If you want a really, really simple rear suspension, look no further than the Veli, a new mountain bike showcasing Vasttech’s Suspension System. Using two oversized pivots about halfway back on the chainstays, they deliver a short arc of rear wheel travel without the complexity of concentric pivot-and-bottom-bracket designs.
They say the design provides optimal chain growth during the part of travel where you’re likely pedaling, then minimizes it deeper in the travel when you’re likely descending rough terrain and don’t want any pedal kickback.
OK, but what’s “optimal chain growth”? It’s basically anti-squat, and when the bike is sagged and sitting in 20-35% of its travel, they say it has 38.7% more chain growth and related anti-squat than a comparable linkage driven single-pivot design.
In other words, it means that your pedaling forces are pulling the chain, which is trying to pull the rear axle downward into the ground. This would maximize traction, but (my assessment) also very likely to reduce small bump compliance. So, traction first in the first third of travel.
After that, they say chain growth is minimal or non-existant.
Another benefit is the angle of the axle path when it hits a bump. Because the axle sits below the main pivot, it first moves up and back, closer to the direction of impact as you roll forward. They say this is far more efficient because it’s not trying to absorb a horizontal impact in a purely vertical fashion.
The other claimed benefit is reduced unsprung mass. The more “stuff” there is between the axle and the shock, the more inertia there is before things get moving, slowing down a suspension’s response to inputs. Reducing that mass lets it react quicker, improving small bump compliance in particular.
While the bike is new, it’s worth noting that the suspension demo videos on Vasttech’s website were uploaded four years ago. Geo chart above, and other pertinent specs are 116mm rear travel with 120mm forks, external routing for everything, and low leverage ratio (.2:1) long stroke shocks for low pressure performance.
More tech specs at Veli.bike and Vasstech.design.