Glossary of Terms
Let’s take a closer look at a few of the factors affecting the way mountain bike suspension performs.
Sag: The percentage of suspension travel compressed by the rider’s weight in a static riding position. Normal rear suspension sag recommendations are 25 to 33 percent, depending on design.
Squat: The force generated when the rider pedals forward must overcome the inertia of the mass of the rider and bike. Invariably, some of this force is displaced through compression of the suspension, essentially resulting in a loss of forward motion, and as most understand it, efficiency. The drive forces pull up on the swingarm, pulling the rider down. This manifests as what is more commonly known as suspension “bob,” or seemingly “over squishy” suspension while pedaling. Squat is common with low single pivot bikes.
Anti-Squat: When certain variables are in place (we’ll get to those in a minute), the increased chain tension resulting from pedaling forces pulls down on the swingarm, extending the suspension, combating the aforementioned “squat.” It’s easy to see why this is generally considered a good thing when it comes to pedaling efficiency. However, too much anti-squat (we’ll say more than 100 percent) can not only make a suspension overly firm, but actually waste power by lifting the rider up rather than propelling him/her forward. The rider is pressing down on the bike and the bike with 100% anti squat will push back the exact same amount, canceling out any motion. Anti-Squat is common with a high single pivot bikes.
Rise: Commonly referred to as “brake jack,” rise occurs when rear braking forces are at odds with the riders forward momentum and the result is stiffening and extension of the rear suspension. This results in two negatives: The rear wheel loses traction and skips over the ground, and with the rise of the suspension, the rider’s weight is transferred forward and over the front wheel while steepening the geometry. Rise is common with high single pivot bikes.
Anti-Rise: Also known as “brake squat,” anti-rise occurs when the rear suspension compresses or “squats” into its travel while braking. This is more favorable than rise because it slackens and lowers the geometry and keeps the rider’s weight back which is more stable when going downhill, however too much anti-rise can result in a less active suspension and loss of traction. Anti Rise is how much the bike pushes back (like statement above) however on the back of the bike during braking you want the bike to squat slightly to counter the riders mass from shifting forward dramatically. 100% would feel like too much when talking about braking. (Chris’s thoughts on too much and too little, % range). Anti-Rise is common on low single pivot bikes.
Pedal Kick: As the rear axle moves throughout travel, its distance in relation to the bottom bracket changes. If this changes too dramatically in any one direction, especially rearward, chain “growth” results, tugging the chain and in turn the cranks back so that the pedals “kick” back as well. In other words, the forces are translated the opposite direction, from the suspension through the drivetrain to the rider, resulting in unwanted pedal movement. I think we can all agree that this is a negative. Pedal kick is common with high single pivot bikes.
Instant Center: Many modern designs—and patents—focus on the Instant Center because as long as drive forces are pointed into the IC, the suspension is balanced. The trouble is that when the IC moves through its path during travel, it will only line up with the chainline once —in other words, it’s very sag dependent. That’s why proper sag is so important. And also why you’re going to give up some pedaling efficiency if you like to run your suspension a bit softer (more sag) for better bump compliance and downhill performance.
Finding the IC – The instant center (IC) is an intangible point found by drawing an imaginary line forward through the two pivots on each link until they intersect. This does not move during compression on a classic single pivot design because it isthe pivot. On modern multi-link designs, it constantly changes location as the suspension cycles through its travel.
Center of Curvature: The Center of Curvature is the virtual pivot in multi-link designs.
Finding the CC – As the rear suspension compresses, the rear axle moves along a given path, or arc, relative to the IC. Of course with different suspensions, this path varies greatly and even changes direction throughout travel. But if you were to connect any single point along the rear wheel’s arc path of a multi-link bike to the aforementioned IC with a straight line perpendicular to each point, and overlayed all those lines for any given point in the travel, the area in which those lines intersect is referred to as the Center of Curvature; the intersection of those two constantly changing points. Though on most multi-link designs, that’s a fairly large area, as the CC is constantly migrating, like the IC.
The CC has largely been overlooked as more of a byproduct in suspension design, rather than a focal point—until now.