-bike like performance gains up the climbs, guaranteeing that Strava KOM.

Image: Craig Kolesky

Balancing descending capabilities with climbing prowess is not unique to mountain biking, it can be one of the more complex. How can it be more complex than something with an engine and a million moving parts? Well… mostly because its engine doesn’t stand still. Why does this matter? Well, when the engine is jumping up and down on the pedals, and moving all about in the turns, it’s a little difficult to design a system that counteracts all of those inputs to make the “ultra-efficient” bike that the marketing loves to preach.

It gets worse… forgetting your e-bike for a second, unlike a car or motorbike, you can’t plug the bicycle’s engine into a power socket, or fill it up at the petrol station. Instead, its engine requires training, nutrition, recovery, and often TLC to get it through a rough day on the trails. Most other forms of transportation are far more simple to refuel (and cheaper) and as such also far easier to keep running.

Why does this matter? Well, what it means is that when designing bicycle suspension, what may seem like a simple task is made infinitely more difficult by the fact that as a rider you are more complex that the most advanced engine. Added to this, engineers need to consider making the bicycle efficient at riding over terrain, but also efficient at transferring the power produced by its owner.

Shredder, Sam Bull's spectacular body position in the turn above perfectly demonstrates one of the many complexities bike engineers are forced to consider when designing suspension systems. Image: Troy Davies

THE COMPLEXITY OF TWO

So lets talk suspension: Front suspension (hardtails) are almost as rare as a bicycle industry “standard”, with fully rigid bikes reserved for beach cruisers and hippies. For these articles we will forget about these two categories so that we can dig into the real “juice” of mountain bikes, full suspension designs.

Front suspension is a simple and concentrated category where almost all linkage style front suspension systems died with good music and dial-up internet. These days “telescopic” designs rule the roost, and are available in a myriad of forms with the same essential, predictable movement up and down.

Image: Craig Kolesky

What is a telescopic design? Well like a pop out umbrella, whether single clamp, triple clamp, upside down or single sided, all telescopic forks allow the front wheel to slide up and down in a single, linear plane. The angle of this is determined by the head angle of the bicycle, the control determined by the damping control of the suspension, and the resistive force provided by the spring (whether air or coil).

Rear suspension is different, and beautifully complex. The way that the motion of the rear wheel is handled, and how that force is transferred into the rear shock can become complicated by many different pivot, linkage and swingarm arrangements.

Image: Craig Kolesky

These “complications” are what give full suspension bicycle designs their “mechanical advantage” over a hardtail, and the performance traits we’ve all come to love. But we don’t fully understand our current suspension design, then how do we know its performing right? And more importantly, how do we know it’s right for us? Well… read on!

Why have suspension? Two reasons: efficiency and control. What’s most interesting about these two things is that they are almost complete opposites. Suspension makes your bicycle more “efficient” by allowing the wheels to move out of the way when they impact an obstacle. This is more energy efficient than the work required to move the entire system (bicycle & rider). On the other hand, suspension improves your “control” by keeping the wheels in contact with the ground thus improving the traction of the tyres giving better cornering, braking and acceleration.

Image: Troy Davies

Well that sounds simple enough right? But just think about that for a second, forget the additional things that are secondary benefits to suspension (comfort, cool factor etc). Ultimately, the purpose of your suspension is to allow your wheels to move out of the way of obstacles, and to keep them in contact with the ground. This may sound like a contradiction, but this is the most important thing to understand, is that it’s this fine balance that is at the very centre of suspension, the ability of the wheels to move out of the way of obstacles must be controlled, and so too must the wheels ability to track the terrain and maintain contact with it.

On rocky terrain, suspension acts to keep the wheels in contact with the ground, maintaining traction. Image: Craig Kolesky

What are the downsides? First up price, weight and maintenance. Full suspension systems tend to weigh more because there are linkages, pivots, shocks, and a more complex set of components needed. Because of this added complexity and the moving parts which make it, the durability of a full suspension bicycle is always going to be less than that of a hardtail because there is more to go wrong. The additional cost of this complexity is well… the cost. These parts are often precision made, and the fact that there are more components, more engineering, more materials means there is more expense.

Arguably though, the main downside of a rear suspension designs for bicycles is energy efficiency. Remember you’re the engine, meaning that we don’t want to waste energy. For other vehicle suspension systems, this isn’t such a problem because there is typically a real engine, however on a bicycle the power comes from the rider, who has a finite amount of energy which we want to conserve.

THE BALANCING OF ODDS

Quick! Get the popcorn! I said “efficiency and control” were reasons to have suspension, then why did I just say there is a loss of efficiency? Well as we learned in physics: “The total energy of an isolated system remains constant, energy can be neither created nor destroyed, rather it can only be transformed from one form into another”. So what does this mean on your bicycle? Well… imagine for a moment that you and your bike have kinetic energy when rolling down a hill. When your wheels hit an object and the suspension allows them to move out of the way of that object some of this energy is transferred into heat as the suspension plays its part in absorbing this impact. The efficiency comes in thanks to the fact that the energy transfer to heat to displace the wheels is less than what would be transferred in order to displace the entire bicycle with you on it. Hence this is more efficient as more of the kinetic energy is maintained. But, when you’re then jumping on the pedals trying to get that KOM your movement up and down, and your force on the pedals also causes movement in the suspension which in turn translates to some conversion into heat as the suspension compresses and extends. This causes some loss of efficiency, and this is the first balancing act!

Go carts vs Dakar trucks: Alongside efficiency is control, and to do this the rear suspension should provide the right amount of ‘support’ so that good handling of the bicycle is maintained through the suspension travel. If the wheel just gave way to the object that it needed to absorb then it would also give way when you needed it to push back into the ground in order to maintain contact and give you control. This is the second balancing act!

Balancing the requirements of bump absorption, energy efficiency and support is part of what makes any vehicle suspension design complex, but what makes a mountain bike suspension design even more complicated is that the physics used to optimise these two balancing acts is an equation with an ever changing variable – you. To spice things up more, we also love to throw in different disciplines, riding styles, terrain etc, making the “perfect” design infinitely complex, and ultimately up to interpretation.

DRAWING A CONCLUSION

What the force? Your back wheel is acted on by 6 main forces, and in most cases these are where energy is transformed / transferred:

  1. Impact - an object moving the wheel upwards
  2. Drive - chain tension wants to pull the rear wheel forwards when pedalling
  3. Squat - acceleration causes the mass of the bicycle, including the rider, to move backwards and downwards over the rear wheel compressing the suspension
  4. Bob - your pedal action and goes upwards and downwards during pedalling, loading and unloading the rear suspension
  5. Braking - under braking the terrain is trying to rotate the wheel around the contact patch with the ground
  6. Mass - your position will change depending on the terrain being ridden, loading the suspension differently as you move.

The diagram above shows the 6 main forces numbered as the relate to the list. Image: Pete Fogden. 

In the simplest terms, the point of a rear suspension system is to maintain contact with the ground be it by absorbing bumps or by tracking the terrain, this is ultimately the requirement. The other aspects are negatives and designs must try to mitigate these without too largely impacting the benefit.

But wait, like any good T.V. series here’s the closing catch: Force 2, the drive force, can be both beneficial and detrimental depending on how it is applied. To find out more, tune into the next episode!