How your bike's suspension design affects its performance on the trail | ENDURO Mountainbike Magazine
In Part I of Underneath the Voodoo, we discussed the fundamental properties of bike suspension. In Part II, we apply this knowledge to understand popular suspension designs and examine how they affect the performance of our bikes on the trail.
All forms of suspension, from the simple Single-pivot used by Orange bikes, Yeti’s innovative Switch Infinity or the wild R3ACT platform used by Marin and Polygon, share the same purpose – to allow the sprung mass (rider and most parts of the bike) to move forwards while keeping the wheel tracking over uneven ground. However, the way in which different designs achieve this is very different.
If you believed the marketing hype you would think there are a million different suspension systems currently being used. However, underneath the sexy acronyms, most bikes use one of just four designs. Single-pivot, Linkage-driven Single-pivot, Twin-link, or Horst-link. There are some rare exceptions to this rule, but these four designs cover most full suspension bikes on the market.
Suspension properties are dynamic not static
Before we dive into the four different suspension platforms, we must first accept that bike suspension is dynamic – the sum of many parts. Suspension properties are not only linked to each other, the geometry and the shock damper, but also to the position of the bike in its travel. Suspension design is a very complex topic, and there’s a huge variation in characteristics not only between suspension designs but also between bike models which share the same design.
Why is this? To get a better understanding we must learn a new term, the ‘instant centre’. When your suspension compresses (or extends), the rear wheel rotates around a ‘centre’ point. That’s easiest to visualise on a Single-pivot bike. The wheel is attached via a solid swingarm to the main pivot, and rotates around the main pivot i.e. the ‘centre’. However, when one or more linkages or rockers are added between the wheel axle and main pivot, the suspension is no longer be rotating around this single fixed point on the frame. The rear swingarm is effectively floating between the different linkage pivots. At any ‘instant’ the ‘centre’ is no longer fixed to one pivot point. Instead, it can be thought of as a virtual point which changes position as the bike moves through its travel. This also means that the linkage can be designed for the ‘instant centre’ to be located almost anywhere, even far outside the confines of the bike.
As the location of the instant centre is directly related to other suspension characteristics, it should be clear that suspension properties are dynamic and can change as the bike moves through its travel. Once we grasp this concept, it’s easier to understand how the addition of additional pivots and linkages can be used to tune suspension performance at different stages of travel.
The most simple suspension design is the Single-pivot, using a solid swingarm to connect the rear axle, main pivot and shock damper.
Popular example: Orange Stage
Single-pivot suspension design is characterised by a rear axle connected directly to the main pivot. On the simplest Single-pivot designs the shock damper is attached directly to the swingarm. The swingarm rotates around an instant centre fixed on the main pivot and the leverage ratio is determined by the position of the shock mount. It can be linear, progressive, digressive or indeed a mixture of these. The fixed instant centre gives predictable suspension properties throughout the travel, but also means designers are limited in their ability to modify suspension characteristics at different stages of travel.
- Single-pivot suspension requires fewer bearings and needs less maintenance
- A simple design means that suspension characteristics are consistent, with smooth curves leading to predictability
- The fixed instant centre leads to high relationships between suspension characteristics which can be undesirable at certain stages of travel
Linkage-driven Single-pivot Suspension
Like the Single-pivot design, the rear axle is connected to the main pivot with a solid swingarm, but in this case, a linkage is added between the swingarm and the shock allowing the leverage-ratio curve to be adjusted.
Popular examples: Cannondale Jekyll, Evils DELTA-System, COMMENCAL META, Kona Process
Linkage-driven Single-pivot designs feature a linkage between the swingarm and shock to change the leverage ratio. The swingarm rotates around the main pivot and the instant centre is fixed throughout the travel. There are many visually different designs on the market, but if there’s a solid chainstay between the rear axle and main pivot they are all variations of a Linkage-driven Single-pivot design. While adding the linkage allows the designer to make the suspension more or less progressive as it moves through its travel, it also increases the complexity of the system and does not guarantee improved performance.
- Linkage-driven Single-pivot suspension provides more freedom to fine-tune the leverage ratio to work in harmony with different shock-damper designs
- More linkages mean more pivots and bearings, increasing maintenance
- The fixed instant centre means tuning anti-squat and anti-rise is just as limited as on a Single-pivot
At first glance, bikes with Twin-link suspension can look a little like Single-pivot bikes with a solid swingarm. However, the solid swingarm is mounted to the frame with two rocker links, connecting the swingarm to a mainframe pivot. The shock can be driven by the rear swingarm or one of the rocker links.
Popular examples:: Giant’s Maestro suspension, Ibis’ DW-link Suspension, Santa Cruz’s VPP
When compared to a Single-pivot design, Twin-link suspension increases the number of pivots from one to four, allowing designers flexibility to modify the position of the instant centre, and thus the suspension characteristics at different points of the travel. There are many Twin-link suspension designs on the market, such as the popular VPP and DW-Link for example, but all feature an instant centre that moves significantly through the travel, changing the properties of the suspension. This allows anti-squat and anti-rise levels to be optimised throughout the travel. However, the large rotation of the short links can lead to some very rollercoaster like curves. The two rocker links can rotate in the same direction, or in opposite directions. In designs like the DW-link used by Ibis, the rocker links rotate in the same direction while brands like Santa Cruz use variations of the VPP system with counter-rotating links.
- Twin-link suspension provides designers with high tunability of all suspension characteristics
- Anti-squat and anti-rise are very consistent throughout the travel, designers argue over whether this is good or not
- It can be hard to design the twin-links, small links packaged into small spaces result in high forces at the pivots and less room for tunability
Made most popular by Specialized with their iconic FSR linkage, many brands now use designs based on a Horst-link suspension design. Horst-link bikes are characterised by a pivot at the axle end of the chainstay, with the rear axle mounted instead to the seat stay.
Popular examples: Canyon Spectral, YT CAPRA, Specialized’s FSR, RAAW Madonna
With the addition of a pivot on the chainstay, Horst-link bikes have a modified axle path rotating around an instant centre that changes position through the travel, allowing anti-squat and anti-rise to be optimised in different stages of the travel. When compared to the Twin-link suspension, the longer links of the Horst-link system generally give smoother curves for more predictable suspension performance. Many brands using the Horst-link design choose to design low levels of anti-rise, isolating the suspension from braking forces and providing a smooth ride under hard braking. With Horst-link suspension, anti-squat and pedal kickback are highly related, many manufacturers prioritise low levels of pedal kickback resulting in low anti-squat values and in turn, poor pedalling efficiency, while others prioritise high anti-squat and accept increased pedal kickback.
- The Horst-link design allows for a high level of tunability on the leverage ratio curve, axle-path, anti-squat and anti-rise characteristics, generally with more predictable curves than a Twin-Link design
- Anti-rise is generally lower than on Single-pivot or Twin-link designs. Designers argue over whether this is good or not
- Many Horst-link designs are designed with low levels of anti-squat to minimise pedal kickback, resulting in poor climbing performance
Exceptions to the rule
While these four designs cover most of the full-suspension bikes on the market, there are exceptions to the rule. Notably, the Marin/Polygon R3ACT system based on a MacPherson strut and Yeti’s Switch Infinity which replaces the lower twin-link with their rail mounted Switch Infinity pivot. However, the latest trend is the resurgence of high-pivot bikes which use chain idlers, allowing the axle-path to be oriented more to the rear for improved impact absorption. The very high instant centre position provides high anti-squat levels, while the chain idler can be used to move the chainline to minimise pedal kickback. The negatives of this system are large amounts of chainstay growth, so the rider needs to get used to the changing geometry as suspension compresses, but from a performance perspective, these new designs are very exciting.
So how does suspension design influence a bike’s performance?
If you’ve read Part I of Underneath the Voodoo, you will have an excellent understanding of the theory of suspension design. But how does that theory manifest itself on the trail? With so much variation between bike designs, putting it into practice is difficult. As examples, we have picked four popular bike models that use the four different suspension designs – the Single-pivot Orange Stage 6, Linkage-driven Single-pivot Evil The Wreckoning, Twin-link Santa Cruz Hightower LT and the Horst-link Specialized Stumpjumper 29. These bikes each highlight the common characteristics of their chosen suspension design.
How does suspension design influence impact absorption?
We learned in Part I that an important characteristic of impact absorption is the wheel rate, a sum of two ratios – the leverage ratio and the shock compression ratio. The leverage ratio is defined purely based on the mechanical construction of the suspension and can change throughout the stroke. Each of our example bikes has a very different leverage ratio curve, as a result of the suspension design used.
