From when rear suspension was first tried, the mountain bike industry has been fighting the problem of pedaling energy being lost into the rear suspension. Known as squat, many systems (each unique in their own way) were invented to counter this by providing anti-squat. These systems have been in use for the last 20 to 30 years and have rightly been celebrated for the improvement they provide to the riders experience.
But too much of a good thing is still too much. High level's of anti squat are provided by these systems where it is needed, but for most, also where it is not needed. Riders have generally accommodated this and become used to it. As a result the benefits of SCD may not be automatically appreciated.
As a very simple way of thinking about it is to consider this - is peak anti squat really needed while pedaling over a bump and when the suspension system is significantly extended above the sag point? What benefit does this provide to the rider? And what difficulty is this creating that the rider has to compensate for?
Testing of an SCD system in comparison with others has shown that high levels of anti squat above the sag point results in unneeded suspension extension when pedaling.
But what about when pedaling over bumps? Testing has also shown that high levels of anti squat above the sag point can actually be unhelpful over bumps. In these situations high levels of anti squat above the sag point force the suspension to extend and lift off the bump instead of absorbing it.
THIS SUSPENSION EXTENSION WASTES ENERGY AND MAKES IT HARDER TO CLEAR OBSTACLES
Anti squat (or rather the provision of it by mountain bike suspension systems) is such a great invention that riders everywhere have come to assume that the rear end standing up a bit when pedaling is the ideal.
However, this is not necessarily the case.
We all know that the suspension compressing due to pedaling forces saps energy from the rider and directs it into the rear shock.
But it is just the same if pedaling forces cause lifting or extension of the rear suspension system (and riders mass) as this also directs energy into the shock and away from the wheels. Excessive anti squat at any given moment can waste energy by lifting the riders mass.
An SCD system provides a different outcome. When riding, VAST feels a lot like a hard tail - not because it feels so firm at the rear end that is actually lifts, but because it is naturally stable around the sag point.
The system neither bobbles upwards or downwards due to pedaling forces. It happily centers itself around the sag point.
Importantly it provides this same outcome during movement of the system above and below the sag point as the rider navigates bumps and obstacles - as chain forces collaborate with the suspension dynamics to settle back to the sag point.
Energy is saved, and the system is more stable and balanced.
When they are big they can be one of the hardest obstacles and take a lot of energy from the rider - getting up over roots and ledges during a climb. Especially in a tight pinch or switch back.
With other systems, as the rear wheel reaches the obstacle, the rider increases chain tension to power over the obstacle. At the same time this high pedaling force generates high anti squat forces which act to extend the suspension "wedging" the rear wheel between the obstacle and the riders mass. It will typically sap forward momentum and can even stall the bike.
(This outcome can easily be experienced - just ride up to a root or ledge and stop with your rear wheel resting onto it. Then try to pedal over the obstacle. If you are paying attention, on most bikes, you will feel the suspension system rise up across the obstacle when you actually need it to compress to let you over the obstacle.)
With an SCD system, the rear suspension is only lightly extended by chain forces when the system is operating around the sag point, so getting over the obstacle is easier. The rider does not experience the rear wheel catching onto the root or ledge and jamming it there.
The scenario above is made worse the steeper the track is or the higher the root or ledge. In these situations the rider often tries to lift their mass up over the root. In the process, the rear suspension of other bikes can extend into a position where it generates even higher levels of anti squat which can "wedge" the rear wheel even harder into the obstacle. In comparison, above the sag point the SCD system has very lower anti squat so it clears these obstacles with ease.
The Dynamic Performance of the Suspension System is improved in a number of ways. Generally these relate to keeping the riders mass low and improving traction as follows: