Shock absorber performance is a multi-physics problem involving fluid dynamics, shim stack structural stiffness and dynamic suspension motion response. Those are hard problems and must be solved simultaneously as a coupled system to obtain a physics-based description of the shock absorber performance.
Shim ReStackor solves those problems using thorough physics based analysis with simple inputs producing practical results describing suspension performance across the entire range of bump velocities and suspension stroke depths.
Shim stack stiffness
- Clamp diameter
- Stack taper
- Shim thickness
- Crossover gaps
Fluid dynamics
- Flow losses
- Oil properties
- Valve port design
- Ideal damping
Suspension response
- Response tuning
- Spring-mass-damper
- Link ratio
- Bump velocity
Shim stack stiffness
Shim ReStackor uses finite element analysis (FEA) to compute the stiffness of shim stacks. The configuration of the shim stack is defined by a simple listing of shim diameters and thickness for each shim used in the shim stack.
Calculations produce a snapshot of the deflected shim stack structure helping to ensure the calculation inputs match the intended shim stack configuration. The deflected stack structure also indicates the closure of crossover gaps and potential contact of the high-speed stack with the shim stack backing washer.
Shim ReStackor outputs plot the face shim edge lift as a function of fluid force on the shim stack. Edge lift indicates changes in shim stack stiffness as crossover gaps close or stack deflection limitations due to contact with the stack backing washer.
The damping force calculations also indicate the clicker tuning range from closed to wide-open. The clicker tuning range provides a simple real-world reference quantifying shim stack changes in terms of clicker settings and the real-world forces you can actually “feel” when you ride.
Fluid dynamics
The flow resistance and pressure drop across the main piston creates the damping force produced by shock absorbers. The pressure drop also creates the fluid forces acting on the shim stack which drives the stack deflection and flow area gap controlling the flow resistance and ultimately the pressure drop itself. Damping force, stack stiffness, flow area and the valve pressure drop form a tightly coupled interactive system requiring simultaneous solution of all components as a coupled system to obtain an accurate description of the fluid dynamics controlling shock absorber performance.
Clicker bleed circuits, valve port leak jets, valve port throat flow restrictions and port entrance restrictions are additional tunable features of the shock used to control shock absorber performance. Those features are modeled in Shim ReStackor using simple geometric dimensions you can easily measure in your garage. Parametric Shim ReStackor calculations varying those fluid circuit dimensions help guide potential modification of the shock absorber valve configuration to optimize shock absorber performance across the entire range of suspension bump velocities.
Shim ReStackor suspension response
Suspension response calculations combine the shock damping force computed by Shim ReStackor with inputs for spring rate, link ratio, bike weight and rider weight to determine the jump landing impact that bottoms the suspension and the bump impact velocity that bottoms the wheels. The example below shows a bump impact producing a suspension velocity of 350 in/sec bottoms the wheels, approximately equivalent to a 3.8 inch bump hit at 50 mph (more info) and a jump landing impact at 120 in/sec bottoms the chassis, approximately equivalent to a freefall height of 18.6 inches (more info).
Response calculations compute the suspension spring and damping force as the suspension decelerates through the compression stroke. Plush setups produce a peak compression damping force that approximately matches the peak spring force. Matched peak force produces a near constant g-force on the rider with compression damping dropping off as the spring force ramps up to keep the overall force approximately constant through the compression stroke. Constant force allows the suspension to absorb the maximum bump energy while transmitting the minimum g-force to the rider (more info).
Spring-mass-damper theory defines rebound damping zeta values of 0.7 to deliver the fastest possible rebound response with damping that is still stiff enough to suppress suspension resonance providing a clear rebound tuning target. However, linked suspension systems produce a continuously changing motion ratio through the stroke generating a nonlinear spring and damping force. Identifying the effective stroke averaged rebound damping requires integration through the stroke to determine the effective zeta coefficient. Shim ReStackor performs that integration through a series of rebound calculations at progressively deeper stroke depths to determine the rebound damping zeta values over the range of suspension stroke depths as displayed below.
Suspension response calculations:
- Defines bump velocities that bottom the wheels
- Jump landing impacts that bottom the chassis
- Detailed mapping of the variation in spring and damping force through the suspension storke
- Stroke averaged rebound damping zeta values needed for tuning of linked suspension systems
