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It should all be so simple. Suspension technology already exists on motorcycles and cars, and it should translate to bikes, right? Wrong. A mountain bike is a totally different beast that requires a unique approach to counter forces no other vehicle has to contend with.
A mountain bike requires its suspension to act differently on descents than it does on climbs and rolling terrain. Your body’s movement — particularly the action of pedaling — constantly activates suspension. Additional forces, such as braking and drivetrain engagement, also affect the suspension’s movement, sometimes simultaneously. Those movements can ruin your pedaling efficiency, particularly on climbs.
“With mountain bikes, we are talking about vehicles that weigh very little, and will deflect much easier [than a car or motorcycle],” says Jose Gonzalez, general manager of the suspension R&D lab at Trek Bicycles. “You can’t introduce that immediate power and acceleration to correct it.”
So engineers attempt to eliminate unwanted ride characteristics like brake jack, braking forces that prevent the suspension from activating; anti-squat, when the chain resists the suspension’s compression; and pedal bob, when the motion of the rider’s pedal stroke activates the suspension. Various manufacturers have different suspension designs, such as Horst link, single pivot, Virtual Pivot Point (VPP), and many more. They are all different paths to the same destination: efficiency.
The difficulty lies in balancing pedaling efficiency with plush, well-damped movement through the shock’s travel.
“Bicycle frame shocks waste energy through excessive damping,” says Mountain Bike Hall of Famer and Shimano product developer Joe Murray. “A full-suspension bike has to absorb bumps. A downhill bike can do this no problem. Yet getting it to climb efficiently? No other vehicle has to deal with pedal-induced forces. The human body only generates at most one-quarter horsepower, so it needs to lose as little energy as possible.”
The shock’s damping is necessary to capitalize on a suspension system’s main function: smooth travel.
If packing both smooth travel and pedaling efficiency into a small, light shock sounds like an impossible task, that’s because it nearly is. Because the bike is subjected to those different forces, engineers can’t simply take a motorcycle shock, make it smaller, and bolt it to a mountain bike.
“You can have two [bikes with] identical geometries statically that will feel very different dynamically, solely based on rear suspension kinematics and shock function,” Gonzalez says. “Getting kinematics right for the specific application is the difference between having an average bike and having a great bike.”
Kinematics? That’s a fancy term that refers to the way each component of a bike’s rear suspension impacts the system’s overall movement. If you’re Googling that now and are getting intimidated, don’t worry; you don’t need a physics degree to buy your next bike.
Assuming your shock is set up properly, you’ll probably notice almost all modern suspension systems work incredibly well. Still, they do have their differences. A single-pivot bike, for example, offers smoother travel. You’ll frequently see this design on downhill bikes that rarely climb because the chain actually lengthens as the shock activates.
DW Link and VPP-equipped bikes are based on a four-bar linkage design, but both systems use dual linkages to combat pedal bob. The idea is to reduce anti-squat, but DW Link differs slightly in where that reduction takes place in the travel. These systems rely heavily on a proper shock tune, and because there are a lot of moving parts, they can be heavy.
The bicycle is unique in that its “engine” is constantly and, at times, dramatically moving. That challenge has meant modern mountain bikes are packed with incredible feats of engineering that aim to make you forget about kinematics and allow you to simply enjoy the ride.