The Single-pivot Orange Stage 6 has the shock damper mounted directly to the swing arm, and the leverage ratio curve can be seen to be very linear throughout the travel, the swing arm applies the same force to the shock at all stages of travel. Adding a linkage to the Evil Wreckoning has allowed the engineers to design a more progressive leverage ratio curve with high initial leverage for small bump sensitivity, but requiring around 30% more force to compress the shock at the end of travel for control during big hits. Engineers using the Twin-link design on the Santa Cruz Hightower LT have taken a different approach, with an initially digressive curve for support, with a linear and predictable midstroke, turning progressive and ramping up firmly at the end for big hits. The smooth and predictable Horst-Link Specialized curve sits in the middle with increasing progression that levels out at 75% of the stroke for predictable suspension performance.
How does suspension design influence pedalling efficiency?
As we pedal and accelerate, the load moves more on to the rear wheel and, unchecked it will compress the suspension. To counter this force, designers can engineer the suspension to mechanically resist this rearward load transfer, resisting this force. Anti-squat is the term describing the amount the suspension mechanism resists compression from the load transfer rearwards. Anti-squat is measured in percentages. 0% anti-squat means there is no resistance to the rearwards load transfer and the rear suspension will compress. 100% anti-squat means that the force counteracts the load exactly, and so the rear suspension will neither extend or compress, more than 100% means the rear suspension will extend under acceleration.
Looking at our examples, the high pivot location of the Single-pivot design of the Orange Stage 6 results in very high levels of anti-squat, around 130% at SAG, giving the Orange its trademark ‘stiffen, lift and accelerate’ under power. However the fixed instant centre means this very high level of anti-squat, and therefore pedal kickback, remains throughout the travel. Moving the main pivot lower on the Evil The Wreckoning results in less anti-squat for a more balanced climbing response, but again the fixed instant centre gives a very linear curve. The moving instant centre of the Twin-link Santa Cruz Hightower LT has a dramatic effect on anti-squat, with high values around the SAG point for maximum pedal efficiency then falling off rapidly deeper into the travel to give less pedal kickback for a ‘chainless feel’ to the suspension. Like many Horst-link bikes, Specialized have prioritized low pedal kickback, resulting in the Stumpjumper having considerably lower levels of anti-squat (around 80% at SAG). This means that pedalling efficiency will be lower and the bike will tend to bob up and down unless platform damping is applied to the shock.
How does suspension design influence braking efficiency?
As we know from Part I, anti-rise is a measure of how much the suspension compresses or extends due to the force of braking. Like anti-squat, anti-rise is also measured in percentages. 0% anti-rise means that when you apply the back brake, the suspension does nothing to counteract the forward load transfer of bike and rider, and the suspension extends. 100% anti-rise means that under rear braking the suspension counteracts exactly all the forward weight transfer of bike and rider, and the suspension does not extend, more than 100% and the suspension will compress. The holy-grail of anti-rise under braking is to have enough to prevent suspension extension but not so much as to create excessive suspension compression.
Anti-rise is directly linked to the location of the suspension instant centre. The high Single-pivot location of the Orange Stage 6 results in the highest levels of anti-rise in our examples, over 100%, meaning the suspension counteracts the forward load transfer from braking, compressing and can feel like it is stiffening up over hard impacts while on the brakes. The lower main pivot position of Evil’s The Wreckoning results in lower levels of anti-rise, but the single-pivot based design still resists forward load transfer under braking. The Horst-link Specialized Stumpjumper has low levels of anti-rise throughout, showing the suspension is largely independent from braking forces, The Twin-link Santa Cruz Hightower LT has high levels of anti-rise initially, dropping to very low (<10% at full travel) and therefore has a suspension action that remains largely independent of braking forces. Some designers like to engineer in more anti-rise at the end of the stroke, as when braking hard, it resists suspension extension and maintains the bike’s geometry.
We have shown four very different bikes with four very different suspension designs, each with their own characteristics and behaviour on the trail. It’s clear that each different suspension design has its own advantages and limitations and while it’s not easy to consider individual characteristics in complete isolation, by changing the pivot locations or orientation they can all can be tuned to meet the designer’s performance priorities. This allows designs to be engineered to balance the different properties in the pursuit of improved performance.
Over these two parts, we hope you have enjoyed a deeper delve into suspension function and performance. Suspension design is an exercise in balance and compromise. There’s no ‘perfect’ solution for everyone, but there’s also no voodoo, just physics.
We would like to thank Ruben Torenbeek, founder of RAAW Bikes and Dan Roberts, Founder of Garage Bike Project, a bike engineering consultancy based out of Champery, Switzerland, for their expert guidance and consultation throughout the Suspension Voodoo series.
Text: Trevor Worsey Illustrations: Julian Lemme