Basic terms

In this section the phrases used in the software will be detailed. These terms are used by mountain bike magazines, advertisements, essays as well. The equivalents of these terms might be different in other areas. Study this section to understand the theory behind the software.

The program simulates a kinematic structure consisting of a non-deforming main frame, a specific kind of suspension linkage (rear suspension) balanced by a telescopic shock and a telescopic suspension (front fork). The main frame and the rear suspension are virtually connected by a drive train, using a chain and variable size cogwheels. Well, basically this is a full-suspension mountain bike.

Regarding physics, the program uses a kinematic model, so no mass is involved, and the force cases are calculated for a static position of the structure (no acceleration). This is adequate for examining the characteristics of the suspension.

We always call “rear” the parts, which are related to the rear wheel in some way, and “front”, the parts related to the front wheel, respectively.

A glossary of the terms used in the software:

Travel – this means how much the suspension compresses as it works. This movement is measured vertically or on the path of the movement. By default it is interpreted vertically. “Bottom out” means when the suspension reaches its maximum travel and no longer works as a sprung suspension, but a rigid structure.

Axle path – the virtual path drawn by the wheel axle as the suspension works.

Leverage ratio – it is the rate between the amounts of wheel travel and the shock compression. Usually values about 2 or 3 are used on current designs. Larger leverage ratios put higher stress on the shock, and probably cause less sensitive tracing of the terrain.

Progressivity – used in two ways – in a geometrical sense and regarding forces. Geometrical progressivity describes the changing of the leverage ratio throughout the travel. A progressive rate suspension means lower values of the leverage ratio for larger travel. A falling rate suspension is the opposite. A linear rate suspension keeps a constant leverage ratio. These affect the “feel” of the suspension. If we take forces into the account as well, we talk about the “real” progressiveness of the suspension. This represents the changing of the rate between the force needed to compress the suspension to a specific travel position, and the balancing force performed by the shock. Thus, a progressive suspension “stiffens up” at larger travel positions, while a falling rate suspension feels really plush and is easier to bottom out. Bike designers tune their suspension for the desired behavior. Short travel cross-country bikes usually use a progressive setup, while downhill bikes usually use a more linear setup to be able to absorb repeated larger bumps. For “freeride” bikes, a progressive setup is optimal at the end of travel to prevent harsh bottom outs on big jumps.

Sag – most suspensions are designed for an initial travel amount in stationary position, which is caused by the weight of the vehicle and the rider. This has an optimal amount given in a percentage of the total travel, usually 20-25%. See the sag calculation section for more.

Chain growth and pedal-kickback – as bikes utilize chains in their drive train, and it is in connection with the engine (the rider), any changes regarding to the chain feeds back to the rider. Compressing the suspension usually causes the wheel to get farther from the bottom bracket, thus the chain is required to “get longer”. This is caused by the geometry of the suspension linkage, and taken up by some chain-tensioning device (or the rear derailleur). Since most bike have a rear hub with a clutch mechanism, which not allows for free forward rotation, the chain lengthening will cause the hub to turn forward (if it can). So the chain length change can be balanced either by wheel rotation forwards, or the cranks turning backwards or the suspension not moving. In real life, if a rider rides over a bump and the suspension is compressing, he might feel his pedals turning backwards to some extents. Or if he’s strong enough to withstand this, either the wheel will have an “extra” rotation forwards or the suspension will not compress that much, as it would without the chain (or drive train). All of these effects work at the same time in different amounts, degrading suspension performance and rider comfort.

Wheel rotation caused by suspension compression – there is another effect affecting wheel rotation besides chain lengthening. For most designs, suspension compression also makes the wheel contact point with the ground getting more rearwards from the main frame, thus if the wheel is not sliding, the rear wheel will turn backwards. Also, as the suspension is compressed and the wheel's ground contact point remains the same (imagine a stationery position), it is turning backwards compared to the main frame. These wheel rotations cause pedal-kickback by the clutch mechanism and the tensioned chain - at the ratio of the rear and front cogwheels. Thus you will feel less pedal-kickback in larger gears.

You can check these previous effects (chain growth, pedal kickback, wheel backwards rotation) on your bike as well. Just pull the front brake and compress the rear of your bike. The wheel and the cranks will turn backwards. Of course, this also happens when you ride over bumps, just you can't separate this feeling this sharply from other effects.

The pedal-kickback values are shown positive counter-clockwise, since this what you really feel as "kickback". Wheel rotation values are shown positive clockwise, thus a positive value means "forward" rotation.
See more on the topic here.

Instant center – this term is used in suspension theory for linkages – in our case - 4-bar linkages. A 4-bar linkage does not have a constant center of rotation regarding the wheel axle (or any points on that linkage element), instead this point is moving. At a given travel position we can calculate the instant center (IC) of the linkage by making the imaginary intersection of the two outermost linkage bars.
“The path tangent of any point in motion (on the mid linkage part) is perpendicular to the line between the IC and the point (obviously all of this is in a single plane).” (Path Analysis by Ken Sasaki)

Center of curvatureThis is also a moving point in connection with the axle path. For a not circular axle path (4-bar linkages), the axle path has more complex shape. For a specific point of this shape we can determine its current curvature center - regarding the curve as if it was circular at that point. The placement of CC and the distance from the curve itself is specific to the curve. To learn more about this topic, please see Ken Sasaki’s “Path analysis” essay.

This image shows the IC and CC for a 4-bar bike (Rocky Mountain ETS-X, image by Peter Ejvinsson). The right most points are the IC’s, the middle numbered points are the CC’s.

Full-suspension bikes have typical designs that determine the way they behave. The most common types are:

Monopivots – frames with a single pivot and swingarm. Example: SantaCruz Bullit, Cannondale Gemini, Scott Octane etc.

“Seatstay” 4-bars, or Monopivots with a shock linkage – a more complex system, the rear wheel is on the swingarm but the shock is activated through a linkage system. This is useful to achieve a tuned progressivity of the suspension. Example: Kona Stinky, Yeti AS-X, Rocky Mountain RM series etc.

“Horst-link” 4-bars, or “Real” 4-bars – these bikes have the rear wheel attached to a rear linkage part, and have a “Horst-link” pivot on the chainstay (named after Horst Leitner of AMP bikes). These bikes can have a more sophisticated axle-path which is not circular arc, it may have positive effects on pedaling effectiveness and reducing pedal kickback. The so-called "VPP" design (stands for Virtual Pivot Point) is a 4-bar linkage too, just the links are short compared to a Horst-link design.

Trail values – The trail, normal trail, rear normal trail and the rate of the last two - these values give an estimate about the steering behaviour of the bike. Bikes for different uses typically show different trail values. A stable at high speeds downhill bike have bigger values, while a nimble quicker handling cross-country bikes shows smaller trail values. This image shows the interpretation of these values (image by Claudio Bosticco).