Motion

Spring presets, easing and duration tokens, and reduced motion.

Principles

Motion explains cause and effect: what opened, what changed, and where it came from.
Animate transform and opacity. Nothing resizes in a single frame; widths and heights follow on a spring.
Actions repeated many times a minute, like keyboard navigation, do not animate.
Every animation keeps working when interrupted and has a reduced motion branch.

Springs

Springs live in components/arc/lib/motion-tokens.ts and come with every Arc install.

PresetValuesUse it for

spring.snappyvisualDuration: 0.2, bounce: 0Presses, toggles, thumbs, and small indicators.

spring.smoothvisualDuration: 0.3, bounce: 0Panels, height changes, and layout shifts. Never overshoots.

spring.morphvisualDuration: 0.3, bounce: 0Shared highlights, shape changes, and widths that follow content.

spring.responsivestiffness: 520, damping: 44Direct manipulation that must track the pointer closely.

spring.gentlestiffness: 340, damping: 34Larger layout changes that should feel calm.

Easing and duration

CSS transitions use the matching variables from the foundation.

TokenUse it for

--ease-standardDefault for UI transitions.

--ease-enterElements arriving on screen.

--ease-springA gentle settle, paired with --duration-spring.

--duration-fast160ms. Hover, color, and small state changes.

--duration-standard240ms. Panels and menus.

--duration-considered480ms. Rare, introductory reveals.

Using the tokens

toggle.tsx

import { motion, useReducedMotion } from "motion/react";import { motionTokens } from "@/lib/motion-tokens"; export function Thumb({ on }: { on: boolean }) {  const reduce = useReducedMotion();  return (    <motion.span      animate={{ x: on ? 20 : 0 }}      transition={reduce ? { duration: 0 } : motionTokens.spring.snappy}    />  );}

card.module.css

.card {  transition: background var(--duration-fast) var(--ease-standard);}

Reduced motion

When someone asks for reduced motion, keep the end state, focus, and feedback, and remove travel, looping, and parallax. A short opacity change is fine. In CSS:

panel.module.css

@media (prefers-reduced-motion: reduce) {  .panel {    transition: opacity var(--duration-fast) var(--ease-standard);  }}