Take a deep breath, hold my hand, here goes ...
Balance is perhaps the single most important consideration of bicycle suspension and suspension setup. The balance that we're talking about here isn’t that which keeps you upright, but rather the relationship between the front and rear wheel, which in a full suspension bike is ultimately controlled by your front and rear suspension.
We want you to imagine for a moment your wheels going up and down over bumps, drops and holes etc. Cast your thought on the front fork that we mentioned in Part 1 as being “telescopic”; moving in a linear motion. Your rear wheel, on the other hand, is attached to your swingarm which either moves in a fixed arc around a single pivot, or in a more complex curve around a series of rotating links. So, the front wheel travel is a linear motion, whereas the rear is rotational (whether single pivot, multi-pivot or other). Now bank this picture and let us get started.
There are a multitude of pivot locations and arrangements in the market currently, with more and more variations coming to market each year by mainstream and fringe brands. Though some are easy to recognise and differentiate from others, some are a little more tricky.
The most prominent designs are:
- Single Pivot
- Virtual Pivot Point (VPP), or Short Dual Link (SDL)
What is important for you to know and understand is that despite what marketing will tell you, the rules on which the performance of all of these bikes is defined is controlled by the laws of physics, and no design can escape the basic principles therein. Another point which we would like to make early is how radically different designs can be very similar in their advantages (or disadvantages), and so to can very similar looking designs differ radically in their ability to deliver on performance advantages.
Axle path and swingarm pivot positioning
Forgetting for a moment how the rear shock is actuated / compressed, or the influence that the tension / torque in the drive-chain has on the suspension, the positioning of the pivot(s) directly affects the motion of the rear wheel when the suspension moves. This movement, or “axle path” as its more commonly referred, is the golden egg, and something that we will refer to constantly in this series.
Before we continue, let’s get a first-hand understanding of what the axle path movement is like on our own bike shall we? If you’re willing, with the bike secured in a repair stand, remove the rear shock, and once removed, apply an upward force to the bottom of the rear tyre (where it would normally contact the ground). By moving your hand up and down vertically you will be able to cycle through the suspension travel, getting an idea for its movement. The only thing resisting its movement currently will be the weight of the wheel. You will immediately see how the swingarm pivot(s) dictate the direction of the axle path.
Let’s start by examining the simplest form of rear suspension movement that is found on “single pivot” designs. For me the easiest bikes to reference here that we should all easily recall is the Morewood Shova and Zula. Both hugely successful bikes at a time, and both making use of a “single pivot” design. On these bikes its clear to see that the swingarm is fixed to the front triangle by a single pivot (hence the name). This is the point about which the swingarm rotates, and therefore the centre of the arc of the axle path is this exact point. Is that it? Well… yes, until we get a little more stuck into the physics part later.
So there are many debates regarding the “ideal” or “optimal” axle path which ultimately as we mentioned is determined by the pivot(s). In an oversimplified ideal world, the rear wheel (much like the front) should move in a direction concurrent to the directional force acting on the wheel by terrain obstacles. The simplest way to understand this is to draw a wheel with an object in front of it, and then draw a line from the top of the object to the centre of the wheel. This is essentially the direction of the force of the impact
You will see that as you adjust the size of the object, the angle of this force line will change – the larger the object the shallower the slope. So what does this mean? Well, this is the direction the wheel should move away from the bump / object in order for the bicycle to maintain optimum forward momentum. It’s important to note that this angle / direction changes based on the size ratio of wheel to object. The larger the object vs the wheel the more shallow the angle, and the more rearward wheel movement required in order to achieve this “optimum forward momentum” that I refer to. Let’s pause for a second for that to sink in, so:
- This is why there is a proven performance advantage in the ability of a bike with a larger wheel to maintain its forward momentum while rolling over an object relative to that of a bike with a smaller wheel rolling over the same object.
- This is why the geometry of a downhill bike has a fork head angle that is much shallower / slacker than that of an XCO or marathon bike – the downhill bike / rider will encounter larger objects during a ride than those encountered on a XC or marathon bike / course.
Arguably, if mountain bikes just went downhill, and there was no need to pedal them, then a high pivot system would be a pretty good solution. Unfortunately for us we must get these bicycles up the hills first before we can enjoy the downhills, and one of the other aspects that must be considered is not only forward momentum and coasting efficiency, but also the efficiency of the power transfer between force applied to the pedals by the rider and its transfer through the drive-chain into the acceleration generated at the rear driving wheel. So… welcome to our first reason for compromise.
Looking at the image above, and remembering the optimal wheel path for maintaining forward momentum while coasting, one may wonder why we don’t see these out winning world cup downhill or marathon events in 2018. We can tell you, it’s not because it’s a 26” wheel. It doesn’t take a genius to see that as the swingarm moves through its travel the distance between the rear cassette and front chainrings increases. What this means is that the chain would essentially be “stretched” between these two points, and when under tension from acceleration this tension would essentially be tugging the two points closer together, impending the free movement of the swingarm. So what does this mean? Well it means that in order for the rear suspension to be able to absorb a bump, it’s force would need to be larger than that of the opposing force that is extending the suspension as a result of the chain tension.
Moving to the opposite end of the scale for demonstration purposes, a low pivot will have minimal effect on “chain growth” and its limitation of suspension movement. The problem here is that a low pivot point produces an axle path that is not conducive to absorption when a terrain obstacle is encountered as the axle path is not in-line with the resultant force direction.
