Full Suspension Mountain Bike Frame Design
There are four main suspension systems that are used today on full suspension bike frames. I will explain a little about each of these systems below.
Single Pivot – This is by far the simplest of all mountain bike rear suspension systems. The rear triangle of the bike is attached to the frame by means of a single pivot which means that the axle will move in a circular motion around the pivot point. The shock can be attached to the swing arm directly which allows for a fairly predictable shock action as the leverage rate is the same for the entire stroke of the shock. The main attraction of a single pivot frame is the simplicity of the design. There are less pivot bearings to maintain and dealing with any squeaking is made easier by the lack of options to examine. These frames used to suffer from quite a lot of feedback from pedalling input but this is not so much of an issue any longer with the improvements in shock design that have allowed the creation of a pedalling platform. A pedalling platform on a full suspension bike essentially locks out the shock from the input of your body pedalling the bike whilst still allowing the shock to react to bumps and jumps. A good example of this system is the Orange Five which is a classic UK trail bike.
Single pivot frames also suffer from brake jack which is when the braking forces of the rear wheel competes with the suspension and stops the system working. This can be quite a pain when you are riding downhill through something super rough and using your brakes.
Single Pivot Linkage Driven (SPLD) – The second type of common linkage found on full suspension mountain bikes is the Single Pivot Linkage Driven system. There are several variations on the single pivot theme which aim to reduce pedal bob and brake jack. This system uses a linkage or linkages to attach the single pivoted swing arm to the shock. This allows suspension designers to tune the leverage ratio that is applied by the rear wheel to the shock during the movement of the bike’s rear wheel. In this system the linkage is only used to actuate the shock, it has no relevance to the path that the axle takes. The axle path remains circular as per the simpler single pivot system.
The four bar system was the next step in suspension design and shares some similarities with the independent suspension designs found on cars. The four bar linkage allows for a huge range of tuning of the shock leverage ratio. This means that designers have the flexibility to design bikes that deal with certain trail conditions better than others, therefore tuning their bikes to be best at one specific thing. The main thing that changes amongst four bar systems is the placement of the pivot near the bike’s rear axle. The placement of this pivot is what determines whether the bike has a split pivot, single pivot or Horst Link.
Many bicycle manufacturers patented their take on the four bar linkage but it’s fair to say that the variations are small. A good example of a four bar system is the Nukeproof Mega AM.
Horst Link – The third type of suspension platform is the Horst Link. The Horst link deals with the circular wheel path that inhibits the effectiveness of the single pivot system by allowing the bike’s wheel to take a path that isn’t a simple circle. Ideally you want your wheel to move backwards and up out of the way of big hits or baby head type rocks. A single pivot won’t allow your wheel to do that as well as a Horst link can. Horst links can also benefit from increased stiffness when compared to other linkages. The Horst link system is active under breaking forces and can be an extremely plush suspension platform when handling big and small hits, depending on the shock set up and the linkage placements.
A good example of a Horst Link bikes are Specialized FSR or Norco Bikes.
Dual Link – The final suspension linkage system is the Dual Link. This system includes the DW Link (designed by Dave Weagle) and Santa Cruz’s VPP (Virtual Pivot Point) system. Both of these suspension systems have an upper and lower link that is attached to a parallelogram shaped rear ‘triangle’. The benefit of this system is that it allows a designer a lot more flexibility to choose the axle travel path of the bike’s rear wheel. These bikes will pedal well uphill and are active under braking forces. They will tend to have a falling rate compression rate which means that the compression rate speeds up through the stroke. This isn’t necessarily a problem with air shocks which are non linear springs as the two effects will cancel each other out giving a fairly linear stroke overall.
A good example of a dual link frame would be the Santa Cruz Nomad.