# Coordinates, units and axes

> The opensus coordinate system for suspension hardpoints: ISO 8855 axes (X forward, Y left, Z up), the origin at the axle line, centreline and ground, millimetres, and how to switch to SAE J670 or CAD axes.

opensus stores every hardpoint in millimetres on ISO 8855 axes: **+X forward, +Y to the left, +Z up**. Each corner's points are measured from **its own axle line** (X), **the car's centreline** (Y) and **the ground** (Z), at the **loaded static position**.

## Origin and axes

| Axis | Positive direction | Zero at |
|---|---|---|
| X | Forward | The axle line of that corner (front points from the front axle, rear points from the rear axle) |
| Y | Left | The car's centreline |
| Z | Up | The ground plane under the car at ride height |

So a front lower ball joint at `[3, 570, 125]` sits 3 mm ahead of the front axle line, 570 mm left of the centreline and 125 mm off the ground. Measuring each axle from its own axle line keeps rear numbers small and readable; the wheelbase on the **Car** panel places the rear axle behind the front one.

## Left and right

A symmetric axle stores only the **left** corner, with positive Y. The right corner is its mirror image. Turn off **Same left and right** on an axle to enter each side separately; right-side points then show their true, negative Y.

## Loaded static position

Points are where they sit **with the car on its wheels at ride height**, driver included. That is where you measure a built car, and where design targets such as static camber apply. opensus then solves each spring's perch so the car holds exactly that position at the mass you entered. Choose **Lock setup** on the Car panel to freeze the perches; the car then settles under load changes like a real car.

## Signs used in results

| Result | Positive means |
|---|---|
| Camber | Top of the wheel leans out (negative camber: top leans in) |
| Toe | Toe-in: the front of the wheel points toward the centreline |
| Wheel travel | Bump: the wheel moves up relative to the body |
| Caster | Steering axis tilts back at the top |
| Kingpin inclination | Steering axis tilts in at the top |
| Scrub radius | The contact patch is outboard of where the steering axis meets the ground |
| Trail | The steering axis meets the ground ahead of the contact patch |
| Anti-dive, anti-lift, anti-squat | The geometry resists that pitch (100 % resists it completely) |

## Other axis conventions and units

Under **Settings**, choose the units (mm or inches, N or lbf, kg or lb, m/s, km/h or mph) and which vehicle direction +X, +Y and +Z point along. Presets cover **ISO 8855**, **SAE J670** (X forward, Y right, Z down), a typical **vehicle CAD** layout and **Y-up**. Fields, charts and exports follow these settings.

Project files and design links always use ISO 8855 in mm, so a design opens the same way whatever settings the next person has. Left-handed axes import into CAD mirrored; opensus warns before you export with them.

Source: https://opensus.app/docs/coordinates/ (last updated 2026-11-07)