But wait, there’s more! Chain growth as a result of a rearward axle path results in what is commonly referred to as “pedal kickback”. This is the rearward rotation of the cranks due to the extension of the chain that is under tension. On a slopestyle bike where riders are taking huge jumps and drops this “kickback” would result in a hugely disturbing sensation when the suspension compresses during impact. Given these two points, the concentric BB / pivot design used in the image above is probably the best possible design for this style of riding as it offers zero pedal kickback (no awkward landings), zero chain growth (ability to run single speed), and a vertical and forward axle path (vertical and rearward movement not required impacts from landing jumps / drops).
With this critical aspect having an effect on suspension, it’s possibly why versions of either extremely high or low pivot bikes haven’t taken prominence despite several attempts at each being taken by brands over the years. Most mainstream consumers purchased bikes in the market today have a pivot location (whether fixed or migrating) that is located somewhere between the high and low extremes, varied based on the objective that is highlighted to be most important by the engineer who designs the bike. This ‘common’ pivot location creates a typical axle path movement as follows:
- Initially backwards
- Slightly forwards at end of stroke
This common axle path offers the best compromise in its juggling the combination of bump compliance (backwards) with beneficial / detrimental drive-chain influence, and also maintaining a stable geometry (both chain force and chain growth). The position of the swingarm pivot, especially the height is critical in this aspect – if you will excuse the pun… “everything literally hangs off this.”
If the pivot is too low (Dobermann image) the wheel will have an axle path not conducive to bump absorption (from a trail obstacle), too high and chain growth forces become too much of an influence (Cannondale image). Typically, for a single pivot the pivot placement is about 55mm in front of the BB centre and 70mm above.
So why have a virtual pivot bike and what is an instant centre?
Good question, we nearly got distracted on axle path before we covered these other two “mainstream” suspension designs. We hope you’ve got your thinking caps on for this one? So, four-bar and VPP/SDL designs are actually very similar because the rear wheel is essentially attached to one side of a quadrilateral with the frame on the other. Good examples of these types of designs are:
- Specialized Stumpjumper (Four-bar)
- Santa Cruz Blur (VPP)
- Giant Anthem (SDL)
Depending on the lengths of the quadrilateral, the axle path can be modified. For a four-bar design the rotating lengths of the quadrilateral are relatively long, whereas a VPP/SDL, the rotating lengths are relatively short. Both four-bar and VPP/SDL systems have an ‘instant centre’ and the difference between these systems is how much and how rapidly the instant centre moves or migrates through the suspension travel.
To understand what an instant centre is, let’s take a step back to the single pivot where the swingarm pivots about a single point. The axle path is very simple to understand in this arrangement as it’s a curve with this point as its centre. For a four-bar, VPP or SDL this centre point changes and is not consistent through the rear wheel travel, hence the name “instant centre” as it’s only the centre point for an instant / moment in time.
How do we find the instant centre?
If you refer to the three examples given above, the Specialized (four-bar), Santa Cruz (VPP) and Giant (SDL), you can determine the instant centre by extending imaginary lines. To determine the location of the instant centre at any given point in the travel of a ‘quadrilateral’ frame design, you do so by intersecting the rotating lengths of the quadrilateral. If you refer to the image of the Specialized with its four-bar design look at the green upper rotating link 1 and the green lower rotating link 2, if you extent imaginary lines from the links of the green rotating links. Where these imaginary lines intersect would be the instant centre at that particular point in the travel. Refer to the .gif images below to get an example of the instant centre (IC) and see how it changes on each design.
FSR: Instant centre begins far ahead of the front wheel (this will give a very gradual, near vertical curve in the axle path). As the suspension is compressed the instant centre migrates down slightly and backwards rapidly (closer to the rear axle). This means that the bump swallowing effectiveness is reduced (lower pivot point) and the chain growth is reduced (tighter curve in wheel path reduces chain growth). It’s clear to see there are benefits to support this suspension design as it offers more or less what you want when its important.
VPP: Instant centre begins a short distance in front of the chainring at a medium height. As the suspension is compressed towards the sag point the instant centre moves up and forward, and continues its forward migration as you progress further into the travel and drops slightly. This means that the initial movement of the wheel is up and rearward, and at the sag point you have the strongest force pulling the suspension into extension which will help improve pedalling efficiency. Towards last two thirds of rear wheel travel as the instant centre drops slightly and continues to move forward the chain growth decreases resulting in less pedal kick back and better compliance for big hits. Again all of these factors are positive.
SDL: In a very similar albeit more compact manner to the FSR, the instant centre begins at the farthest point from the rear wheel axle and moves down and towards the rear axle as the suspension is compressed. Again, the benefits here are the initial high instant centre provides for a rearward axle path for the first part of the suspension travel, and as one moves further into the travel the rate of chain growth is decreased as the instant centre moves down and closer to the rear axle.
What's the difference between Four Bar, VPP and an SDL?
Well, they are all quadrilateral designs as we have seen. The movement path of the instant centre on four-bar and SDL designs are very similar with SDL designs simply having a shorter, tighter migration curve (down and back) due to the shorter links. VPP bikes with their short opposite rotating links have the opposite migration curve starting lower and closer to the rear axle, moving up first, reaching the highest point in the “sag” zone, and then moving down and forward in the last part of the suspension travel. So what does this mean out on the trails, and how does this impact your ride? Well… you’ll have to check back for more in the next episode