Welcome to our 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.
If you missed PART 2 where we discussed the purpose of rear suspension then check it out HERE. If that wet your appetite and you’ve come back for more, then pop the kettle on and strap yourself in.
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 lets 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.
AXEL 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.
Tangent warning! For those of you who are super geeks, you will now better understand the importance of having a low “un-sprung mass”. Why? Well if we think about the most basics of physics, in order for the bump to have any influence on the wheel(s), the force of the bump impacting the wheel (pushing up) must be greater than the force of gravity acting on the wheels (pulling down). So the heavier the wheel, the greater the force required to move it (…and the more damping required to control the movement).
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.
Above: The swingarm and axle path of a single pivot Orange 5. (sorry we couldn’t find a Morewood image)
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 over simplified 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.
It’s safe to say that the most efficient “coasting” suspension is one which has an axle path most similar to the slope of the resultant force – in almost all cases this is a rearward axle path. This rearward axle path is achieved using a high pivot point, i.e where the location of the pivot point is forward of the wheel, and higher than that of the horizontal line linking the front and rear wheel axles. So why aren’t we all riding these? Well because we’re forgetting about a few things here, most important of which is the chain, and we will tackle this a bit later.
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.
Above: A Dobermann dirt jump bike with a pivot at / concentric to the BB
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.
Tangent warning! The pic above shows a dirt jump / slopestyle bike that would be used to take huge jumps and drops, if this “low pivot” design isn’t optimal for bump absorption then why would they use it? Well, remember when we talk about the optimal wheel path as being up and rearward this is to absorb the impact of an obstacle in the trail, not to absorb the impact of a drop or jump. Hence, it’s not only the size of the impact to be absorbed, but also the type of impact that is to be absorbed. 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 extreme high or low pivot bikes haven’t taken prominence despite several attempts at each being taken by brands over the years. Most mainstream consumer 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
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?
- Specialized Stumpjumper (Four-bar)
- Santa Cruz Blur (VPP)
- Giant Anthem (SDL)
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.
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 ;)
OH, ONE LAST THING ... WIN WITH FAUX BAR!
What on earth is faux bar?! Well, herein lies the challenge ... once you've researched what faux bar is, we'll give a Santa Cruz jumper to the person who can list the highest number of faux bar bike models (any incorrect model will automatically disqualify you) -- ENTER by clicking on the red button below and commenting with your list of Faux Bar bike models on the specific facebook post on our page.
Images by: Pete Fogden, Desmond Louw and Troy Davies