Shock Leverage

__Ratio__The Shock leverage ratio is the relationship between the vertical distance the rear wheel travels, divided by the distance the shock compresses. It has a large bearing on performance. This is a simple formula. Its the total vertical travel of the rear wheel, divided by the total travel of the shock that is fitted to the bike. In telescopic suspension forks this is a whole lot simpler where a 100mm travel fork actually slides 100mm. With rear suspension it is more complicated and it is not uncommon to find a bike with rear wheel travel of 250mm, using a shock with 70mm travel. The resultant shock leverage ratio is thus 3.57:1 In other words, for ever 3.57mm the rear wheel moves, the shock correspondingly moves just 1mm.

To understand whether a high or low shock leverage ratio is best, we shall exaggerate the problem. Imagine a bike with 100mm of travel and an air shock that moves just 10mm. Its a 10:1 shock leverage ratio (very high). A second bike has 100mm of wheel travel and a shock that moves 50mm. It thus has a 2:1 shock leverage ratio – relatively low and five times less than our first bike. The air pressure in the first bikes shock would have to be five times higher to achieve the same result. It would also be far less sensitive to small bumps. Small amounts of stiction in the shocks seals would make a large impact on performance and its damping would be less controlled. It would however have a lighter air shock as the shock would be smaller but with 5 times more air pressure inside, the seals will likely blow out faster and the shock will have a much reduced life span.

Low shock leverages generally perform better. In the current suspension climate a very low shock leverage ratio is 2:1, while a very high one is 3.6:1. The majority of designs fall within this number. The leverage ratios of three top selling bikes are shown below:

Giant Anthem

Trek Fuel EX 29

Canyon Nerve 29 AL

To understand whether a high or low shock leverage ratio is best, we shall exaggerate the problem. Imagine a bike with 100mm of travel and an air shock that moves just 10mm. Its a 10:1 shock leverage ratio (very high). A second bike has 100mm of wheel travel and a shock that moves 50mm. It thus has a 2:1 shock leverage ratio – relatively low and five times less than our first bike. The air pressure in the first bikes shock would have to be five times higher to achieve the same result. It would also be far less sensitive to small bumps. Small amounts of stiction in the shocks seals would make a large impact on performance and its damping would be less controlled. It would however have a lighter air shock as the shock would be smaller but with 5 times more air pressure inside, the seals will likely blow out faster and the shock will have a much reduced life span.

Low shock leverages generally perform better. In the current suspension climate a very low shock leverage ratio is 2:1, while a very high one is 3.6:1. The majority of designs fall within this number. The leverage ratios of three top selling bikes are shown below:

Giant Anthem

__2.67__:1Trek Fuel EX 29

__2.52__:1Canyon Nerve 29 AL

__2.5__:1Shock Leverage

__Curve__The second thing to look at is the bikes shock leverage curve. Is it a rising rate or a falling rate design? Or does it rise and then fall? Consistency is what is important here. If the leverage ratio were gently rising rate or gently falling rate throughout, a shock can be custom tuned to suit it. Perfectly linear is better still. Any major spikes or dips in the curve however will be detrimental to performance and a very steep rising or falling rate will not work well. A graph showing the shock leverage curve of three bicycles is shown below.

The Trek Fuel EX has a shock leverage ratio of 2.52:1 and a curve that is falling and fairly consistent. There is a 9% difference between the extremes of its shock leverage ratio.

The Canyon Nerve looks fairly similar and also has a falling rate curve with about a 9% difference between its highest and lowest leverage rates.

With the Rumblefish its a bit more complicated to look at as it starts its travel as rising rate and then turns to a rapidly falling curve. But like the other two, it also has approximately 9% of variation in its curve.

Since we have already learned that lower shock leverage ratios are better, the question can be asked as to why these bikes don't use longer travel shocks to achieve a lower shock leverage ratio? Regarding this there are many factors a suspension bike designer has to consider including frame space, but most important is the relationship between the curve the shocks upper eyelet moves on in relation to the shocks lower eyelet. The greater that angle of change throughout the shocks movement the less linear the bikes shock leverage curve will be.

The Canyon Nerve looks fairly similar and also has a falling rate curve with about a 9% difference between its highest and lowest leverage rates.

With the Rumblefish its a bit more complicated to look at as it starts its travel as rising rate and then turns to a rapidly falling curve. But like the other two, it also has approximately 9% of variation in its curve.

Since we have already learned that lower shock leverage ratios are better, the question can be asked as to why these bikes don't use longer travel shocks to achieve a lower shock leverage ratio? Regarding this there are many factors a suspension bike designer has to consider including frame space, but most important is the relationship between the curve the shocks upper eyelet moves on in relation to the shocks lower eyelet. The greater that angle of change throughout the shocks movement the less linear the bikes shock leverage curve will be.

VAST Link

__Optimum shock leverage ratio:__

The VAST suspension system is advantageous in that it can provide a very low shock leverage ratio combined with a more linear shock leverage curve throughout suspension travel. The VAST suspension equipped bike pictured features 118mm vertical wheel travel and a 57mm travel shock. The shock leverage ratio is therefore

__2.07__:1 (very low). This is a full 22.5% lower then that of GIANTS premier race bike, the Anthem 29. The rate of change of the shock leverage curve undergoes throughout its suspension travel is determined by the path prescribed by upper pivot point on the VAST link relative to the fixed position of the upper eyelet of the shock at various points of the suspension travel. For example, with the pictured bike, as the rear wheel axle begins to move upward, the upper pivot point moves forward and upward towards the shocks upper eyelet (but not directly towards it). As the upper pivot point moves further into its travel, its path curves more directly toward the shocks upper eyelet until at bottom out the upper pivot point is on a trajectory directly towards the shocks upper eyelet. The shock leverage curve is therefore slightly progressive throughout suspension travel before leveling out to be linear at bottom out.

On the prototype VAST link bike the variation in the shock leverage curve is less than 8% and it can be made less than this also. This ideal shock leverage ratio and curve can be achieved without the need for a rocker links (and weight and complexity associated with it) found in most contemporary suspension systems.

__Easily Tunable shock leverage ratio:__

The shock leverage ratio may be easily tuned by adjusting the position of the upper pivot point. For example, if the upper pivot point is moved forward closer to the seat tube (while the distance between the upper pivot point and the main pivot point remain constant) then the path prescribed by the upper pivot point would begin with an upward and forward trajectory at the beginning of suspension travel, transition in a direction more directly towards the shocks upper eyelet before finally ending on a downward and forward trajectory away from the shocks upper eyelet. The shock leverage ratio curve would therefore be slightly progressive to begin with before flattening out to linear and then falling slightly toward bottom out. This may be advantageous with certain shock configurations. The overall shock leverage ratio is tuned by simply shifting the upper pivots points location closer or further from the main pivot point. Designing a bike with dual travel options is likewise simple with very little difference in shock leverage ratio curve to be felt between the different travel options.

The benefits to be felt on the trail are substantial. The VAST link can provide a very good shock leverage curve while not sacrificing the benefits of a low shock leverage ratio and all the extra suppleness it provides.