Suspension frame design

specialized FSR

Frame design throughout the years has evolved massively to allow us to take off road riding to its limits and beyond. From the downright wacky to inspired race machines, suspension designers have always been chasing the elusive “perfect bike”.
But what does the perfect bike do? This is a very difficult question to answer. With every design there are limitations and at some point you will have to make a compromise.



For instance a bike with 8 inches of travel is never going to win the tour de France and Steve Peat is never going to win another world champs riding the latest Pinnerello. But somewhere in the middle these two extremes in design meet and demand a bike that handles well but also peddles well. And this “middle ground” has been the target for many suspension designers in the last few years.
The 6 inch travel market has exploded and brought with it a host of technology advances giving us light bikes that pedal well and handle well when the going gets tough.
The first thing we need to look at is the types of suspension linkage used on bikes today



The single pivot linkage compromises of a single pivot point at which the wheel axle rotates. This very simply design requires a large pivot and swing arm assembly that gives us a very stable and strong frame that requires little maintenance.
The most famous example of a single pivot design would be the Orange 222 or Santacruz heckler.























SPLD systems can be used to fine tune the suspension system to give various different feels. It does this by taking a single pivot system and adding a linkage to push the shock. This means that the designers have the opportunity to control how much of the shock is compressed at any given point in the travel of the system.
It also means that you can do away with the bulky swing arm and add more pivots giving you more freedom of design.





















The "FAUX BAR" linkage is basically an SPLD system, with the wheel attached to the swing arm and a linkage pushing the shock.

The wheel is still attached to the swing arm that is connected to the main pivot giving the system superior stiffness and responsiveness. The shock is then compressed using a series of linkages.




The true "Four bar" system has been around for many years and has been used in engineering to provide leverage to many applications. One of the most common uses is on racing car suspension systems. But we can also see this system being used on mountain bikes.

The most common system is the Specialised FSR system. The designers at specialised found out after a lot of R&D that the path that the rear wheel axles takes as it compresses through the suspension travel is incredibly important as it determines how the system reacts to inputs from the ground.
The problem with a single pivot system is that the wheel rotates around a fixed point limiting its wheel path arc capabilities.






















Specialised found that if you mount the wheel on the seat stay of the bike and add a pivot in front of the wheel, you can tune the arc path that the wheel takes ultimately changing the way the system reacts to inputs from the ground.

This system also had some advantages in minimizing an effect called “brake jack”. this is an issue that affects single pivot system the most and it occurs when the brake is applied at the back wheel locking the suspension system. For a suspension system to work it needs to rotate around the wheel axle, but if the wheel is being held still by the brake it simply builds tension between the wheel and the main pivot and starts a tug of war along the swing arm locking the system out.
Single pivot designs get around this by fitting floating calipers that allow the swing arm to rotate around the wheel axle when the brake is applied. Trek use their ABP system to get around this.

The Specialised FSR system worked so well that specialised patented the system. The systems worked so well that other manufacturers started asking to use their design and would pay royalties to Specialised just to get this design on their frame.
The best examples of this are, YETI DHI frames, Intense M1 frames, Azonic recoil frames and many many others.



The dual link system has been home to many marketing spiels and claims. None less than Sanatcruz who claimed their VPP (variable pivot point) system has an S shaped wheel path. Something that baffled the rest of the industry and an idea they later withdrew.

A dual link system uses a parallelogram swing arm design. This design compromises of a rear triangle supported by two swing links. One at the top and one at the bottom. This gives the designers even more freedom to determine the axle path and the rate at which the shock is compressed.

Here is probably the most famous dual link system. The Santacruz VPP

There are many other designs that manufacturers claim to be the best and unlike anything else on the market.





























single pivot suspension system
specialized FSR
single pivot suspension system
VPP system
VPP system

Axle path

​Simply described as the path drawn by the wheel axle as the suspension system is compressed.



















The perfect path for absorbing a bump is one that moves backwards and then up raising the wheel up and over the obstacle.
However to achieve this means you have to have a system where the distance between your main pivot and your wheel increase at some point. This can be termed as “growth”. This growth can cause some problems in terms of the way the bike handles and pedals.

The bigger the growth in the system the more the drive train has to react to counteract the rotating forces of the suspension and the virtual increase in chainstay length. This manifests’ itself in an affect called “pedal kick back”. Pedal kickback is where the cranks rotate against the direction the wheel travel rotation and actually move a few degrees. This increases during travel and affects bikes with high growth more than those with smaller more horizontal axle’s paths.

You can see this effect by watching the rear mech movement when compressing the suspension.

Pedal kick back can be reduced by using a bigger chain ring and manufacturers will optimise their designs around a certain chaining size. For example the Commencal DH frames have been optimised to use a either a 36 or 38 tooth chain ring.















So a compromise must be used to optimise the frame for its intended use. If you design the suspension to be very efficient at soaking up bumps, chances are its not going to pedal well and suffer from pedal kickback. But if you design the bike to minimise pedal kick back you suffer bump response.

vpp axle path

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