How to Build a Motorcycle Suspension Shim Stack
A shim stack controls damping by restricting oil flow through a piston. Designing one properly is the difference between controlled suspension and unpredictable behaviour.
Basic Structure
A typical shim stack includes four main components, assembled in order on the valve shaft:
Key Principles
Larger diameter = more leverage
A bigger face shim has a longer lever arm from the valve seat to its outer edge. Oil pressure acts on this larger area, so a bigger shim provides more resistance at the same thickness.
Thicker shim = much stiffer (cube relationship)
Stiffness increases with the cube of thickness. A 0.20mm shim is not twice as stiff as 0.10mm — it's roughly eight times stiffer. Small thickness changes have massive effects on damping.
More shims = more support
Additional backing shims prevent the main stack from deflecting too easily at high velocities. They control the "high-speed" portion of the damping curve without affecting low-speed behaviour much.
You are shaping a force vs velocity curve — not just "making it stiffer or softer". Every shim in the stack affects a different part of the curve.
What You're Really Doing
When you design a shim stack, you're defining how much force the suspension produces at every shaft velocity:
- Low velocities (0.01–0.15 m/s) — controlled by bleed holes, face shim diameter, and initial stack compliance
- Mid velocities (0.15–0.50 m/s) — controlled by the main taper and crossover point
- High velocities (0.50+ m/s) — controlled by support shims and total stack deflection
The goal is a smooth, progressive curve that matches the ideal damping for your rider weight, spring rate, and application.
The Biggest Mistake
Copying stacks without understanding them
Every setup depends on rider weight, spring rate, application (MX, enduro, road race), valve geometry, oil viscosity, and bleed specification. A stack that works perfectly for a 70kg enduro rider on a KTM 350 will be completely wrong for a 95kg motocross racer on a YZ450F. Always design for your specific combination.
Changing too many things at once
If you change the face shim, add a crossover, swap oil weight, and adjust oil level all at once, you have no idea what actually helped. Change one variable at a time and test. Use the calculator to predict the effect of each change individually.
Ignoring stack height
Every shim adds height. If the stack gets too tall, there's insufficient thread on the valve shaft to secure it properly. The nut won't have enough engagement, or it crushes the stack. Always check that your total stack height stays within the valve shaft allowance.
Better Approach: Model First
Instead of building stacks blind, model them digitally:
- Build stacks digitally — enter shim diameters, thicknesses, and quantities
- Test changes instantly — swap a 40mm face for a 38mm and see the curve shift immediately
- See real damping effects — force-velocity curves, not guesswork
- Compare recommendations — three stacks per circuit (baseline, softer, stiffer) calibrated to your specific setup
Build Your Stack Digitally
Enter your shim stack and see the force-velocity curve. Test changes before you rebuild. Get three recommended stacks calibrated to your rider weight and discipline.
Use the Shim CalculatorGet the Handbook
The Suspension Engineer's Handbook covers shim stack theory, design methodology, and practical tuning workflows in detail. A$69.95 PDF download.
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